KR100426605B1 - Switching Power Supply Device Having Series Capacitance - Google Patents

Abstract

High efficiency is achieved with switching power supplies that can reduce switching losses. In addition, through the compact and lightweight transformer, it is possible to lower the number of components and the cost of switching elements of the control circuits. In this switching power supply, one end of a series circuit composed of the primary winding of the transformer, the inductor and the capacitor is connected to the junction of the first switching circuit and the second switching circuit. The other end of the series circuit is connected to an input power source. The secondary winding of the transformer is connected to a rectifying and flattening circuit comprising a rectifying element. Also, in order to perform self oscillation, first and second control circuits are arranged. The first control circuit controls the elapsed time until the first switching circuit is turned on after the generation of the voltage in the first drive winding of the transformer and the elapsed time until the first switching circuit is turned off. Similarly, the second control circuit controls the elapsed time from the generation of the voltage at the second drive winding of the transformer to the turn-on of the second switching circuit and the elapsed time until the turn-off of the second switching circuit.

Description

Switching Power Supply Device Having Series Capacitance

The present invention relates to a switching power supply for supplying a DC stabilization voltage. More specifically, the present invention provides a switching power supply that stores energy in the primary winding and capacitor of a transformer during the on-period of the switching element and supplies the stored energy to the load from the secondary winding of the transformer during the off-period of the switching element. It is about.

Japanese Patent Application No. In 9-352696, a first switching circuit is connected to a second switching circuit on the primary winding side of transformer T, and switching control circuits are provided before and after the period in which the two switching elements are turned off. Disclosed is a switching power supply in which self-excited oscillation is performed by alternately turning on and off switching elements included in second switching circuits. 1 is a block diagram showing the switching power supply.

In the switching power supply, the input power source E, the inductor L and the first switching circuit S1 are connected in series with the primary winding T1 of the transformer T. In addition, a series circuit composed of the primary winding T1 and the inductor L is connected in parallel to the series circuit composed of the capacitor C and the second switching circuit S2. The first drive winding T3 generates a voltage that is approximately proportional to the voltage of the primary winding T1. The voltage of the first drive winding T3 is input to the control circuit 11. Similarly, the second drive winding T4 generates a voltage that is approximately proportional to the voltage of the primary winding T1. The voltage of the second drive winding T4 is input to the control circuit 12. The voltage of the control circuit 11 is input to the control terminal of the first switching element Q1 of the first switching circuit S1. The voltage of the control circuit 12 is input to the control terminal of the second switching element Q2 of the second switching circuit S2. The first switching circuit S1 is formed by a parallel connection circuit composed of the first switching element Q1, the first diode D1, and the first capacitor C1. The second switching circuit S2 is formed by a parallel connection circuit composed of the second switching element Q2, the second diode D2, and the second capacitor C2.

The rectifying element Ds is connected in series with the second winding T2 of the transformer T. The rectifying and planarizing circuit is composed of a rectifying element Ds and a capacitor Co connected to the output of the rectifying element Ds. The rectifying element Ds is connected in parallel to a capacitor (capacitive impedance Cs). A detection circuit 14 for detecting the voltage of the load is connected between the output of the rectification and flattening circuit and the load. The output feedback of the detection circuit 14 is sent to the first control circuit 11.

In addition, U.S. Patent No. 3,596,165 discloses a switching power supply in which two switching circuits are connected to each other on the primary winding side of a transformer to perform separate-excited oscillation and a full-wave rectifier is connected to the secondary winding. It is starting.

Japanese Unexamined Patent Application Publication No. 5-328719 and No. 11-136940 also discloses a switching power supply. In each device, two switching circuits are connected to each other in the primary winding of the transformer and the secondary winding is formed in the circuit configuration shown in FIG. In this case, the inductor and the capacitor are connected in series with the primary winding. The second switching element is connected in parallel to the series circuit.

However, each of the switching power supplies described above has the following problems.

(1) US Patent No. 3,596,165

The switching power supply is a circuit referred to a resonant half-bridge circuit (ON-ON type). In this type of circuit, when each switching element is turned on, energy is transferred from the primary winding to the secondary winding. The on-time of each switching element is almost fixed and the switching frequency is changed to change the impedance of the LC resonant circuit connected in series with the primary winding. In other words, when the LC resonant frequency and the switching frequency are close to each other, the impedance of the LC resonant circuit becomes smaller, so that a large current flows in the transformer, thus obtaining a large output power. On the contrary, when the LC resonant frequency and the switching frequency are far from each other, a small output power can be obtained. With this configuration, depending on the output power, the switching frequency changes considerably. If the frequency changes significantly, the output planarization circuit and the filter circuit are enlarged. As a result, problems such as an increase in interference to electronic components and loss of control circuits occur.

In addition, since the switching power supply is an oscillating type device, the number of components increases, which hinders the miniaturization of the device and the cost reduction. Therefore, at least two diodes are required on the secondary side of the transformer in order to perform full-wave rectification.

(2) Japanese Unexamined Patent Application Publication No. 5-328719 and Japanese Unexamined Patent Application Publication No. 11-136940

Each switching power supply disclosed in the publication has an on-off switching power supply in which energy is stored in the primary winding during the on-time of the switching elements and the stored energy is released into the secondary winding during the off-time of the switching elements. to be. However, each of these devices is not oscillating but is oscillating or synchronous oscillation. Therefore, since the apparatus requires an oscillator, a driving circuit, or the like, the number of parts is increased, which hinders the miniaturization and cost reduction of the apparatus. Japanese Unexamined Patent Application Publication No. In 5-328719, since a synchronous oscillator circuit is used, no oscillator is required. Nevertheless, the power supply includes an IC including a MOS-FET having high voltage breakdown characteristics to drive a high-side switching element. In addition, a pulse transformer for driving with insulation is required. As a result, also in the said switch power supply apparatus, the size of a switching control circuit and manufacturing cost increase.

(3) Japanese Patent Application No. 9-352696

The switching power supply provided in this publication is an on-off type that stores energy in the primary winding during the on-time of the first switching circuit and releases the stored energy from the secondary winding during the off-period of the first switching circuit. It is a self-oscillating type of switching power supply. As shown in FIG. 1, since a voltage equal to the sum of the input voltage Vin and the capacitor voltage Vc is applied to the switching element, the switching element requires a high breakdown voltage element. In addition, since the power supply device has a configuration in which an input voltage Vin is directly applied to the primary winding T1 of the transformer T, the voltage applied to the primary winding T1 becomes higher, which leads to a smaller size of the device. Disturbs.

