Analysis of compensation network of voltage-type high-power switching power supply

introduction

Voltage-type single-loop control is simple and widely used in various fields. When applied to low-power switching power supplies, the compensation network can be simply realized by voltage-dividing feedback and reference amplification comparison. However, it is very difficult to calibrate in high-power circuits and the accuracy is insufficient. At present, it is only satisfied with repeated debugging, which is time-consuming and laborious. This article discusses this issue.

1. Voltage-type PWM inverter control system structure and principle

The final output of the inverter control system can be DC voltage, AC voltage, DC current, AC current, frequency or power, and filtering is required at the output. Most inverter systems output DC voltage, that is, the system outputs and adjusts the DC voltage. Of course, there is also a rectifier circuit on the secondary side of the inverter transformer. Pulse width modulation (PWM) type switching regulated power supply is to only sample the output voltage and implement closed-loop control. This control method belongs to voltage control type and is a single-loop control system. For these systems, the feedback amount is a certain proportional value of the output voltage, and the error signal between the given voltage and the feedback voltage is used to adjust the width of the PWM pulse. We usually call this inverter control system a voltage-type PWM control system. For most voltage-type PWM inverter control systems, whether it is DC output or AC output, the structural block diagram of the control system can be uniformly drawn in the form shown in Figure 1.

Figure 1 Structural block diagram of voltage-type PWM inverter control system

Figure 2 Voltage-type PWM inverter control system

The open-loop transfer function G(S)H(S) of the inverter system is given by:

Figure 3 Amplitude-frequency characteristic curve of voltage-type PWM inverter control system

Its biggest disadvantage is that the current value in the power circuit is not involved in the control process. As we all know, the output current of the switching power supply must flow through the inductor, so there is a 90-degree phase delay for the voltage signal. However, for the voltage-stabilized power supply, the current should be considered to adapt to the change of the output voltage and the demand of the load, so as to achieve the purpose of stabilizing the output voltage. Therefore, only the output voltage sampling method is used, which has a slow response speed and poor stability. Even when the large signal changes, it will produce oscillation, causing power tube damage and other faults.

2. System analysis and design

If the error amplifier (or regulator) is a proportional link, equations (6.12) and (6.13) are both second-order, that is, the system is a second-order system. A second-order system is a conditionally stable system. In addition, since the output filter parameter LC is generally large, the frequency parameter

It is relatively low. Therefore, the system crosses the line L(ω)=0 at a slope of -40dB/dec in the mid-frequency band. In this system, even if a PI regulator is used, it is only to reduce the steady-state error. Therefore, the zero point is also very low, and the mid-frequency band still crosses the zero line at a slope of -40dB/dec, as shown in Figure 3.

In order to make the system meet the requirements of steady-state performance, dynamic performance and stability, correction must be made in the system. Obviously, in the mid-frequency band, adding a series advance correction link (active or passive) as shown in Figure 4 can make the low, medium and high frequency bands of the open-loop amplitude-frequency characteristics meet the requirements, as shown in Figure 4.

Figure 4 Correction network

3. Join the compensation network

At the voltage divider feedback, due to the lag of the inductor voltage, a branch of resistor and capacitor is connected in parallel at both ends of R1 at the feedback point, so that the feedback can instantly reflect the change of the output voltage through the lead of the capacitor voltage. Since the ratio of R1 and R2 is large, a capacitor is connected in parallel at both ends of R2 to sense the slight change of the output.

Its amplitude-frequency characteristic curve is as follows:

Amplitude-frequency characteristic curve of voltage-type PWM inverter control system with correction link.