Snubber or freewheeling diode

[carlz0r]

Hey all. I'm working on a circuit that will allow for switched 12V or ground, as well as a high impedance off state. The end goal is to be able to use this in general purpose automotive circuits, 15-20A continuous. Given that, I'm assuming I will want some sort of snubber circuit, or at the very least some higher power rated diodes between the output and the 12V and ground rails.

Any pointers are greatly appreciated!

[David Hess]

The body diodes built into the power MOSFETs in parallel between the drains and sources already serve as freewheeling diodes and have the same current and voltage ratings as the power MOSFETs.  If for some reason the body diodes are unacceptable, then external schottky diodes with their lower forward voltage drop will need to be used to prevent the MOSFET body diodes from turning on.

A snubber is as simple as a series RC circuit in parallel with the output to ground.

[CopperCone]

I'm guessing you want to take advantage of the internal diode and perhaps a RC snubber to take the edge off?

Is this typically done?

[David Hess]

I'm guessing you want to take advantage of the internal diode and perhaps a RC snubber to take the edge off?

Is this typically done?

The snubber both lowers EMI and limits dV/dT.  Whether this is required or not depends on the application.  If I was laying out a printed circuit board, I would at least include spots for the snubbing capacitor and resistor.

Power MOSFETs have dV/dT limits because capacitive coupling between the collector and base of the parasitic bipolar transistor may turn it on destroying the power MOSFET.  This is not nearly as much of a problem now as it was in the past so it can usually be ignored.  Thyristors also have dV/dT limits to prevent false triggering so snubbers are almost always used with them in applications which include commutation.

[carlz0r]

The snubber both lowers EMI and limits dV/dT.  Whether this is required or not depends on the application.  If I was laying out a printed circuit board, I would at least include spots for the snubbing capacitor and resistor.

Power MOSFETs have dV/dT limits because capacitive coupling between the collector and base of the parasitic bipolar transistor may turn it on destroying the power MOSFET.  This is not nearly as much of a problem now as it was in the past so it can usually be ignored.  Thyristors also have dV/dT limits to prevent false triggering so snubbers are almost always used with them in applications which include commutation.

Perfect thanks, I think my confusion around the diodes was based on seeing BJT based circuits employing an external freewheeling diode because, of course, they don't have the same internal diode as a MOSFET. I'll add RC snubbers to the design so that, as you've said, at least the option is there.

[David Hess]

Perfect thanks, I think my confusion around the diodes was based on seeing BJT based circuits employing an external freewheeling diode because, of course, they don't have the same internal diode as a MOSFET. I'll add RC snubbers to the design so that, as you've said, at least the option is there.

The situation is different with bipolar transistors for the reason you identify; they do not include a built in freewheeling diode by default.  Some are manufactured with this diode deliberately added and bipolar darlington transistors intended for switching applications often include it.

There is usually nothing wrong with using the body diode of a power MOSFET as a freewheeling diode and it has the same voltage and current specifications as the power MOSFET itself.  An external schottky freewheeling diode in parallel with the power MOSFET might be used if an inductive load is switched back on before the magnetizing current decays to prevent the reverse recovery time of the body diode from causing excessive shoot through current.  The lower forward voltage drop of the schottky diode prevents the body diode from turning on.

In fast switching applications like switching regulators, the body diode's reverse recovery time may be too long but they make power MOSFETs with pretty fast body diodes now.  In h-bridge applications, the reverse recovery time may contribute to excessive shoot through currents.