In addition, only excitation energy stored in the primary winding of the transformer is output to the secondary side of the transformer. The energy of the capacitor C is not output to the secondary side. As a result, the peak current value of the primary winding becomes large, and the conduction loss increases.

1 is a view schematically showing the configuration of a conventional switching power supply.

2 is a circuit diagram of a switching power supply according to an embodiment of the present invention.

3 shows waveforms of the present switching power supply.

4A to 4B are diagrams illustrating primary current waveforms obtained by various control methods.

Fig. 5 is a diagram showing a circuit configuration of main parts of the switching power supply.

6 is a diagram illustrating a circuit configuration of a main part of a switching power supply according to another embodiment.

FIG. 7 is a diagram illustrating a circuit configuration of a main part of a switching power supply according to still another embodiment.

8 is a circuit diagram illustrating a first control circuit of a switching power supply according to still another embodiment.

9 is a circuit diagram illustrating a second control circuit of a switching power supply according to still another embodiment.

10 is a circuit diagram illustrating a second control circuit of a switching power supply according to still another embodiment.

11 is a circuit diagram illustrating a startup circuit of a switching power supply according to still another embodiment.

Accordingly, an object of the present invention is to provide a switching power supply having high efficiency and low loss. In addition, in the present device, the stress of the switching element can be reduced and the size and weight of the transformer can be reduced.

In order to solve the above problems, according to an aspect of the present invention, the first switching circuit consisting of a parallel connection circuit of the first switching element (Q1), the first diode (D1) and the first capacitor (C1), and A second switching circuit comprising a parallel connection circuit of a second switching element Q2, a second diode D2, and a second capacitor C2, wherein the first and second switching circuits form a series circuit, and A power source is connected to the series circuit, the transformer T comprises a primary winding and a secondary winding, the primary winding, the leakage inductor L and the capacitor C constitute a series circuit, and the series circuit One end of is connected to a connection point of the first switching circuit and the second switching circuit, the other end of the series circuit is connected to the input power supply, the rectifying and planarizing circuit comprises a rectifying element (Ds), the rectifying and The planarization circuit is connected to the secondary winding of the transformer T. Connected, energy is accumulated in the primary winding and the capacitor C during the on-period of the first switching element Q1 and from the secondary winding during the off-period of the first switching element Q1. Obtaining an output, controlling an on-time of the first switching element Q1 to control an output power, and wherein the first driving winding included in the transformer T turns on the first switching element Q1. Generate a voltage that is approximately proportional to the voltage of the primary winding, and a second drive winding included in the transformer generates a voltage that is approximately proportional to the voltage of the primary winding that turns on the second switching element Q2, Switching control circuits alternately turn on and turn off the first and second switching elements Q1 and Q2 before and after a period in which the switching elements Q1 and Q2 are both turned off, and the first switching The element Q1 is the second switching element Q2 and The switching power supply is turned on after all of the rectifying elements Ds are turned off, and as a result, self-oscillation is performed.

With the above configuration, the following advantages can be obtained.

(1) Since a voltage is applied to each of the first and second switching elements Q1 and Q2, semiconductor elements having a low voltage rating can be used as the switching elements Q1 and Q2. For example, the on-resistance of a typical MOS-FET increases approximately in proportion to the square of the withstand voltage. However, if a switching element with a low voltage rating is used, the on-resistance is small and the conduction loss can be reduced. Also, devices with low voltage ratings are typically inexpensive. Thus, by reducing the heat generation of the switching elements, the entire switching power supply can have high efficiency and can be produced at low cost, and its weight and size are also reduced.

(2) The voltage applied to the primary winding of the transformer T is approximately half of the voltage of the conventional switching power supply shown in FIG. As a result, the number of turns of the primary winding can be reduced and the core gap can be made small. In addition, the transformer T having the desired voltage breakdown properties can be easily designed, and the transformer can be miniaturized.

(3) Since the switching elements Q1 and Q2 of the first and second switching circuits are connected in parallel to the diode and the capacitor, the switching elements Q1 and Q2 are turned on at zero voltage and the switching element Q2 ) Is turned off at zero current. As a result, switching losses can be greatly reduced and heat generation can be prevented.

(4) The secondary rectifier Ds is turned on at zero current and the current waveform rises relatively rapidly from zero current to reach a peak point at which the rate of current change is zero. Thereafter, the current waveform falls back to zero current when the rectifying element Ds is turned off. Compared with the conventional inverted triangle waveform, the waveform is similar to a rectangle, so that the peak current value can be lowered. As a result, the effective current value is small and the conduction loss can be reduced.

(5) No insulation is required when using pulse transformers or photocouplers. In the present invention, two switching elements Q1 and Q2 having different ground levels can be driven. In addition, since the switching elements Q1 and Q2 have a self-oscillating configuration, it is not necessary to use a control IC including another oscillator. Therefore, since the switching elements are not complicated configurations, the entire apparatus can be miniaturized at low cost.

In the switching power supply according to the present invention, each of the switching control circuits may include a delay circuit formed by a series circuit composed of a resistor or a resistor and a capacitor, wherein the resistor or delay circuit comprises the first driving winding and the first driving winding. And between the control terminals of the switching element and between the second drive winding and the control terminal of the second switching element, respectively. In this switching power supply, each of the first and second switching elements has a voltage in the first and second drive windings that is approximately equal to the voltage of the primary winding that turns on the first and second switching elements. After it is turned on with a delay.

Therefore, before and after the period in which the two switching elements Q1 and Q2 are both turned off, the switching elements Q1 and Q2 may be alternately turned on and off. With this configuration, it is possible to prevent an increase in damage and breakage caused by simultaneously turning on the two switching elements Q1 and Q2.

Further, in the present switching power supply, any one of the switching control circuits is approximately equal to the voltage of the switching device for turning off the first switching element and the primary winding for turning on the first switching element in the first drive winding. And a time constant circuit controlling the first switching element by the switching device after the predetermined voltage has elapsed and a predetermined time has elapsed.

Since the switching speed of the first switching element Q1 can be increased by the switching device which turns off the first switching element Q1, the switching loss caused by the switching element Q1 can be reduced. In addition, by the time constant circuit for setting the on-time of the switching element Q1, the on-time of the switching element Q1 may be set or controlled to stabilize the output voltage.

