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A realization method of flyback soft switching

Abstract: a realization method of ZVS soft switching of flyback circuit is proposed, that is, by adding a winding, the exciting inductance current is reversed, so as to create the ZVS soft switching condition of the main switch of flyback circuit; The working principle and the design of circuit parameters are analyzed; Finally, the experimental results verify the working principle and effectiveness of the circuit

key words: flyback circuit; Soft switch; Auxiliary winding

0 introduction

light and small is the goal of current power products. Increasing the switching frequency can reduce the volume of inductance, capacitance and other components. However, the bottleneck of increasing switching frequency is the switching loss of switching devices. So soft switching technology came into being

this paper presents a flyback zero voltage soft switching method with auxiliary winding. Through the analysis of the working principle of the circuit and the experimental results, it is verified that the circuit is feasible. Amputees wearing artificial limbs bdi800 l 36 can achieve the same safety perception

1 working principle

Figure 1 shows the soft switching circuit proposed in this paper. The number of turns of the auxiliary winding is the same as that of the output winding. Switch tubes S1 and S2 are complementary to each other, and there is a certain dead zone between them to prevent common state conduction, as shown in Figure 2. The value of excitation inductance LM in the circuit is small, so that the current ILM can be reversed to meet the ZVS soft switching condition of main switch S1, as shown in Fig. 2 (a) and Fig. 2 (b). Since the working conditions of the circuit under light load and full load are slightly different, the working principle of the circuit under light load will be analyzed in detail below, and the working principle under full load will be briefly described. Considering the junction capacitance and dead time of the switch, a cycle of the circuit under light load can be divided into seven stages, and the equivalent circuit of each stage is shown in Figure 3. Its working principle is described as follows

Figure 1 flyback converter with auxiliary winding

(a) circuit working waveform under light load

(b) circuit working waveform under full load

Figure 2 main circuit working waveform

(a) stage1[t0, t1] (b) stage2[t1, t2]

(c) stage3[t2, t3] (d) stage4[t3, t4]

(E) stage5[t4, t5] (f) stage stage6[t5, t6]

(g) stage7[t6, t7]

Figure 3 equivalent circuit diagram of each stage

1) stage 1 [T0, T1] S1 in this stage is on, LM bears the input voltage, and the excitation current ILM increases linearly in the positive direction, from negative to positive. At time T1, S1 turns off, ILM reaches the maximum value, and this stage ends

2) after S1 in phase 2 [T1, T2] is turned off, the exciting inductor current begins to drop, part of which charges the output junction capacitance of S1, and the drain source voltage of S1 rises linearly; At the same time, the other part is coupled to the secondary side through the transformer to discharge the output junction capacitance of S2. The drain source voltage of S2 can be approximately considered as a linear decrease. At T2, the drain source voltage of S2 drops to zero, and this stage ends

3) stage 3 [T2, T3] when the drain source voltage of S2 in China has been completely "reduced to" a supplier of mineral raw materials, the parasitic diode of S2 will be turned on, clamping the drain source voltage of S2 at zero voltage, which creates conditions for the zero voltage conduction of S2. At the same time, diode D is also on

4) stage 4 [T3, T4] at T3, the gate of S2 changes to high level and S2 zero voltage is turned on. The exciting inductor LM bears the reverse voltage NVO (n is the turns ratio of the original and secondary sides of the transformer), the current on LM decreases linearly, and at T4, it drops to zero, and the current through the switch S2 and diode D also drops to zero at the same time. This stage ends

5) in stage 5 [T4, T5], after the current through diode D drops to zero, diode D turns off naturally. While S2 continues to be on, LM bears the voltage NVO, and the current flowing through LM increases inversely and linearly from zero. At T5, S2 is turned off, and this stage ends

6) stage 6 [T5, T6] at this time, the current direction on the excitation inductance LM is negative, and part of this current discharges the output junction capacitance of S1, so that the drain source voltage of S1 can be approximately considered as a linear decrease; At the same time, the other part is coupled to the secondary side through the transformer to charge the output junction capacitance of S2, so that the drain source voltage of S2 rises linearly. At time T6, the drain source voltage of S1 drops to zero, and this stage ends

7) stage 7 [T6, T7] when the drain source voltage of S1 drops to zero, the parasitic diode of S1 turns on, clamping the drain source voltage of S1 at zero voltage, which creates conditions for the zero voltage conduction of S1. At T7, S1 is then turned on under zero voltage conditions to enter the next cycle. It can be seen that both switches S1 and S2 realize soft switching

the above analysis is the working principle of the circuit under light load. The working principle of the circuit under full load is slightly different from that under light load, that is, there is no link that the diode D current drops to zero and turns off naturally. The diode D current gradually drops to zero after the switch S2 is turned off, as shown in Figure 2 (b)

2 parameter design of soft switch

here, the parameter design of soft switch is mainly the design of transformer excitation inductance

the peak and peak values of the exciting inductor current can be expressed as

ilm= (vindt)/lm (1)

where: D is the duty cycle

t is the switching cycle

then the maximum and minimum values of the exciting inductance current can be expressed as:

ilmmax= (vindt)/2lm + IO/N (2)

ilmmin= (vindt)/2lm - IO/N (3)

where: IO is the load current

from the above principle analysis, we can see that the soft switching condition of S1 is created by | ilmmin | discharging the output junction capacitance of S1 and charging the output junction capacitance of S2 through a transformer; The soft switching condition of S2 is created by | ilmmax | charging the output junction capacitance of S1 and discharging the output junction capacitance of S2 through the transformer

s1 and S2 soft switching limit condition is that the energy stored on LM charges and discharges the output junction capacitance of S1 and S2, which is enough to discharge one junction capacitance to zero and charge the other junction capacitance to the maximum

in this way, the limit condition of S1 is

(NVO + VIN) 2 (=) LM (4)

the limit condition of S2 is

(NVO + VIN) 2 (=) LM (5)

where: C1 and C2 are the output junction capacitance of S1 and S2 respectively

since the dead time is relatively small in the actual circuit, it can be approximately considered that the current on the inductance LM remains unchanged during the dead time, that is, a constant current source discharges the junction capacitance of the switch. The soft switching condition in this case is called the margin condition

s1 margin condition is

(NVO + VIN) | ilmmin | tdead1 (6)

s2 margin condition is

(NVO + VIN) | ilmmax | tdead2 (7)

, where: tdead1, tdead2 are the dead time before S1 and S2 are opened respectively

since the energy is transmitted from the power supply to the load, that is, the load current IO 0, we can see | ilmmax | ilmmin | by comparing equation (2) with equation (3), especially at full load, | ilmmax | ilmmin |. Therefore, the soft switch implementation of S2 is much easier than that of S1. Therefore, in the specific experimental design, the key is to design the soft switching condition of S1. First, determine the maximum dead time that can be borne, and then calculate the IP inductance LM according to equations (6) and (3). On the premise of realizing soft switching, LM should not be too small, so as not to cause excessive current effective value on the switch tube and excessive conduction loss of the switch

3 experimental results

a flyback circuit model with auxiliary winding with 48V input and 5v/5a output is designed, and the experimental results are given to further verify the correctness of the above soft switching implementation method. The specifications and main parameters of the converter are as follows:

input voltage Vin 48V

output voltage Vo 5V

Output Current IO 0 ~ 5A

working frequency f 100kHz

main switches S1, S2, IRF730, irfz44

excitation inductance LM 70 h

the turns ratio of the primary and secondary sides of the transformer and the auxiliary winding is 26: the national building materials transaction is active 4:4

Figure 4 shows the experimental waveforms at light load (1a) and full load (5a) respectively. From Figure 4 (g) to figure 4 (J), it can be seen that switch tubes S1 and S2 realize soft switching at light load and full load

（a）Current of D(1A) （b）Current of D(5A)

（c）Current of S2(1A) （d）Current of S2(5A)

（e）Current of S1(1A) （f）Current of S1(5A)

（g）Soft switching of S1(1A) （h）Soft switching of S1(5A)

（i）SoftswitchingofS2(1A) （j）SoftswitchingofS2(5A)

Figure 4 experimental waveform

4 Conclusion

this paper analyzes the situation of the circuit working under light load and full load, that is, the output rectifier diode is in intermittent and continuous states, which have their own advantages and disadvantages. The intermittent state can realize the zero current shutdown of the diode, but its current stress is high, while the continuous state is just the opposite. Therefore, the circuit can be designed in one state or across two states according to specific needs

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