DC-AC power supply based on SHE-PWM control

1 Introduction :

The inverter power supply with a control circuit composed of a single-chip microcomputer and DSP as the core can realize full digital control of the power supply, but the system is relatively complex, the software workload is large, and the development cycle is long. This design uses a high-performance dedicated control chip based on SHE-PWM (selective harmonic elimination pulse width modulation) and a driver chip with a bootstrap circuit, combined with an inverter bridge composed of IGBT, to form a highly cost-effective inverter power supply design. The output waveform is good and the performance is stable and reliable. And it has been verified through experiments.

2. The structure and working principle of the inverter circuit

2.1 System structure and block diagram

The whole system consists of the following parts: main circuit, PWM generation circuit, drive circuit and feedback circuit. As shown in Figure 1

Figure 1 Inverter circuit system block diagram

The AC input voltage is rectified and filtered before being used as the input of the inverter, which uses a single-phase full-bridge circuit. The output voltage is fed back to the control chip to adjust the output to achieve voltage stabilization.

2.2 Functions and pins of automatic voltage stabilizing inverter chip

The use of dedicated integrated circuits can simplify the control circuit and reduce the size. This paper uses the single-phase 50H _Z automatic voltage-stabilized inverter power supply SHE-PWM control chip of Hongxin Taiye Company. The chip has few peripheral circuit components, high modulation ratio, good output waveform, no need to compile software, and stable performance. SHE-PWM control ^[1] is a calculated PWM control strategy. The basic method is: through the Fourier series analysis of PWM control, the Fourier series expansion is obtained. With the pulse phase angle as the unknown number and certain specific harmonics set to zero, a nonlinear equation group is obtained. This equation group is the detuning PWM model. Control is performed according to the results of the model solution, and the output does not contain these specific low-order harmonics.

The pin diagram and peripheral circuit of the voltage-stabilizing chip shown in Figure 2, the chip 7-10 pins output PWM signals PWM1 ~ PWM4, after the chip is powered on, PWM1 ~ PWM4 output PWM signals normally, when the 11th pin is low level, the signal output by PWM1 ~ PWM4 is 0000 (1 is high level, 0 is low level), when the 11th pin is high level, PWM1 ~ PWM4 output PWM signals normally. The output PWM signal is positive logic. The 12th pin is feedback control, the feedback voltage reference is 0.7V _CC , the feedback voltage is compared with the reference voltage to adjust the PWM output, so as to achieve the purpose of voltage regulation.

Figure 2 Automatic voltage stabilizing inverter chip pins and peripheral circuits

2,3 pins are connected to external crystal oscillator 12MHz. Among them : +3.5V~5V. : 22~33pF. : .

2.3 IR2101

2.3.1 IR2101 performance and structure

IR (International Rectifier) ​​provides a variety of bridge driver integrated circuit chips, such as R2110, IR2101, IR2102, IR2181, etc. This series of chips are dual-channel, gate-driven, high-voltage and high-speed power drivers. They use highly integrated level conversion technology in the chip, which greatly simplifies the control requirements of the logic circuit for power devices and improves the reliability of the drive circuit. At the same time, the upper tube uses an external bootstrap capacitor to power up, which greatly reduces the number of drive power supplies compared to other IC drivers, greatly reduces the volume of the control transformer and the number of power supplies in engineering, reduces product costs, and improves system reliability.

This article uses IR2101, which adopts HVIC and latch anti-interference manufacturing process and integrates DIP and SOIC packages. Its main features include: the floating channel power supply adopts a bootstrap circuit; the gate drive voltage range of the power device is 10V~20V; the logic power supply range is 5V~20V, and a +5V offset is allowed between the logic power ground and the power ground; the COMS Schmitt input with a pull-down resistor can be easily matched with LSTTL and CMOS levels; independent low-end and high-end input channels ^[2] . Its internal structure is shown in Figure 4:

Figure 3 IR2101 internal structure diagram

Figure 4 IR2101 pinout

Pins 7 and 5 are two independent outputs, LO (low-end output) and HO (high-end output), pins 1 and 8 are VCC (low-end power supply voltage) and VB (high-end floating power supply voltage), pin 4 (COM) is the low-end power common terminal, pin 6 is VS (high-end floating power common terminal), pin 2 (HIN) is the logic input control terminal, and pin 3 (LIN) is the low-end logic input.

2.3.2 Typical drive circuit applications ^[2]

Figure 5 IR2101 typical drive circuit

In Figure 5, C2 is a bootstrap capacitor, and VCC charges C1 through D2, C2, load, and S2 to ensure that when S2 is closed and S1 is turned on, the gate of S1 is driven by sufficient energy stored on C1.

2.4 Voltage Feedback

The voltage feedback circuit is shown in Figure 6. When the inverter works normally, the output voltage of the inverter is connected to the feedback transformer, and its secondary voltage is rectified, filtered, and divided to obtain the feedback voltage. The voltage is proportional to the output voltage of the inverter. Adjusting R3 can adjust the feedback voltage, thereby adjusting the inverter output voltage.

Figure 6 Voltage feedback circuit

3 Test results and test analysis

Based on the work done in the previous article, a test prototype was built to verify the performance of the inverter. The main technical parameters are as follows: input voltage 70~80V; output voltage 55V; switching frequency 12.8kH _Z ; filter inductor 2mH ; filter capacitor 16uF; load 100 ohms; output frequency 50H _Z.

A

B

C

Figure 7 Experimental waveform

The experimental results are shown in Figure 7. Figure A shows the voltage waveform when the load is on; Figure B shows the voltage and current waveforms when the load is suddenly increased; and Figure C shows the voltage and current waveforms when the load is suddenly reduced. The experimental analysis shows that the inverter power supply can still maintain a stable voltage output when the load changes suddenly.

4 Conclusion

This paper introduces a design of a DC-AC power supply with few components, high cost performance and no programming required. The test results show that the inverter power supply can maintain a good output waveform and stable operation under the condition of drastic load changes.

References:

[1] Zhou Zhimin. Practical Technology of Inverter Power Supply[M]. Beijing: China Electric Power Press, 2005

[2] Data Sheet No.PD60043-M International Rectifier