Design and implementation of low power inverter based on single chip microcomputer

    An inverter is an electrical device that converts steam power into AC power, usually implemented with high-power and high-reverse voltage power electronic devices. In solar power generation, the electricity generated by the photovoltaic array is DC. However, most electrical equipment is powered by AC, so an inverter that converts DC into AC is often required in the power system. In addition, inverters are also widely used in industrial control, communications, transportation and other fields. Sinusoidal Pulse Width Modulation (SPWM) refers to using a sine wave as a modulating wave (Modulating Wave) and a triangular wave F times the frequency of the sine modulating wave as a carrier (Carrier Wave). After waveform comparison, a set of rectangular pulse sequences with equal amplitude and width proportional to the sine modulating wave are generated to equal the sine modulating wave. This paper takes the STC12C5A60S microcontroller as the core, uses its internal two-way programmable counter array (PCA) module to simulate the pulse width modulation method, and 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 schematics and PCB diagrams. 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 The power module
    uses the DC to DC conversion chip MC34063 in combination with LM7805 and LM7812 to obtain 12 V and 5 V DC power, providing the required power for each module of the hardware circuit.
1.2 The pre-stage boost module
    generates two complementary high-frequency PWM (Pulse Width Modulation) pulse waves through the SG3525 chip and its peripheral circuits. These two high-frequency pulse waves are used to control the single-side bridge high-frequency inverter composed of two MOS (IRF 3205) tubes, and realize the pre-stage boost together with the high-frequency transformer. Through the pre-stage boost, the 6 V DC power is increased to a high-frequency AC power 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 filters it through the LC filter as the input of the power frequency inverter bridge circuit.
1.4 Power frequency inverter MOS bridge circuit driver module
    The four MOS tubes that drive the power frequency inverter bridge in this design are completed using the IR2110 chip. The two SPWM control signals generated by the microcontroller are used as the logic input of the two IR2110 chips after the dead time. The drive circuit composed of two IR2110 chips outputs four complementary signals, thereby controlling the on and off of the upper and lower bridge arms of the full-bridge inverter circuit to achieve the inverter function.
1.5 SPWM generation module
    The minimum system built with the 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 read the voltage value on the potentiometer to achieve adjustable inverter output amplitude. The two-way SPWM signal is output from the STC12C5A60S single-chip microcomputer 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 single-chip microcomputer 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-way SPWM waves is shown in Figure 2.

2 System Software Design
    The control chip of this design is STC12C5A60S MCU of 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 MCU in the form of an array. Then use this data table to dynamically set the value of the PCA module comparison register to realize the dynamic change of 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 Test
3.1 SPWM waveform generation module test
    After the SPWM waveform generation module with the microcontroller as the core is run under program control, two complementary SPWM waveforms can be obtained, 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 test
    Input 5 V DC at the input end, and after the system inversion, you can get an AC with an effective value of 110 V and a frequency of 50 Hz 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 arm of the full-bridge circuit, and the other one just corresponds to the upper right arm.

4 Conclusion
    This design is based on the realization of the digital/analog hybrid circuit controlled by the single-chip microcomputer, which greatly reduces the size of the entire hardware circuit. Due to the introduction of the digital single-chip microcomputer, the controllability is greatly enhanced.