The reason for the negative voltage spike of the switching node is that: in the process of turning off the upper tube and turning on the lower tube, the current of the upper tube gradually decreases, and the current of the lower tube gradually increases, so the parasitic inductance of the lower tube and its loop will To prevent the increase of its current, the direction of the induced voltage is up negative and down positive, so the voltage of the measured switch node appears negative voltage.
The negative pressure peak of the switching node is directly related to the overall parasitic inductance LTL of the lower tube loop and the di/dt of the upper tube turned on. The larger the LTL and di/dt, the larger the negative pressure peak of the switching node:
-VSW= LTL*di/dt
Usually, when measuring the voltage of the switching node, the probe ground of the oscilloscope will be placed on the source S pin of the lower tube. In the actual PCB design, the ground of the PWM or driver IC is usually connected to the ground of the input large capacitor. If the probe ground of the oscilloscope is placed on the ground of the input capacitor, the inductance of this loop will be larger, and the measured negative pressure The spikes will be bigger.