CN107196516B

CN107196516B - Flyback switching power supply circuit

Info

Publication number: CN107196516B
Application number: CN201710519863.3A
Authority: CN
Inventors: 王志燊; 王保均
Original assignee: Mornsun Guangzhou Science and Technology Ltd
Current assignee: Mornsun Guangzhou Science and Technology Ltd
Priority date: 2017-06-30
Filing date: 2017-06-30
Publication date: 2020-02-14
Anticipated expiration: 2037-06-30
Also published as: WO2019001242A1; CN107196516A

Abstract

A flyback switching power supply circuit keeps N in a transformer B on the basis of an LCL flyback converter_P1The same name is connected with a power supply, and a second primary winding N_P2End of same name is grounded, N_P1And N_P2One end of a capacitor C1 is connected with N in a double-wire parallel winding manner_P1The different name end is connected with the N_P2Synonym ends are connected, N_P2The dotted terminal is connected with a power supply through a capacitor C3, so that the following steps are realized: when Q1 is saturated and turned on, N_P1And N_P2Are all excited, when Q1 is switched off, the secondary side N_SThe energy is output, the primary side is in series connection with a leakage inductor in a voltage source mode, C3 resonates with the leakage inductor, and zero voltage switching-on of Q1 is achieved; or after the D2 is turned off, zero voltage switching-on of the Q1 is realized through reverse excitation; the duty ratio can be larger than 0.5, the energy recovery of the demagnetization circuit is realized, and particularly, the conversion efficiency is improved under light load.

Description

Flyback switching power supply circuit

Technical Field

The invention relates to the field of switching power supplies, in particular to a flyback switching power supply circuit using resonance demagnetization.

Background

The switching Power supply is widely applied, and the flyback switching Power supply has attractive advantages in the occasions that the input Power is below 75W and the Power Factor (PF) is not required: the circuit topology is simple, and the input voltage range is wide. The reliability is relatively high due to few components, so the application is wide. For convenience, many documents are also called flyback switching power supply, flyback power supply, and flyback converter, and are also called flyback converter, flyback switching power supply, and flyback power supply in taiwan of japan and china. A common topology for AC/DC converters is shown in fig. 1-1, which is originally from pages 60 of switching power converter topology and design, bib number ISBN 978-7-5083-. The flyback power supply comprises a rectifier bridge 101, a filter circuit 200 and a basic flyback topology unit circuit 300, wherein the basic flyback topology unit circuit 300 is also called as a main power stage, and a practical circuit is additionally provided with a voltage dependent resistor, an NTC thermistor, an EMI (electromagnetic interference) and other protection circuits in front of the rectifier bridge so as to ensure that the electromagnetic compatibility of the flyback power supply meets the use requirement. In general, the flyback switching power supply requires that the smaller the leakage inductance between the primary and secondary windings, the better the leakage inductance is, so that the conversion efficiency is high, the withstand voltage borne by the primary main power switching tube V is also reduced, and the loss of the RCD network is also reduced for the flyback converter using the RCD network as demagnetization. Note: the RCD absorption refers to an absorption circuit consisting of a resistor, a capacitor and a diode, the national literature is the same as that in the world, the resistor is generally numbered by a letter R and represents the resistor, the capacitor is numbered by a letter C and represents the capacitor, the diode is numbered by a letter D and represents the diode, and the resistor and the capacitor are connected in parallel and then connected in series with the diode to form an RCD network.

The rectifier bridge 101 generally consists of four diodes, and when the rectifier bridge 101 does not exist, the rectifier bridge 200 and the rectifier bridge 300 can form a DC/DC switching power supply or a converter, because the DC power supply does not have the requirement of power factor, and the power can be more than 75W. In fact, the adoption of a flyback topology in a low-voltage DC/DC switching power supply is not the mainstream, because at low voltage, the input current of the flyback power supply is discontinuous, the ripple is large, and the requirement on the power supply equipment at the previous stage is high; the output current is also discontinuous, the ripple is large, and the requirement on the capacity of a filter capacitor behind is high; especially when the input voltage is lower, the primary winding adopts the parallel winding of a plurality of strands because the exciting current is increased; two parallel primary windings are generally adopted to be applied to low-voltage DC/DC, and the low-voltage DC/DC switching power supply generally means that the input voltage is below 48V, and part of low-voltage DC/DC switching power supplies for application can work to 160V direct current, such as railway power supplies.

