How to control the switch of power supply

The control circuit outputs the driving signal of the switching tube by sampling the state signal of the power stage. The duty cycle of the switch is adjusted by using the current state of the power stage, so that the system can ensure relatively stable output under various disturbances. It can be seen that the control circuit of switching power supply is also very important in the whole system. Other functions of switching power supply, such as remote control and over-current protection, are realized by control circuit.

The control modes of switching power supply mainly include voltage type, current type, charging type, average current type and V2 type.

First, the voltage control method

The traditional PWM DC-DC converter usually adopts voltage mode control. The voltage mode control method only samples the output voltage and uses it as a feedback signal to realize closed-loop control to stabilize the output voltage. Because it is single loop control, it is easy to analyze and design. However, because the basic switching converter is a second-order system with inductance and capacitance, we know that the second-order system is a conditionally stable system, and it is usually necessary to add a compensation circuit to the control to make the system work stably.

Second, the current control method

According to the control theory, the N-order system needs N independent feedback variables to achieve optimal control, so the current-mode control method which simultaneously introduces two state variables, namely capacitor voltage and inductor current, improves the performance of PWM control strategy. Compared with voltage mode control method, current mode control method has the advantages of automatic current limiting, faster response speed to input voltage change, high stability of closed-loop system and so on. Therefore, the current mode control method has been widely used. However, when the duty cycle of current mode control method is greater than 50%, it will produce open-loop instability and subharmonic oscillation, which needs to be eliminated by adding slope compensation. Because the current ripple amplitude of current mode control method is relatively small, the anti-interference is poor, and there is an error between peak current and average current, so the control accuracy of current is not high.

Three. V2 control method

V2 control method is still double-loop control, but the current ripple in current mode control method is replaced by the ripple of output voltage. Therefore, V2 control method not only has the advantages of fast dynamic response of input voltage and high stability of closed-loop system, but also has ultra-fast response to load changes. However, V2 control method does not have automatic current limiting function, and slope compensation is needed to eliminate open-loop instability and subharmonic oscillation when the duty cycle is greater than 50%.

At present, the information industry has sprung up suddenly, and the microprocessor chip technology has developed rapidly. Microprocessor chips have more and more strict requirements for power performance indicators (especially transient response). V2 control mode is expected to become the first choice of the new generation power supply because of its super-fast response speed to load changes.

Excerpt from: Big Bit forum.