Power supply design based on high performance AD ​​converter and DSP

1 Overview

In the case that the traditional inverter power supply adopts analog control and cannot overcome its inherent shortcomings, people are increasingly turning to digital solutions to reduce the complexity of the control circuit, improve the flexibility of power supply design and manufacturing, and adopt more advanced control methods to improve the output waveform quality and reliability of the inverter power supply system. Therefore, the transition from analog control to digital control is an inevitable trend in the development of inverter power supply.

With the development and application of industrial high-speed digital signal processors (DSP), the transformation of inverter power control from analog control to digital control has become possible. Due to its super data processing capability and fast processing speed, combined with high-performance AD ​​converters, DSP can instantly read the output of the inverter power supply and calculate the output PWM value in real time. It is the adoption of DSP that has solved many problems existing in analog control, and some advanced control strategies have gradually been applied to the control of inverter power supplies. In this way, for the uncertainty of the inverter power load, the digital system can dynamically compensate for the harmonics generated by the dynamic changes of the load without human intervention, so that the output waveform quality of the inverter power supply reaches an acceptable level.

Starting from the structure of SPWM inverter power supply, this paper uses the ancient PID control and proposes a digital solution based on the instantaneous value of voltage, which has passed the simulation.

2 Inverter Power Supply Physical Model

In the inverter system, full-bridge or half-bridge structures are often used. Figure 1 is a main circuit diagram of a single-phase full-bridge inverter with an LC filter.

The state equation with Vc and iL as state variables is:

Then the transfer function of Vi to the output voltage V0 is:

Thus, the principle block diagram of the inverter can be obtained, as shown in Figure 2.

3 Digital Control Solution

This system uses dual-loop control PID regulation. PID control is widely used in engineering practice due to its simplicity, easy parameter setting and mature development. The control of inverter power supply is no exception. Dual-loop control not only ensures the steady-state characteristics of the system, but also improves the dynamic performance of the system.

3.1 Digital PID Algorithm

PID control is the most widely used control law. PID stands for proportional-integral-differential. Assume that the PID regulator is shown in Figure 3.

The relationship between the output and input of the regulator is proportional-integral-differential, that is:

If expressed in the form of a transfer function:

Among them: Ti is the integral time constant; Td is the differential time constant; Kp is the proportional coefficient; Kd=Kp/Ti is the integral coefficient; Kd=KpTd is the differential coefficient.

The numerical PID regulator is used in the computer control system, which is the discretization of equation (1). When discretizing, let:

Where: D is the sampling period.

Obviously, in the above discretization process, the sampling period T must be short enough to ensure sufficient accuracy. From equations (4) and (7), we can get:

Formula (8) is the output-input relationship of the digital PID regulator.

The PID algorithm contains the main information of the past, present and future in the dynamic process. Among them, the proportion (P) represents the current information, plays the role of correcting the deviation, and makes the process respond quickly. The differential (D) has a leading control effect when the signal changes, and represents the future information. The integral (1) represents the past information, which can eliminate the static error and improve the static characteristics of the system. Therefore, a well-designed PID controller has the advantages of fast dynamic response, high steady-state accuracy, and strong robustness. It is the most widely used type of controller in engineering practice. For the inverter power supply, since the unloaded SPWM inverter is similar to the critical oscillation link, the integral action will increase the phase lag, which will have a negative impact on the steady-state performance of the system. Therefore, when designing a PID controller with instantaneous value feedback, proportional control (P) or proportional differential (PD) control is always used.

3.2 Digital Control Scheme

The block diagram of the control system is shown in Figure 4.

The system includes two control loops: the outer loop is the voltage effective value control loop, and the inner loop is the output voltage instantaneous value feedback loop. The outer loop performs digital filtering to obtain the effective value of the output voltage; it is compared with the output effective value given Vrms, and its error signal is then adjusted by the PI controller to control the amplitude of the given value of the standard sine wave signal. The inverter power supply is controlled by the effective value outer loop, and theoretically, the output voltage effective value can be steady-state without difference. The purpose of this control loop is to ensure that the steady state of the output voltage effective value remains unchanged when the load changes and the system is disturbed, that is, to ensure the steady-state accuracy of the system's output voltage. The inner loop is the output voltage instantaneous value feedback control loop, which controls the instantaneous value of the output voltage so that the output voltage tracks the given sine wave and maintains the good sinusoidal property of the output. In order to ensure that the system has sufficient stability margin, the controller of this loop mostly adopts a proportional (P) controller or a proportional differential (PD) controller. The main function of this loop is to ensure the sinusoidal property of the output voltage, overcome the influence of interference on the output voltage waveform, and improve the dynamic performance and steady-state performance of the control system.

4 Simulation Results

The various characteristics of the inverter power supply with instantaneous value feedback digital PID control shown in Figure 1 are analyzed in Matlab's graphical simulation environment Simulink. The circuit parameters are: rated power: P = 6kVA; filter inductance: L = 1 mH; filter capacitor: C = 25μF; equivalent series resistance rL = 0.6Ω, rc = 0.1Ω; DC bus voltage: E = 360V; switching frequency: fc = 20kHz; output voltage: 220V/50Hz; the simulation waveforms of the system at no load and different loads are shown in Figure 5.

5 Conclusion

The physical structure and digital PID algorithm of SPWM are analyzed in detail, and on this basis, a dual-loop inverter power supply digital control system based on the feedback of the instantaneous value of the output voltage is proposed. It can effectively reduce the total harmonic distortion (THD) of the output waveform and improve the output waveform quality of the inverter power supply system. The reasonable use of the digital PID control algorithm ensures the stable accuracy of the system output voltage and good dynamic and static performance.