Development of a new 2kW push-pull forward DC converter
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The working principle of a new push-pull forward circuit is introduced, the circulation process is thoroughly analyzed, and the influence of clamping capacitor and transformer primary leakage inductance on the circuit operation is analyzed. The theoretical analysis is verified by simulation and experiment. Based on this, a 2kW DC converter prototype with input voltage DC24~32V and output voltage DC120V is developed, with a typical efficiency of 93.2%, indicating that the circuit is reliable and efficient, and is suitable for medium and high power applications with low voltage and high current input.Keywords: push-pull; push-pull forward; DC converter 0 Introduction In low-voltage and high-current applications, push-pull circuits have been widely used due to their simple structure and high core utilization. However, traditional push-pull circuits have the following disadvantages: 1) Due to the existence of primary leakage inductance, a large voltage spike is generated at the drain-source when the power tube is turned off; 2) The ampere-second integral of the input current ripple is large, so the input filter is larger in size. This article adds a clamping capacitor to the traditional push-pull circuit, which can solve the two shortcomings of the above traditional circuit. 1 Working principle of push-pull forward circuit As shown in Figure 1, the two main power switches V1 and V2 of the converter and two primary windings Tp1 and Tp2 with Np turns are alternately connected to form a loop, and a clamping capacitor C is connected between the two midpoints of the loop . Cin is the input capacitor, Dv1 and Dv2 are the parasitic anti-parallel diodes of the two main power switches. D1 and D2 form a double half-wave rectifier circuit. 
Figure 1 Schematic diagram of push-pull forward circuit The positive pole of the power supply → T p2 → C → T p1 → the negative pole of the power supply forms a loop. Ignoring the leakage inductance of the transformer, the sum of the voltages applied to the two windings on the primary side of the transformer is zero. The voltage on C is U in , which is positive at the bottom and negative at the top. The other loop is the positive pole of the power supply → V 1 → C → V 2 → the negative pole of the power supply. According to Kirchhoff's circuit law, we can get U ds1＋U ds2 = U in＋U c =2 U in Therefore, when one switch is turned on, the voltage of the other switch is clamped at 2 U in ; when both switches are turned off, the voltages of the switch are U in . Before analyzing the working mode of the push-pull forward circuit, make the following settings: 1) V 1 , V 2 , D 1 , and D 2 are all ideal devices, and the on-state voltage drop is negligible; 2) C is relatively large, and the voltage across the terminals remains basically unchanged during operation ; 3) The filter inductor Lf is relatively large and can be regarded as a constant current source in a short period of time, and the current remains unchanged; in steady state, the output current Io = Uo / R ; 4) The number of turns of the primary winding is the same as Np , the excitation inductance and leakage inductance are the same as Lm and Lσ , the number of turns of the secondary winding is the same as Ns , and the turns ratio n = Ns / Np ; 5) Switching cycle Ts , the on-time of each cycle of V1 and V2 is t on , and the duty cycle of V1 and V2 is D = t on / Ts ; Figure 2 is a waveform diagram of the working principle of the push-pull forward circuit, which is divided into 8 working modes in total. 
Figure 2 PPF working principle waveform 1 ) [ t1 - t2 ] Before t1 , V1 and V2 are both off, and the input current circulates along the positive pole of the power supply → Tp2 → C → Tp1 → negative pole of the power supply, and the circulating current is Ia = nDIo [ 1 ] ( detailed analysis is given in Section 2). The voltage of the primary and secondary windings is 0, and D1 and D2 are turned on at the same time. At t1 , V1 is turned on, Uin is added to the leakage inductance of Tp1, and i1 increases rapidly ; Uc is added to the leakage inductance of Tp2, and i2 decreases rapidly and increases in the opposite direction. Correspondingly, the current iD1 flowing through D1 on the secondary side increases, and the current iD2 flowing through D2 decreases. At t2 , D2 is turned off and iD2 = 0. The equivalent circuit diagram of this mode is shown in Figure 3 (a), and the duration is Δt 1－2 = （ 1） Where: i L fmin is the filter inductor current at time t 1 . 2) [ t2 - t3 ] When D2 is turned off, the working mode starts to work. Uin is added to the excitation inductance and leakage inductance of Tp1, and Uc is added to the excitation inductance and leakage inductance of Tp2 , each bearing half of the excitation current and load current change rate. At this time, the primary is equivalent to two single- ended forward circuits working in parallel [2][3][4]. i1 increases, and i2 increases in the opposite direction. The working mode is shown in Figure 3 (b), and the duration is Δt2-3 = DTs- ( 2 ) 3) [ t 3 - t 4 ] At t 3 , V 1 is turned off and this working mode starts to work. Before this, i 1 is always greater than i 2 , so at the moment V 1 is turned off, the anti-parallel diode D v2 of V2 is turned on. At the same time, the current i D1 flowing through D 1 decreases, and the current i D2 flowing through D 2 increases from zero, and the secondary winding works in a short circuit. The capacitor voltage U c is added to the leakage inductance of Tp1 , and U in is added to the leakage inductance of T p2 , i1 decreases rapidly, and i 2 increases rapidly. The working mode ends when i 1 = i 2. The equivalent working mode circuit is shown in Figure 3 (c) and the duration is Δ t 3－4 =  （3） Where: i L fmax is the filter inductor current at time t 3 . 4) [ t4 - t5 ] During this period, both V1 and V2 are turned off. The average leakage inductance current (circulating current) Ia flows through the positive pole of the power supply → Tp2 → C → Tp1 → negative pole of the power supply. Since the power supply voltage and the clamping capacitor voltage are equal, the voltages applied to the two primary windings are both zero, and the circulating current Ia remains unchanged. The equivalent working mode is shown in Figure 3 (d), and the duration is Δt 4－5 = (1－2 D ) T s - ( 4)  5) [ t5 - t9 ] At t5 , V2 turns on and starts the second half cycle. The working mode is the same as the first half cycle, except that the direction of the excitation current is opposite, completing the demagnetization of the transformer . 
