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PICList Thread
'PIC MOSFET control'
1995\12\01@185115 by mlk

picon face
Hi all,

       I have a puzzler which is related to PICs only because I use a PIC to
control the circuit.  If you want to save your mail reading bandwith
for PIC only questions skip to the next message now.....

       I am using a PIC to switch a full-wave
rectified signal to a load through a MOSFET.  The actual circuit is a PIC
16C57 output driving a CMOS 4050 buffer which drives the gate of an IRF520
n-channel MOSFET.  The switched signal is a full-wave rectified 24vac
(rms) so that means roughly 12volts rms (upper half of the waveform).
Here is the circuit:

                                  ^         _      _      _
                                  |        / \    / \    / \
                               12vrms     /   \  /   \  /   \
                              ____|______/_____\/_____\/_____\______
                                  24vac (rms), full-wave rectified
                                                |
                                                |
                                   load (4 ohm, pure resistance)
                                                |
                                                |
                                              drain
                                                       }
       PIC-------->MHC14050------->IRF520 gate         } rds(on)=0.27ohm
                                                       }
                                              source
                                                |
                                                |
                                  ground (common to PIC and rms waveform)


I use the PIC to sense the zero of the rectified signal and turn on the
MOSFET at low voltage (approx. 1 volt).  The MOSFET turns off at the
appropriate point to provide the desired power to the load.  This all
works like a champ.

       My problem is that the MOSFET is getting very hot.  It is rated
at 8 amps and 40 watts with rds on of 0.27 ohm (at 5 amps).  Thus far I have
only used about 30 percent duty cycle to the load.  By my calculations that
should be just over 2 watts in the MOSFET even at 100% duty cycle.  But
that little puppy is cooking.

       I hesitate to display my ignorance but am I just missing
something basic about the use of MOSFETs?  I did not expect to be near
the limit for power usage at all.  If any of you see an obvious solution
please let me know.

       It is probably most appropriate to respond directly to me at
spam_OUTmlkTakeThisOuTspamasu.edu.  This is not just PIC related so let's not clog the list
bandwidth with discussion of MOSFET physics.

Thanks all for your patience,

Martin Kirk
Arizona State University
.....mlkKILLspamspam@spam@asu.edu
(602) 582-5718

1995\12\02@070308 by Conny Andersson

flavicon
face
At 16.48 1995-12-01 -0700, Martin Kirk wrote:

   -- chop -- chop --

>        My problem is that the MOSFET is getting very hot.  It is rated
>at 8 amps and 40 watts with rds on of 0.27 ohm (at 5 amps).  Thus far I have
>only used about 30 percent duty cycle to the load.  By my calculations that
>should be just over 2 watts in the MOSFET even at 100% duty cycle.  But
>that little puppy is cooking.

The thermal resistance of air is rather high so I suggest you put the MOSFET
onto a heat sink with some "silicon grease" between the MOSFET and the heat
sink.

It was a while since I did some calculations with power dissipation but one
guideline is:

  Tj - Tamb = P * (Kjm + Kmh + F * Kh)

where

  Tj = temperature inside MOSFET.

  Tamb = ambient temperature (air).

  P = power dissipation from MOSFET.

  Kjm = thermal resistance between MOSFET (inside) and it's package,
        internal, may be found in the MOSFET:s data sheet.

  Kmh = thermal resistance between the package and the heat sink
        depends on what you put between your MOSFET and the heat sink,
        may also be found in the MOSFET:s data sheet.

  Kh = thermal resistance between the heat sink and the ambient air,
       may be found in the heat sink data sheet.

  F = factor representing air flow onto the heat sink.
      F = 1 if air velocity = 0 and decreases exponentially with increasing
      air velocity.



-- Conny

1995\12\02@075157 by ken

flavicon
picon face
Look at the gate of the MOS FET with a scope and see how fast you are turning
the device on, it may have an ON resistance of 0.27 ohms but this is when it is
fully ON, if the rise time of the gate drive voltage is slow then you will be
disipating a lot of power during this time. It may only be for a few mS but you
would not believe how much power can be disipated in that time. MOS FET's have
a gate capacitance of about 1600pf and you need to be able to drive them with a
current source capable of supplying 1.5A+for a few nS.

Hope this helps.

Ken.

