Power rectifier module using zero voltage switching PWM three-level DC converter

1 Introduction

In the past, phase-controlled power supplies were mostly used in power DC power supply systems, but the efficiency, ripple, noise and other aspects of phase-controlled power supplies were not satisfactory; especially now that maintenance-free batteries are widely used, the ripple of phase-controlled power supplies is large, which makes the floating charge voltage easy to fluctuate, and the battery pulsating charge and discharge phenomenon occurs, which damages the battery and shortens the battery life. Therefore, after the emergence of high-frequency switching power supplies, due to its small size, light weight, superior technical indicators, modular design, and N+1 hot backup, it has been widely used in many fields. After the 1990s, all new or renovated power plants and substations in advanced industrial countries abroad have adopted high-frequency switching power supplies. Today in China, the use of high-frequency switching modular power DC power supply systems has been recognized in the industry and promoted.

With the advancement of high-frequency switching power supply technology, the technology of high-frequency switching rectifier modules for power is also constantly improving. The 3KW power rectifier module introduced in this article uses a zero-voltage switching three-level DC converter with a clamping diode (2). The circuit structure is simple and compact. The control circuit uses a dedicated full-bridge phase-shift control chip. The module has multiple protection functions such as input and output over-voltage protection, short circuit and over-temperature protection.

2 Main circuit design

The main circuit consists of input EMI filter, rectifier filter, three-level DC converter, output rectifier filter, output EMI filter and other units, as shown in the following block diagram:

Main circuit diagram

Among them, the EMI filter plays a role in suppressing high-frequency electromagnetic interference. In order to achieve a good filtering effect, a secondary common-mode inductor is used in this power rectifier module.

[page] In the above figure, the three-level DC converter circuit consists of MOS switch tubes Q1-Q4, freewheeling diodes D1, D2, clamping diodes D3, D4, etc. The resonant inductor L3 (including transformer leakage inductance) and the junction capacitance of the MOS tube form a resonant circuit to realize the zero voltage switch (Zero Voltage Switch, referred to as ZVS) of the MOS tube.

The converter adopts phase shift control, Q1 and Q4 are 180º complementary conduction, and Q2 and Q3 are also 180º complementary conduction. The driving signals of Q1 and Q4 lead Q2 and Q3 by a phase angle, namely the phase shift angle, respectively. Q1 and Q4 are called leading tubes, and Q2 and Q3 are called lagging tubes. The output voltage can be adjusted by controlling the size of the phase shift angle.

The conditions for achieving ZVS: there must be enough energy to draw away the voltage on the junction capacitor of the MOS tube that is about to be turned on, and to charge the junction capacitor of the MOS tube that is about to be turned off. In the leading tube switch, the resonant inductor L3 and the output filter inductor L4 are connected in series, and L4 is generally very large, so the leading tube is easy to achieve ZVS. The lagging tube only relies on the energy of the resonant inductor L3 to achieve ZVS. Increasing the resonant inductance or increasing the current flowing through L3 can ensure the ZVS of the lagging tube, but the resonant inductor causes the loss of effective duty cycle, and the larger its inductance, the greater the impact of duty cycle loss; and the increase in current flowing through the resonant inductor also means that the conduction loss of the MOS tube and the loss of the transformer increase, so it is difficult for the lagging tube to achieve ZVS under light load. Selecting appropriate resonant inductor parameters is the key to the optimal operation of the converter.

Design of resonant inductance Lr:

This power rectifier module has three-phase input of 304V---456V, output of 198V---300V, 10A; switching frequency f=50KHz,

transformer secondary voltage:

Vo_min=(Vo_max+V_d+V_l4)/D_max=320.895

In the above formula, Vo_max is the maximum output voltage of 300V, V_d is the output rectifier tube voltage drop, which is 1.5V, V_l4 is the output filter inductor voltage drop, which is 0.5V, and D_max is the maximum effective duty cycle, which is 0.95.

Transformer primary voltage: Vi_min=268.5V.

Transformer turns ratio: K_cal=Vo_min/Vi_min=1.539

Considering the ZVS condition of the switch tube and the loss of the secondary duty cycle, the following formula is obtained [2]

Lr=8.Cmos.(Vi/2) 2 /3.(Ip_pk_max) 2

Ip_pk_max=Io_max.K_cal, because Io_max=10, so Ip_pk_max=15.39 (A)

In this power rectifier module, the MOS tube uses IRF360, the junction capacitance Cmos=1300pF, Lr=5.53uH,


[page]3 Control circuit design

The control circuit composed of the dedicated phase shift control device UC3879 is as follows:

The two drive signals OUTA and OUTB pass through the push-pull circuit and the isolation pulse transformer to control the two MOS tubes Q1 and Q2. The other two drive circuits OUTC and OUTD are the same.


4 Experimental results

The experimental waveform of this power rectifier module at AC input 380V, DC output 286V, 10A is as follows:

Main transformer primary voltage and current waveform

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5 Conclusion

After the power rectifier module adopts the phase-shifted three-level converter technology, it can run stably for a long time and has the following advantages:

1. The voltage stress of the MOS switch tube is reduced, making the selection requirements of the switch tube low.

2. The voltage spikes at both ends of the rectifier tube on the secondary side of the transformer are suppressed, and the absorption network can be omitted.

3. The switch tube achieves ZVS, which improves efficiency.


References:

1. Ruan Xinbo, Phase-shifted zero voltage switching three-level DC converter, Proceedings of the 14th National Power Technology Annual Conference

2. Ruan Xinbo, Zero voltage switching PWM three-level DC converter with clamping diode, Power World 2003 Issue 2