WO2021051858A1

WO2021051858A1 - Control method for active clamp flyback converter

Info

Publication number: WO2021051858A1
Application number: PCT/CN2020/092469
Authority: WO
Inventors: 尹向阳; 王海洲; 袁源; 刘湘
Original assignee: 广州金升阳科技有限公司
Priority date: 2019-09-20
Filing date: 2020-05-27
Publication date: 2021-03-25
Also published as: CN110677045B; CN110677045A

Abstract

Disclosed is a control method for an active clamp flyback converter. In the flyback converter, a main switch tube controls the current of a primary winding of the flyback converter, a clamping switch tube clamps a node voltage of a primary side of the flyback converter, and a controller generates control signals for controlling the main switch tube and the clamping switch tube by detecting a feedback voltage; when the converter works under a light load, and when the main switch tube is turned off, the clamping switch tube is immediately turned on after a period of fixed deadtime, the converter charges a clamping capacitor by means of the clamping switch tube to recover leakage inductance energy, wherein a turn-on time period of the clamping switch tube is shorter than a resonance period of the leakage inductance and output junction capacitance of the main switch tube and the clamping switch tube. In the present invention, a large resistor does not need to be connected to the clamping capacitor in parallel, and the light load efficiency is improved.

Description

A control method of active clamp flyback converter

Technical field

The invention relates to a flyback converter, in particular to a control method of an active clamp flyback converter.

Background technique

Flyback converters are widely used in small and medium power switching power supplies due to their low cost and simple topology. In the actual working process, due to the leakage inductance of the flyback converter, all the energy of the primary side cannot be transferred to the secondary side. The leakage inductance energy remaining on the primary side and the junction capacitance of the MOS tube resonate to cause the drain of the main switch tube. Generate high-frequency voltage spikes. In order to reduce the voltage stress of the switch tube when making products, the usual method is to add a suitable absorption circuit. Common absorption circuits include RCD absorption circuit, LCD absorption circuit and active clamp circuit. Among them, the active clamp circuit adds an additional clamp switch tube and a larger clamp capacitor, which can save the leakage inductance energy in the clamp capacitor and recover this energy to the input end of the converter. In addition, due to the electrical inertia of the leakage inductance, the active clamp circuit extracts the charge on the junction capacitance of the drain terminal of the main switching tube through the reverse excitation current after the recovery process of the leakage inductance energy is completed, so that the drain voltage of the main switching tube is reduced to Zero, so as to realize the zero voltage turn-on (ZVS) of the main switch tube, reduce the turn-on loss of the main switch tube, and further improve the power density of the product.

As shown in Figure 1, 100 is a circuit diagram of a typical active clamp flyback converter. In the figure, LK is the leakage inductance, LM is the magnetizing inductance, C_CLAMP is the clamping capacitor, S2 is the clamping switch, S1 is the main switch, C _{OSS is the} main switch junction capacitance, RCS is the excitation inductance current sampling resistor, NP Is the number of turns of the primary winding of the transformer, NS is the number of turns of the secondary winding of the transformer, DR is the rectifier diode, C _OUT is the output capacitor of the converter, and the unit 120 is the controller of the converter (that is, the main control chip of the converter), The unit 130 is an isolated feedback circuit. The main control chip realizes dual-loop peak current mode control by sampling the output voltage of the converter and the voltage drop on the current sampling resistor RS, and determines when the main switching tube S1 is turned on and when it is turned off. In order to realize the ZVS turn-on of the main switch S1, it is necessary to reasonably control the turn-on time of the clamp switch S2. In fact, it is difficult to pull the voltage of the switch node to the ground potential only by relying on the leakage inductance, and the inductance of the magnetizing inductance LM needs to be appropriately reduced, so that the magnetizing inductance also has a negative current. After the clamp switch is turned off, the magnetizing inductance and leakage inductance still flow negative currents, extracting energy from the switch junction capacitance, so that the switch node voltage is pulled to the ground potential.

Figure 1 shows the schematic diagram of a typical active clamp flyback converter. Figure 2 shows the key signal waveforms of a typical complementary mode active clamp flyback. Among them, S1 is the gate drive waveform of the main switch. , S2 is the gate drive waveform of the clamp switch tube, VSW is the main switch tube drain voltage waveform, ILM is the excitation inductance current waveform, and ILK is the leakage inductance current waveform. Assuming that the inductance of the magnetizing inductance is L _M , the inductance of the leakage inductance is L _K , the positive peak value of the magnetizing inductance current is I _PKP , the negative peak value is I _PKN , the drain terminal voltage of the main switch is V _SW , and the switch The parasitic capacitance of the node is C _OSS . In order to reliably turn on the main switch tube ZVS, the above power level parameters need to meet:

Among them, L _M and C _OSS are fixed. From this formula, it can be seen that in order to realize the ZVS of the main switch, a certain magnitude of negative inductor current must be guaranteed, and the negative current required as the input voltage increases is also Bigger. When the output load decreases, the peak value of the positive inductor current begins to decrease, so the on-time of the main switch tube and the on-time of the clamp switch tube should be reduced accordingly to ensure that the peak value of the negative excitation current is a constant value. Therefore, the switching frequency of the complementary mode active clamp flyback converter increases as the load decreases, and the switching loss and driving loss of the switching tube do not decrease when the output load decreases. In addition, there is still a large circulating energy in the clamp switch tube path in the complementary mode at light load, which will also cause the light load efficiency to decrease.