Further, in the present switching power supply, the switching control circuit is a voltage that is approximately proportional to the voltage of the switching device for turning off the second switching element and the primary winding for turning on the second switching element in the second drive winding. This may include a time constant circuit for controlling the second switching element to be turned off by the switching device after a predetermined time has elapsed.

Similar to the above case, since the switching speed of the second switching element Q2 can be increased, the switching loss caused by the switching element Q2 can be reduced. In addition, by the time constant circuit for setting the on-time of the switching element Q2, the on-time of the switching element Q2 may be arbitrarily set or controlled to stabilize the output voltage.

In addition, in the switching power supply of the present invention, the switching device may be composed of a transistor connected to the first switching element or the second switching element, and the control terminal of the transistor is connected to the first impedance circuit and the charge / discharge capacitor. It can be connected to the configured time constant circuit.

Thus, there is no need to use a MOS-FET or IC having a high voltage rating to drive the high-side switching element Q2. With the simple configuration including the transistor and the time constant circuit, the switching element Q2 can be driven. Thus, the size and weight of the switching power supply of the present invention can be reduced, and the device can be manufactured at low cost.

Further, in the switching power supply according to the present invention, the impedance of the first impedance circuit forming the time constant circuit is changed in accordance with the output power or in response to an external signal.

Depending on the output power or in response to a signal from the outside, the impedance value of the impedance circuit constituting the time constant circuit is changed. With this configuration, the time for charging and discharging the capacitors included in the time constant circuits is changed. As a result, it is possible to control the on-times of the respective switching elements Q1 and Q2 so that the switching elements Q1 and Q2 perform the switching operation at the optimum on-time according to the output voltage.

In addition, the switching power supply may further comprise a second impedance circuit comprising a resistor, which is connected to both ends of the capacitor (C) or the primary winding of the capacitor (C) and the transformer (T). It is connected to both ends of the series circuit consisting of a, characterized in that for applying an input voltage to the first switching circuit via the second impedance circuit.

By connecting an impedance circuit comprising a resistor across the capacitor C or across the series circuit consisting of the primary winding of the capacitor C and the transformer T, the starting voltage is switched by means of the first switching circuit through the impedance circuit. May be applied to the circuit. Without the impedance circuit, since the input voltage is applied to the capacitor C, oscillation may not start even if a voltage is applied to the control terminal of the switching element Q1. The impedance circuit can be connected at both ends of the series circuit consisting of the primary winding of the capacitor C and the transformer T. However, it is preferable that the impedance circuit is connected to both ends of the capacitor (C). Because this arrangement lowers the voltage applied to the impedance circuit, the loss can be further reduced.

In addition, the switching power supply further includes a third impedance circuit including a resistor for dividing an input voltage applied to the switching circuit through the second impedance circuit and applying it to the control terminal of the first switching element to start oscillation. can do.

In this case, the voltage applied to the first switching circuit is divided by the third impedance circuit and applied to the control terminal of the switching element Q1 to start oscillation. At this time, the voltage-dividing resistors are not connected to the input power source but to the first switching circuit. As a result, oscillation can be started only when a voltage is applied to the first switching circuit. This prevents starting failures.

Also, since it becomes unnecessary to arrange the one-shot pulse generation circuit for starting the oscillation, the switching control circuits can be simplified. Therefore, the whole apparatus can be miniaturized and manufactured at low cost.

In addition, the switch power supply may further include a capacitor (Cs) connected in parallel to the rectifying element (Ds), the capacitive impedance value of the capacitor (Cs) is the second switching element (Q2) and the rectifying element ( When all of Ds) is turned off, the capacitor Cs resonates with the inductor of the transformer T so that the voltage waveform across the capacitor Cs exhibits the same waveform as that of a sine wave so that it rises at zero voltage or falls at zero voltage. Is set.

In the on-time of the switching element Q1, the charge accumulated in the capacitor (or capacitive impedance element Cs) can be output without flowing to the rectifying element Ds when the conduction of the rectifying element Ds starts. Therefore, the conductive loss of the rectifying element Ds can be reduced. In addition, when the reverse recovery loss of the rectifying element Ds is reduced and the sudden voltage change is controlled, the noise can be reduced. In addition, since the waveform of the current flowing through the rectifying element Ds rises rapidly and the current waveform is similar to the rectangular waveform, the effective current can be reduced.

Further, in the present invention, the rectifying element Ds may be a switching element that performs switching with a control signal.

In this case, for example, the rectifying element Ds is not composed of a general diode but is composed of a switching element such as a MOS-FET having a small on-resistance. When such a switching element switches with a control signal, the conduction loss in the on-time of the switching element can be reduced, and the conduction loss caused by the secondary rectifying element can be reduced.

In addition, in the switching power supply of the present invention, the switching element may be a field-effect transistor.

When the first or second switching element is a field-effect transistor such as a MOS-FET, parasitic diodes and parasitic capacitors can be used. Therefore, when the parasitic diode is used as the first diode (D1) or the second diode (D2) and the parasitic capacitor is used as the first capacitor (C1) or the second capacitor (C2), the diode (D1) or diode (D2) and No capacitor C1 or capacitor C2 is needed. Thus, the number of component parts can be reduced.

The switching power supply also includes at least one of a leakage inductor L of the transformer T and an external inductor L connected in series with the primary winding, the inductor L of the first switching element Q1. During the off-period, it resonates with the capacitor C causing the waveform of the current flowing in the primary winding to be part of the sinusoidal wave.

The inductor L resonates with the capacitor C in the off-period of the first switching element Q1 so that the waveform of the current flowing in the primary winding becomes part of the sinusoidal wave. As a result, the peak current value of the switching element Q2 and the peak value of the current flowing through the rectifying element Ds become smaller, and the zero current turn-off operation of the switching element Q2 can be performed. In addition, when the leakage inductor L of the transformer T is used as the inductor L, the external inductor L is not necessary. Thus, the number of components can be reduced, and the energy loss due to leakage inductance of the transformer can be reduced.

Further, in the present switching power supply, the switching control circuit causes the first or second switching element to be turned on after the voltage of the first or second capacitor drops to zero or near zero.

The zero voltage switching operation is performed by setting the delay time in such a manner that the switching control circuit turns on the switching element Q1 or Q2 after the voltage across the first or second capacitor drops to zero or around zero. With this configuration, the turn-on loss can be reduced and switching noise can be prevented.

