Circuit explanation of phase shifting full bridge soft switch for electroplating
2023-04-14 09:19
Due to the special nature of the industry, the power supply for electroplating is significantly different from the ordinary power supply, which requires higher current output and lower output voltage. The current requirement of electroplating power supply ranges from several kilowatts to tens of kilowatts, and most of this relatively high-power electroplating power supply uses thyristor phase-controlled rectification method.
This article will introduce a switching power supply for electroplating, whose output voltage is adjustable from 0 to 12V, current from 0 to 5000A, and full-load output power is 60kW, using technologies such as ZVT soft switching and better heat dissipation structure.
Main Circuit Topology
Considering the need for high-current output, the design uses a full-bridge topology with IGBT as the power switching device in the high-frequency inverter part of the main circuit. The entire main circuit is shown in Figure 1, including: frequency three-phase AC input, diode rectifier bridge, EMI filter, filter inductor capacitor, high-frequency full-bridge inverter, high-frequency transformer, output rectifier link, and output LC filter.
To prevent bias magnetization, the circuit includes a spacer capacitor Cb, which is used to balance the volt-second value of the transformer. Considering the efficiency, the resonant inductor LS utilizes only the leakage inductance of the transformer itself. This is because if the inductor is too large, it will result in excessive turn-off voltage spikes, which are extremely detrimental to the switching tube and also increase the turn-off losses. On the other hand, it will also cause severe duty cycle loss, causing a higher peak current in the switching device and making the system performance lower.
Zero-Voltage Soft Switching
The control method of the high frequency full bridge inverter is phase shifted FB2ZVS control method, and the control chip is UC3875N from Unitrode. zero voltage soft switching is achieved in the full load range for the over-bridge arm, and in the lagging bridge arm in more than 75% load range. Figure 2 shows the drive voltage and collector voltage waveforms of the hysteresis bridge arm IGBT, which can be seen to achieve zero voltage turn-on.
The switching frequency of 20kHz is chosen to reduce the turn-off loss of the IGBT on the one hand, and to reduce the size of the power transformer and output filtering link by taking into account the high frequency on the other hand.
The output current is limited in order to protect the switching components in the power supply from overload capacity. Figure 3 shows the schematic diagram of the control circuit. After adding the current inner loop, not only can the output current be limited, but also can improve the dynamic response of the output and help reduce the ripple of the output voltage.
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