Research on inductor split push-pull commutation soft switching technology

    Abstract: This paper proposes an inductor split push-pull commutation soft switching circuit, analyzes the working principle of the circuit and the conditions for realizing soft switching. The simulation results show that the circuit has simple control and reliable performance, and is especially suitable for medium and small power applications.

    Keywords: split inductor, push-pull commutation, soft switching, simulation

1 Introduction

　　The push-pull circuit is widely used in medium and low voltage input converters due to its advantages of simple control and no shoot-through phenomenon. However, the two switching tubes in the push-pull circuit are in a hard switching state. As the switching frequency increases, the turn-on power consumption decreases. Larger[1]. Bridge arm commutation soft switching is widely used in bridge converters (including half-bridge converters), such as phase-shifted full-bridge soft switching circuits [2], but bridge arm commutation soft switching cannot be applied in push-pull circuits. Although push-pull soft switching has been discussed in literature [3], [4], it can only be applied to two-tube push-pull circuits, and increases the conduction loss of one switch tube, which is a problem for low-voltage input push-pull circuits. is inappropriate.

　　In response to this situation, this article realizes the zero-voltage turn-on soft switching technology of the push-pull circuit by splitting the secondary DC filter inductor and then equivalently converting it to the primary.

2 Circuit topology and working principle

2.1 Main circuit topology

　　The main circuit structure of the inductor split push-pull commutation soft switch and the control method of the switch tube are shown in Figure 1 (a) and Figure 1 (b). It is different from the general push-pull circuit in two points: first, the secondary filter inductor is split into two ( L 1 = L 2 = L ), and second, the transformer secondary provides two demagnetization windings L N3 and L N4 , which together with V 3 , V 4 , L 1 , and L 2 constitute a demagnetization circuit.

2.2 Working principle

　　Its circuit can be divided into the following working processes:

　　(1) From the t 0 to t 1 stage, that is, when the switch S 1 is on, the transformer secondary L N2 generates a positive and negative induced voltage, and the transformer transmits energy to the load. At this time, the transformer secondary L N2 and the diode V 2 , inductor L 2 and load form a loop. At time t 1 , S 1 is turned off, and the voltage across S 1 is clamped by capacitor C 1 and cannot change suddenly, so S 1 is softly turned off.

　　(2) In the t 1 to t 2 stage, at t 1 moment, S 1 is turned off, and L P2 , GB, and V 6 form a primary excitation leakage circuit. At the same time , an upper positive and lower negative induced voltage is generated at both ends of L N3 . At this time, the diode V 3 is turned on, and the inductor L 2 forms an i L2 freewheeling loop with the load R and the transformer secondary L N3 . At time t 2 , S 2 is driven. Assume that the i L2 current is not discharged to zero at this time, the primary V 6 is still in the conducting state, the voltage across S 2 is zero, and it is a zero-voltage turn-on.

　　(3) The stage from t 2 to t 3 is still the freewheeling stage of i L2 . At t 3 , the discharge of i L2 is completed.

　　(4) In the t 3 to t 4 stage, at t 3 , i L2 = 0, the current on L P2 is reversed, an upper positive and lower negative induced voltage is generated at both ends of the transformer secondary winding L N1 , and the diode V 1 is turned on.

After 　　t 4 , repeat the working process of the first half of the week.

　　Based on the above process, we can see the basic principles and characteristics of soft switching in this circuit:

　　(1) During the period when neither S 1 nor S 2 is conducting (t 1 ~ t 2 , t 4 ~ t 5 ), the freewheeling current of L 1 (L 2 ) is coupled to the primary, forming V 5 and V 6 commutation Current, this current can be maintained until the driving pulses of S 1 and S 2 arrive, and zero-voltage turn-on can be achieved.

　　(2) The superposition of the split L 1 and L 2 currents is continuous to the load. Therefore, the combined effect of L 1 and L 2 is a DC filter inductor, but the current of a single inductor L 1 or L 2 is discontinuous throughout the cycle. During the turn-on period of S 1 or S 2 , it has the nature of an AC inductance, so it is similar to the bridge Arm commutation in series with the primary inductance plays the same role.

