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As the global energy crisis intensifies, electric vehicles as green vehicles will become the trend of future car development. At present, China has completed the research and development of electric large and medium-sized passenger cars, and has been put into demonstration operation as an ideal daily public transport in some cities. The opening of electric bus lines in the bustling area of ​​the city can effectively solve the problems of automobile exhaust emissions and shortage of petroleum raw materials. Therefore, charging technology has become one of the key technologies for the development of electric vehicles. It is of great significance to develop high-power high-frequency intelligent chargers for building electric bus public charging stations.
The main circuit of the charger adopts the phase shift control ZVZCS PWM full-bridge converter. The capacitor C and two diodes Dc and Dh are added to the secondary side of the transformer. The simple auxiliary circuit reset current is used to realize the ZVS of the leading bridge arm and the lag bridge arm. ZCS.
1 charger main circuit topology
The traditional phase shift control full-bridge soft-switching circuit realizes zero-voltage switching by using the transformer leakage inductance or the resonance of the primary series inductance and the parasitic capacitance of the power switch tube. The lag bridge arm mainly relies on the leakage energy storage of the transformer, which causes the lag bridge arm to not easily meet the conditions of zero voltage switching. This paper uses a circuit topology of ZVZCS, as shown in Figure 1.
Vs is the DC voltage obtained by rectifying and filtering the single-phase or three-phase AC input. S1, S2, S3, and S4 are power switching devices, C1 and C3 are parallel capacitors of the leading bridge arm, Llk is the leakage inductance of the transformer, and T is Transformer, D1, D2, D3, D4 are freewheeling diodes, the auxiliary circuit is composed of clamp capacitor C and two diodes Dc, Dh, Lo is the output filter inductor, and Co is the output filter capacitor.
First, S1 and S4 are turned on, the primary side outputs energy to the secondary side, and the clamp capacitance Cc is charged to the maximum value. Turn off S1, the primary current Ip charges C1, and discharges to C3. Because of the existence of C1, S1 is zero voltage off. At this time, the leakage inductance and the output filter inductor Lo are connected in series to provide energy together; the primary voltage and the secondary voltage Both fall, when the secondary voltage drops to the clamp capacitor voltage, due to the action of Cc, the transformer secondary voltage drops faster than the primary side, resulting in a voltage difference, acting on Llk causes the primary current to drop. C3 discharges to zero, providing S3 with zero voltage turn-on conditions. The secondary side induced voltage acts on Llk, accelerating the drop of the primary current Ip until Ip is completely reset. The switch mode is +1/0, and the 0 state is in the current reset mode. The clamp capacitor Cc provides the load current and the secondary voltage drops. Cc discharge is complete, rectifier diodes D1 ~ D4 all conduct freewheeling, during the freewheeling period, because the primary current has been reset, at this time off section S4, open S2, due to leakage inductance Llk primary current can not be abrupt, S4 zero current off , S2 zero current is turned on.
2 main circuit working process analysis
The full-bridge converter has nine operating states in half a cycle, which are recorded as mode l to mode 9.
2.1 mode l
S1 and S4 are turned on, the primary current flows through S1, Llk, the primary winding, and S4; the secondary current flows through D1 and L. , R. , D4 and the secondary winding, Cc is charged by Dc, Co, and the input side transmits energy to the output side. The circuit is simplified. As shown in Fig. 3, since the output filter inductor Lo is larger than Llk, it is regarded as a constant current source, and the equivalent circuit is shown in Fig. 4.
2.2 Mode 2
When cosωat=-l, VCc(t) reaches the maximum value, then sjmωat=o, Ip(t)=nIo, Ic(t)=0, diode Dc turns off, and the secondary current of the transformer flows through D1 and L. , Co, R. , D4 and secondary winding, simplified circuit shown in Figure 5. at this time:
2.3 Mode 3
S1 is turned off, the primary current is transferred from S1 to C1 and C3, C1 is charged, and C3 is discharged. The simplified circuit is shown in Fig. 6. Due to the presence of C1, S1 is zero voltage off. Transformer primary leakage inductance Llk and output filter inductance L. In series, the Llk value is small, and the Lo value is large, which can be regarded as the primary current Ip is basically unchanged, Ip(t)=nIo. The transformer primary voltage Vab and the rectifier bridge output voltage Vrec decrease linearly with the same slope:
2.4 Mode 4
When the rectifier bridge output voltage Vrec linearly falls to the clamp voltage VCc=2 (nVs-Vo), Dh is turned on, and the simplified circuit is as shown in FIG. Since Cc is much larger than C1+C3, Cc keeps the voltage at both ends constant, so that the output voltage of the rectifier bridge drops slower than the primary side voltage, causing the voltage difference to act on Llk, causing the primary side current Ip to start to fall, the equivalent circuit such as Figure 8 shows.
2.5 mode 5
C3 is discharged to O, D3 is turned on, and the simplified circuit is turned on as shown in Fig. 9 at this time. Due to the presence of D3, S3 is turned on at zero voltage. The primary voltage Vab=O. The equivalent circuit is shown in Figure 10.
At the end of this mode, the primary side current is reduced to zero and the rectified side voltage is Vβ.
2.6 Mode 6
The primary current is reset to zero, and the simplified circuit is shown in Figure 11. Cc provides the load current, and the secondary side rectifier bridge output voltage drops rapidly. The equivalent circuit is shown in Figure 12.
at this time,
2.7 Mode 7
Cc is discharged to zero, rectifier diodes D1 to D4 are all turned on, and the load current is freewheeled through the rectifier diode. The simplified circuit is shown in FIG. Section S2 can be turned off during freewheeling, at which point S2 is zero current off.
2.8 Mode 8
S4 is turned on, and the simplified circuit is as shown in FIG. At this time, the zero current is turned on. Due to the presence of the leakage inductance Llk, the primary current cannot be abruptly changed, and the Ip line shape increases.
3 Simulation and experimental results and conclusions
In this paper, the saber simulation software dedicated to power electronics is used to build the model and simulate it. The simulation parameters are as follows:
The simulation waveform is as shown in FIG.
In the experimental system, the 380 V three-phase AC is rectified to supply DC voltage, a charger, a purely resistive load, and an oscilloscope. The test waveforms of the transformer primary voltage, primary current, and secondary rectifier output voltage are shown in Figures 16 and 17.
Fig. 16 is a waveform of the primary side current Ip and the primary side voltage Uba (-Uab) at an input voltage of 508V.
Figure 17 shows the primary side current Ip and the transformer secondary side rectifier bridge output voltage Vrec waveform when the input voltage is 508V. The developed electric vehicle charger adopts a full-bridge converter, and the main current is reset by adding a clamp capacitor and a freewheeling diode on the secondary side of the transformer, so that the power switching device of the main circuit operates in a zero voltage and zero current state. The switching loss is reduced, and the soft switching of the power supply is realized.
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