Patent US9991800B2 provides a multi-mode control active clamp flyback controller to improve light-load efficiency and reduce no-load power consumption. This patent works in normal flyback mode at light load or no load, and the clamp switch does not work. However, during each switching period, the converter charges the clamping capacitor through the body diode of the clamping switch. Since the clamping switch is not working, the energy on the clamping capacitor cannot be released.

Patent US10243469B1 provides a burst mode control method to achieve light load efficiency improvement. When a light load is detected, it also works in normal flyback mode, but only changes the number of driving signals in the burst pulse group. Reduce the equivalent frequency, so as to achieve the improvement of light load efficiency. However, during each switching period, the converter charges the clamping capacitor through the body diode of the clamping switch. Since the clamping switch is not working, the energy on the clamping capacitor cannot be released. All of the above patents consume the energy of the clamp switch tube by connecting a large resistor in parallel with the clamp capacitor.

Summary of the invention

In view of the shortcomings of the prior art, the purpose of the present invention is to provide a control method of an active clamp flyback converter to improve the light load efficiency.

In order to achieve the above-mentioned purpose of the invention, the present invention provides a control method of an active clamp flyback converter. In the flyback converter, the main switch tube controls the current of the primary winding of the flyback converter, and clamps the pair of switching tubes. The node voltage on the primary side of the flyback converter is clamped, and the controller detects the feedback voltage to generate a control signal for controlling the main switch and the clamp switch; when the converter is working at light load, when the main switch is turned off After a fixed dead time, the clamp switch tube is turned on immediately, and the converter charges the clamp capacitor through the clamp switch tube to recover the leakage inductance energy. The on time of the clamp switch tube is less than that of the leakage inductance and the main switch tube. The resonant period of the output junction capacitance and the output junction capacitance of the clamp switch tube.

This avoids the leakage inductance and the current of the clamp capacitor from flowing through the body diode of the clamp switch tube. At the same time, the opening of the clamp switch tube will also be released through the clamp switch tube when the clamp capacitor has stored energy to a certain extent. Therefore, there is no need to connect a large resistor in parallel with the clamp capacitor, which improves the light-load efficiency.

Preferably, the dead time is set between 100ns and 200ns.

Preferably, the on-time of the clamp switch tube of the converter is fixed and can be set at 100ns±10%, so that the energy stored in the clamp capacitor is small and can be quickly released when the load is light.

Preferably, when the converter is working under light load, the switching frequency of the main switch tube and the clamp switch tube is between 20KHz-35KHz, which reduces driving loss and switching loss, and does not produce audible noise.

Preferably, when the converter is working at no load, the converter enters a burst mode, and the equivalent switching frequency of the main switch tube and the clamp switch tube is less than 600 Hz, which further reduces the driving loss and switching loss of the two switching tubes. It enters the burst mode at no-load, reduces the equivalent operating frequency of the main switch, limits the peak value of the excitation current of the primary side, avoids the generation of audio noise, and optimizes the no-load power consumption.

Preferably, when the converter is working at no load, the peak current of the primary side of the converter should be less than 0.5A, reducing the peak current to further reduce the no-load loss. At this time, the switching frequency is in the audible range of human ears, and the audible noise is eliminated by reducing the peak current.

Compared with the prior art, the control method proposed by the present invention has the following effects:

1. At light no-load, the traditional method consumes the energy of the clamping capacitor by connecting a large resistance in parallel with the clamping capacitor. This method effectively solves the clamping circuit at light no-load by turning on the clamping tube for a short period of time. The problem that the energy of the clamping capacitor cannot be recovered;

2. In the traditional complementary mode active clamp flyback, due to the existence of negative current, the primary side circulating current is large, and the no-load power consumption is large. There is no negative current on the primary side of the present invention, so the primary side of the converter has no Circulating current

3. High light load efficiency;

4. Low no-load power consumption.

Description of the drawings

Figure 1 is a block diagram of the existing typical ACF circuit principle;

Figure 2 is a waveform diagram of key signals of an existing typical complementary mode active clamp flyback converter;

Figure 3 is a key waveform diagram of the present invention at light load;

Figure 4 is a key waveform diagram of the present invention at no-load;

detailed description

In one embodiment, the active clamp flyback converter is used to adjust the input voltage and output the desired voltage. The active clamp flyback converter includes a main switch tube that controls the current of the primary winding of the flyback converter, And a clamp switch that clamps the node voltage on the primary side of the flyback converter. The controller generates a control signal for controlling the main switch tube and the clamp switch tube by detecting the feedback (FB) voltage.

