System Overall Scheme
1. Overall Design Framework
The inverter uses an ARM controller as its control core. The feedback signals of output voltage and current are processed by the feedback circuit and then enter the on-chip AD converter of the ARM processor. After AD conversion and digital PI operation, the corresponding SPWM pulse signal is generated. Changing the modulation ratio of SPWM can change the magnitude of the output voltage, thereby completing the closed-loop control of the entire inverter.
2. SPWM Scheme Selection
2.1 PWM Power Supply Chip Solution
Using common PWM power control chips, such as SG3525, TL494, KA7500, etc., has the advantage of being able to directly generate pulse width modulation signals. However, its disadvantage is that the waveform linearity is not good, and the oscillator relies on the charging and discharging circuit to generate the waveform. When the PWM chip is to generate SPWM signals, many additional circuits are required.
2.2 CPU Software Solution
Using a CPU to generate SPWM pulses, such as a microcontroller, ARM, or DSP, has the advantage that the pulse width can be adjusted by software, resulting in high precision and simple, inexpensive peripheral circuitry.
In conclusion, the STM32F107 (ARM) was selected to generate the SPWM pulses and control the entire inverter.
3. System Hardware Circuit Design
3.1 CPU Controller
The CPU is the core of the entire inverter, primarily responsible for feedback signal acquisition, digital PI closed-loop calculation, PWM waveform output, parameter setting, and external communication. The CPU uses STMicroelectronics' latest STM32F107 series ARM chip. This series uses ARM's 32-bit Cortex M3 core, with a maximum clock frequency of 72MHz. The Cortex core has single-cycle hardware multiplication and division units, making it suitable for high-speed data processing. The chip features three independent conversion cycles, with a minimum high-speed analog-to-digital converter of 1μs. Each of the three independent digital-to-analog converters has its own independent sample-and-hold circuit, making it particularly suitable for three-phase motor control, digital power supplies, and network applications. The chip also includes rich communication units, including one Ethernet interface, five asynchronous serial interfaces, one USB slave device, one CAN device, I2C, and SPI modules.
3.2 Drive and Inverter Circuits
The main inverter circuit, as shown in Figure 2, employs a single-phase full-bridge inverter circuit based on an H-bridge. This single-phase full-bridge inverter circuit mainly consists of four MOSFETs: Q1, Q2, Q3, and Q4. Adding a load between AC and OUT forms an inverter loop. By controlling Q1, Q2, Q3, and Q4 to conduct and cut off in a specific sequence, the desired sinusoidal waveform can be obtained.
For this design, the selection of the switching transistor is primarily based on its rated voltage and rated current. Here, the IRFP460N channel enhancement-type MOSFET with a rated voltage of 500V and a rated current of 20A is selected as the switching transistor, which meets the design requirements. To limit the MOSFET gate drive current, a current-limiting resistor needs to be connected in series with the gate to prevent device damage due to overcurrent.
3.3 Filtering Circuit
The voltage waveform generated across the load resistor by the two SPWM signals is a square wave that changes sinusoidally. It is a bipolar SPWM waveform. What is actually needed is a 50Hz sine wave, so the SPWM wave needs to be filtered. A typical PWM inverter uses an LC low-pass filter. For the design of the LC filter, the cutoff frequency of the filter is considered first. The cutoff frequency of the LC filter is shown in equation (1).
Taking into account factors such as the harmonic distortion of the filter output voltage, the dynamic response of the system, and size and weight, the cutoff frequency is selected.
3.4 Push-Pull Boost Circuit
The push-pull boost circuit consists of two MOSFETs with identical parameters and a boost transformer. Push-pull transformers are characterized by high efficiency and low loss, making them suitable for low input and high output. As shown in Figure 3, the push-pull boost circuit uses a structure where the two MOSFETs are turned on separately. The IPRF250 MOSFET is selected, with a rated current of 30A and a rated voltage of 250V. This design meets the requirements while having low internal resistance, making it the most reasonable choice.
4. System Software Design
The CPU's main functions are to implement closed-loop PI control algorithms, send SPWM pulses, provide fault protection, display data, and facilitate remote communication. The system software primarily involves programming the STM32 chip, using Keil Vision4 software from Germany as the development environment, and C language as the programming language.
The program consists of a main program and several subroutines: a communication program, a sampling subroutine, a PWM interrupt program, and a display program. Upon entering the PWM interrupt, the program first acquires and processes the feedback signals from each channel, as shown in the flowchart in Figure 4. Then, after calculation by a digital PI regulator, a PWM pulse output is generated. This output is then isolated and amplified by the drive circuit to drive the MOSFET, thus achieving closed-loop control of the entire inverter power supply system.
The inverter adopts fully digital control, and all parameters can be set through the display panel. The digital tube can display the inverter system's input voltage, input current, output current, output voltage, operating status, fault information, etc. in real time. When a fault occurs, the CPU blocks all PWM pulses, then displays fault information such as overvoltage, overcurrent, and overload, and the buzzer sounds an alarm.
Experimental results
Figure 5(a) shows the waveforms of two complementary and symmetrical SPWM pulses generated by the CPU, with a dead time of 3µs; Figure 5(b) shows the drive waveforms of the upper and lower MOSFETs in one of the bridge arms of the full-bridge inverter circuit; Figure 5(c) shows the inverter output AC sinusoidal voltage waveform; and Figure 5(d) shows the inverter current output waveform. From the figures, we can see that the inverter output voltage waveform is almost undistorted, and the output current THD is controlled within 5%, achieving excellent control performance.
Summarize
This paper proposes a design scheme for an ARM-controlled inverter, which is a fully digital inverter based on ARM (STM32F107). It features high precision, small size, and full digital control. All power parameters are directly set and stored through a human-machine interface, and it also has the function of remote communication with a host computer. Experiments show that the inverter designed in this scheme can achieve soft-start functionality. When overcurrent, overvoltage, or overload conditions occur, it can quickly block PWM pulses and turn off MOSFETs, and promptly display fault information, thus realizing the intelligence of the inverter.
