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PICList Thread
'using logic level hexfet'
1998\06\01@212602 by andre

Hi to all engineers.

I made PWM  based on 16c71 connection between
pic output to  logic level hexfet  gate  I put 300 ohms resistor.
I am driving  10 A fuel pump. the problem is that ones in a while
when this hexfet gets  hot  PWM stops working until it
cools down. my question is using 300 ohm between pic and FET is
it correct.

any Idea.


1998\06\02@002039 by Bob Blick

Hi Andre,

300 Ohms is OK. You might want to put a pinch-off resistor 10K or so
gate-source in your prototype so if you sit in reset or run the circuit
without a PIC in it the gate will not float.

If you are doing PWM you need a freewheel diode across the fuel or snubber
of some sort to keep the drain-source voltage from exceeding the voltage
rating of the hexfet. Otherwise it will zener and make lots of heat and
also put voltage into the gate and into your PIC. Maybe that is what's

Cheerful regards,
Bob Blick

At 04:36 PM 6/1/98 -0700, you wrote:
{Quote hidden}

1998\06\02@130643 by Chris Eddy


I side with bob on the free wheel diode for sure.

In addition, you must check to ensure that the FET is being driven into and
out of the on condition very quickly.  ( a scope helps with this).  When a FET
is turning on or off, it is in the linear region, and will generate large
waste.  I would check two things..  That the 300 ohm resistor in conjunction
with ?? 1000 pF of gate capacitance is rising slow enough to add a linear
region.  Also, is the pic truly going to 5.00 VDC?  I think that you will find
that it is not.  The question is then whether the PIC's on state is really
enough to do the job.  Also, you might check the current drive of the PIC pin
against the charge time of the FET.  The high end FET driver chips will drive
over an amp at the start of the on signal to overcome the carge time issue.
All of this is more important for PWM, since due to the duty cycle you
experience, a continuous percentage of your time is spent turning on and off.
You can calculate this linear versus overall period by looking at the scope.
The moral of the story is, if you reduce the PWM frequency, you reduce this
duty cycle of waste time and improve waste heat.

On the off chance that you cannot get 5V to drive the part right, mail back
and I will throw some gate drive ideas at you to try out.

Last question (serious one, I would have to check), are the logic level FETS
more expensive?  There may be a trade off.

Chris Eddy

Bob Blick wrote:

{Quote hidden}

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