Design of a high performance DC switching power supply based on a single chip microcomputer

DC regulated power supplies have been widely used in many industrial fields. In industrial production (such as electric welding, electroplating or DC motor speed regulation, etc.), a large number of DC power supplies with adjustable voltage are needed. They generally require a DC power supply that can easily adjust the voltage output. At present, due to the high efficiency and miniaturization of switching power supplies [1], traditional linear regulated power supplies and thyristor regulated power supplies are gradually replaced by DC switching regulated power supplies. The main control method of switching power supplies is to use pulse width modulation integrated circuits to output PWM pulses and use analog PID regulators for pulse width modulation. This control method has certain errors and the circuit is relatively complex [2]. This paper designs a power switching power supply with a wide range of continuously adjustable output voltage based on ST's high-performance microcontroller μPSD3354 as the control core. The microcontroller directly generates PWM waves and performs digital control on the main circuit of the switching power supply. The circuit is simple and powerful [3].

2 Power DC power supply system principle and overall design

2.1 System Principle

This power DC power supply system consists of two parts: the main circuit and the control circuit of the switching power supply. The main circuit mainly processes electric energy, and the control circuit mainly processes electrical signals. Negative feedback is used to form an automatic control system. The switching power supply adopts PWM control mode, and the deviation is obtained by comparing the given amount and the feedback amount, and the PWM output is controlled by the digital PID regulator, thereby controlling the output of the switching power supply. Among them, PID regulation and PWM output are both controlled by software by the single-chip microcomputer system.

2.2 Overall system design

The hardware part of the system consists of input and output rectification and filtering circuits, power conversion part, drive circuit, single chip microcomputer system and auxiliary circuits. Figure 1 is a block diagram of the single chip microcomputer controlled power DC power supply.

Figure 1 MCU controlled power supply structure diagram

As can be seen from Figure 1, the 50Hz, 220V AC power passes through the power grid filter to eliminate interference from the power grid, and then enters the input rectifier filter for rectification and filtering, and is converted into a DC voltage signal. The DC signal is converted into a high-frequency AC signal through the power conversion circuit, and the high-frequency AC signal is converted into a DC voltage output through the output rectifier filter circuit [1]. The control circuit adopts PWM pulse width modulation. The PWM control signal with adjustable pulse width generated by the single-chip microcomputer is processed by the drive circuit to drive the power conversion circuit to work. The output voltage is collected regularly using the high-speed ADC conversion channel of the single-chip microcomputer, and compared with the expected value, and PID adjustment is performed based on the error. The voltage acquisition circuit realizes the acquisition of the DC voltage V0 and matches it with the analog input voltage range of the A/D converter. When the switching power supply has overvoltage, overcurrent and short circuit faults, the protection circuit protects the power supply and load. The auxiliary power supply provides DC power for the control circuit, drive circuit, etc.

3. Switching power supply main circuit design

The main circuit of the switching power supply is used to complete the DC-AC-DC conversion. The main circuit of the system adopts a full-bridge DC-DC converter, as shown in Figure 2. The power switching device used in this system is the BSM 50GB120DN2 series IGBT module of EUPEC. Each module is a half-bridge structure, so two modules are required in the full-bridge system. A fast freewheeling diode is embedded in each module.

Figure 2 Main circuit diagram of power DC power supply

4. Control circuit hardware design

4.1 Control circuit structure diagram

The control circuit of the power DC power supply uses the μpsd3354 microcontroller of ST Company as the core. The control circuit mainly completes the following functions: voltage acquisition, A/D conversion, closed-loop regulation, PWM signal generation, IGBT drive and protection, keyboard input and output voltage display. The control circuit mainly includes: microcontroller system, voltage acquisition circuit, IGBT drive circuit and keyboard, display circuit, etc. The structural block diagram is shown in Figure 3. The system controls the on and off time of the power conversion switch through PWM output to complete the stable control of the output voltage, completes the sampling of the output voltage of the switching power supply through A/D conversion, and adopts voltage closed-loop control. When the switching power supply is working, the microcontroller realizes PID adjustment of the PWM duty cycle according to the deviation between the expected value and the voltage feedback value.

