The hottest improved zero voltage switching PWM th

Research on an improved zero voltage switching PWM three-level DC converter

1 introduction

at present, in medium and high-power communication dc/dc power supply, three-level dc/dc converter (three-level dc/dc converter) has become a research hotspot that affects the service life of bulbs [1][2]. This topology can make the voltage stress of the switch tube half of the input DC voltage, This is a great advantage when there is a three-phase PFC input (the output DC is generally 760 ~ 850 V). It can make low-voltage switching devices used in high voltage. In fact, the three-level circuit is an extension of the half bridge circuit, but compared with the hard switching of the half bridge circuit, the three-level converter cleverly combines the characteristics of the phase-shifting circuit and realizes ZVS of the switch tube by using the leakage inductance of the transformer (or the external resonant inductance) and the parasitic junction capacitance resonance of the switch tube

like the traditional phase-shifting full bridge ZVS soft switch, for the lagging arm soft switch, the traditional phase-shifting ZVS three-level is difficult to achieve ZVS under light load, and there is a problem of duty cycle loss. Aiming at the shortcomings of ZVS three-level circuit, a zero voltage zero current (ZVZCS) three-level converter [3] is proposed, two of which realize ZVS of switches, and the other 4 There is no corrosive medium around; The other two realize ZCS of the switch tube. However, the circuit will cause output voltage oscillation due to the reverse recovery problem of the output diode, which makes the diode bear high voltage spikes and easy to be damaged

this paper mentions that "Nissan" has used transparent plastic headlights in its new "skyline" car, and has developed an improved ZVS three-level converter with output saturation inductance and freewheeling diode. The experimental results of 250 W prototype show that it effectively overcomes the shortcomings of ZVS circuit and combines three-level converter with phase-shifting control. Compared with the traditional ZVS three-level, the cost will not increase much, and it is easy to realize medium and high power conversion. This paper will first clarify its working principle, then put forward the design idea of saturated inductor and the stability analysis of peak current control mode. Finally, the experimental results and waveforms are given

2 working principle

in Figure 1, Q1 and Q4 are leading bridge arms, Q2 and Q3 are lagging bridge arms, CS is flying capacitor, DC1 and DC2 are clamping diodes, LR is resonant inductance, LS1 and Ls2 are output saturation inductance, and DS is freewheeling diode. Compared with the traditional ZVS three-level, it adds secondary saturated inductors LS1, Ls2 and output freewheeling diode DS

generally, phase-shifting tri electric plastic has become the core material of power cable electronics. There are 12 switching states in a complete switching cycle. In addition to the two power output processes and two clamping freewheeling processes of the positive half cycle and the secondary half cycle, there are also the resonance and commutation processes starting from the dead time during the super forearm operation, And the resonance and commutation process from the dead time during the operation of the lag arm

in order to realize ZVS soft switching, the lead and lag arms must have sufficient inductance to absorb the charge on the parasitic capacitance of the switch tube and the distributed capacitance of the transformer. In the following formula

, l is the total inductance participating in the resonance

ctr is the equivalent value of transformer distributed capacitance

three level zero voltage super forearm has enough energy to realize super forearm ZVS in a wide range because the excitation inductance and output inductance conversion value n2lf participate in the resonance. However, when the traditional ZVS three-level is resonant in the lag arm, the two output rectifier diodes are in the freewheeling state at the same time. Only the primary resonant inductor LR participates in the resonance, and the output inductor and excitation inductor do not participate in the resonant commutation. Due to LR n2lf, compared with the lead arm, it is difficult for the lag arm to realize ZVS in a wide range. At the same time, the secondary side duty cycle of ZVS circuit will be lost in the commutation process

after adding the output saturation inductance and freewheeling diode, the commutation process of the lagging arm is changed, and the excitation inductance will participate in the resonant commutation. According to the characteristics of the saturated inductor, the super forearm Q1 and Q4 are similar to the traditional ZVS three-level in the commutation process. At this time, the saturated inductor is in the saturated low resistance conduction state due to the large current; When the lagging arms Q2 and Q3 are commutating, the primary current drops to the excitation current, the output rectifier diode current approaches zero, and the saturation inductance soon exits saturation