MOSFET conducting when Vgs=0V

Thread Starter

buryerohan

Joined Mar 16, 2023
2
I have a circuit as shown here and the expected waveforms are shown besides. When iL>0, the mosfet Q1 and diode D2 conducts the current and vice versa.

doubt.jpg

I see in the practical circuit implementation that the switch Q1 conducts current when no turn on voltage (Vgs1) is provided to Q1. Can someone please explain this behavior happening here. It would be really helpful. I have attached the waveforms obtained from oscilloscope to show this behaviour. The blue color waveform is a current iL through the branch shown in the circuit diagram . Ideally when iL<0, switch Q2 and diode D1 is conducting. As soon as iL=0 the switch Q1 block the voltage Vac, but what is seen from the oscilloscope is that Q1 conducts for some duration the current even when Vgs1=0V. What is the explanation for this? The diodes used here are Schottky barrier diodes.

doubt2.png
 

RoofSheep

Joined Mar 7, 2023
36
I suggest that you look at the drain-source voltage of Q1. Remember that the inductor current will always find a path, even if it means driving a MOSFET into breakdown.
 

Thread Starter

buryerohan

Joined Mar 16, 2023
2
I suggest that you look at the drain-source voltage of Q1. Remember that the inductor current will always find a path, even if it means driving a MOSFET into breakdown.
Thanks for your response. I am attaching here the Vds waveform for your reference. The only thing I could infer from Vds waveform is that the time duration for which Vds makes a transition from zero to some finite voltage is when the mosfet conducts. could it be related to the turn off speed of the mosfet? Please comment? But if Vgs is zero there shouldn't be channel formed and no path for the current to flow.
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RoofSheep

Joined Mar 7, 2023
36
Another way the MOSFET can conduct is when the parasitic diode goes into breakdown. I suggest that you take a look at the following document: https://www.infineon.com/dgdl/mosfe...BVDSS, is the,and source are shorted together.

What is the breakdown voltage of your MOSFETs and diodes?

The inductor will manipulate the voltage across itself in such a way that it finds a current path. This path can even be trough air. This is what happens in the sparkplug of a car.
 

crutschow

Joined Mar 14, 2008
34,281
You need a diode across the inductor (cathode to positive) to suppress the inductive voltage spike that can cause voltage breakdown in the MOSFETs.
 
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