In addition, when the current flowing in the second switching element Q2 drops to zero or near zero, the switching control circuits may turn off the second switching element Q2.

With this configuration, the switching element Q2 performs a zero-current turn off operation, so that switching losses and switching surges occurring when the switching element is turned off can be reduced.

Further, in the present invention, the capacitor so that the waveform of the current flowing through the rectifying element (Ds) rises from zero to reach a peak point at which the current change rate becomes zero, and then the waveform falls to the zero current at which the rectifying element (Ds) is turned off again. The values of (C) and inductor (L) can be set.

Since the peak current value of the current flowing through the rectifying element Ds is lowered and its waveform is similar to the rectangular waveform, the effective current is reduced and the conduction loss of the rectifying element Ds is reduced. In addition, since the current flowing through the rectifying element Ds does not change rapidly, the generation of switching noise is suppressed and the rectifying element Ds is turned off at zero current, thereby reducing the reverse recovery loss.

The switching control circuit also controls in such a way that the ratio of the excitation amount in the reverse direction of the transformer to the excitation quantity in the forward direction of the transformer changes in accordance with the magnitude of the load connected to the output terminal of the rectifying and flattening circuit. .

By changing the on-time of the switching element Q1, the output voltage of the rectifying and flattening circuit is controlled to provide a stabilized output voltage to the load. Further, for example, while the on-time of the switching element Q2 is almost fixed, the ratio of the reverse excitation amount and the forward excitation amount changes according to the magnitude of the load connected to the output of the rectification and flattening circuit. With this configuration, the change in the switching frequency can be suppressed, so that interference with the electronic device can be prevented and the loss in the control circuit can be reduced.

The switching control circuit also controls in such a way that the excitation amount in the reverse direction of the transformer is zero or a predetermined fixed value regardless of the magnitude of the load connected to the output terminal of the rectifying and leveling circuit.

In the present switching power supply, the stabilized output voltage can be supplied to the load by changing the on-time of the switching element Q1 which controls the output voltage of the rectifying and flattening circuit. In addition, the on-time of the switching element Q2 is controlled in such a way that the amount of excitation in the reverse direction of the transformer becomes zero or a predetermined fixed value irrespective of the magnitude of the load connected to the output terminal of the rectifying and leveling circuit. With this configuration, conduction losses in the transformer and the switching circuit due to the regeneration of the current can be reduced.

In addition, one of the switching control circuits sets the on-time of the switching element to a minimum value or more, so that switching can be performed even when the load connected to the output terminal of the rectifying and leveling circuit is shorted.

In this case, the switching operation can be continued even in the state of the short circuit by setting the on-time of the switching element to the minimum value so as to perform the switching even in a state where the load connected to the output terminal of the rectifying and leveling circuit is shorted. . Thus, when the short circuit condition is removed, the output is automatically applied back to the load. In this way, an auto-recovery overcurrent protection circuit capable of recovering the output can be constructed. When the on-time below the minimum value is set, under a short load, an input voltage is applied to the capacitor C and oscillation stops, so that a latch type overcurrent protection circuit is constructed.

2 is a circuit diagram of a switching power supply according to an embodiment of the present invention.

Basically, the switching power supply of this embodiment includes a series circuit formed by connecting a primary winding T1, an inductor L, and a capacitor C of a transformer T, and one end of the series circuit is It is distinguished from the conventional switching power supply in that it is connected to the connection point of a 1st switching circuit and a 2nd switching circuit, and the other end of the said series circuit is connected to an input power supply. Hereinafter, a circuit configuration of the switching power supply device will be described in detail.

The first switching circuit S1 includes a parallel connection circuit of the first switching element Q1, the first diode D1, and the first capacitor C1. The second switching circuit S2 includes a parallel connection circuit of the second switching element Q2, the second diode D2, and the second capacitor C2. The first and second switching circuits S1, S2 are connected in series with each other, and the series circuit is connected in parallel with the input power source E. FIG. The first and second switching elements Q1 and Q2 used in this embodiment are field-effect transistors (hereinafter referred to as FETs).

The primary winding of the transformer T is connected in series to the inductor L and the capacitor C. One end of the series circuit is connected to the connection point of the first switching circuit S1 and the second switching circuit S2, and the other end of the series circuit is connected to the input power source E.

The drive winding T3 of the transformer T generates a voltage approximately proportional to the voltage of the primary winding T1. The voltage generated in the drive winding T3 is input to the first control circuit 11. The first control circuit 11 has a delay formed by connecting a resistor R3 and a capacitor C3 disposed in series between the first driving winding T3 and the control terminal gate of the first switching element Q1. A circuit for receiving a delay circuit, a transistor Tr1 which is a switching device for turning off the first switching element Q1, and a feedback signal from the detection circuit 14; And a time constant circuit composed of a photo-coupler (PC) and a capacitor (C4), which are one impedance circuits. The time constant circuit is connected to the control terminal base of the transistor Tr1. The control circuit 11 turns the first switching element Q1 ON after a delay after the voltage is generated in the first driving winding T3. In addition, the control circuit 11, after the voltage is generated in the first drive winding (T3), when the time set by the time constant circuit composed of the impedance of the photo-coupler (PC) and the capacitor (C4) has elapsed, the transistor The first switching element Q1 is turned off quickly by turning on (Tr1). In this way, the control circuit 11 may randomly change the on-time of the first switching element Q1.

The transformer T comprises a second drive winding T4. The voltage generated in the second drive winding T4 is applied to the second control circuit 12. The second control circuit 12 turns on the delay circuit formed by connecting the resistor R5 and the capacitor C5 connected in series to the second driving winding T3 in series, and the second switching element Q2. Transistor Tr2, which is a switching device for turning off, and a time constant circuit composed of a resistor R6 and a charge / discharge capacitor C6, which are first impedance circuits. The time constant circuit is connected to the control terminal base of the transistor Tr2. The delay circuit, the first impedance circuit, and the transistor Tr2 included in the second control circuit are similar to the configuration of the first control circuit described above.