The inductance of primary winding is also lower, often appear the turn of calculating and can not tile the left side to the right of the wire casing of full skeleton, can adopt the scheme of sandwich series winding method when operating voltage is higher, and compelled to adopt the scheme of sandwich parallel winding method under low operating voltage, because two primary windings are not in the same layer, there is the leakage inductance between these two primary windings, this leakage inductance can produce the loss, thereby let switching power supply's efficiency step-down, the loss problem that the leakage inductance between two parallelly connected primary windings causes: this is present both when exciting and when demagnetizing; if a third winding is used for demagnetization, who is the third winding in parallel with the two primary windings and around? Only two third windings can be adopted and are respectively wound around two parallel primary windings and then are connected in parallel to form the third winding, the process is complex, unequal voltages can be induced by the two parallel third windings, and therefore loss and larger electromagnetic interference are caused.

In fact, for common third winding demagnetization, the advantage is lossless demagnetization, the efficiency is higher, but the line diameter selection of the third winding is also a problem: the selection is thin, the parallel winding with the primary winding is troublesome, and the thin wire is easy to break; if the wire diameter is the same as that of the primary winding, the cost is high. The third winding demagnetizes the flyback converter, and also acts as a three-winding absorption flyback converter.

The Chinese application numbers are respectively: 201710142832.0 and 201710142797.2 are named as "a flyback switching power supply", and the technical solutions of fig. 1-2 and fig. 1-3 are respectively shown, so as to solve the above problems, that is: the primary winding can be connected in parallel without two parts, namely, the leakage inductance between the primary winding and the secondary winding is allowed to be larger, the third winding is not used for demagnetization, the conversion efficiency is not reduced, and the loss during excitation and demagnetization is reduced. In the two schemes, the demagnetization mode shown in fig. 1-2 has strict requirements on leakage inductance, otherwise, the excitation energy can be directly returned to the direct current power supply U from the D1_DCAnd does not occur in the secondary winding N_SIn this case, no current flows in the secondary side D2, so that the output voltage is low or no output is generated; and the reflected voltage generated when D2 is conducted is required to be not larger than the DC power supply U_DCFor example, the duty ratio cannot be larger than 0.5, so that the power density cannot be further increased. For fig. 1-3, the demagnetization circuit is a more classical topology, the duty cycle can be greater than 0.5, and the energy of the leakage inductance is not recycled.

For convenience, the inventor defines topologies used by flyback switching power supplies with chinese application numbers 201710142832.0 and 201710142797.2 respectively, including forward topologies, and basic topologies without demagnetization modes are defined as follows: the LCL converter is derived from two primary side excitation inductors and a capacitor connected in series with the primary side excitation inductors.

Disclosure of Invention

In view of the above, the present invention provides a flyback switching power supply circuit using resonant demagnetization, which has a duty ratio greater than 0.5, and simultaneously implements energy recovery of a demagnetization circuit, further implements zero voltage turn-on of a main power switching tube, further reduces loss, and improves conversion efficiency.

The invention aims to realize the purpose, and the flyback switching power supply circuit comprises a transformer, an N-channel field effect transistor, a first capacitor, a second capacitor and a first diode, wherein the transformer comprises a first primary winding, a second primary winding and a secondary winding, the different name end of the secondary winding is connected with the anode of the first diode, the cathode of the first diode is connected with one end of the second capacitor and forms an output positive, and the same name end of the secondary winding is connected with the other end of the second capacitor and forms an output negative; the positive end of an input direct-current power supply is connected with the homonymous end of the first primary winding, and the synonym end of the first primary winding is connected with the drain electrode of the N-channel field effect transistor; the source electrode of the N-channel field effect transistor is connected with the homonymous end of the second primary winding, and the connection point is simultaneously connected with the negative end of the input direct-current power supply; the grid electrode of the N-channel field effect transistor is connected with a driving control signal; the primary winding of first and second is two-wire parallel winding, and the one end of first electric capacity links to each other with first primary winding synonym, and the other end of first electric capacity links to each other with second primary winding synonym, its characterized in that: one end of the third capacitor is connected with the positive end of the input direct-current power supply, and the other end of the third capacitor is connected with the different name end of the second primary winding.