+-----------------------------+----------------------------------+
|   ken hewitt                |   Email kenspamKILLspamwelwyn.demon.co.uk   |
+-----------------------------+----------------------------------+

1995\12\02@143515 by Scott Stephens

picon face
>        My problem is that the MOSFET is getting very hot.  It is rated
>at 8 amps and 40 watts with rds on of 0.27 ohm (at 5 amps).  Thus far I have
>only used about 30 percent duty cycle to the load.  By my calculations that
>should be just over 2 watts in the MOSFET even at 100% duty cycle.  But
>that little puppy is cooking.
>
>        I hesitate to display my ignorance but am I just missing
>something basic about the use of MOSFETs?

I think so. I think this is like a problem I had using the same MOSFET to
drive a transformer primary. You need to turn the gate on with a voltage
higher than the circuit Vcc. Try creating a seperate power supply a volt or
so higher to power the PIC and the 4050. See if that reduces the voltage
drop across the MOSFET to a level that would result in the .3 ohm rds on
range. I think this is whats called a "high-side" drive.

1995\12\02@194513 by mlk

picon face
Thanks to all who have responded to my PIC drives MOSFET question.  Some
of the best info was regarding the voltage drive levels to the MOSFET
gate.  It appears that 5V may not be enough gate drive.  It also appears
that I may need enough current drive to insure that the gate turns ON and
OFF (source and sink) very quickly so that I spend very little time in the
high resistance region.  That could also account for excessive MOSFET heating.

       So here is the prize question... the winner will have my eternal
gratitude (I'll pull a thorn out of your foot someday):

       Does anyone know of a single-chip solution to interface the PIC
output (5V) to the MOSFET gate (around 12V) with plenty of source and
sink capability.  I hear the MOSFET gate capacitance can be as high as
1600pf (I do not have a data sheet on the IRF520) so current drive should be:

       I = C*(deltaV/deltaT) = 1600pf * (12V/100nsec) = 192 mA

       A bipolar transistor voltage translation could work.  I need 8 of
these for 8 channels of MOSFET drive.  So single-chip solution means all
8 channels.

Thanks all,

Martin Kirk
Arizona State University
.....mlkKILLspamspam.....asu.edu
(602) 582-5718

1995\12\02@201706 by Henry Carl Ott

picon face
{Quote hidden}

Maxim has a few high side mosfet driver chips (some with internal charge
pumps). I think the MAX620 series might be what you are looking for.  You
can call Maxim at 800-998-8800, they have been extremely helpful to me in
the past supplying literature and samples.

On another note. I believe there was a fairly comprehensive article in
Circuit Celler Ink in the last few months doing something very similar to
what you want to do. I've lent out all my CCI for the last year to a friend,
so I can't look up the article myself. Some other pic-list reader might have
a better memory.


Hope this helps.

carl

----------------------------------------------------------------
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EraseMEcarlottspam_OUTspamTakeThisOuTinterport.net    | http://www.interport.net/~carlott/
----------------------------------------------------------------
"On justice and on friendship, there is no price,
but there are established credit limits."

1995\12\02@213320 by AV Presentations

picon face
On Sat, 2 Dec 1995 mlkspamspam_OUTASU.EDU wrote:

>
>         Does anyone know of a single-chip solution to interface the PIC
> output (5V) to the MOSFET gate (around 12V) with plenty of source and
> sink capability.  I hear the MOSFET gate capacitance can be as high as
> 1600pf (I do not have a data sheet on the IRF520) so current drive should be:
>
>         I = C*(deltaV/deltaT) = 1600pf * (12V/100nsec) = 192 mA
>
>         A bipolar transistor voltage translation could work.  I need 8 of
> these for 8 channels of MOSFET drive.  So single-chip solution means all
> 8 channels.
>
Electronics Now magazine has a column called Hardware Hacker by Don
Lancaster.  A while back, maybe a year ago, he had some info on "high
side drivers".  He is very good with cross references and
bibliographies.  He is on the Genie service in the PSRT (postscript round
table) also.  Good luck to you, I spent a lot of time pulling my hair out
over a similar, though higher current circuit.

morris beverly   @spam@avpresKILLspamspamworld.std.com

1995\12\03@152103 by Henry Carl Ott

picon face
NOTE:  This is the second attempt to post because the list bounced the first
one.

