Design of high-power variable-frequency adjustable power supply

　　1 Introduction

　　The application of sinusoidal pulse width modulation and variable frequency speed regulation technology in the field of industrial control is becoming more and more extensive. Many power test instruments require high power and high performance to meet the test requirements of power equipment. At present, the core power devices of high-power switching power supplies on the market mostly use MOSFET semiconductor field effect transistors and bipolar power transistors, which cannot meet the requirements of small size, high frequency and high efficiency. MOSFET field effect transistors have the characteristics of fast switching speed and voltage type control, but their on-state resistance is large and it is difficult to meet the requirements of high voltage and high current; although bipolar power transistors can meet the requirements of high withstand voltage and high current, they do not have fast switching speeds and are current-controlled devices, requiring a large power drive. Insulated gate bipolar power crystal IGBT integrates MOSFET field effect transistors and bipolar power transistors, and has the advantages of voltage type control, large input impedance, small driving power, fast switching speed, high operating frequency, and large capacity. The inverter power supply developed with high-performance insulated gate bipolar power crystal IGBT as the switching inverter element and variable frequency amplitude modulation technology has the advantages of high efficiency, reliable performance, and small size.

　　2 Working Principle

　　The power supply adopts high-frequency inverter technology, digital signal generator, sinusoidal pulse width modulation and variable frequency amplitude modulation, timing control power-on and series resonant output. The power supply has the advantages of high efficiency, large output power and small size. Its overall principle block diagram is shown in Figure 1.

　　The sine wave generated by the digital signal generator is modulated by a 25kHz triangular modulation wave to obtain a sine pulse width modulation wave, which drives the inverter element IGBT through the drive circuit. By changing the frequency and amplitude of the sine wave, the frequency modulation and amplitude modulation output can be achieved. The inverter output is a series resonant output, which filters out the high-frequency carrier signal to obtain a sine signal of the required frequency. The timing control circuit is used to control the power supply to power on slowly when it is powered on, ensuring that the current is stable when the power is powered on, and at the same time avoiding the impact caused by non-zero-crossing switches; a fault locking function is also designed in the control circuit. Once the power fails, the locking function will prohibit the opening of the IGBT. When a fault occurs, the IGBT is locked and opened, and the large-capacity filter capacitor will store a high amount of electrical energy. Therefore, the power supply part has fault protection to automatically cut off the working power supply and automatic discharge functions. The whole machine is designed with perfect protection functions such as double overcurrent, overvoltage and overheating.

　　3 Control and drive circuit

　　The control circuit refers to the main control circuit, including the generation of sinusoidal pulse width modulation waves, duty cycle adjustment and fault lockout circuit. The sinusoidal modulation wave of the control circuit can adjust its frequency according to the actual application. The drive circuit uses the IGBT dedicated drive module EXB840 produced by Mitsubishi. This drive module can drive IGBTs up to 150A/600V and 75A/1200V. The internal drive circuit of this module makes the signal delay ≤1μs, so it is suitable for switching operations up to 40kHz. When using this module, please note that the IGBT gate-emitter loop wiring must be less than 1M, and the gate-emitter drive wiring should use twisted wire. The drive circuit of EXB840 is shown in Figure 2. 　　4 Inversion and buffer circuit

　　The power supply adopts a half-bridge series resonant inverter circuit, and the principle of the main circuit is shown in Figure 3. In the high-power IGBT resonant inverter circuit, the structural design of the main circuit is very important. Due to the parasitic inductance of the leads in the circuit, the surge peak voltage Ldi/dt excited on the inductor when the IGBT switches cannot be ignored. Since this power supply adopts a half-bridge inverter circuit, it will produce a larger di/dt than the full-bridge circuit. Correctly designing the overvoltage protection, that is, the buffer circuit, is very important for the normal operation of the IGBT. If the buffer circuit is not designed properly, the loss of the buffer circuit will increase, which will cause serious heating of the circuit and easy damage to the components, which is not conducive to long-term operation.

　　The process is: when VT2 is turned on, as the current rises, under the action of the line stray inductance Lm, Uab drops to Vcc-Ldi/dt. At this time, the buffer capacitor C1, which was charged to Vcc in the previous working cycle, discharges through the anti-parallel diode VD1, VT2 and the buffer resistor R2 of VT1. In the buffer circuit, the instantaneous conduction current ID1 flowing through the anti-parallel diode VD1 is the sum of the current IL flowing through the line stray inductance and the current IC flowing through the buffer capacitor C1. That is, ID1 = IL + IC, so IL and di/dt are much smaller than the unbuffered circuit. When VT1 is turned off, due to the action of the line stray inductance Lm, Uce rises rapidly and is greater than the bus voltage Vcc. At this time, the buffer diode VD1 is forward biased, and the energy storage (LmI2/2) in Lm is transferred to the buffer circuit. The buffer circuit absorbs the energy storage and will not cause a significant increase in Uce.

　　5 Calculation and selection of buffer components

　　Where: f is the switching frequency; Rtr is the switching current rise time; IO is the maximum switching current; Ucep is the transient voltage peak.

　　When selecting components for the snubber circuit, capacitors with higher withstand voltage should be selected, diodes with high performance and fast recovery should preferably be selected, and resistors with no inductance should be used.

　　6 Conclusion

　　This power supply has been successfully applied to high-power electrical test instruments. Compared with traditional methods, it not only has high measurement accuracy, but also improves work efficiency, increases work safety, and reduces labor intensity.

　　References

　　1 Li Mengjin. Insulated gate of power electronic devices - bipolar transistor and its application. Electrical Measurement and Instrumentation, 1997 (10)

　　2 Ren Tianliang. Design of 300W zero current quasi-resonant DC power supply. Power Electronics Technology, 2000 (3)

　　3 Tian Jian et al. Research on transient protection of high power IGBT. Power Electronics Technology, 2000 (4)