Design of low power inverter based on STC12C5A60S

This paper takes the STC12C5A60S microcontroller as the core and uses its internal two-way programmable counter array (PCA) modules to simulate the pulse width modulation method. It designs and implements a small inverter with adjustable output voltage amplitude.

1 System Hardware Design

This paper uses AltiumDesigner6.9 to complete the design of hardware circuit schematic and PCB diagram. Figure 1 is the overall circuit structure diagram of the design.

The function implemented by this design is to convert 6 V DC power into industrial frequency AC power with a frequency of 50 Hz and an amplitude of 110 V through three-stage power conversion (DC-HFAC-DC-LFAC) for use by AC loads. The specific design and functions of each part of the hardware circuit are described as follows.

1.1 Power module

The DC to DC conversion chip MC34063 is combined with LM7805 and LM7812 to obtain 12 V and 5 V DC power to provide the required power for each module of the hardware circuit.

1.2 Pre-stage boost module

The SG3525 chip and its peripheral circuits generate two complementary high-frequency PWM (Pulse Width Modulation) pulse waves, which are used to control the single-side bridge high-frequency inverter composed of two MOS (IRF 3205) tubes, and realize the front-stage boost together with the high-frequency transformer. Through the front-stage boost, the 6 V DC is increased to a high-frequency AC of about 300 V, preparing for the subsequent industrial frequency inverter.

1.3 Rectifier and filter module

The rectifier bridge circuit composed of four diodes rectifies the high-frequency AC output by the front-stage boost module, and is filtered through an LC filter as the input of the industrial frequency inverter bridge circuit.

1.4 Power frequency inverter MOS bridge circuit drive module

In this design, the 4 MOS tubes driving the power frequency inverter bridge are completed by the IR2110 chip. The two SPWM control signals generated by the single chip microcomputer are used as the logic input of the two IR2110 chips after the dead time. The driving circuit composed of two IR2110 chips outputs 4 complementary signals, thereby controlling the on and off of the upper and lower bridge arms of the full-bridge inverter circuit to realize the inverter function.

1.5 SPWM generation module

The minimum system built with STC12C5A60S microcontroller as the core is used as the control part of the module. At the same time, an analog/digital conversion circuit is added to realize the adjustable output amplitude of the inverter by reading the voltage value on the potentiometer. The two SPWM signals are output from the STC12C5A60S microcontroller PCA module P1.3 and P1.4. The principle is to use the sine table data to set the value of the comparison register of the STC12C5A60S microcontroller PCA module to simulate the pulse width modulation method, and finally obtain a rectangular pulse sequence with a width proportional to the sine modulation wave to equal the sine modulation wave. The principle of generating two SPWM waves is shown in Figure 2.

2 System Software Design

The control chip of this design is STC12C5A60S microcontroller from Hongjing Company, which has two PCA modules inside. The standard industrial frequency AC sine table is calculated in advance by Matlab and stored in the extended data memory inside the microcontroller in the form of an array. Then this data table is used to dynamically set the value of the PCA module comparison register to achieve dynamic changes in the counting cycle, and the output high-level pulse width changes with the sine rule.

The main program flow chart is shown in Figure 3, and the PCA interrupt subroutine flow chart is shown in Figure 4.

3 System Testing

3.1 SPWM waveform generation module test

The SPWM waveform generation module based on the single-chip microcomputer can obtain two complementary SPWM waveforms after being run under program control, as shown in Figure 5. Several detailed fragments of the single-channel SPWM of the module were captured by an oscilloscope, as shown in Figure 6, and the pulse width is constantly changing.

3.2 Overall system testing

A 5 V DC power is input at the input end, and after the system is inverted, an AC power with an effective value of 110 V and a frequency of 50 Hz can be obtained at the output end of the system. The output voltage is attenuated 10 times by the oscilloscope probe and then connected to the oscilloscope to obtain the graph shown in Figure 7. Another input of the oscilloscope is to detect the SPWM waveform of the upper left bridge arm of the full-bridge circuit, and the other input corresponds to the upper right bridge arm.

4 Conclusion

The design is based on the realization of digital/analog hybrid circuit controlled by single chip microcomputer, which greatly reduces the volume of the whole hardware circuit. Due to the introduction of digital single chip microcomputer, the controllability is greatly enhanced.