- Detail

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

1 overview

when the traditional inverter power supply can not overcome its inherent shortcomings by using analog control, people increasingly turn 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 transformation from analog control to digital control is the inevitable trend of inverter development

with the development and application of industrial high-speed digital signal processor (DSP), the transformation of inverter control from analog control to digital control has become possible. Due to the strong data processing ability and fast processing speed of the overpressure experimental machine, combined with the high-performance AD converter, DSP can instantly read the output of the inverter and calculate the output PWM value in real time. It is the adoption of DSP that solves many problems in analog control, and some advanced control strategies are gradually applied to the control of inverter power supply. In this way, for the uncertainty of the inverter load, the digital system can dynamically compensate the harmonic generated by the dynamic change of the load without human intervention, so that the output waveform quality of the inverter can reach an acceptable level

starting from the structure of SPWM inverter power supply, this paper uses the ancient. A digital scheme based on the instantaneous value of voltage is proposed, which has passed the simulation

2 physical model of inverter power supply

in inverter systems, full bridge or half bridge structure is often used. Figure 1 shows the main circuit structure of a single-phase full bridge inverter with 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:

from this, the principle block diagram of the inverter can be obtained, as shown in Figure 2

3 digital control scheme

this system adopts PID regulation of double loop control. PID control is widely used in engineering practice because of its simplicity, easy parameter setting and mature development. The control of inverter power supply is no exception. Double 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 represents proportion - integral (stable and reliable performance) differential. Set PID regulator as 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 transfer function:

where: Ti is the integral time constant; TD is the differential time constant; KP is the proportion 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 discretization, make:

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:

equation (8), that is, the output input relationship of the digital PID regulator

pid algorithm contains the main information of the past, present and future in the dynamic process. The proportion (P) represents the current information, plays the role of correcting the deviation, and makes the process respond quickly. Differential (d) has the function of leading control when the signal changes, and represents the future information. Integral (1) represents the past information, which can eliminate static error and improve the static characteristics of the system. Therefore, the designed PID controller has the advantages of fast dynamic response, high steady-state accuracy and strong robustness. It is the most widely used controller in engineering practice. For inverter power supply, because the no-load 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, proportional control (P) or proportional derivative (PD) control is always used in the design of PID controller with instantaneous value feedback

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 RMS control loop, and the inner loop is the output voltage instantaneous value feedback loop. The outer loop is digitally filtered to obtain the effective value of the output voltage; Compared with the given VRMs of the output effective value, the error signal is adjusted by the PI controller to control the amplitude of the given value of the standard sine wave signal. Through the effective value outer loop control, the inverter can theoretically achieve the steady state of the effective value of the output voltage without error. The purpose of this control loop is to ensure that the steady state of the effective value of the output voltage remains unchanged when the load changes and the system is disturbed, that is, to ensure the steady state accuracy of the output voltage of the system. The inner loop is the feedback control loop of the instantaneous value of the output voltage, which controls the instantaneous value of the output voltage to make the output voltage track the given sine wave and maintain the good sinusoidal output. In order to ensure that the system has sufficient stability margin, the controller of this loop mostly adopts proportional (P) controller or proportional differential (PD) controller. The main function of this loop is to ensure the sinusoidal output voltage, overcome the influence of interference on the output voltage waveform, and improve the dynamic and steady-state performance of the control system. 4 simulation results

next, the various characteristics of the instantaneous value feedback digital PID controlled inverter power supply shown in Figure 1 are analyzed in the Matlab graphical simulation environment Simulink. Circuit the board mentioned here does not mean that the board parameters of the computer are: rated power: p= 6KVA; Filter inductance: l = 1 MH; Filter capacitance: c=25 μ F； Equivalent series resistance rl=0.6 Ω, rc=0.1 Ω; DC bus voltage: e ＝ 360V; Switching frequency: fc=20khz; Output voltage: 2203. Latent injury: the heat generated by electrostatic discharge electric field or current will damage the component V/50Hz; The simulation waveform of the system under no-load and different loads is shown in Figure 5

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