Noise comparions between Schottky and MOSFET

Question

I have a circuit that uses a Schottky diode to provide reverse voltage protection.

However, the diode chosen is on a long lead time and an alternative that is immediately available is required.

I've been looking for an alternative Schottky, but due to a number of factors (e.g. a specific footprint size) I'm struggling to find a suitable device.

I was wondering whether I might be able to use a P-N junction diode, but these will likely add more noise into the system which I want to avoid.

Another idea was to try to use a P-type MOSFET instead. However, I'm not sure if these are noisier than than a Schottky. Does anyone know if there is any work comparing and constrasting the noise between these two devices? Or is there a general rule of thumb that suggests that one is generally considered quieter than the other?

Answer 1

If the diode is used for protection against wrong polarity, then I assume it is connected in the power line, not the signal line, so noise wouldn't be any of my concern. Schottky diodes have a lower voltage drop during conduction than Si diodes. Before considering using a Si replacement I advise you to check whether your circuit still works correctly at the slightly (?) lowered supply voltage. Even if the Si diode would be noisier than the Schottky type, I would assume there is a big fat capacitor across the power lines that will absorb most of the noise.

Answer 2

Reverse voltage protection, so then I assume (you show no schematic!) that you want a diode in series with your supply.

What makes you think that noise on the supply caused by the diode is an issue? Only in few scenarios I can imagine that to be a problem and then I would simply use decoupling capacitors after the diode to short any remaining noise to ground.

A circuit with a PMOS can also work but in general you will need to pay attention to the fact that all MOSFETs have a build-in Drain-Source diode and you do not want that diode conducting the inversed supply voltage. Look for examples how to do this.

As a MOSFET can have a very low Rdson when used properly, it will not add much noise but again, noise should not be an issue and if it is, use supply decoupling.

If you do not use supply decoupling then the variations on the supply voltage caused by the circuits operating will very likely cause more noise than the diode!

If you're desperate about your supply (noise) consider this circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Here if you reverse power it, the fuse will blow.

You can also use a (self recovering) poly fuse.

You can use almost any diode as it just needs to conduct when power is reversed.

The diode is in reverse mode, no current flows in normal operation.

It is still a good idea to add a supply decoupling capacitor.