How to find out the heating problem of MOS tube through measurement?

Recently, I made a low-power switching power supply. During debugging, it was found that the MOS tube heats up very seriously. In order to solve the MOS tube heating problem, it is necessary to accurately determine whether these causes are caused. It is important to conduct correct tests before Found the problem. Through solving this MOS heating problem this time, it is found that whether the test of correct selection of key points is consistent with the analysis is the key to solving the problem.
In the switching power supply test, in addition to using a three-meter to measure the pin voltage of other components of the control circuit, it is more important to use an oscilloscope to measure the relevant voltage waveform. When judging whether the switching power supply is working normally, where to test can reflect the working status of the power supply, whether the primary and secondary of the transformer and the output feedback are reasonable, whether the switching MOS tube is working normally, whether the output of the PWM controller is normal, including the pulse amplitude And the duty cycle is normal, and so on.
The reasonable selection of test points is very important. The correct selection is not only safe and reliable measurement, but also can reflect the cause of the failure, and find the cause quickly.
Analyze the reasons for the failure of the MOS tube. According to the previous understanding of the switching power supply, the general reasons for the heating of the MOS tube are:
1: The driving frequency is too high.
2: The driving voltage of G pole is not enough.
3: The Id current through the drain and source is too high.
Therefore, the focus of the test is on the MOS tube, and accurate testing of its working conditions is the root of the problem. Select the test point as shown in the figure:

Q1 is a power switch MOS tube, point A is the drain, point B is the source, R is the current sampling resistor, and point C is the ground terminal. Connect the two probes of the dual trace oscilloscope to points A and B respectively, and the ground terminals of the two probes are clamped at the ground terminal C of the resistor R at the same time.
MOS tube drain test point A waveform
And from the waveform at point B, it can be seen that the source voltage waveform of the MOS tube, this waveform is the voltage waveform on the sampling resistor R, which can reflect the drain current and the information such as the on and off time, as shown in the following analysis:

It can be seen that in each cycle, when the switch MOS tube is turned on, the drain current is from the beginning to the peak current.
Sampling resistor R's voltage waveform at test point B
Points A and B, these are the two key test points, which basically reflect the working status and faults of the switching power supply. The peak voltage and peak current when it is turned on are very large. If the peak voltage and peak current can be turned on If the current is eliminated, the loss can be reduced by more than half, and the problem of MOS heating can be solved. Of course, it is also a direct reflection of whether the MOS tube is working normally or not.
After analyzing the test results, change the gate drive resistance value, select the appropriate frequency, and create conditions for the MOS tube to be fully turned on. After the MOS works, the peak voltage is effectively reduced, and the MOS tube with a smaller internal resistance is selected to make The pressure drop of the tube itself decreases during the switching process. At the same time, a reasonable choice of radiator. After this treatment, re-experiment, let the entire power supply work normally, increase the load to full load work, the MOS tube heat never exceeds 50 °, it should be ideal.
In the process of testing with an oscilloscope, pay special attention to the waveforms of these two test points. When gradually increasing the input voltage, if the peak voltage or peak current is found to exceed the design range, pay attention to the heating of the MOS tube. If it is abnormal, you should immediately Turn off the power and find the cause to prevent damage to the MOS tube.