This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CSD18510Q5B: body diode clamping behavior for switch node

Part Number: CSD18510Q5B

Hi,

I'm working on a H-bride application with the FET.  The switch node is known to have ringing due to the parasitic inductance.  My question is why the switch node voltage can go higher than Vbus+0.7V when you have the FET body diode? Wouldn't the body diode act as a voltage clamp?

-Kelvin

  • Hi Kelvin,

    Thanks for the question. Normally in a half bridge, ringing occurs at the switch node after the bottom FET turns off and the top FET turns on. The body diode of the high side FET is effectively shorted by the conducting channel of the FET. The ringing can be anywhere from a few volts up to 2x the input voltage. There is really no clamping mechanism unless the voltage exceeds BVDSS and the FET goes into avalanche. When this occurs, the normally damped sinusoid will have a flat top at the peak. If for some reason, the voltage at the switch node exceeds the input voltage by ~1V when the high side FET is off, the body diode will conduct and clamp the voltage at the switch node. There are many articles and app notes on switch node ringing in synchronous buck converters. Check this one: