Simple mosfet question

Thanks! As you can tell, I'm a novice, especially in the area of transistors, sorry for a couple more questions as to the how and why?

I had thought about adding a series resistor at the gate, but I didn't realize it was necessary with a MOSFET?

I'm not sure how you mean to hook up the diodes or zener? The regular diodes would be hooked up cathode at the drain (positive rail) and anode at the gate? And would the zener be hooked up cathode at the gate and anode at the source (ground)? I'm picturing it that way with a ~4V zener? And I would only need one or the other? Do either of the diodes need special ratings to achieve their function?

Argh! Right after I sent this, I realized the zener would be better rated > 4.5 and < max Vgs (~ 20V), so a ~12V zener would be good, right? Not a ~4V which would eliminate the input from ever reaching the gate!

Yeah, that is way lower than the worry zone. I withdraw my comment.

Yes it would limit spikes to some extent, providing it is close to the gate physically, and a low inductance type - ceramic disc or stacked foil/glass or monolithic ceramic type.

With a single cap on the gate you'd be using the inductance of the wire up to the gate as an LC low pass filter and that might eliminate any fast transients present by dropping them across the inductance.

It takes very little energy to punch through a mosfet gate insulator - A zener or mov will protect against higher energy transients than a small cap on the gate.

A fast oscilloscope with a high impedance probe will tell you a lot about what is going on there.

Roger that. 4 volts is too low 10-12 ideal.

Yes.

In a likely real situation it will reduce them, the reduction will depend on the nature of the source. It may not be sufficient. Human body ESD model is around 100pF so with a 20V gate rating (?) you could have 2000V ESD without exceeding the 20V. But a bit (<100uA) of 60Hz leakage and you could exceed the 20V.

You should not bring an unprotected gate lead out. A series R (eg.

10K) from the input and diodes to the supply rails (at the gate) or a zener at the gate should work for you.

Best regards, Spehro Pefhany

Yes, it was a ceramic cap, and yes it was close to the gate. Sounds like the types of transients that are killing this new circuit were the type that such a cap could handle.

At any rate, I'm going to try the resistor/zener combo for the next revision. The more I think about it, the more it makes sense, but I hope I'm not all wrong about why and how it hooks up. A 12V zener, cathode at the gate and anode at the source, with a 10k resistor between input and gate is the way to hook it up, and should protect the transistor gate very well, right? Any transients or spikes above

12V should be dumped directly to the source (ground). Schematic below for this change:

Thanks again guys, I really love having the ability to talk to some people smarter than me about this stuff. Unfortunately, even as novice as I am, I'm about the highest technical level EE here.

And the power rating of the zener should be very insignificant with a

10k resistor there, right? 200mW should be plenty I'm guessing? Or is there another reason why I should look at a higher rating?

Yes, a 12V zener usually is about perfect.

--[10K]---x---------gate | \----\ /\ -- source | | GND GND

If the gate connection could be disconnected at any time, and especially if that could cause harm, it's often a good idea to put something like 100K from the input to ground to ensure things don't float around.

Best regards, Spehro Pefhany

Well with a 10K you'd be charging the gate capacitance slowly - you'd want to look at the time constant and see if the 10K plus your gate capacitance will have a negative impact on the rise time. Unless I botched the arithmetic (double check it) - the shift in phase with 470 pf would be about 5 milliseconds so your 1-2 millisecond pulse would probably be greatly attenuated (there and gone before the transistor ever turned on)

Most folks would probably opt for a lot lower resistance - but it is a trade off. You improve the resistance to a gate spike at the expense of spending more time in the linear region (not a concern with your load) and shift the phase of the signal and delay it (turns on later and slower).

If the 470 cap stays in the circuit, how about a 100 ohm series resistance plus the zener? That way there'd still be some signal there to turn on the mosfet.

Yeah, the zener is only there to clamp the input and probably won't see anything that will harm it - certainly not through a series resistor. (short of a lightening strike)

Anode goes to the source, cathode goes to the gate. Resistor is in series with the signal to the gate and zener is on the gate side of that resistor.

Another thing about the circuit in the schematic - there's no resistor from gate to source to drain the gate voltage. If the signal source is doing that - no problem.

With mosfets (particularly when you are using larger ones with lots of gate capacity) you apply a voltage to the gate, and remove the connection and the mosfet stays on for minutes while the gate charge slowly bleeds off through dirt on the circuit board or through the air.

Helps to know how the thing turns off - if you just got lucky this time and have enough conductive dirt or humidity to turn it off . . . you'd want to know that so when the weather dries or a new board is less conductive it would still work.

If, for instance, your signal source is a open collector pulled high with a resistor - that's no problem because the collector will be low most of the time and give the gate a path to discharge back into the transistor driving it.

Wouldn't it be ~5 microseconds instead of milliseconds?

I think it was 5usec which would be no problem, but I have no reason to use higher resistance that I can think of. I'll lower it down to the 100 to 200 ohm region, I don't see a problem with that either way.

I may end up removing the 470pF cap as it was originally there to filter a noisy signal that should no longer be noisy due to better PCB layout/design. But I will design as if it is staying.

That's what I was thinking. I found a nice 12V zener @ 200mW that I'm going to use.

Because I'm not sure about the design of the servo motor driver, I will probably add a 100k resistor from gate to source. Even if it IS an open collector pulled high, this wouldn't hurt anything, would it?

I hadn't considered all of this! Thanks for the info!

Yes. milliseconds didn't look right to me either. Made the same mistake twice apparently.

Probably not. Last time I took a receiver apart it had a 4.7K pull up for the servo drive.