FIG. 1 shows an active clamp flyback power supply 100 according to some embodiments in the form of a schematic diagram. 100 includes an active clamp flyback (ACF) converter 160 and a controller 120 for adjusting the input voltage of the voltage source 170 and outputting a desired output voltage V _out .

The ACF converter 160 includes a primary side circuit 110, a flyback transformer 140, and a secondary side circuit 150. Both the primary winding and the secondary winding of the flyback transformer 140 have the same-named end and the different-named end, and a magnetic core coupled with the primary and secondary windings.

The primary side circuit 110 includes a clamp capacitor 111, a leakage inductor 112, an excitation inductor 113, a clamp switch 114, a main switch 115, and a sampling resistor 116. The first terminal of the capacitor 111 is connected to the output terminal of the input power source 170. The first terminal of the inductor 112 is connected to the output terminal of the input voltage source 170, and the second terminal of the inductor 112 is connected to the opposite end of the primary winding of the flyback transformer 140. The first terminal of the inductor 113 is connected to the opposite end of the primary winding of the flyback transformer 140, and the second terminal of the inductor 113 is connected to the same end of the primary winding of the flyback transformer 140. The drain of the switching tube 114 is connected to the second terminal of the capacitor 111, and the source of the switching tube 114 is connected to the end of the primary winding of the flyback transformer 140 with the same name. The drain of the switching tube 115 is connected with the end of the same name of the primary winding of the flyback transformer 140, and the source of the switching tube 115 is connected with the first terminal of the resistor 116. The second terminal of the resistor 116 is connected to the ground. The

switch tubes

114 and 115 are both N-channel metal oxide semiconductor (MOS) transistors.

The secondary circuit 150 includes an output rectifier diode 151 and an output capacitor 152. The anode of the rectifier diode 151 is connected to the end of the same name of the secondary winding of the flyback transformer, and the cathode of the rectifier diode 151 is connected to the first terminal of the output capacitor 152. The second terminal of the output capacitor 152 is connected to the ground. In some embodiments, the rectifier diode can also be replaced by an N-channel metal oxide semiconductor (MOS) transistor.

The controller 120 includes a feedback signal input port FB connected to the second port of the isolation feedback 130, a second output port D2 connected to the gate of the switch tube 114 for providing a driving signal to it, and the gate of the switch 115 It is connected to the first output port D1 for providing a driving signal to it. As shown in FIG. 1, the controller 120 is implemented by an integrated circuit, and other components of the multi-mode power supply are discrete components. In other embodiments, some discrete devices can also be integrated into an integrated circuit.

In actual work, the controller 120 controls the switching actions of the

switch tubes

115 and 114 through the GS_1 and GS_2 driving signals sent from the D1 and D2 ports, and is used to control the ACF converter 160 to adjust the output voltage to a preset value. The isolated feedback circuit provides the feedback signal FB to the controller 120. The controller 120 compares the FB signal with the primary peak current sampling voltage. When the primary peak voltage sampling value is greater than the FB voltage sampling value, the main switch is turned off. The switch tube 115 adjusts the output voltage to a desired value. After the switching tube 115 is turned off, the switching tube 114 is turned on after a fixed dead time, and the on-time of the switching tube 114 is set to a fixed value by the controller 120. The traditional complementary mode is used as an active clamp flyback light no-load control strategy. First, the switching frequency will increase as the load decreases in the complementary mode, which increases the switching loss and driving loss; at the same time, due to the negative direction in the complementary mode The existence of current causes the circulating current of the converter to be large, which results in low light load efficiency of the converter.

Fig. 3 shows the control method provided by the present invention in a graphical way. The key waveforms of the converter when working at light load. The principle analysis of Fig. 3 according to different moments is as follows:

Stage 1 t ₀ ～t ₁ : This stage is the dead time. At t _{0, the} main switch tube drive signal S1 is switched from high to low level, and the primary excitation current charges the output junction capacitance of the main switch tube, and the leakage sense and clamp capacitor to the clamp capacitor is charged by resonance current clamp diode switch body, when the voltage on the junction capacitance of the main switch rises to V _in + nV _out, the clamp switch drain-source voltage drops to zero at both ends, start time t ₁ of the transformer to transfer energy to the secondary side. Since the dead time is affected by the turn-off delay of the MOS tube, the dead time is set between 100ns and 200ns, which is controlled by the controller pin

Stage two t ₁ ～t ₂ : At t ₁ , since the voltage across the clamp switch tube drops to zero, the clamp switch tube realizes zero-voltage turn-on, the leakage inductance and the clamp capacitor resonate through the clamp switch tube, and the resonance current is given to The clamp capacitor continues to charge, and the energy stored in the leakage inductance is transferred to the clamp capacitor for storage. At this time, the transformer still transfers energy to the secondary side. Among them, the on-time of the clamp switch is fixed, and the on-time of the clamp switch is less than the resonant period of the leakage inductance and the output junction capacitance of the main switch and the clamp switch. At this time, the clamp switch is turned on. The time can be set at 100ns (plus or minus 10% error).