The invention also provides an equivalent scheme of the first scheme, and the second scheme is as follows: the invention also can realize the aim that a flyback switching power supply circuit comprises a transformer, an N-channel field effect transistor, a first capacitor, a second capacitor and a first diode, wherein the transformer comprises a first primary winding, a second primary winding and a secondary winding; the positive end of an input direct-current power supply is connected with the drain electrode of the first N-channel field-effect tube and the synonym end of the second primary winding at the same time, and the source electrode of the first N-channel field-effect tube is connected with the synonym end of the first primary winding; the different name end of the first primary winding is connected with the negative end of the input direct-current power supply; the grid electrode of the first N-channel field effect transistor is connected with a driving control signal; first primary winding and second primary winding are two-wire parallel winding, and the one end of first electric capacity links to each other with first primary winding dotted terminal, and the other end of first electric capacity links to each other with second primary winding dotted terminal, its characterized in that: the direct current power supply further comprises a third capacitor, one end of the third capacitor is connected with the negative end of the input direct current power supply, and the other end of the third capacitor is connected with the homonymous end of the second primary winding.

The improvement of the two schemes is characterized in that: and zero voltage switching-on of the N-channel field effect transistor is realized through reverse excitation.

The improvement of the two schemes is characterized in that: the wire diameters of the first primary winding and the second primary winding are the same.

Preferably, the physical paths of the excitation currents of the first primary winding and the second primary winding are opposite in direction when the PCB is wired.

The working principle will be explained in detail with reference to the embodiments. The invention has the beneficial effects that: the zero voltage switching-on of the main power switch tube is realized, the duty ratio can be larger than 0.5, the energy recovery of the demagnetization circuit is realized, the loss is further reduced, and the conversion efficiency is improved.

Drawings

Fig. 1-1 is a schematic diagram of a conventional flyback switching power supply for converting ac current into dc current;

FIG. 1-2 is a schematic diagram of the technical solution disclosed in Chinese application No. 201710142832.0;

FIGS. 1-3 are schematic diagrams of the technical solution disclosed in Chinese application No. 201710142797.2;

fig. 2 is a schematic circuit diagram of a flyback switching power supply circuit according to a first embodiment of the present invention;

fig. 2-1 is a schematic diagram of two

excitation currents

41 and 42 generated when Q1 is in saturation conduction according to the first embodiment of the present invention;

fig. 2-2 is a schematic diagram of the first embodiment of the present invention in which Q1 is turned off to generate the freewheeling current 43 and the demagnetization current 44;

fig. 3 is a schematic circuit diagram of a flyback switching power supply circuit according to a second embodiment of the present invention.

Detailed Description

First embodiment

FIG. 2 shows a resonant demagnetization flyback switching power supply circuit according to a first embodiment of the present inventionThe schematic diagram comprises a transformer B, a first N-channel field effect transistor Q1, a first capacitor C1, a second capacitor C2 and a first diode D2, wherein the transformer B comprises a first primary winding N_P1A second primary winding N_P2And secondary winding N_SSecondary winding N_SThe different name end is connected with the anode of a first diode D2, the cathode of a first diode D2 is connected with one end of a second capacitor C2, the positive output end is the + end of Vout in the figure, and a secondary winding N is arranged_SThe end with the same name is connected with the other end of the second capacitor C2, and forms an output negative, which is the-end of Vout in the figure; input DC power supply U_DC(hereinafter also referred to as DC power supply U)_DCPower supply U_DCOr U_DC) Positive terminal + and first primary winding N_P1The terminals of the same name are connected, and a first primary winding N_P1The synonym end is connected with the drain electrode d of the N-channel field effect transistor Q1; the source s of the N-channel field effect transistor Q1 is connected with the second primary winding N_P2The homonymous terminal and the connection point are simultaneously connected with an input direct current power supply U_DCNegative terminal of (c); the grid g of the N-channel field effect transistor Q1 is connected with a driving control signal; one end of the first capacitor C1 and the first primary winding N_P1The other end of the first capacitor C1 is connected with the second primary winding N_P2The different name end is connected with each other, the circuit also comprises a third capacitor C3, one end of the third capacitor C3 is connected with an input direct current power supply U_DCAnd the other end of the third capacitor C3 is connected to the second primary winding N_P2And (4) a synonym terminal.

And the end with the same name: one end of the winding is marked with a black dot in the figure;

a synonym terminal: the end of the winding not marked with black dots in the figure;

the drive control signal: various square waves including PWM pulse width modulation signals, PFM pulse frequency modulation and the like;

and a transformer B: first primary winding N_P1And a second primary winding N_P2In the figure, the cores are connected by a dotted line, which means that the cores are wound around a single transformer, the same core is used, and the drawing in the figure is used only for the sake of clarity of the drawing and simplicity of connection, and the transformer is not independent.