In each of the control circuits 11 and 12, the delay time is the manner in which the switching elements Q1 and Q2 are turned on after the voltage across each of the capacitors C1 and C2 drops to zero or near zero. Is set. In this arrangement, the zero voltage switching operation is performed. As a result, turn-on loss is reduced, and the occurrence of switching noise is prevented. In addition, the control circuit 12 controls the second switching element Q2 to turn off when the current flowing through the second switching element Q2 becomes zero or near zero. Through this control, switching element Q2 performs a zero-current turn-off operation, thereby reducing switching losses and switching surges that occur when the element Q2 is turned off. In addition, the closer the waveform of the current flowing through the rectifying element Ds to the rectangular form, the smaller the loss in the rectifying element Ds. Therefore, in order to obtain such a waveform, the values of the capacitor C and the inductor L and the ON-period of the second switching element Q2 set by the switching control circuit are determined.

The detection circuit 14 includes a voltage dividing resistor R9 and R10 and a shunt regulator IC1 having a connection point of the resistors R9 and R10 connected to a reference voltage input terminal Vr. And a photodiode (PC) connected in series with the shunt regulator IC1. The shunt regulator IC1 controls the current flowing between the cathode and the anode in order to keep the voltage of the reference voltage input terminal Vr constant. This change in current is converted into the intensity of light of the photodiode PC and input to the phototransistor PC connected to the first drive winding T3 of the transformer T. In such a circuit, in accordance with the change of the current flowing in the photodiode PC, the turn-on time of the transistor Tr1 is controlled through the phototransistor PC, and consequently the on-time of the first switching element Q1. This is controlled. Specifically, when the output voltage increases and the current of the photodiode PC starts to increase, the on-time of the first switching element Q1 becomes shorter, thereby lowering the output voltage. On the contrary, when the output voltage decreases and the current of the photodiode PC begins to decrease, the on-time of the first switching element Q1 becomes longer, thereby increasing the output voltage. In this operation, the output voltage can be stabilized.

A resistor R1, which is a second impedance circuit, is connected in parallel to a capacitor connected in series to the primary winding T1 of the transformer T. By connecting the resistor R1 in parallel to the capacitor C, when the power is turned on, a starting voltage can be applied to the first switching circuit S1 through the resistor R1. If the resistor R1 is absent, the input voltage Vin is applied to the capacitor C. In this case, even if a voltage is applied to the control terminal gate of the first switching element Q1, current cannot flow through the transformer. Thus, oscillation cannot be started. The resistor R1 may be connected across the series circuit consisting of the capacitor C, the primary winding T1 of the transformer T and the inductor L. However, as shown in this embodiment, the resistor R1 is connected in parallel to the capacitor C, so that the voltage applied to the resistor R1 is lowered, and thus the loss is reduced.

The voltage applied to the first switching circuit S1 through the resistor R1 is a third impedance circuit connected to both ends of the first switching element Q1, and is a series circuit composed of a resistor R2 and a resistor R7. The voltage is divided by and applied to the control terminal gate of the first switching element Q1. In this arrangement, when the input voltage Vin is applied, self-excited oscillation can be started. In this embodiment, the resistor R2 is connected to the first switching circuit S1 without connecting to the input power source. By this connection, only when the voltage is applied to the first switching circuit S1, the voltage can be input to the control terminal of the switching element Q1 to start oscillation. As a result, start-up failure can be prevented.

Next, the operation of the switching power supply will be described.

FIG. 3 shows the waveform of the circuit shown in FIG. 2. Hereinafter, the operation of the circuit will be described with reference to FIGS. 2 and 3.

In FIG. 3, Q1 and Q2 represent signals showing ON and OFF times of the switching elements Q1 and Q2, and Vds1, Vds2 and Vds represent waveform signals of voltages applied to the capacitors C1, C2 and Cs. , id1, id2 and id represent the current waveform signals of the switching circuit S1, the switching circuit S2 and the rectifying element Ds.

The switching performed after the circuit is started can be divided into four operating states, mainly from time t1 to t5, in one switching period Ts. First, the state at the time of start-up (when oscillation starts) is demonstrated, and next, the remaining state is demonstrated.

Start up

When the input voltage Vin is applied, the voltage is applied to the drain of the first switching element FET Q1 via the resistor R1, the inductor L, and the primary winding T1. The input voltage is divided by the resistor R2 and the resistor R7 and applied to the gate of the FET Q1. When the voltage is greater than the threshold voltage of the FET Q1, the FET Q1 is turned ON and an input voltage is applied to the capacitor C and the transformer T1. Next, a voltage is generated in the first driving winding T3, and the generated voltage is applied to the gate of the FET Q1 through the resistor R3 and the capacitor C3. As a result, the FET Q1 is turned ON.

Next, four operating states from the time t1 to t5 in one switching period Ts under the optimum rated condition from the state where the FET Q1 is turned ON will be described.

State 1-t1 to t2

The FET Q1 is in an on state. A voltage obtained by subtracting the voltage of the capacitor C from the input voltage Vin is applied to the primary winding T1 of the transformer T. Next, the current flowing in the primary winding increases linearly so that excitation energy is stored in the transformer T. In addition, the capacitor C is charged by this current, so that the electrostatic energy is stored in the capacitor C. FIG.

At this time, the capacitor C4 is charged through the phototransistor PC. When the voltage of the capacitor C4 reaches the threshold voltage (about 0.6 V) of the transistor Tr1, the transistor Tr1 is turned on and the FET Q1 is turned off at time t2. State 2 is followed.

State 2-t2 to t3

When FET Q1 is turned off, primary winding T1 and inductor L resonate with capacitor C1 and capacitor C2, so that capacitor C1 is charged and capacitor C2 is discharged. . On the secondary side, secondary winding T2 resonates with capacitor Cs, so that capacitor Cs is discharged. The curve of the rising portion of Vds1 and the curve of the falling portion of Vds2 are part of the sine wave generated by the resonance of the inductor L, the primary winding T1, and the capacitors C1, C2.

When the voltage Vds2 of the capacitor C2 drops to zero, the diode D2 becomes conductive. The voltage generated in the driving winding T4 is slightly delayed after the turn-off of the FET Q1 and is applied to the gate terminal of the switching element Q2 via the capacitor C5 and the resistor R5, and the switching element Q2. ) Is on. As a result, zero-voltage switching is performed and goes to state 3.

At this time, on the secondary side, the voltage Vs of the capacitor Cs drops to zero and the rectifying element Ds is conducted so that a zero-voltage turn-on operation is performed. The curve of the rising portion of Vs is part of the sinusoidal wave caused by the resonance between the capacitor Cs and the secondary winding T2.