 ---snip ---snip

{Quote hidden}

Maxim has a few high side mosfet driver chips (some with internal charge
pumps). I think the MAX620 series might be what you are looking for.  You
can call Maxim at 800-998-8800, they have been extremely helpful to me in
the past supplying literature and samples.

On another note. I believe there was a fairly comprehensive article in
Circuit Celler Ink in the last few months doing something very similar to
what you're attempting. I've lent out all my CCI for the last year to a
friend, so I can't look up the article myself. Some other pic-list reader
might have a better memory.


Hope this helps.

carl

----------------------------------------------------------------
Henry Carl Ott   N2RVQ   | talk/chat  carlott@204.74.7.186
KILLspamcarlottKILLspamspaminterport.net    | http://www.interport.net/~carlott/
----------------------------------------------------------------
"On justice and on friendship, there is no price,
but there are established credit limits."

1995\12\04@054120 by Markus Imhof

flavicon
face
...
>        So here is the prize question... the winner will have my eternal
>gratitude (I'll pull a thorn out of your foot someday):
>
>        Does anyone know of a single-chip solution to interface the PIC
>output (5V) to the MOSFET gate (around 12V) with plenty of source and
...

Well, close, but I won't claim the price since you'll need two of them:
Maxim has a few chips that do exactly that, e.g. the MAX62x series (there
may be others, but I just had a look in the '94 databook). These contain 4
drivers and a charge pump to increase the driver voltage to 11 Volts above
supply (IIRC, MOSFET supply).

Hope this Helps.
Bye
 Markus

1995\12\04@065835 by Ken Pontbriant

picon face
International Rectifier makes a line of MOSFETs designed specifically to be
driven at logic levels. They have a much lower gate threshold voltage than
their "regular" counterparts. The equivalent device to the IRF520 would be
the IRL520. The gate capacitance for the IRL520 is 490 pF.
Hope this helps-

Ken Pontbriant

1995\12\04@075804 by Robert Schwenke

flavicon
face
cut...cut
>        Does anyone know of a single-chip solution to interface the PIC
>output (5V) to the MOSFET gate (around 12V) with plenty of source and
>sink capability.  I hear the MOSFET gate capacitance can be as high as
>1600pf (I do not have a data sheet on the IRF520) so current drive should be:
>
>        I = C*(deltaV/deltaT) = 1600pf * (12V/100nsec) = 192 mA
>
>        A bipolar transistor voltage translation could work.  I need 8 of
>these for 8 channels of MOSFET drive.  So single-chip solution means all
>8 channels.
>
cut...cut

Try a photo FET optocoupler from QT Optoelectronics, H11F1 or H11F2.
Digikey sells these for about $2.00.

Rob

1995\12\04@123449 by Ken Pontbriant

picon face
In a message dated 95-12-03 15:21:41 EST, you write:

>>        So here is the prize question... the winner will have my eternal
>>gratitude (I'll pull a thorn out of your foot someday):
>>
>>        Does anyone know of a single-chip solution to interface the PIC
>>output (5V) to the MOSFET gate (around 12V) with plenty of source and
>>sink capability.  I hear the MOSFET gate capacitance can be as high as
>>1600pf (I do not have a data sheet on the IRF520) so current drive should
>be:
>>
>>        I = C*(deltaV/deltaT) = 1600pf * (12V/100nsec) = 192 mA
>>
>>        A bipolar transistor voltage translation could work.  I need 8 of
>>these for 8 channels of MOSFET drive.  So single-chip solution means all
>>8 channels.
>>
>
International Rectifier makes a line of MOSFETs designed specifically to be
driven at logic levels. They have a much lower gate threshold voltage than
their "regular" counterparts. The equivalent device to the IRF520 would be
the IRL520. The gate capacitance for the IRL520 is 490 pF.
Hope this helps-

Ken Pontbriant

1995\12\04@153530 by Scott Stephens

picon face
You may be able to use a few diodes in series with the load to drop the
voltage at the MOSFET's drain. Or toggle a couple port lines for a charge
pump to raise the Vcc of the 4050 driver chip.

1995\12\04@191536 by John Payson

flavicon
face
> Electronics Now magazine has a column called Hardware Hacker by Don
> Lancaster.  A while back, maybe a year ago, he had some info on "high
> side drivers".  He is very good with cross references and
> bibliographies.  He is on the Genie service in the PSRT (postscript round
> table) also.  Good luck to you, I spent a lot of time pulling my hair out
> over a similar, though higher current circuit.

