Flyback switching power supplies are widely used due to their simple circuits and relatively low electromagnetic interference. They also put forward higher requirements for the output voltage spikes and EMI of switching power supplies. The usual method to reduce EMI is to use a self-excited flyback switching power supply, using a bipolar transistor with a relatively slow switching speed as the main switch; increasing the capacitance of the buffer circuit to reduce the EMI generated by the dz/dt and di/dt of the shutdown process, and reducing the turn-on EMI by slowing down the conduction process. The price paid is the decrease in power supply efficiency, high heat generation, and decreased reliability. Therefore, a low-EMI, high-efficiency flyback switching power supply is needed, and a soft-switching flyback switching power supply is a more ideal solution.

Zero Voltage Switching

The main circuit of the zero voltage switching flyback switching power supply is shown in Figure 1

The main waveform is shown in Figure 2. The circuit working process is divided into four stages: the switch tube is turned off and the buffer circuit acts, the transformer releases the stored energy, the buffer circuit resets, and the switch tube is turned on.

1. Switching tube shutoff and buffer circuit action stage

In the waveform of Figure 2, the period from t1 to t2 is the stage of the switch tube being turned off and the buffer circuit acting. The equivalent circuit is shown in Figure 3. At t2, the control circuit turns off the switch tube, and the primary current of the transformer is transferred from the switch tube to the buffer capacitor. The switch tube current decreases, the buffer capacitor current increases, and the switch tube current decreases until the zero transformer primary current is completely transferred to the buffer capacitor. The equivalent circuit is shown in Figure 3, and the switch tube turn-off process ends. The length of the switch tube turn-off process depends on the characteristics of the switch tube itself and the control circuit, which is generally 1/100 - 1/201 of the switching cycle or about 100 nanoseconds. Since the voltage on the buffer capacitor cannot change suddenly, the drain and source voltages are very low and close to zero during the shutdown process of the switch tube, achieving "zero voltage". Shutdown. To ensure "zero voltage" shutdown, the buffer capacitor should take a larger value, so that the buffer capacitor voltage is still very small at the end of the shutdown process of the switch tube, the polarity of the transformer primary voltage has not changed, the reverse voltage of the output rectifier diode anode cannot be turned on, and the transformer primary current still needs to flow through the buffer capacitor until the buffer process ends. The duration of the buffer process is about 1/20 of the switching cycle, which is relatively short compared to the switching cycle. The transformer primary current changes very little. For the convenience of analysis, it can be considered that the transformer primary current remains unchanged, so the buffer capacitor voltage is:

Where Ics is the transformer primary current value at time t1, which can be approximated to the value at time t0.

(VR is the voltage reflected from the output voltage of the regulated power supply to the primary side of the transformer), at time t2, the output rectifier diode is turned on, the transformer energy storage is released to the output end through the output rectifier diode, and the transformer primary current is zero. The circuit enters the transformer energy storage release stage.

2. Transformer releases stored energy stage

The transformer releases stored energy to the output end through the secondary winding and output rectifier diode. The transformer secondary current is:

The transformer secondary current drops to zero, the transformer energy storage is completely released, the output rectifier diode is naturally turned off, and the circuit enters the buffer circuit reset stage.

3. Buffer circuit reset stage

The reset phase of the snubber circuit corresponds to the period t3-t4. In order to enable the snubber capacitor to play a buffering role in the next switching cycle and ensure the "zero voltage" shutdown and "zero voltage" opening of the switch tube, the snubber capacitor needs to be discharged and all the charges are released, that is, reset. Different from the lossy snubber circuit, the lossless snubber circuit uses LC resonance to reset the snubber capacitor. The reset inductor of the circuit in this article is the primary inductance of the transformer. The circuit is shown in Figure 5.

When the transformer energy storage is fully released, since the voltage vcn on the buffer capacitor is higher than the power supply voltage Ein, the buffer capacitor resets the buffer capacitor voltage in LC resonance through the transformer primary inductance. Due to the reset process, the buffer capacitor voltage will be lower than Ein+VR, and the output rectifier diode will be naturally turned off. The equivalent circuit is:

The switch tube conduction stage is the t3-t4 stage. When the voltage on the buffer capacitor drops to zero or the lowest voltage, the switch tube is turned on at zero voltage or the lowest voltage, and the transformer current increases. The equivalent circuit is shown in Figure 6.

The transformer primary current is

2. Control method

When the flyback switching power supply works in the discontinuous current mode, the voltage regulation process complies with the energy conservation principle, that is, equation (7). From equations (7) and (8), we can get

From (9) and (10), it can be seen that the output power can be adjusted and the output voltage can be stabilized by adjusting the on-time or duty cycle or frequency modulation mode or a combination of several methods. When the output power decreases or the power supply voltage increases, the on-time decreases, and vice versa; as the output power increases or decreases, the peak values ​​of the primary and secondary currents of the transformer increase or decrease. From formula (3), it can be seen that the release energy storage time of the transformer also increases or decreases. In order to achieve "zero voltage" switching, the transformer cannot work in an intermittent state. Therefore, the F%IM control method cannot meet the "zero voltage" opening requirements of the circuit in this paper. Based on the constraints of the above working conditions, the circuit in this paper should use output voltage feedback to control the on-time, and use "zero voltage" detection to control the switch tube conduction time, that is, frequency modulation and duty cycle adjustment working mode. The specific circuit can use a combination of general devices, as shown in Figure 7

Circuit Performance Analysis

The " zero voltage " switching mode proposed in this paper has no loss in the reset process, which basically eliminates the switching loss in the switching process, so the efficiency is high, usually higher than 85%, which is 5-10% higher than the lossy buffer circuit. Not only that, because the "zero voltage" switch basically realizes zero voltage turn-on during the turn-on process, and the inductor current is also zero, the turn-on process has no energy exchange (including the energy exchange of parasitic parameters), and the output rectifier diode has sufficient time and slow reverse recovery under the slow-changing voltage during the reset process of the buffer circuit. The output voltage spike and EMI generated by parasitic oscillation at the turn-on time are greatly reduced. Due to zero voltage turn-off and large-capacitance buffer capacitor, the turn-off process avoids large dv/dt, suppresses the parasitic oscillation caused by transformer leakage inductance and diode turn-on, so the output spike voltage and EMU at the turn-off time of the switch tube are also very small, which basically eliminates the phenomenon of conventional lossy buffer circuit suppressing the switch voltage spike.

Although the circuit principle analysis can achieve "zero" or extremely low output voltage spikes and EMU, in fact, parasitic oscillations due to various reasons still exist, and different degrees of output voltage spikes and EMU will be generated during the switching process. Therefore, it is sometimes necessary to appropriately slow down the switching process, and the proportional drive of the opening process can also be used. Since the zero voltage switch eliminates the parasitic oscillation of the buffer capacitor and the primary inductance of the transformer after the transformer energy storage is released, it is beneficial to reduce the loss of the transformer. The overcurrent protection of the zero voltage switch circuit of the flyback switching power supply proposed in this paper should adopt the cycle-by-cycle peak current limiting method. In the overcurrent state, it will not be a zero voltage switch, and the switching loss will increase, so it should be supplemented by the "break" protection method.