The original factors that caused the instability of the switching power supply loop

In the design process of the switching power supply, the design of the control loop is very important, and even the success or failure of the power supply can be determined. Therefore, the loop problem encountered in the design is mostly a core problem. Power loop compensation design is often seen as a difficult task, especially for inexperienced power supply designers. In the actual compensation design, in order to adjust the value of the compensation component, it is often necessary to perform numerous iterations.

Let's take a look at the problem below.

A 4-cell lithium battery charging and system powered module was used with the Linear Technology IC. Input 14~30V, output 16.8V, VBAT maximum charging current 3.3A. The problem with VOUT output 3A (measured 4A is no problem, efficiency 91%) is loop compensation - loop instability.

Detailed description: The test just arrived after the test, input 15V, output 16.8V, the output current is more than 2A, the inductor whistle, and the inductance is hot, the efficiency at this time is about 80%. Basically, it can be judged as loop instability, but at that time, I didn't know how to solve it. I measured the compensation pin ITH or VC directly in the 2A or 3A state. After the oscilloscope probe was touched, the inductor no longer screamed and was no longer hot.

Check out the relevant information, mainly TI: loop compensation is easy, and Sprinter: switching mode power supply modeling and loop compensation design.

One of them is mentioned: the general behavior of loop instability is: instability

Typical symptoms of a power supply include: audible noise from magnetic components or ceramic capacitors, jitter in the switching waveform, output voltage oscillations, power FET overheating, and more.

Judgment mode use: A method to quickly determine if run instability is caused by loop compensation is to place a large 0.1μF capacitor between the feedback error amplifier output pin (ITH) and IC ground. (Or, in the case of a voltage mode power supply, this capacitor can be placed between the amplifier output pin and the feedback pin.)

This 0.1μF capacitor is generally considered large enough to extend the loop bandwidth to low frequencies, thus ensuring voltage loop stability. If the power supply becomes stable after using this capacitor, the problem may be solved by loop compensation.