3 basic relations

3.1 Main relations

　　Based on the working process of the above circuit, the equivalent circuit of each commutation stage is obtained, as shown in Figure 2.

　　(1) t 0 ~ t 1 stage: The transformer transmits energy stage. The equivalent topology structure at this time is shown in Figure 2(a).

　　The figure shows the equivalent inductance of the primary leakage inductance and secondary inductance of the transformer. It is the secondary load of the transformer converted to the equivalent load of the primary. It is the secondary capacitance of the transformer converted to the equivalent capacitance of the primary. Then the equivalent load and equal The current i L2 of the effective leakage inductance satisfies the equation:

the initial condition of the circuit is: i L2 (t 0 ) = 0 U C (t 0 ) = U min

The solution of this circuit can be obtained as:


　　(2) When t 1 ~ t 3 stage: freewheeling stage, the equivalent circuit is shown in Figure 2(b).

    The state equation of the secondary loop is:

    Boundary conditions: i L (t 2 ) = I LP , uC (t 2 ) = U OP (I LP is the peak current, U OP is the peak voltage). If the circuit is ideal, that is, U OP =N LN3 / N P ·Ui, assuming uC (t) = constant, then solving equation (3) we get:

    i L2 =I LP -1/L(U C (t )-U LN3 )t (t>t 1 ) (4) The freewheeling time of

i L2

is: 3.2 Problems and analysis

　　(1) It can be seen from equation (5) that as long as △t>(t 3 - t 1 ), the circuit can realize zero-voltage soft switching technology.

　　(2) It can be seen from the equivalent circuit diagram 2(b) and equation (2) that L 1 (L 2 ) essentially plays the role of the commutation AC inductor in the bridge arm commutation in the circuit, so it also has a duty cycle than the loss problem [4].

　　(3) Since the equivalent circuit parameters of the turn-on current of L 1 (L 2 ) and the freewheeling current are basically the same, when N LN3 =N LN1 =N LN2 , it is easy to cause the primary S 1 (S 2 ) current to have a sawtooth waveform. current, which increases the current stress of the switching device. Although the freewheeling time Δt can be changed by changing the ratio of (N LN1 /N P ), it is not an ideal method. How to construct a controllable inductor L 1 (L 2 ) is a method for further improvement of this circuit.

4 Simulation results

　　In order to test the working condition of the inductor split push-pull commutation soft switching circuit proposed in this article, PSPICE electronic circuit simulation software was used for simulation.

　　The electrical parameters used in the simulation are:

　　Inductor L 1 =L 2 =0.1mH, capacitor C 5 =C 6 =0.6μF, load R=10Ω.

　　Figure 3 shows the voltage, current and driving waveform of S2 of the switching tube during simulation. Figure 4 shows the power loss waveform and output average power waveform of the switching tube.

　　It can be seen from Figure 3(d) that the switch tube is turned on when the voltage of the switch tube is zero, that is, zero voltage is turned on. At this time, the current of the switch tube is zero as shown in Figure 3(b), and Figure 3(c) is the transformer. The voltage waveform of the secondary inductor L N3 , Figure 3 (a) shows the current waveforms of the inductors L 1 and L 2. It can be seen that the secondary voltage of the transformer changes during the commutation of the inductor current.

　　Figure 4 is the power consumption analysis of the switch tube. It can be seen from Figure 4 (a) and (b) that when the switch tube is turned on, the power consumption of the switch tube is zero. Calculate the power consumption of Figure 4 (a) and (b) , the power consumption of its switching tube is very small, which shows that its efficiency is very high.

5 Conclusion

　　The inductor split push-pull commutation soft switching technology proposed in this article has its advantages and is especially suitable for medium and low power applications. However, due to the loss of duty cycle, how to form a controllable inductor L 1 (L 2 ), Awaiting further research.