State 3-t3 to t4

In state 3, on the primary side, diode D2 or switching element Q2 is turned on and inductor L and capacitor C start to resonate with each other. In this period, the capacitor C is discharged. At this time, on the secondary side, the rectifying element Ds is turned on to discharge the excitation energy stored in the transformer T and the electrostatic energy stored in the capacitor C from the secondary winding T2, and the rectification / flat circuit Output via (rectifying / smoothing circuit). At this time, the waveform of the current (is) flowing through the rectifying element (Ds) linearly decreases the excitation current (excitation current) from the value of the resonance current (id2) generated by the inductor (L) and the capacitor (C) on the primary side is similar to a waveform representing a value obtained by subtracting the value of im). Thus, the waveform rises relatively rapidly from zero to become a waveform having a sinusoidal curve. Next, after reaching the peak point at which the rate of change of current reaches zero, the waveform falls to zero current. When the exciting current im of the transformer T becomes zero, the rectifying element Ds performs a zero-current turn-off operation, so that the secondary side current is zero.

On the primary side, the discharge of the capacitor C reverses the direction of the excitation current im, and excites the transformer T in the direction opposite to that in the state 1. The voltage generated in the secondary drive winding T4 is charged to the capacitor C6 through the resistor R6. When the voltage reaches the threshold voltage (about 0.6V), transistor Tr2 is turned on and FET Q2 is turned off near zero current at time t4, thereby performing a zero-current turn-off operation. When FET Q2 is turned off, a reverse voltage is applied to the secondary rectifier diode so that capacitor Cs starts to resonate. As a result, the winding voltage of the transformer begins to reverse.

At this time, there is a difference in the order of the time when the exciting current im is zero and the time when the FET Q2 is turned off, depending on the magnitude of the load connected to the output terminal. In other words, when the load is small, the FET Q2 is turned off after the exciting current im becomes zero, and the reverse voltage is applied to the rectifying element Ds. In contrast, under heavy load, the excitation current im becomes zero and the reverse voltage is applied to the rectifying element Ds after the FET Q2 is turned off. Under any load condition, the reverse voltage is applied to the rectifying element Ds at the time t4 when both the FET Q2 and the rectifying element Ds are turned off, and the state t4 is shifted.

State 4-t4 to t5

In state 4, the secondary winding T2 of the transformer T resonates with the capacitor Cs, whereby the capacitor Cs is charged. On the primary side, primary winding T1 and inductor L resonate with capacitor C1 and capacitor C2 so that capacitor C1 is discharged and capacitor C2 is charged.

When the voltage Vds1 of the capacitor C1 drops to zero, the diode D1 becomes conductive. At this time, the voltage generated in the primary driving winding T3 is slightly delayed through the resistor R3 and the capacitor C3 and applied to the gate of the switching element Q1. Next, at time t5 FET Q1 is turned on and a zero-voltage switching operation is performed to end state 5. On the secondary side, the voltage Vs of the capacitor Cs rises from zero and is fixed to the sum of the secondary winding voltage and the output voltage.

The above-described operation will be performed for every switching period and thus repeated for a series of switching periods.

In the above operation, while the first switching element Q1 is in the on-state, the excitation energy is stored in the primary winding T1 of the transformer T and the electrostatic energy is stored in the capacitor C. When the switching element Q1 is turned off, the excitation energy and the electrostatic energy are released. As a result, in the conventional switching power supply shown in FIG. In comparison, the current peak value can be lowered and the conduction loss can be reduced.

In the switching power supply shown in Fig. 2, as in the case of the conventional switching power supply, the switching element Q1 and the switching element Q2 are turned on at zero voltage and the switching element Q2 is turned off near the zero current. do. Thus, switching losses and switching surges can be significantly reduced. In addition, the secondary rectifier Ds is turned on at zero current, and its current waveform rises relatively rapidly from zero current. After the waveform reaches the peak point at which the current rate of change becomes zero, the waveform falls back to zero current to turn off the rectifying element Ds. Therefore, the waveform of the current flowing through the rectifying element becomes a rectangular shape, thereby reducing the peak current value. As a result, since the effective current value becomes small, the conduction loss can be reduced.

In addition, since the leakage inductor L of the transformer is arranged, no switching surge occurs. Thus, a semiconductor device having a low voltage rating can be used. In addition, since sudden changes in current and voltage flowing through the switching element are reduced, switching noise can also be reduced.

Regarding the control of the on / off-time of the switching elements Q1 and Q2 by the control circuit, the following three methods can be applied.

4A-4C show the waveform of "id1" obtained by the three control methods.

In the method shown in FIG. 4A, the switching control circuit controls the on-times of the switching elements Q1 and Q2, and zeros or zero-voltage switching operation an excitation quantity in the reverse direction of the transformer T. The output voltage is stabilized by setting it to the minimum value necessary to realize. This method changes the switching frequency by fixing the ratio of the on-time Ton and the off-time Toff of the switching element Q1 in accordance with the amount of load. Therefore, since the amount of load is approximately inversely proportional to the switching frequency, the smaller the load, the higher the switching frequency. For example, the control of the output voltage Vo can be performed during the on-time of the switching element Q1. In addition to the output voltage Vo, the output current Io may also be detected to set the conduction time of the switching circuit S2 equal to the sum of the reset time of the transformer T and a predetermined reverse excitation time. .

In the method shown in FIG. 4B, at light load, regenerative current is generated on the primary side of transformer T. The symbol Ton2 represents the time when the regenerative current is generated. In this method, the switching control circuit constantly controls the on-time of the switching element Q2 and controls the on-time of the switching element Q1 so that the excitation amount in the forward direction and the reverse excitation in the transformer T is reversed. By varying the ratio between the quantities, the output voltage is stabilized. This method almost fixes the switching frequency, regardless of the amount of load. For example, in this type of control, the conduction time of the switching circuit S2 can be set to be equal to the sum of the maximum transformer-reset time and the predetermined reverse excitation time.

On the other hand, this method has the advantage that the switching frequency is almost fixed, but even at light loads, the peak value of the current flowing through the switching elements and the transformer is increased, so that the switching loss and the conduction loss are increased and the magnetic flux of the transformer T is increased. Change is maximal. As a result, significant transformer losses occur.

