Electronic – Level Shifting a MOSFET

level-shiftingmicrocontrollermosfet-driverpowermosfet

I have a high current low voltage MOSFET I'd like to drive with a higher voltage and a micro to reduce power losses through it. I built this circuit based off the typical level shifting designs. The MOSFET does not turn off. The circuit has been reconstructed twice, and both times the MOSFET works fine before and after, so I don't think I'm breaking anything. Any ideas as to why its not working?

schematic

simulate this circuit – Schematic created using CircuitLab

I am a bit new to circuits, so please correct me if I've made some very silly mistake.

edit: source and drain of the 2N7000 should not be connected together like they are in the picture, not sure what happened there – source should connect to the logic signal, drain should connect to the power mosfet

Best Answer

What might be happening is that, during turn-off, voltage drop across 2n7000 might be bigger due to 3.3V dc source being near to 2n7000 threshold voltage(check in datasheet) when it is applied across VGS.

Edit:

Explanantion: During turn-off, the charged Q1 gate capacitor has to be discharged and the path following is through the Q2 and logic controller ground. Looking at Q2 datasheet, at VGS = 3.3V, you can get Id = 0.04A(approx, from the ID vs VDS curve). Since Q1 gate current would be discharged at 0.04A causing the turn-off process to be slow. Also the VDS Q2 would increase for cap to discharge slowly since the IdQ2 current cannot increase beyond 0.04A.

Kindly check the signal on oscilloscope at Q1 drain when applying signal. If it follows the same thing, then, we can extend the discussion.

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