I don't have that magazine; I'm trying to figure a good way to switch as
many outputs per chip as I can, at least 20 mils per and preferably (at
least for some) 50 [driving IR led's].  Currently I'm using 5484's for
low side and 5484's with external pullups for high side.  Unfortunately,
sinking all the unused pullups uses an objectionable amount of current and
causes the power supply to sag.

If I can't find a good chip, I'm thinking of using NPN transistors as
emitter-followers for the high-side drive.  These would have the advantage
that I could hook the collector up to an unregulated supply and the output
of the transistors would still be stable [so my regulator wouldn't have to
supply too much current].  Any thoughts on this approach [good/bad/indiffer-
ent?]

1995\12\04@230153 by Martin J. Maney

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face
On Mon, 4 Dec 1995, John Payson wrote:

> low side and 5484's with external pullups for high side.  Unfortunately,
> sinking all the unused pullups uses an objectionable amount of current and
> causes the power supply to sag.
>
> If I can't find a good chip, I'm thinking of using NPN transistors as
> emitter-followers for the high-side drive.  These would have the advantage
> that I could hook the collector up to an unregulated supply and the output
> of the transistors would still be stable [so my regulator wouldn't have to
> supply too much current].  Any thoughts on this approach [good/bad/indiffer-
> ent?]

I don't see any need for a tightly regulated drive voltage: you need to
turn them on, but you aren't switching anywhere near fast enough to need
great concern over the rise and fall times.  For that matter, if I recall
this application correctly, you're always turning the gate drive on at
the zero crossing, aren't you?  In that case the gate risetime is
unimportant: there won't be any current flowing until the unfiltered
supply voltage rises, so FET turn-on time should have negligible effect
on power dissipation.

However, turn-off will matter.  So I would suggest that you should use the
open-collector drivers with pullups to the higher voltage (to insure full
turn-on) but you can use relatively large resistors - the time when you
need to move a lot of charge on the gate is at turn off, when the
open-collector drivers should turn on and do the job very nicely.

Luck!

1995\12\05@022243 by Martin J. Maney

flavicon
face
On Mon, 4 Dec 1995, John Payson wrote about driving MOSFETs:

> low side and 5484's with external pullups for high side.  Unfortunately,
> sinking all the unused pullups uses an objectionable amount of current and
> causes the power supply to sag.
>
> If I can't find a good chip, I'm thinking of using NPN transistors as
> emitter-followers for the high-side drive.  These would have the advantage
> that I could hook the collector up to an unregulated supply and the output
> of the transistors would still be stable [so my regulator wouldn't have to
> supply too much current].  Any thoughts on this approach [good/bad/indiffer-
> ent?]

I don't see any need for a tightly regulated drive voltage: you need to
turn them on, but you aren't switching anywhere near fast enough to need
to be greatly concerned about the the rise and fall times.  For that
matter, if I recall this application correctly, you're always turning the
gate drive on at the zero crossing, aren't you?  In that case the gate
drive risetime is unimportant: there won't be any current flowing until
the unfiltered supply voltage rises, so FET turn-on time should have
negligible effect on power dissipation.

However, turn-off will matter.  So I would suggest that you should use the
open-collector drivers with pullups to the higher voltage (to insure full
turn-on) but you can use relatively large resistors - the time when you
need to move a lot of charge on the gate is at turn off, when the
open-collector drivers should turn on and do the job very nicely.

Luck!

1995\12\06@081434 by Alexander Colquhoun

flavicon
picon face
> >Thanks to all who have responded to my PIC drives MOSFET question.  Some
> >of the best info was regarding the voltage drive levels to the MOSFET
> >gate.  It appears that 5V may not be enough gate drive.  It also appears
> >that I may need enough current drive to insure that the gate turns ON and
> >OFF (source and sink) very quickly so that I spend very little time in the
> >high resistance region.  That could also account for excessive MOSFET
heating.
{Quote hidden}

Linear Technology make a LTC1155 DUAL High Side Driver chip.
This allows using low cost N channel FET's for High Side
switching applications.

These chips can operate off a 5V supply and safely drive the gates
of virtually all FET's.

Ideally suited for low voltage applications,and compatible with
Standard logic families.

Available in 8-Pin SO Package.

Hope this helps.



-Sandy.

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