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'[EE]: Help with FET circuit'
2002\10\15@083101 by Ray Gallant

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I would like to drive a SFP9520 FET (or alternate) (24VDC, 800mA) I'm considering adding a small FET stage if required to drive this larger FET.
I have omitted the filtering op-amps circuit to try to get this circuit functioning. The input to this FET circuit now is 5volts rail to rail PWM 20kHz from the opto-coupler (pic driven) . I will test with just static condition at first. I am requesting
your help with a circuit that will drive this large FET from the opto-coupler's output on. Tx again Richard, that opto is great. Tx in advance, {slewrate}
http://www.nve.com/isopdf/il710.pdf
http://www-fa.upc.es/websfa/lab/111/irf9520.pdf

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2002\10\15@104713 by Alan B. Pearce

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>I would like to drive a SFP9520 FET (or alternate)
>(24VDC, 800mA) >I'm considering adding a small FET
>stage if required to drive this larger FET.

>I have omitted the filtering op-amps circuit to try
>to get this circuit functioning. The input to this FET
>circuit now is 5volts rail to rail PWM 20kHz from the
>opto-coupler (pic driven) . I will test with just static
>condition at first. I am requesting

I take it that the IRF9520 is an equivalent of the part number you quote.


First thing is that the FET is not going to saturate with a 5V gate drive.
On page 2 of the data sheet, check the Rds(on) value, it is given with 10V
gate drive. This is typical gate drive for most FET's unless they are
classed as logic level devices. Logic level devices tend to have other
limitations such as higher Rds(on) values.

To check if you can get a low enough Rds(on) check figure 6. This would
suggest you are going to have about 2V across the FET at 0.8A current with
5V gate voltage.

The other thing that I suspect is stopping your circuit is how you drive the
gate. This looks like a large capacitor to your circuit, so if you are
trying to switch it fast, you will need a heavier current drive than the
opto can give you.

You may need to give a reasonable sketch of your circuit for further help.

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2002\10\15@115650 by Ray Gallant

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First of all tx for the response, it's appreciated!


> >I would like to drive a SFP9520 FET (or alternate)
> >(24VDC, 800mA) >I'm considering adding a small FET
> >stage if required to drive this larger FET.
> >I have omitted the filtering op-amps circuit to try
> >to get this circuit functioning. The input to this FET
> >circuit now is 5volts rail to rail PWM 20kHz from the
> >(pic driven)
>>-------------------------------
Alan B. Pearce wrote:
<<<<< First thing is that the FET is not going to saturate with a 5V gate
drive. On page 2 of the data sheet, check the Rds(on) value, it is given
with 10V gate drive. This is typical gate drive for most FET's unless they
are  classed as logic level devices. Logic level devices tend to have other
limitations such as higher Rds(on) values.

To check if you can get a low enough Rds(on) check figure 6. This would
suggest you are going to have about 2V across the FET at 0.8A current with
5V gate voltage.

The other thing that I suspect is stopping your circuit is how you drive the
gate. This looks like a large capacitor to your circuit, so if you are
trying to switch it fast, you will need a heavier current drive than the
opto can give you.>>>>>
>-------------------------------------------
My new question:  The input (available) to this driving [FET(s)] circuit now
is 5 volts rail to rail from the opto-coupler.  This is the stage I need
help with.  Perhaps an IRLD110 FET as a intermediary stage between the opto
and the main FET could be used.
From the opto coupler's output, can I drive a smaller FET to provide proper
gate drive to the larger FET, then in turn offering 24VDC out.
Still require help with the driving circuit for this valve.
Regards, {slewrate}
http://www.irf.com/product-info/datasheets/data/irld110.pdf

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2002\10\15@121708 by Alan B. Pearce

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>My new question:  The input (available) to this driving [FET(s)] circuit
now
>is 5 volts rail to rail from the opto-coupler.  This is the stage I need
>help with.  Perhaps an IRLD110 FET as a intermediary stage between the opto
>and the main FET could be used.
>From the opto coupler's output, can I drive a smaller FET to provide proper
>gate drive to the larger FET, then in turn offering 24VDC out.
>Still require help with the driving circuit for this valve.

I think you will have trouble if you try and use a pull up (or pull down)
resistor, with a single transistor. Normally a push pull pair of transistors
is used, a bit like the output stage of an audio amplifier. The data sheet
gives a Cgs of 300pF, and a gate charge of 16 to 22nC, which is the item
that really gets you. I am not an expert on driving these, but have watched
"over the shoulder" of colleagues who have.

I would suggest you look at some application notes on the Intersil web site.
Search for HIP4080, there is a family of devices where just the last digit
of the part number varies, and check the application notes for these parts.
IIRC there is a reasonable amount of information on how to drive FET gates
in there. Just use this link and you will see all of them.
http://www.intersil.com/design/parametric/partsearch.asp?txtprodnr=hip408

Also you should be able to find some application notes on the IR web site
where you found the transistor data sheet.

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2002\10\15@130436 by Ray Gallant

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face
> >My new question:  The input (available) to this driving [FET(s)] circuit
> now
> >is 5 volts rail to rail from the opto-coupler.  This is the stage I need
> >help with.  Perhaps an IRLD110 FET as a intermediary stage between the
opto
> >and the main FET could be used.
> >From the opto coupler's output, can I drive a smaller FET to provide
proper
> >gate drive to the larger FET, then in turn offering 24VDC out.
> >Still require help with the driving circuit for this valve.
>
> I think you will have trouble if you try and use a pull up (or pull down)
> resistor, with a single transistor. Normally a push pull pair of
transistors
> is used, a bit like the output stage of an audio amplifier. The data sheet
> gives a Cgs of 300pF, and a gate charge of 16 to 22nC, which is the item
> that really gets you. I am not an expert on driving these, but have
watched
> "over the shoulder" of colleagues who have.
>
> I would suggest you look at some application notes on the Intersil web
site.
> Search for HIP4080, there is a family of devices where just the last digit
> of the part number varies, and check the application notes for these
parts.
> IIRC there is a reasonable amount of information on how to drive FET gates
> in there. Just use this link and you will see all of them.
> http://www.intersil.com/design/parametric/partsearch.asp?txtprodnr=hip408
>
> Also you should be able to find some application notes on the IR web site
> where you found the transistor data sheet.
../\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
I checked the site, Tx.  Are you suggesting that I develop an H-bridge type
to drive this solenoid?  I did not understand the avenue you offered in
regards to driving the main FET.  Could you please re-iterate or simplify
your feedback.  Thank you. {slewrate}

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2002\10\16@035556 by Alan B. Pearce

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>Are you suggesting that I develop an H-bridge type
>to drive this solenoid?

No, but your original email talked about driving the FET with 20kHz PWM.

>I did not understand the avenue you offered in regards to
>driving the main FET.  Could you please re-iterate or
>simplify your feedback.  Thank you.

Well I figured by looking at the way they are driving a transistor within
the H bridge driver, you would be able to see how they achieve reasonable
FET switching times at high switching speeds.

>{slewrate}

If you want to achieve reasonable slewrates then you will need to do a
reasonable driver circuit. However if all you do is turn a solenoid off and
on then that is a whole different ball court.

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