MOSFET body diode as flywheel

Hey,

I'm building a small board that drives an inductive load (solenoid) using an N-Channel MOSFET.

Since I have an **broken link removed** lying around (which is a double MOSFET SO-8 chip) I was thinking of using one of the MOSFETs as the driver, while using the other's body diode as the flywheel.

I know how bad it sounds, but this is a one-build project, and board-estate is tight... (plus, getting SMD components isn't easy for me)

I am looking for your opinion, because it feels quite wrong, and I wanted to know if it's just "ugly", or fundamentally broken as a concept.

Thanks!

Hello,

Using a body diode as freewheel diode can be done. When the switching mosfet goes on AND the freewheel diode is still conducting, then you might have a problem.

Many mosfets have slow body diodes. When you drive the switching mosfet aggressively, you will get a high current through your switch and the body diode that you use as freewheel diode. Just after driving your switch, the conducting freewheel diode appears as a short circuit. It takes time before it starts blocking (reverse recovery behavior).

When you only switch when the current through the solenoid is zero (hence freewheel diode current is zero also), you only have to check current, voltage rating and dissipation. You may add a series resistor to avoid/reduce ringing and to increase the current decay in your solenoid. Doing this, increases the voltage stress on your switch.