In FIG. 2, the source of the N-channel FET Q1 is connected to the second sourceSide winding N_P2The homonymous terminal and the connection point are simultaneously connected with an input direct current power supply U_DCThe negative terminal of (1), i.e. the source of the FET Q1, is connected to the input DC power supply U_DCThis is not directly true in practical applications, since in the field of switched-mode power supplies, unnecessary factors are omitted from the fundamental topology analysis. In practical application, the source of the fet is connected to a current detection resistor or a current transformer to detect an average current or a peak current to implement various control strategies, and the current detection resistor or the current transformer is connected to the source and is connected to the source in a similar manner, which is a known technique in the art. If the current transformer is used, the current transformer can be arranged at any place of the excitation loop, such as the drain electrode of a field effect tube, for example, the homonymous end or the synonym end of the first primary winding, and the current transformer can be a Hall sensor besides the traditional magnetic core type transformer of which the primary side is a turn of 'conducting wire' and the secondary side is a turn of coil.

The working principle is as follows: referring to fig. 2, when the capacitor C3 is replaced by a diode, the prior art circuit of fig. 1-2 is shown, but the working principle of the circuit is completely different from that of the prior art circuit after the capacitor C3 is added;

when the circuit of fig. 2 is powered on, Q1 does not work because the driving control signal is not received, which is equivalent to open circuit, and then the power supply U_DCBy a first primary winding N_P1Charging C1, the current passing through the secondary winding N_P2Back to power supply U_DCNegative terminal of (1), first primary winding N_P1The charging current of (a) is: flowing from the homonymous end to the synonym end; second primary winding N_P2The charging current of (a) is: flowing from the different name end to the same name end; n is a radical of_P1And N_P2The two currents are in parallel connection, the two currents are equal in magnitude, the generated magnetic fluxes are opposite and completely cancel, namely, when the power supply U is powered on_DCThe C1 is charged by two windings of the transformer B, the magnetic fluxes of the two windings are counteracted by mutual inductance and do not work, and the C1 is equivalent to the step of passing through N_P1And N_P2Direct current internal resistance and power supply U_DCIn parallel, C1 still functions as power supply filtering and decouplingThe function of (1); over time, the terminal voltage of C1 equals U_DCLeft positive and right negative. Meanwhile, the terminal voltage of the capacitor C3 is positive, negative and equal to U_DCThe voltage of (c).

When the control signal is normally received by the Q1, for example, in a period in which the gate of the Q1 is high, the Q1 is in saturation conduction and has an internal resistance equal to the on-state internal resistance R_ds(ON)For convenience of analysis, the condition is regarded as a through and is a wire, as shown in fig. 2-1, C3 will bring loss to the conduction of Q1 during the turn-on of Q1, since C1 has large capacity, C3 is equivalent to being connected in parallel with Q1, and the terminal voltage change is realized by the conduction of Q1, so that loss is generated, wherein the analysis is the period of normal operation, and the terminal voltage of C3 is up-positive-down-negative and is twice U3 during the saturated conduction of Q1_DCVoltage of (d); at this time N_p1The resulting excitation current is shown at 41 in FIG. 2-1; if the circuit is an ideal model circuit, the excitation current 42 should not be present, since N is_P2The induced voltage of C1 is equal to the terminal voltage of C1, but the excitation current 42 is true because the actual circuit is not an ideal model.

In the excitation process, the excitation voltage is U_DCWinding N_P2Induced voltage of U_DCThe homonymous terminal induces a positive voltage, and the synonym terminal induces a negative voltage; secondary winding N_SThe induced voltage is also generated according to the turn ratio, and the induced voltage is as follows: the homonymous terminal induces a positive voltage, and the synonym terminal induces a negative voltage equal to U_DCMultiplication by the turn ratio N, i.e. N_SInducing a lower positive voltage and an upper negative voltage, wherein the lower positive voltage and the upper negative voltage are connected in series with the terminal voltage of C2 and are added at two ends of D2, D2 is reversely biased and is not conducted, and the secondary side is equivalent to no-load and has no output;

during the excitation process, the current of 41 is increased linearly upwards; the current direction flows from the homonymous terminal to the synonym terminal in the primary inductor;

