David Meiklejohn wrote 2012-11-16 12:09:
> Jan-Erik Soderholm wrote:
>> alan.b.pearce@stfc.ac.uk wrote 2012-11-16 10:57:
>>>>
>>> Have a look at Microchips TC442x range of FET driver chips. They have
>>> 1A and 3A chips available in 8 pin DIL (as well as surface mount for
>>> some of them) that will operate to 18V. I am looking at using them to
>>> drive small motors for intermittent use to control model railroad
>>> point motors. They are specifically spec'd to allow some back current
>>> such as would come from FET Miller Effect, which I think will allow
>>> driving small motors without clamp diodes. You get two drivers in one
>>> package, so can do a full bridge.
>>>
>>
>> Maybe it was ment so, but you need a standard MOSFET also. The 1-3 A
>> spec is only for very short times during the actual switch of the
>> driven MOSFET.
>>
>> But yes, they are better to drive power MOSFET's then the PIC pins
>> themself.
>
> Thanks Alan and Jan-Erik!
>
> To be clear, my emphasis is on illustrating the operation of the "P" (PWM=
)
> in the mid-range PIC ECCP module.  That's why I used the [PIC] tag, but
> maybe this should be [EE] if the discussion is mostly about FET drive
> methods.
>
> Anyway, the point is to keep the electronics as basic as practical, so th=
at
> the emphasis in my examples is on explaining how to configure and operate
> the PIC's PWM module, and everything else serves to illustrate that, with=
out
> "muddying the waters".  Not always an easy line to tread, I know, because=
 at
> the end of the day if the FETs die, the PWM hasn't been illustrated...
>
> So, to keep it as simple as possible, I wasn't thinking "power MOSFETs".
> More like a 2N7000, which is rated at only 200 mA drain current (500 mA
> pulsed), for use with a tiny motor with a rated load current of only 70 m=
A.
> As far as I can see, 20 mA gate current is enough to switch those fast
> enough.  But obviously I'll need to test!  :-)
>
>
> David
>

Yes, low power FET's can usualy be switched fast enought.
Check the datasheet and look the the gate capacitance. Note
that during the actual switch you have a short-curcuit for
a short time until the gate capacitor is charged or
decharged.

Use a gate resistor between the PIC-pin and the gate to limit
the current to whatever is specified for the PIC pin, usualy
20 mA.

Another problem is that the PIC probably runs at 5V and you
should preferable use a "logic-level" FET that opens fully
at 5V Vgs voltage (or enought). Now, the 2N7000 has specs
at Vgs 5.0V also, so it might be OK...

Jan-Erik.
--=20
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