The method shown in FIG. 4C is the same as the method of FIGS. 4A and 4B combined. At light loads, the ratio between the excitation amount in the forward direction and the excitation amount in the reverse direction of the transformer T is varied to stabilize the output voltage, and thus the switching frequency can be lowered. Under heavy load, as the load increases, the on-time of the switching element Q1 is lengthened to stabilize the output voltage. For example, the conduction time of the switching circuit S2 may be set to be equal to the sum of the transformer reset time at the rated load and the predetermined reverse excitation time.

Such a system can control with high efficiency between light and heavy loads while controlling fluctuations in the switching frequency.

In the above-described embodiment, the first switching element Q1 and the second switching element Q2 are composed of field-effect transistors FETs. Alternatively, other semiconductor elements such as transistors may also be used as the first and second switching elements.

FIG. 5 shows the main part of the switching power supply shown in FIG. 2. In this figure, one end of the series circuit composed of the capacitor C, the inductor L and the primary winding T1 of the transformer T is connected to the junction of the switching elements Q1, Q2, The other end is connected to the positive terminal of the input power supply.

6 shows a main part of a switching power supply according to another embodiment. In this embodiment, the capacitor connection position and the polarity of the input voltage Vin are changed, but the series circuit composed of the capacitor C, the inductor L and the primary winding T1 of the transformer T is not changed. . Similarly, one end of the series circuit is connected to the connection point of the switching elements Q1 and Q2, and the other end thereof is connected to the input power source. The circuit operation of the main part is also the same as that of the circuit shown in FIG.

7 shows a main part of a switching power supply according to another embodiment. In this configuration, the capacitor C includes the capacitors C1, C2. Therefore, the circuit shown in FIG. 7 is an example in which the capacitor C is divided into a capacitor C1 and a capacitor C2. The combined capacitance of the capacitor C1 and the capacitor C2 is the same as that of the capacitor C, and the operation of the circuit is the same as the circuit shown in Figs.

In the above embodiments, the input power source is a direct current power source. However, the input power supply of the present invention may be a power supply rectified and planarized. Also, a capacitor or other component may be connected between the primary and secondary windings of transformer T. Instead of a transformer, inductance elements can be used in the circuit. Even in this case, the basic operation of the circuit is similar to the circuit operation described above.

8 is a circuit diagram of a first control circuit 11 included in a switching power supply according to another embodiment.

In this embodiment, the resistor Ra is connected in series to the source of the first switching element Q1, and the capacitor C4 is connected to the connection point of the source and the resistor Ra. One end of the time constant circuit composed of the resistor R4, the impedance of the phototransistor PC and the capacitor C4 is connected to the base of the transistor Tr1. In the time constant circuit, a resistor is connected in parallel to the capacitor C4. In this control circuit, after the switching element Q1 is turned on, the current id1 increases. As the current id1 increases, the voltage applied to the resistor Ra also increases. At this time, the charging of the capacitor C4 of the time constant circuit is continued. When the sum of the charging voltage of the capacitor C4 and the voltage applied to the resistor Ra reaches the threshold voltage (about 0.6V) of the transistor Tr1, the switching element Q1 is turned off. Therefore, in the circuit of this embodiment, the current flowing through the switching element Q1 is detected by the resistor Ra so that the on-time of the switching element Q1 is controlled.

9 is a circuit diagram of a second control circuit for controlling the switching element Q2 included in the switching power supply according to another embodiment.

In this embodiment, the transistor Tr2 which is a switching device is a pnp type transistor.

10 is a circuit diagram of a second control circuit included in a switching power supply according to still another embodiment.

In this embodiment, phototransistor PC2 is connected to the base of transistor Tr2. The impedance of the phototransistor PC2 is changed by an output signal or a signal input from the outside, so that the on-time of the switching element Q2 is changed. In this arrangement, by controlling the on-time of the switching element Q2, the switching of the switching element Q2 can be performed at an optimal on-time according to the output power.

11 is a circuit diagram of a starter circuit included in a switching power supply according to another embodiment.

In this embodiment, the resistor R1 is connected in parallel to the series circuit composed of the capacitor C, the inductor L and the primary winding T1 of the transformer T. One end of the starting resistor R2 for starting the switching element Q1 is connected to the input power supply via the zener diode DZ.

In the above embodiments, a diode is used as the rectifying element Ds in the rectifying / flattening circuit. However, instead of using a diode that is a rectifying element Ds, a switching element such as a MOS-FET having a low ON-resistance may also be used as the element Ds. When such a switching element performs switching by a control signal generated when the voltage of the secondary winding rises, the conduction loss in on-time is reduced. As a result, the conductive loss of the secondary rectifier circuit can be reduced.

When the first and second switching elements Q1 and Q2 are configured as FETs, parasitic diodes of the FET can be used in place of the first and second diodes D1 and D2, and the parasitics of the FETs. Capacitors may be used in place of the first and second capacitors C1 and C2. In such a configuration, since the first and second diodes and the first and second capacitors shown in FIG. 2 are not necessary, the number of components can be reduced.

Similarly, the inductor L may also consist of only the leakage inductance of the transformer T. In this case, no external inductor L would be needed. Thus, the number of parts can be reduced.

Thus, the advantages of the present invention can be summarized as follows.

The switching power supply of the present invention is a switching control for alternately turning on and off the drive windings for driving the first and second switching elements of the transformer and the switching elements before and after a period during which the two switching elements are turned off. Circuits, and as a result oscillation is performed. With this configuration, the number of parts, the size and the weight of the device can be reduced. In addition, by allowing the switching elements to perform a zero-voltage switching operation, switching losses can be significantly reduced. In addition, since the voltage applied to the switching elements Q1 and Q2 is an input voltage Vin, the switching elements Q1 and Q2 may be constituted by semiconductor devices having a low voltage rating. Therefore, the switching power supply of the present invention can have a high efficiency and can be manufactured in a compact and lightweight device.

In addition, since capacitor C is connected in series to the primary winding of the transformer, energy can be stored in both the primary winding and capacitor C. As a result, the conductive current can be reduced by reducing the peak current. In addition, the voltage applied to the primary winding is approximately half of the voltage of the ringing choke converter (RCC) shown in FIG. Accordingly, the number of primary windings can be reduced and a low breakdown voltage transformer and a small transformer having a lower voltage rating can be manufactured.

While preferred embodiments of the present invention have been described above, those skilled in the art will understand that various modifications and changes can be made without departing from the spirit and scope of the invention.