Figure 3 Control circuit structure diagram

4.2 IGBT drive circuit design

In order to accurately control the voltage output of the switching circuit, this system uses pulse width modulation to adjust the working state of the switch tube. According to the voltage control algorithm (an improved PID control algorithm can be used), the microcontroller is set to generate square wave signals with different duty cycles, and the switching device is controlled through the photocoupler to adjust the circuit output set voltage value. For the normal operation of the IGBT, appropriate drive is crucial. The task of the drive circuit is to convert the signal sent by the control circuit into a signal that is added between the control terminal and the common terminal of the power electronic device to turn it on or off. At the same time, the drive circuit usually also has functions such as electrical isolation and protection of power electronic devices. This system uses the EXB841 integrated driver of the EXB series of Fuji Electric Company to drive the IGBT [4].

4.3 Sensor Input Channel and A/D Conversion

The system collects voltage signals through voltage sensors, which are received by the microcontroller after A/D conversion. This system uses CHV series Hall voltage sensors to collect voltage, and uses the A/D converter inside the μpsd3354 microcontroller for analog-to-digital conversion. The line connection is simple, and the maximum accuracy is 5mV. It can basically meet the control requirements.

4.4 Keyboard and Display Circuit

The keyboard and display circuit of the power DC power supply are installed on the operation panel and controlled by the single-chip microcomputer. This system uses a self-made 4×4 matrix keyboard, with PB4~PB7 of the single-chip microcomputer as output lines and PB0~PB3 as input lines. The display part uses dynamic digital display, driven by a dedicated digital tube display driver chip MAX7219.

4.5 Other auxiliary circuits

In order to make the DC power supply work reliably and safely, there are some auxiliary circuits in the power supply system, such as overheating, overcurrent and short circuit protection. In addition, there is an auxiliary power supply part to provide the power required by the system.

5. System software design

The system software is mainly composed of the main program and the interrupt service program, which are mainly used to realize the following functions: keyboard scanning, digital display, A/D conversion, digital PID adjustment and PWM waveform generation, etc. Keyboard scanning and digital display are not introduced here. This design mainly uses software to realize the digital control of power DC power supply.

5.1 Main program design

The main program flow chart of this system is shown in Figure 4. After completing the initialization of various variables and I/O, the main process can input the expected voltage value and store it in the register. When the start button is pressed, the power supply system is started. Here, the PWM output duty cycle is set to the minimum value, that is, 0.1%. After starting, the A/D conversion subroutine is called and the key value is read in. After comparing the feedback voltage value with the given voltage value, the PID adjustment operation is called to update the duty cycle of the drive waveform, and then the PWM generation subroutine is called to output the PWM signal, and the output voltage is displayed through the display subroutine.

Figure 4 Main program flow chart Figure 5 PID adjustment subroutine flow chart

5.2 A/D conversion subroutines

Directly using the 10-bit ADC port of the microcontroller, the A/D conversion part of the program is relatively simple. The program only needs to complete the following functions: select the analog input channel and preset the division number; configure the control register ACON; read the value after A/D conversion and return the data in ADTA0 and ADTA1.

5.3 PID adjustment subroutine

PID regulation is implemented by a single-chip microcomputer. The single-chip microcomputer calculates the adjustment amount by subtracting the error obtained by subtracting the given signal from the feedback signal to control the duty cycle of the switch. In the algorithm, a correction is made. When the deviation and the integral sign are opposite, the integral is cleared. Because if the signs are opposite, it means that the integral term has a counter-effect, so the integral term is cleared [5]. The PID control flow chart is shown in Figure 5. The parameters KP, KI, and KD are set during the debugging process.

6. Conclusion

This system combines the switching power supply with the single-chip system to design a power switching power supply with continuously adjustable output voltage. The power supply has high precision, simple circuit, flexible operation and good application prospects. The single-chip microcomputer-controlled DC power supply meets the requirements of the development of new power electronics products in the "four directions", namely, high frequency of application technology, modularization of hardware structure, digitalization of software control and green product performance.