Analog MOSFET Driver

[Dave]

Need an analog driver for a big mosfet. Not sure if there might be an
appropriate power op-amp out there or not. Linearity is not too
important as long as I can drive the mosfet to different pulse
amplitudes. Seems that most mosfet drivers are digital. Right now I'm
considering a wimpy op-amp driving a PNP/NPN pair. The mosfet itself
would be a IRFZ44 or similar. I'd like to get a rise-time on the gate
down to a few uS.

Thanks,
Dave

 

[John Larkin]

On 24 Feb 2005 13:33:46 -0800, "Dave" <galt_57@hotmail.com> wrote:

Need an analog driver for a big mosfet. Not sure if there might be an
appropriate power op-amp out there or not. Linearity is not too
important as long as I can drive the mosfet to different pulse
amplitudes. Seems that most mosfet drivers are digital. Right now I'm
considering a wimpy op-amp driving a PNP/NPN pair. The mosfet itself
would be a IRFZ44 or similar. I'd like to get a rise-time on the gate
down to a few uS.

Thanks,
Dave

What's the peak current requirement work out to be?

How about four sections of a quad opamp? Run them as followers, with
each having a, say, 100 ohm resistor to the gate.

John

 

[Pooh Bear]

Dave wrote:

Need an analog driver for a big mosfet. Not sure if there might be an
appropriate power op-amp out there or not. Linearity is not too
important as long as I can drive the mosfet to different pulse
amplitudes. Seems that most mosfet drivers are digital. Right now I'm
considering a wimpy op-amp driving a PNP/NPN pair. The mosfet itself
would be a IRFZ44 or similar. I'd like to get a rise-time on the gate
down to a few uS.

Usual technique is to use an NPN/PNP complementary pair driving the gate.
Watch out for driver dissipation btw.


Graham

 

[Ken Smith]

In article <1109280826.629435.209980@o13g2000cwo.googlegroups.com>,
Dave <galt_57@hotmail.com> wrote:

Need an analog driver for a big mosfet. Not sure if there might be an
appropriate power op-amp out there or not. Linearity is not too
important as long as I can drive the mosfet to different pulse
amplitudes. Seems that most mosfet drivers are digital. Right now I'm
considering a wimpy op-amp driving a PNP/NPN pair. The mosfet itself
would be a IRFZ44 or similar. I'd like to get a rise-time on the gate
down to a few uS.

How about an LT1206 with the capacitive load compensation connected?

It will drive about 250mA. The overshoot and ringing issues normal to
driving capacitive loads are taken care of.



--
--
kensmith@rahul.net forging knowledge

 

[Dave]

Ken Smith wrote:

In article <1109280826.629435.209980@o13g2000cwo.googlegroups.com>,
Dave <galt_57@hotmail.com> wrote:
Need an analog driver for a big mosfet. Not sure if there might be
an appropriate power op-amp out there or not. Linearity is not too
important as long as I can drive the mosfet to different pulse
amplitudes. Seems that most mosfet drivers are digital. Right now
I'm considering a wimpy op-amp driving a PNP/NPN pair. The mosfet
itself would be a IRFZ44 or similar. I'd like to get a rise-time on
the gate down to a few uS.

How about an LT1206 with the capacitive load compensation connected?

It will drive about 250mA. The overshoot and ringing issues normal
to driving capacitive loads are taken care of.

Yeah I wonder. Seems like there are numerous approaches. I guess I'll
have to experiment. Thanks.

 

[Dave]

Can anyone offer a reason why slower rise-times might be seen at the
output of a power FET being turned on only partially? I 'm trying to
diagnose the issue remotely, so I don't have it in front of me, but it
doesn't make sense. I can't see why it would ever take a driver circuit
longer to drive a FET to produce 5 amp pulses than to produce 30 amp
pulses. Am I forgetting some devious issue?

Thanks,
Dave

 

[John Woodgate]

I read in sci.electronics.design that Dave <galt_57@hotmail.com> wrote
(in <1110229345.328999.111240@z14g2000cwz.googlegroups.com&gt about
'Analog MOSFET Driver', on Mon, 7 Mar 2005:

Can anyone offer a reason why slower rise-times might be seen at the
output of a power FET being turned on only partially? I 'm trying to
diagnose the issue remotely, so I don't have it in front of me, but it
doesn't make sense. I can't see why it would ever take a driver circuit
longer to drive a FET to produce 5 amp pulses than to produce 30 amp
pulses. Am I forgetting some devious issue?

If Rds,on = 2 ohms, you get a delay of half a mho. (;-)

--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Rich Grise]

On Mon, 07 Mar 2005 22:12:10 +0000, John Woodgate wrote:

I read in sci.electronics.design that Dave <galt_57@hotmail.com> wrote
(in <1110229345.328999.111240@z14g2000cwz.googlegroups.com&gt about
'Analog MOSFET Driver', on Mon, 7 Mar 2005:
Can anyone offer a reason why slower rise-times might be seen at the
output of a power FET being turned on only partially? I 'm trying to
diagnose the issue remotely, so I don't have it in front of me, but it
doesn't make sense. I can't see why it would ever take a driver circuit
longer to drive a FET to produce 5 amp pulses than to produce 30 amp
pulses. Am I forgetting some devious issue?

If Rds,on = 2 ohms, you get a delay of half a mho. (;-)

Well, AFAICS, Mho will never be as funny as Zepto. ;-p

Cheers!
Rich

 

[Ken Smith]

In article <pan.2005.03.07.23.01.08.867388@example.net>,
Rich Grise <richgrise@example.net> wrote:
[...]

If Rds,on = 2 ohms, you get a delay of half a mho. (;-)

Well, AFAICS, Mho will never be as funny as Zepto. ;-p

Q: If you make a device the reports the natural log of the conductivity
what do you call it?


A: A Ln(Mho)er



--
--
kensmith@rahul.net forging knowledge

 

[Yzordderrex]

Yes, and I would suppose that if you offered this device as an option
which the customer would have to specificially ask for. Then much like
an insurance policy he would have to add a 'Rider' to his
specification.

So now do we have a Rider Ln(Mho)er ?

regards,
Bob