the gate of Q1 changes from high level to low level, Q1 also changes from saturation conduction to cut-off, because the current in the inductor can not change suddenly, although Q1 is cut off at this time, the current of 41 and 42 still flows from the same name end to the different name end, because the primary current loop is cut off, the magnetThe energy in the core flows from the homonymous terminal to the synonymous terminal on the secondary side, see fig. 2-2, the primary winding N_P2And secondary winding N_SA current flow occurs from the homonymous terminal to the heteronymous terminal, as shown at 44 and 43 in fig. 2-2, and current 44 discharges C3 until its voltage reaches U_DC-n Vout. The current 43 causes D2 to conduct in the forward direction and charge the capacitor C2 through the forward conducting D2, Vout builds up the voltage or continues to output energy. At this time, the output voltage Vout is applied to the winding N_P2The voltage is clamped so that the C3 voltage is unchanged and the current 44 becomes 0 and the transformer is demagnetized by Vout. This process is a demagnetization process.

When the current 43 drops to 0, the reverse excitation process is entered. This process D2 is turned off. Winding N_P2When the voltage with the negative end of the same name and the positive end of the different name is borne, reverse excitation is started, and the excitation current is C3 for charging. In the process, N_P2The voltage gradually decreases and reverses to become positive at the homonym terminal and negative at the synonym terminal, and the voltage of Q1 gradually decreases. When the voltage of C3 becomes twice U_DCThe voltage of Q1 is 0. The voltage of C3 becomes twice U due to the clamping action of the Q1 body diode_DCThe voltage of Q1 is 0. When the drive voltage of Q1 is high before the field current is reversed, ZVS is realized.

The output end of the flyback switching power supply obtains energy when a primary winding is disconnected with the power supply, and the transformer B is not used for converting voltage, but used for freewheeling through a magnetic core and is an isolated version of a Buck-Boost converter; so transformer B is also commonly referred to as a flyback transformer;

because the primary winding and the secondary winding cannot be bifilar and parallel winding under normal conditions, leakage inductance is certain to exist. The energy stored in the primary winding exciting inductor is transmitted to the secondary winding N through the transformer B after Q1 is switched off_SThe output end but the energy on the leakage inductance is not transferred, so that the two ends of the Q1 tube are over-pressurized and the Q1 tube is damaged. The circuit for demagnetizing the leakage inductance consists of C3, and the working principle is as follows:

first primary winding N_P1And a second primary winding N_P2Is wound in two wires and one of the two windingsThe leakage inductance between the primary winding and the secondary winding is zero, the energy on the leakage inductance is not transferred to the secondary side at the moment of Q1 turn-off and after the moment, the energy on the leakage inductance is not transferred to the secondary side, and the secondary winding is connected with the secondary winding_P2The electric energy of the medium leakage inductance, the current direction of which is the same as that of the excitation, flows from the end with the same name to the end with the different name, namely from bottom to top in fig. 2-2, and the electric energy discharges C3 to form leakage inductance demagnetization current shown by 44;

it is apparent that the output voltage Vout is divided by the turn ratio N, which is the secondary winding N_SThe reflected voltage formed on the primary side when D2 is on is greater than the DC power supply U due to the presence of C3 DC blocking_DCThe circuit can also work normally. When D2 freewheels, C2 corresponds to a voltage source that feeds the secondary winding N_SAnd (3) excitation is carried out, wherein a reflected voltage is formed on a primary side, at the moment, a primary side winding is equivalent to a voltage source with the voltage equal to the reflected voltage and is connected with a leakage inductor in series, and the primary side winding is recovered to be connected with the excitation inductor and the leakage inductor in series only when the current in D2 is reduced to zero and D2 is turned off.

Then, during the period of D2 turning on and freewheeling, there are many operation modes, i.e. after C3 absorbs the energy of the leakage inductance, there are many operation modes of the circuit, and the following operation principle is stated here:

during the conduction period of D2, the primary winding N_P2And N_P1The leakage inductance and the C3 are in resonance when the leakage inductance and the C3 are connected in series, the value of C3 is calculated when the leakage inductance and the C3 are designed, and the voltage of the end of C3 is close to or equal to twice U at a specific time in the resonance process_DCAt voltage, when the voltage is up-positive, down-negative, the terminal voltage of C1 is always left-positive, right-negative and equal to U_DCAt this moment, the Voltage of the left terminal of the C1 is Zero volt, that is, the terminal Voltage of the Q1 is also Zero volt, and if the Q1 is in saturation conduction at this moment, Zero Voltage switching of the Q1 is realized (Zero Voltage Switch is abbreviated as ZVS, and only Zero Voltage switching is realized here, and it is quasi-ZVS), which is also called soft switching technology, so that the recycling of the primary side leakage inductance energy is realized, and the energy on the output capacitor Coss of the Q1 is also transferred due to resonance, so that the recycling of the primary side leakage inductance energy is realized.

