Another N channel enhancement mode MOSFET driven directly by a PIC12F629

[Dave]

At least, you should have a pull down resistor of about 10K ohms so to give the FET gate some kind of state during startup or if by change in your code the pin is made an input.

[vintagecarguy]

At least, you should have a pull down resistor of about 10K ohms so to give the FET gate some kind of state during startup or if by change in your code the pin is made an input.

Thanks for the input; that makes sense to keep the gate low at startup. The system turns on from a cold start so it's good to
set the state low with a pulldown.

[andywpg]

I have been told (by an electronics engineer that I know) that, along with the 10K pull down resistor, you should have something in series with it. His explanation was that a MOSFET can occasionally 'run wild' and the gate essentially shorts to source and will draw amazing amounts of current.

His suggestion was 1K to 5K or so, try it, if it works, great. Since then, I've used a 2.2K and have never had any problems.

Just my 2 cents

[vintagecarguy]

I have been told (by an electronics engineer that I know) that, along with the 10K pull down resistor, you should have something in series with it. His explanation was that a MOSFET can occasionally 'run wild' and the gate essentially shorts to source and will draw amazing amounts of current.

His suggestion was 1K to 5K or so, try it, if it works, great. Since then, I've used a 2.2K and have never had any problems.

Just my 2 cents

The series resistor is between the I/O line from the PIC and the gate of the MOSFET? That makes
sense but I'm thinking it shouldn't be too large because that would limit the charge current to the gate. A 220 ohm
limits the current to 23mA which shouldn't be a problem for the PIC.

Thanks for your comments.

[Dave]

Well actually, The resistor should be rather small, in the order of 10 to 100 ohms Steven. When switching the gate at high frequencies you need to overcome the gate to source capacitance issue. Even at low frequencies if you take a scope and look at the gate, you will notice a delay when switching it on when using a resistor. The larger the resistor the slower the turn on time as well as discharge time. In fact when switching the gate at frequencies of approx.15+ KHz. you should be using some kind of push/pull driving circuit. In the past I have used the IR2183 mosfet driver ic's on all of my PWM full bridge drivers. They will deliver up to 1 amp to the gate. There are others available. Microchip also makes single mosfet drivers.

[andywpg]

Well actually, The resistor should be rather small, in the order of 10 to 100 ohms Steven. When switching the gate at high frequencies you need to overcome the gate to source capacitance issue. Even at low frequencies if you take a scope and look at the gate, you will notice a delay when switching it on when using a resistor. The larger the resistor the slower the turn on time as well as discharge time. In fact when switching the gate at frequencies of approx.15+ KHz. you should be using some kind of push/pull driving circuit. In the past I have used the IR2183 mosfet driver ic's on all of my PWM full bridge drivers. They will deliver up to 1 amp to the gate. There are others available. Microchip also makes single mosfet drivers.

Hmmmm - I guess the fact that I'm just switching relatively slow stuff is the reason I haven't run into trouble with 2.2K. My engineer friend was aware of what I was building, so that's probably why he suggested the fairly large values.

And, of course there's always the possibility that I misunderstood him, and, in my ignorance, went with what I 'remembered'.

Thanks for the information!

[Ioannis]

There are cases where the larger gate resistor is necessary (although rare). If there is an EMC interference, large gate resistor makes the switching more rounded and no high frequency harmonics are generated.

In general we need fast switching mosfets to make losses as low as possible and the efficiency as high as possible.

So, yes, a low resistor in the order of 10-100 ohms max. is what you need. Or even better no resistor. Mosfets have insulated gate and no current flows through the gate (besides the spike to charge gate capacitance).

Ioannis