An inverter power supply circuit design based on ARM control

　　System overall plan

　　1. Overall design diagram

　　As shown in Figure 1, the inverter system consists of a boost circuit, an inverter circuit, a control circuit, and a feedback circuit. The low-voltage DC power supply DC12V is boosted, rectified, and filtered by the boost circuit to obtain a high-voltage DC power of about DC170V, and then converted by the full-bridge inverter circuit DC/AC and filtered by the LC filter to obtain a sinusoidal AC power of AC110V.

　　The inverter uses the ARM controller as the control core. The feedback signals of the output voltage and current are processed by the feedback circuit and enter the on-chip AD of the ARM processor. After AD conversion and digital PI operation, the corresponding SPWM pulse signal is generated. Changing the SPWM modulation ratio can change the output voltage, thereby completing the closed-loop control of the entire inverter.

　　2. SPWM solution selection

　　2.1、PWM power chip solution

　　Ordinary PWM power supply control chips are used, such as SG3525, TL494, KA7500, etc. The advantage of such chips is that they can directly generate pulse width modulation signals, but their disadvantage is that the waveform linearity is not good, and the oscillation generator relies on the charge and discharge circuit to generate the waveform. When the PWM chip generates SPWM signals, many additional circuits need to be added.

　　2.2 CPU software solution

　　The SPWM pulse is generated by CPU, such as single-chip microcomputer, ARM or DSP. The advantage of this method is that the pulse width can be adjusted by software. It not only has high accuracy, but also the peripheral circuit is simple and cheap.

　　To sum up, STM32F107 (ARM) is selected to complete the generation of SPWM pulses and the control of the entire inverter.

　　3. System hardware circuit design

　　3.1 CPU Controller

　　The CPU is the core of the entire inverter, and is mainly responsible for the collection of feedback signals, digital PI closed-loop calculation, PWM wave output, parameter setting and external communication. The CPU uses the latest STM32F107 series ARM chip launched by ST. This series of chips uses ARM's 32-bit Cortex M3 as the core, with a maximum main frequency of 72MHz. The Cortex core has a single-cycle hardware multiplication and division unit, so it is suitable for high-speed data processing. The chip has three independent conversion cycles, a high-speed analog-to-digital converter with a minimum of 1μs, and three independent digital-to-analog converters with their own independent sampling and holding circuits, so it is particularly suitable for three-phase motor control, digital power supply and network applications. The chip also has a wealth of communication units, including 1 Ethernet interface, 5 asynchronous serial interfaces, 1 USB slave device, 1 CAN device, I2C and SPI modules.

　　3.2 Drive and Inverter Circuit

　　The inverter main circuit adopts a single-phase full-bridge inverter circuit based on H bridge as shown in Figure 2. The single-phase full-bridge inverter circuit is mainly composed of four MOSFETs Q1, Q2, Q3, and Q4. If a load is added between AC and OUT, an inverter circuit is formed. The desired sine waveform can be obtained by controlling Q1, Q2, Q3, and Q4 to turn on and off in a certain order.

　　For this design, the selection of the switch tube is mainly based on its rated voltage and rated current. Here, the IRFP460N channel enhancement MOS tube with a rated voltage of 500V and a rated current of 20A is selected as the switch tube. It can meet the design requirements. In order to limit the driving current of the MOSFET gate, a current limiting resistor needs to be connected in series with the gate to prevent device damage caused by overcurrent.

　　3.3 Filter Circuit

　　The voltage waveform generated on the load resistor by the drive of two SPWM signals is a square wave that changes according to the sine law. It is a bipolar SPWM waveform. What is actually needed is a sine wave with a frequency of 50Hz, so the SPWM wave needs to be filtered. The general PWM inverter uses an LC low-pass filter. For the design of the LC filter, the cutoff frequency of the filter is first considered. The cutoff frequency of the LC filter is shown in formula (1).

　　Taking into account the harmonic distortion of the filter output voltage, the dynamic response of the system, as well as factors such as volume and weight, the cutoff frequency is selected.

　　3.4 Push-Pull Boost Circuit

　　The push-pull boost circuit is composed of two MOSFET tubes with the same parameters and a boost transformer. The push-pull transformer is characterized by high efficiency and low loss, and is suitable for low input and high output. The push-pull boost circuit is shown in Figure 3. It adopts a structure in which two MOS tubes are turned on separately. The IPRF250 field effect tube is selected, with a rated current of 30A and a rated voltage of 250V. It can meet the requirements and has a small internal resistance, which is the most reasonable choice.

 4. System software design

　　The main functions of the CPU are to complete the closed-loop PI control algorithm, send SPWM pulses, fault protection, data display and remote communication. The system software is mainly for programming the STM32 chip. The development environment uses KeiluVision4 software from Keil, Germany, and the programming language is C.

　　The program consists of a main program and several subprograms: communication program, sampling subprogram, PWM interrupt program, display program, etc. After entering the PWM interrupt, the feedback signals of each channel are first collected and processed, as shown in the flow chart in Figure 4, and then the PWM pulse output is generated after the digital PI regulator is operated, and the MOSFET is driven after isolation and amplification by the drive circuit to realize the closed-loop control of the entire inverter power supply system.

　　The inverter adopts full digital control, and all parameters can be set through the display panel. The digital tube can display the input voltage, input current, output current, output voltage, operating status, fault information, etc. of the inverter system in real time. When a fault occurs, the CPU will block all PWM pulses, and then display the fault information such as overvoltage, overcurrent, overload, etc., and the buzzer will sound an alarm.

　　Experimental Results

　　Figure 5 (a) shows the two complementary symmetrical SPWM pulse waveforms sent by the CPU, with a dead time of 3us; Figure 5 (b) shows the driving waveform of the upper and lower MOSFETs in one of the bridge arms of the full-bridge inverter circuit; Figure 5 (c) shows the AC sinusoidal voltage waveform output by the inverter; and Figure 5 (d) shows the inverter current output waveform. From the figure, we can see that the inverter output voltage waveform is almost undistorted, and the output current THD is controlled within 5%, achieving a good control effect.

　　Summarize

　　This paper proposes a design scheme of an ARM-controlled inverter, which is a fully digitally controlled inverter based on ARM (STM32F107). It has the characteristics of high precision, small size, and full digital. All power parameters are set and stored directly through the human-machine interface, and it has the function of remote communication with the host computer. Experiments show that the inverter designed in this scheme can realize the soft start function. When overcurrent, overvoltage, and overload occur, it can quickly block the PWM pulse and turn off the MOSFET, and display the fault information in time, realizing the intelligence of the inverter.