To achieve longer resonance times, C3, due to less leakage inductanceThe capacity is relatively large. Just as C3 is larger and the terminal voltage can rise, the terminal voltage and the DC power U_DCThe voltage of the transformer is in a series relation, the duty ratio can be larger than 0.5 by utilizing a volt-second balance law, and the transformer can normally work.

In this way, it is obvious that in the current continuous mode, the time for the Q1 to be turned on again is very difficult to be grasped, and if the D2 is turned off, the C3 and the primary inductor resonate, and the terminal voltage of the C3 approaches or equals to twice the U voltage at a specific time_DCAt voltage, when the voltage is up-positive, down-negative, the terminal voltage of C1 is always left-positive, right-negative and equal to U_DCAt this moment, the voltage of the left terminal of the C1 is zero volt, that is, the terminal voltage of the Q1 is also zero volt, if the Q1 is in saturation conduction at this moment, then zero voltage switching-on of the Q1 is realized, and this way is certainly in a current interruption mode, and the time for the Q1 to be switched on again is very easy to detect and realize.

The

currents

41 and 42 are the same, and the wire diameters of the first primary winding and the second primary winding are the same, so that the winding is convenient, the wire diameters are the same, the litz wires are the same in size, the colors of the litz wires can be different, namely the litz wires are stranded, and the colors of the litz wires in the same size can be different for convenience of identification. As the operating frequency increases, the high frequency current tends to flow more on the surface of the enameled wire, in which case the litz wire can solve this problem. Of course, the litz wire is made by using two enamelled wires with different colors, the enamelled wires are directly wound, and then the first primary winding and the second primary winding are separated according to the colors, or the wire diameters and the strand numbers of the two windings are different, so that the invention aims are also realized.

The flyback switching power supply circuit keeps N in the transformer B on the basis of the LCL flyback converter_P1The same name is connected with a power supply, and a second primary winding N_P2End of same name is grounded, N_P1And N_P2One end of a capacitor C1 is connected with N in a double-wire parallel winding manner_P1The different name end is connected with the N_P2Synonym ends are connected, N_P2The dotted terminal is connected with a power supply through a capacitor C3, so that the following steps are realized: when Q1 is saturated and turned on, N_P1And N_P2Are all excited, when Q1 is switched off, the secondary side N_SOutput energy, sourceThe side is connected with a leakage inductor in series through a voltage source, and C3 resonates with the leakage inductor to enable Q1 to realize zero voltage switching-on; or after the D2 is turned off, zero voltage switching-on of the Q1 is realized through reverse excitation; the duty ratio can be larger than 0.5, the energy recovery of the demagnetization circuit is realized, and particularly, the conversion efficiency is improved under light load.

It can be seen that compared to existing LCL converters, the present invention is many different, mainly: the duty ratio can be more than 0.5, the zero voltage switching-on of the main power switch tube is realized, the energy recovery of the demagnetization circuit is realized, the loss is further reduced, and the conversion efficiency is improved.

Second embodiment

The present invention further provides an equivalent solution of the first embodiment, and referring to fig. 3, an equivalent solution of the first embodiment, a resonant demagnetization flyback switching power supply circuit includes a transformer B, a first N-channel fet Q1, a first capacitor C1, a second capacitor C2, and a first diode D2, where the transformer B includes a first primary winding N_P1A second primary winding N_P2And secondary winding N_SSecondary winding N_SThe different name end is connected with the anode of a first diode D2, the cathode of a first diode D2 is connected with one end of a second capacitor C2, the positive output end is the + end of Vout in the figure, and a secondary winding N is arranged_SThe end with the same name is connected with the other end of the second capacitor C2, and forms an output negative, which is the-end of Vout in the figure; input DC power supply U_DCThe positive end of the primary winding is simultaneously connected with the drain electrode of the N-channel field effect transistor Q1 and the second primary winding N_P2The different name end is connected, and the source electrode of the N-channel field effect transistor Q1 is connected with the first primary winding N_P1The terminals with the same name are connected; first primary winding N_P1The different name end is connected with an input direct current power supply U_DCA negative terminal of (a); the grid electrode of the N-channel field effect transistor Q1 is connected with a driving control signal; first primary winding N_P1And a second primary winding N_P2Double wires are wound in parallel; one end of the first capacitor C1 and the first primary winding N_P1The other end of the first capacitor C1 is connected with the second primary winding N_P2The same name end is connected with the input direct current power supply U, the input direct current power supply U also comprises a third capacitor C3, and one end of the third capacitor C3 is connected with the input direct current power supply U_DCThe other end of the third capacitor C3 is connected to the second primary winding N_P2The same name endAnd (4) connecting.

In fact, the second embodiment is a variation of the first embodiment: on the basis of FIG. 2 of the first embodiment, the series devices of the excitation loop are interchanged, i.e. N_P1And Q1, the positions of which are interchanged, C3 and N_P2The circuit of the second embodiment of fig. 3 is obtained by interchanging the position of C1 and connecting it between the two series devices, and since the source voltage of Q1 is varied, the circuit is driven in floating ground, which is more costly.

The working principle is briefly described as follows:

referring to FIG. 3, when the circuit is powered on, the power supply U_DCThe C1 is charged by two windings of the transformer B, the magnetic fluxes of the two windings are counteracted by mutual inductance and do not work, and the C1 is equivalent to the step of passing through N_P2And N_P1Direct current internal resistance and power supply U_DCIn parallel connection, C1 still plays the role of power supply filtering and decoupling;

over time, the terminal voltage of C1 equals U_DCRight positive and left negative; c3 is positive-top, negative-bottom;

when Q1 is saturated and turned on, its internal resistance is equal to on-state internal resistance R_ds(ON)The magnetic field generator is regarded as a wire as in the above, and then generates a path of excitation current;

the first path is as follows: power supply U_DCThe positive terminal is connected in through the drain of the Q1 and the source of the Q1 and then passes through the first primary winding N_P1End of same name of (N)_P1The synonym of (2) is sent back to the power supply U_DCA negative terminal; note: when the circuit is an ideal device, the second path of excitation current does not exist.

The second path is: the right positive end of the capacitor C1 passes through the second primary winding N_P2End of same name of (N)_P2The synonym end of Q1 goes in, the drain of Q1 goes out, and returns to the left negative end of the capacitor C1;

it can be seen that the first path and the second path of excitation current are in parallel connection, because N is_P1And N_P2The inductance is the same, the excitation voltage is the same, and the inductance and the excitation voltage are all equal to U_DCThe two paths are completely equal, and in the excitation process, the winding N_P2Induced voltage of U_DCThe homonymous terminal induces a positive voltage, and the synonym terminal induces a negative voltage; secondary sideWinding N_SInduced voltage is generated according to turn ratio, positive voltage is induced at the same-name end, negative voltage is induced at the different-name end, and the magnitude of the induced voltage is equal to U_DCMultiplication by the turn ratio N, i.e. N_SInducing a lower positive voltage and an upper negative voltage, wherein the lower positive voltage and the upper negative voltage are connected in series with the terminal voltage of C2 and are added at two ends of D2, D2 is reversely biased and is not conducted, and the secondary side is equivalent to no-load and has no output;

in the excitation process, the excitation current is increased upwards in a linear mode; the current direction flows from the homonymous terminal to the heteronymous terminal in the inductor;

when Q1 is cut off, the current in inductor can not change suddenly, the energy in magnetic core flows from the same name end to different name end at secondary side, and the secondary winding N_SA current flow occurs from the dotted terminal to the dotted terminal, which charges the capacitor C2 through the forward conducting D2, Vout builds up the voltage or continues to output energy. This process is also a demagnetization process.

D2 is switched off and then enters the reverse excitation process. In the process, the winding Np2 bears the voltage with the same name end being negative and the different name end being positive, reverse excitation is started, the excitation current is C3 for charging, the voltage at the two ends of Q1 is gradually reduced to 0, and conditions are provided for ZVS.

In the second embodiment, the circuit for demagnetizing the leakage inductance comprises C3 and the second primary winding N_P2The composition and the working principle are the same as those of the first embodiment.