Phase three t ₂ ～t ₃ : At _{time t 2} , the clamp switch is turned off, the excitation current does not drop to zero, and energy continues to be transferred to the secondary side until the excitation current is zero.

Phase 4 t ₃ ～t ₄ : At t ₃ , the excitation current is zero, the primary side no longer transfers energy to the secondary side, the voltage across the transformer winding is zero, and the transformer leakage inductance and excitation inductance are output together with the main switching tube. The junction capacitance resonates until the main switch is turned on at _{t 4 and enters the next cycle.}

When working under light load, the converter will charge the clamp capacitor every cycle. When the energy on the clamp capacitor is charged to a certain level, the energy on the clamp capacitor will be charged by the clamp switch in a certain cycle. It is released and recovered, which avoids connecting a large resistor in parallel with the clamping capacitor to consume the energy on the clamping capacitor, thereby improving the light-load efficiency. When working under light load, the switching frequency of the main switch tube and the clamp switch tube is between 20KHz-35KHz, which will reduce the driving loss and switching loss, and will not produce audible noise.

Figure 4 graphically shows the key waveforms of the converter when working at no-load. As shown in the figure, when the load is no-load, the converter works in Burst mode (burst mode). Entering the burst mode is realized by detecting the FB voltage. The controller implements the burst mode by setting two threshold voltages Burst_L and Burst_H, and the two threshold voltages are driven and set by the controller 120 according to the existing burst mode. When the FB voltage is less than the burst mode low threshold Burst_L, the controller turns off the pulse signals of the main switch tube and the clamp switch tube, the input terminal does not transfer energy to the output terminal, and the output terminal energy is provided by the output capacitor; when the FB voltage is greater than the burst mode When the high threshold Burst_H is in the transmission mode, the controller starts to pulse signals to the main switch tube and the clamp switch tube, and the primary side starts to transfer energy to the secondary side.

When the above-mentioned operation is at no load, the equivalent switching frequency of the main switch tube and the clamp switch tube is less than 600 Hz, which can further reduce the driving loss and switching loss of the two switching tubes. It enters the burst mode at no-load, reduces the equivalent operating frequency of the main switch, limits the peak value of the excitation current of the primary side, avoids the generation of audio noise, and optimizes the no-load power consumption. In addition, when the above-mentioned work is at no-load, the peak current of the primary side of the converter should be less than 0.5A. Reducing the peak current can further reduce the no-load loss. At this time, the switching frequency is in the audible range of human ears, which can be eliminated by reducing the peak current. Audible noise.

The embodiments of the present invention are not limited to this. According to the above content, according to the common technical knowledge and conventional means in the field, without departing from the above basic technical ideas of the present invention, the control method of the present invention has other embodiments; therefore, the present invention The invention can also be modified, replaced or changed in various other forms, all of which fall within the protection scope of the invention.

Claims

A control method for an active clamp flyback converter. In the flyback converter, a main switch tube controls the current of the primary winding of the flyback converter, and the clamp switch tube is used for the primary side node of the flyback converter. The voltage is clamped, and the controller generates a control signal for controlling the main switching tube and the clamping switching tube by detecting the feedback voltage; it is characterized in that: when the converter is working at a light load, when the main switching tube is turned off, it passes through a fixed period of time. The dead time turns on the clamp switch tube immediately, and the converter recovers the leakage inductance energy by charging the clamp capacitor through the clamp switch tube. The on time of the clamp switch tube is less than the leakage inductance and the main switch tube and the clamp switch tube. The resonant period of the output junction capacitance.
The control method according to claim 1, wherein the dead time is set between 100 ns and 200 ns.
The control method according to claim 1, wherein the on-time of the clamp switch tube of the converter is fixed and set at 100ns±10%.
The control method according to claim 1, characterized in that: when the converter is working at a light load, the switching frequency of the main switch tube and the clamp switch tube is between 20KHz and 35KHz.
The control method according to claim 1, characterized in that: when the converter is working at no load, the converter enters a burst mode, and the equivalent switching frequency of the main switch tube and the clamp switch tube is less than 600 Hz.
The control method according to claim 1, wherein when the converter is working at no load, the peak current of the primary side of the converter should be less than 0.5A.