# MOSFET working in saturation region @ low Vgs

I am trying to regulate a "low current" (20A) by the controlling the voltage between the source and the gate of a MOSFET SiC. I want to make the MOSFET work into this region :

Nevertheless it is not able to regulate 20A. The setup is the following :

I set the power supply in voltage mode and limit it current to 21A. At the beginning I set the voltage VGS to 15V. And I slowly decrease the voltage VGS to hope that at a certain voltage it will limit the current to 20A. At a certain VGs around 3V, the current starts to oscillate. I was not expecting this. I was thinking it will stop conduct if the vgs was too low but I wasn't thinking that it will oscilatte. Then toward 2V, it stops conducting.

Is it normal to see the current oscillating when Vgs is near the threshold voltage ?

Thank you :)

• Is your observation of the LTspice simulation only, or a test-bench physical power supply? Mar 8, 2023 at 12:47
• test bench physical power supply
– Jess
Mar 8, 2023 at 13:04

Is it normal to see the current oscillating when Vgs is near the threshold voltage ?

It's oscillating because the power supply is current limited and, it's the current limiting circuit that is defining the frequency of oscillation. Be very careful that you don't damage the MOSFET in this region; the MOSFET will have a temperature coefficient that will cause really rapid thermal runaway as documented in google and on some of my answers: -

• But why it enter into oscillation ? At Vgs = 15V, the power supply is limiting the current and it does not oscillate ? I agree on the thermal runaway
– Jess
Mar 8, 2023 at 12:38
• The current limit circuit in the power supply works on a knife-edge; at 20 volts and 20.99 amps it isn't current limiting whereas at 21.00 amps, the current limit circuit will potentially drop the output voltage to 0 volts. If you had a 1 ohm power resistor in series with the drain (also in series with the supply) it might help stop the oscillation because it blunts the "knife edge" a little. At a Vgs of 15 volts the MOSFET is thoroughly in the ohmic region and, no matter what you do with the power supply, it'll remain in the ohmic region and not cause the power supply to oscillate. Mar 8, 2023 at 12:47
• However, if you dropped the power supply voltage to around 0.2 volts, it might start oscillating a little bit with Vgs = 15 volts. Mar 8, 2023 at 12:48
• But if the MOSFET begin to limit the current rather than the power supply it should stabilize.
– Jess
Mar 8, 2023 at 12:59
• @Jess it's worth trying just to prove the mechanism. Mar 8, 2023 at 13:21