The second embodiment is a modification of the first embodiment, and the working principle is equivalent, and the object of the invention is also achieved. As a technical solution of using an N-channel fet, it can also be implemented by using a P-channel fet, which has a relatively low cost at a low operating voltage, and at this time, on the basis of the first embodiment, the polarities of the power supply, the diode and the same-name terminal are reversed, and the output rectifying part is not reversed, so that the third and fourth embodiments are obtained.

The foregoing is merely a preferred embodiment of the present invention, and it should be noted that the above-described preferred embodiment should not be construed as limiting the present invention. For those skilled in the art, it is obvious to those skilled in the art that several improvements and modifications can be made without departing from the spirit and scope of the present invention, such as adding a control loop to realize the voltage stabilization of the output, which is obtained by the prior art, for example, using a switching tube Q1 with other symbols, adding a plurality of outputs to the secondary side output, and using pi-type filtering for filtering; such modifications and decorations shall be considered as the protection scope of the present invention, which shall not be described herein by way of example, and shall be subject to the limitations defined by the claims.

Claims

1. A flyback switching power supply circuit comprises a transformer, an N-channel field effect transistor, a first capacitor, a second capacitor and a first diode, wherein the transformer comprises a first primary winding, a second primary winding and a secondary winding, the different name end of the secondary winding is connected with the anode of the first diode, the cathode of the first diode is connected with one end of the second capacitor and forms an output positive end, and the same name end of the secondary winding is connected with the other end of the second capacitor and forms an output negative end; the positive end of an input direct-current power supply is connected with the homonymous end of the first primary winding, and the synonym end of the first primary winding is connected with the drain electrode of the N-channel field effect transistor; the source electrode of the N-channel field effect transistor is connected with the homonymous end of the second primary winding, and the connection point is simultaneously connected with the negative end of the input direct-current power supply; the grid electrode of the N-channel field effect transistor is connected with a driving control signal; the primary winding of first and second is two-wire parallel winding, and the one end of first electric capacity links to each other with first primary winding synonym, and the other end of first electric capacity links to each other with second primary winding synonym, its characterized in that:

one end of the third capacitor is connected with the positive end of the input direct-current power supply, and the other end of the third capacitor is connected with the different name end of the second primary winding.

2. The flyback switching power supply circuit of claim 1, wherein: and zero voltage switching-on of the N-channel field effect transistor is realized through reverse excitation.

3. The flyback switching power supply circuit of claim 1, wherein: the wire diameters of the first primary winding and the second primary winding are the same.

4. A flyback switching power supply circuit according to any one of claims 1 to 3, wherein: when the PCB is wired, the directions of the physical paths of the excitation currents of the first primary winding and the second primary winding are opposite.

5. A flyback switching power supply circuit comprises a transformer, an N-channel field effect transistor, a first capacitor, a second capacitor and a first diode, wherein the transformer comprises a first primary winding, a second primary winding and a secondary winding, the different name end of the secondary winding is connected with the anode of the first diode, the cathode of the first diode is connected with one end of the second capacitor and forms an output positive end, and the same name end of the secondary winding is connected with the other end of the second capacitor and forms an output negative end; the positive end of an input direct-current power supply is connected with the drain electrode of the first N-channel field-effect tube and the synonym end of the second primary winding at the same time, and the source electrode of the first N-channel field-effect tube is connected with the synonym end of the first primary winding; the different name end of the first primary winding is connected with the negative end of the input direct-current power supply; the grid electrode of the first N-channel field effect transistor is connected with a driving control signal; first primary winding and second primary winding are two-wire parallel winding, and the one end of first electric capacity links to each other with first primary winding dotted terminal, and the other end of first electric capacity links to each other with second primary winding dotted terminal, its characterized in that:

the direct current power supply further comprises a third capacitor, one end of the third capacitor is connected with the negative end of the input direct current power supply, and the other end of the third capacitor is connected with the homonymous end of the second primary winding.

6. The flyback switching power supply circuit of claim 5, wherein: and zero voltage switching-on of the N-channel field effect transistor is realized through reverse excitation.

7. The flyback switching power supply circuit of claim 5, wherein: the wire diameters of the first primary winding and the second primary winding are the same.

8. The flyback switching power supply circuit of any of claims 5-7, wherein: when the PCB is wired, the directions of the physical paths of the excitation currents of the first primary winding and the second primary winding are opposite.