Electronic – I need some help understanding overshoot and ringing in MOSFET half bridges


From FDA75N28 datasheet:

What I am having difficulty understanding are the reasons behind overshoot and ringing in V_DS.

Now, to what I already know:

  • The current that passes in an inductor cannot change abruptly, but has to change gradually.

  • Initially current is flowing through the Driver FET, but then this transistor turns off.

  • The current has to keep going somewhere, and the easiest way is to flow through the freewheeling diode of DUT transistor. This makes V_DS negative, as the drain pin has a higher potential than the source pin, thus forward biasing the body diode.

  • Before all the excess current is dissipated in whatever parasitic resistances the circuit has, the driver transistor turns on again.

(I need help undestanding what happens between these two steps and am not 100% sure of the next)

  • The body diode of DUT transistor becomes reverse biased right after it was forward biased. Because the depletion region takes some time to reestablish, current starts flowing in opposite direction to what is normal in the diode. From the datasheet, this phenomenon does not take too much long, 320ns which is the reverse recovery time. The current passing in the inductor keeps flowing in the same direction but this time through the driver transistor instead of through the body diode of the DUT transistor.

Additional remark:

As far as I can tell, the potential of the positive probe, V_D, never changes, so for an overshoot to happen, the negative probe has to go below the negative pin of the power supply. I cannot see what can cause this.

Why the ringing after overshoot? I have talked with a teacher that only has a general knowledge in electronics and he suggested that it could be a parasitic capacitance somewhere that was auto-oscillating with the inductance. I would like to know what exactly causes this.

Best Answer

What I am having difficulty understanding are the reasons behind overshoot and ringing in V_DS.

The ringing is due to the parasitic capacitance of the MOSFET reacting with the inductor as an underdamped 2nd order system. This capacitance can be in the range of 100 pF to 1nF. Read the MOSFET data sheet - it looks like it will be about 900 pF.