Claims (21)

As a switching power supply, A first switching circuit composed of a parallel connection circuit having a first switching element, a first diode and a first capacitor; A second switching circuit constituting a series circuit with the first switching circuit, the second switching circuit comprising a parallel connection circuit having a second switching element, a second diode, and a second capacitor; An input power source connected to the series circuit; A transformer comprising a primary winding and a secondary winding; A rectifying and flattening circuit comprising a rectifying element, said rectifying and flattening circuit connected to said secondary winding of said transformer; A first drive winding included in the transformer and generating a voltage substantially proportional to the voltage of the primary winding turning on the first switching element; A second drive winding included in the transformer for generating a voltage that is substantially proportional to the voltage of the primary winding that turns on the second switching element; And Switching control circuits including a first switching control circuit formed between the first drive winding and the first switching circuit and a second switching control circuit formed between the second drive winding and the second switching circuit; , The primary winding, inductance and series capacitor constitute a series circuit, one end of the series circuit is connected to a connection point of the first switching circuit and the second switching circuit, and the other end of the series circuit is connected to the input power source. Become; Energy is accumulated in the primary winding and the series capacitor during the on-period of the first switching element, and output is obtained from the secondary winding during the off-period of the first switching element, and the on of the first switching element The time is controlled so that the output power is controlled; The switching control circuits alternately turn on the first and second switching elements after a period in which both the first and second switching elements are turned off, and the first and second switching elements are both turned off. The first and second switching elements are alternately turned off before a period of time, and after the second switching element and the rectifying element are both turned off, the first switching element is turned on, so that oscillation is performed. Switching power supply. 2. The switching circuit of claim 1, wherein each switching control circuit comprises a delay circuit formed by a series circuit composed of a resistor or a resistor and a capacitor, wherein the resistor or delay circuit comprises a control terminal of the first drive winding and the first switching element. And between the second drive winding and the control terminal of the second switching element, respectively; Each of the first and second switching elements exhibits a delay after a voltage is generated in the first and second drive windings, the voltage being approximately proportional to the voltage of the primary winding turning on the first and second switching elements, respectively. Switching power supply, characterized in that turned on. 2. The circuit of claim 1, wherein the first switching control circuit comprises: a first switch for turning off the first switching element, and approximately a voltage of the primary winding for turning on the first switching element in the first drive winding. And a first time constant circuit for controlling the first switching element so that the first switching element is turned off by the first switch after a predetermined time has elapsed due to a proportional voltage. . 2. The circuit of claim 1, wherein the second switching control circuit comprises: a second switch for turning off the second switching element and approximately the voltage of the primary winding for turning on the second switching element in the second drive winding. And a second time constant circuit for controlling the second switching element such that the second switching element is turned off by the second switch after a predetermined time has elapsed since a predetermined voltage has been generated. . 4. The first time constant of claim 3, wherein the first switch includes a transistor connected to a control terminal of the first switching element, and the control terminal of the transistor includes a first impedance circuit and a charge / discharge capacitor. Switching power supply, characterized in that connected to the circuit. The second time constant according to claim 4, wherein the second switch includes a transistor connected to a control terminal of the second switching element, and the control terminal of the transistor includes a first impedance circuit and a charge / discharge capacitor. Switching power supply, characterized in that connected to the circuit. 6. The switching power supply according to claim 5, wherein an impedance of the first impedance circuit constituting the time constant circuit is changed according to one of an output power level from the secondary winding and a response to an external signal. The switching power supply according to claim 6, wherein the impedance of the first impedance circuit constituting the time constant circuit is changed according to one of an output power level from the secondary winding and a response to an external signal. 2. The apparatus of claim 1, wherein the switching power supply further comprises a second impedance circuit comprising a resistor, wherein the second impedance circuit is connected to at least one of both ends of a series capacitor of the series circuit and both ends of the series circuit. And applying an input voltage to the first switching circuit via the second impedance circuit. 10. The device of claim 9, further comprising: a resistor that divides an input voltage applied to the first switching circuit through the second impedance circuit and applies the divided voltage to a control terminal of the first switching element to initiate oscillation. The switching power supply further comprises a third impedance circuit. The switching power supply of claim 1, further comprising an output capacitor connected in parallel to the rectifying element, wherein the capacitive impedance value of the output capacitor is such that both the second switching element and the rectifying element are turned off. Wherein the output capacitor resonates with the inductance of the transformer so that the voltage waveform across the output capacitor is set to rise from zero voltage or fall to zero voltage, exhibiting a waveform approximately equal to a portion of a sine wave. Power unit. The switching power supply according to claim 1, wherein said rectifying element is a switching element which performs switching by a control signal. The switching power supply of claim 1, wherein the switching elements are field-effect transistors. The inductance of claim 1, wherein the inductance is comprised of at least one of a leakage inductor of the transformer and an external inductor connected in series with the primary winding, wherein the inductance resonates with the series capacitor during an off-period of the first switching element. Switching the waveform of the current flowing through the primary winding to a waveform approximately similar to a part of the sine wave. 2. The switching power supply of claim 1, wherein the first switching control circuit turns on the first switching element after the voltage of the first capacitor drops to approximately zero. 2. The switching power supply of claim 1, wherein the second switching control circuit turns on the second switching element after the voltage of the second capacitor drops to approximately zero. The switching power supply according to claim 1, wherein said second switching control circuit turns off said second switching element when the current flowing in said second switching element becomes approximately zero. The zero current value according to claim 1, wherein the series capacitor and inductance values have a zero current at which the rectifier is turned off again after reaching a peak point at which the waveform of the current flowing through the rectifier rises from zero and the current change rate becomes zero. Switching power supply, characterized in that the waveform is set to fall to. 2. The method of claim 1, wherein the ratio of the excitation amount in the reverse direction of the transformer to the excitation quantity in the forward direction of the transformer is the magnitude of the load connected to the output terminal of the rectifying and flattening circuit. Switching power supply characterized in that the control to change. 2. The switching control circuit of claim 1, wherein the switching control circuits control the excitation amount in the reverse direction of the transformer to be zero or a predetermined fixed value, regardless of the magnitude of the load connected to the output of the rectifying and leveling circuit. Switching power supply. 2. The switching circuit of claim 1, wherein one of the switching control circuits is set so that the on-time of the first switching element is at least a minimum value, so that switching is performed even when a load connected to the output of the rectifying and leveling circuit is shorted. Switching power supply, characterized in that to perform.