Another N channel enhancement mode MOSFET driven directly by a PIC12F629

I have had success using the 12F629 at 5v to replace the 555 and now am looking at driving the
load - 30 red LED's at 20mA for a total of 600mA - with a logic level power MOSFET - Fairchild
FDD8778 in a TO252 package. The gate capacitance is nominally 700pF and nominal gate charge is 9.4nC.
Running through the math the PIC should be able to switch the MOSFET in less than 10 microseconds. The
equivalent capacitance of the LED's is ~600pF so this should not be a problem.

I plan on turning the LED's on for 1 milli-second and off for 1 millisecond so effectively
at a 500Hz rate and am wondering if there will be any issues directly driving the FET.
Some have mentioned needing to have a pull down resistor and a series limiting resistor; looking
at the I/O driver for the 12F629 it looks like it should be able to source/sink 25mA without
an issue. The MOSFET is placed close to the PIC and I'n running wide traces to the drain.

The question is will the PIC be able to directly drive the FET and have I missed anything with my
calculations?

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.

Quote:

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Originally Posted by 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.

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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.

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

Quote:

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Originally Posted by 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

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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.

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.

Quote:

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Originally Posted by 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.

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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'. :confused:

Thanks for the information!

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

Quote:

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Originally Posted by Ioannis

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

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Hi, Ioannis

Without a series resistor, you could have some surprises ( the RMW well known "feature" ) ... try i.e. an IRFZ44 or IRL2203 - or some nF+ range gate capacitance device - ...

tested ( and debugged ) !

Alain

Yes you are right. Thats is why I spoted the "spike to charge gate capacitance". You cannot leave it to luck and some claculations need to be done after the study of the datasheets.

I had to be more careful to my statement and think that many are not engineers, but more like enthusiasts about electronics.

Also this "great" feature, RMW, can bite experienced people too.

Ioannis

in fact, it's quite simple : try not to drain more than the 20 ma max current ...
so, a 220 to 270 Ohm resistor can be considered as the minimum value.

that gives +/- 2/3 µs min turn on/off time ...

and if not fast enough, a mosfet driver has to be used ...

Alain

Or a faster Mosfet with less capacitance or both. All depends.

Ioannis