(a) [ t 1－t 2 ] 
(b) [ t 2－t 3 ] 
(c) [ t 3 t 4 ] 
(d) [ t 4 t 5 ] Figure 3 PPF working mode diagram 2 Circulation analysis Assuming the power loss of the push-pull forward converter is zero, according to the law of conservation of system energy, the power input power in half a cycle T s /2 is W in(Δt 1－5 ) =  U in i in d t (5) In order to simplify the analysis of the problem, we assume the following ideal conditions: 1) The commutation of the two windings on the primary side is completed instantly, that is, Δt 1-2 =0, Δt 3-4 = 0 ; 2) The excitation inductance Lm and the filter inductance Lf are large, the excitation current is zero, and Lf can be regarded as a constant current source. W in(Δt 1－5 ) = U in ( I a + nDI o )  (6) Output power is W out(Δt 1－5 ) =2 nDU in I a (7) Combining equations (6) and (7), we get Ia = nDIo ( 8 ) It can be seen from this that when the circuit's working duty cycle D is large, the primary side circulation time is short and the circulation value is large; as the output power increases, the circulation value also increases. 3 Analysis of the influence of main parameters on circuit operation 3.1 Function and selection of clamping capacitor C 3.1.1 Two main functions of clamping capacitors 1) Suppressing the voltage spike when the switch is turned off As shown in Figure 1, when V1 is turned off, C provides a low-impedance energy release loop Dv2 → C → Tp1 for the primary leakage inductance of the transformer. The drain-source voltage of V1 is clamped at Uin + Uc , so that the voltage spike of the switch is effectively suppressed. The clamping capacitor C stores electrical energy when the switch is fully turned off, and releases the energy to the load when it is turned on . In theory, there is no energy loss. 2) Reduce the volume of the input filter. Compared with the traditional push-pull circuit, the clamping capacitor in the push-pull forward circuit provides a freewheeling loop for the switch tube during the off period. It is precisely because of the existence of the freewheeling loop that the ampere-second integral of the input current ripple of the push-pull forward circuit is smaller than that of other topologies. Therefore, the volume of the input filter can be reduced. 3.1.2 Selection of clamping capacitor According to the analysis in the previous article, the voltage ripple Δuc of the clamping capacitor C is determined by the charge amount during the circulation period, that is, Δuc = ( 9 ) The circuit duty cycle Ts , maximum load current Io , and transformer turns ratio n are determined before design. In engineering practice, Δuc = 20% Uin is selected . Therefore, the required capacitance value can be calculated based on the working range of the duty cycle D. At the same time, in order to reduce the influence of capacitor ESR , a solution of connecting multiple film capacitors in parallel is generally adopted. 3.2 Impact of transformer leakage inductance on PPF operation For an ideal transformer, the transformer leakage inductance L σ = 0. No matter which power tube is turned off, the transformer winding current is instantly reduced to 0, and there is no circulating current when both switch tubes are turned off. In fact, any transformer has leakage inductance. In the push-pull forward circuit, when both switch tubes are turned off, the energy of the primary leakage inductance charges C through the U in positive pole → T p2 → C → T p1 → U in negative pole loop to form a circulating current, which generates voltage pulsation on the clamping capacitor. At the same time, reducing the primary leakage inductance can reduce the commutation time when the power tube is turned on, that is, reducing the loss of duty cycle, thereby improving the utilization rate of the transformer and reducing the loss of circuit operation. From the above analysis, it can be seen that reducing leakage inductance can improve system efficiency. Therefore, transformers often use the method of winding between the primary and secondary sides to reduce the value of leakage inductance. 4 Simulation and Experiment 4.1 Simulation Analysis Based on the above analysis, the principle simulation of the PPF operation is carried out, and the simulation main circuit is shown in Figure 1. The main simulation parameters are: U in =28V, C =70μF, n =6, I o =10A, L f =160μH, C f =680μF/400V×2, T s =20μs. Figure 4 is a simulated waveform diagram of the voltage ripple Δuc of the clamping capacitor C corresponding to the output current Io = 10A and the duty cycle D is 0.1 , 0.25, and 0.4 respectively . It can be seen from Figure 4 that Δuc is the largest when D = 0.25 . 
Figure 4 Duty cycle D and Δ u c relationship simulation waveform Figure 5 is the input current simulation waveform, where Figure 5 (a) is the simulation waveform when the primary magnetizing inductance L m = 12μH and the leakage inductance L σ = 0.05μF; Figure 5 (b) is the simulation waveform when the primary magnetizing inductance L m = 12μH and the leakage inductance L σ = 0. The simulation results show that when L σ = 0, there is no circulating process of the input current. 
(a) Lσ = 0.05μF input current simulation waveform 
(b) Lσ = 0 input current simulation waveform Figure 5 Input current simulation waveform 4.2 Experimental Results According to the relevant technical requirements, a 2kWDC/DC converter with input DC24V~32V and output DC120V was developed. The system parameters are: switching frequency fs = 50kHz; main power switch tube is IXTK180N15; rectifier diode is DSEP60-06A; clamping capacitor C = 70μF; filter inductor Lf = 160μH; filter capacitor Cf = 680μF/400V×2; main transformer turns ratio n = 6, magnetic core is EE55×2. Figure 6 is the experimental waveform under rated load, where Figure 6 (a) is the primary winding current waveform (ch1 is the switch tube V1 drive signal waveform, ch2 is the switch tube V2 drive signal waveform, ch3 is the winding Tp1 current waveform i1 , ch4 is the winding Tp2 current waveform i2 ) ; Figure 6 (b) is the switch tube drain-source waveform (ch1 is the switch tube V1 drive signal, ch2 is the switch tube V1 source -drain voltage waveform, ch3 is the switch tube V2 drive signal, ch4 is the switch tube V2 source -drain voltage waveform). The experimental waveforms in Figure 6 verify the correctness of the above theoretical analysis. 
ch1:20V/div, ch2:20V/div, ch3:15A/div, ch4:15A/div (a) Driving signal and primary winding current waveform 
ch1:50V/div, ch2:50V/div, ch3:50V/div, ch4:50V/div (b) Power tube drain-source voltage waveform Figure 6 Rated working waveform Figure 7 shows the waveforms of the primary winding current and clamping capacitor voltage pulsation at the same Io ( =16A), different Uin and different D (ch3 is the winding Tp1 current waveform i1 , ch4 is the winding Tp2 current waveform i2 , ch1 is the clamping capacitor voltage pulsation Δuc waveform ). The experimental waveforms fully illustrate the correctness of the circulating current analysis in Section 2 and the clamping capacitor selection principle in Section 3.1.2. 
ch3:80A/division, ch4:80A/division, ch1:5V/division (a) U in =24V, I o =16A, D =0.45 
ch3:80A/division, ch4:80A/division, ch1:5V/division (b) U in =32V, I o =16A, D =0.325 Figure 7. Primary circulating current and clamping capacitor voltage pulsation waveforms at different D values Figure 8 is the efficiency distribution curve of a 2kW DC/DC converter, the efficiency of which can reach 93.2%. Figure 9 is a physical picture of the converter. 
Figure 8 Efficiency distribution curve 
Figure 9: 2kW DC/DC converter physical picture 5 Conclusion The simulation analysis and experimental results verify the correctness of the theoretical analysis and formula derivation, indicating that the push-pull forward circuit applied to the converter has the following advantages: 1) It suppresses the drain-source voltage spike of the switch tube, reduces the voltage stress and power loss of the switch tube[5], and improves the overall efficiency; 2) The transformer has bidirectional magnetization and high core utilization; 3) The input current ripple ampere-second integral is smaller than other topologies, reducing the size of the input filter. The converter has high engineering practical value, especially in low voltage and high current situations. About the Author Wang Qi (1980-), male, master student, research direction is power electronics technology. Gong Chunying (1965-), female, professor, is mainly engaged in the research of power electronics technology and applications.
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