Groups > sci.electronics.design > #429714

Re: Gate resistor for lateral power MOSFET?

From	Frank Miles <fpm@u.washington.edu>
Newsgroups	sci.electronics.design
Subject	Re: Gate resistor for lateral power MOSFET?
Date	2016-09-28 22:22 +0000
Organization	A noiseless patient Spider
Message-ID	<nshfqg$fss$1@dont-email.me> (permalink)
References	<nsgvlp$i4p$1@dont-email.me> <lu2oublpqceu8gviluq1av52258htckcfb@4ax.com>

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On Wed, 28 Sep 2016 11:35:23 -0700, John Larkin wrote:

> On Wed, 28 Sep 2016 17:46:33 -0000 (UTC), Frank Miles
> <fpm@u.washington.edu> wrote:
> 
>>I'm designing a power amplifier using some Exicon lateral 
>>MOSFETs as the final common-source stage.  These will be
>>used in various experimental systems.  Each only needs
>>to run around +/-50V out, up to 4A, and driving sinusoidally
>>between 50 and 400kHz.  The load can be approximated as a
>>lossy series LC circuit, driven near resonance with R 
>>generally 10-20 ohms.  There's an additional >200pF of
>>capacitance on each output source to ground due to the 
>>capacitance to the heat sink.
>>
>>These MOSFETs have huge capacitances:
>>	N ch: Ciss 900pF, Coss 500pF
>>	P ch: Ciss 1.8nF, Coss 850pF
>>
>>MOSFET "lore" (particularly from audio power land) says that
>>you need fairly substantial series gate resistors to protect
>>these FETs.  Unfortunately the dominant pole in the amplifier
>>is in the preceding voltage-gain stage, so I'm having some
>>HF stability issues when I use resistors of the recommended
>>value (>300 ohms).  I'd like these amplifiers to tolerate
>>a variety of loads, not require tweaking for particular 
>>systems.
>>
>>QUESTION : are these resistors necessary?  How can a proper
>>value be determined?
>>
>>Thanks for any insights!
>>   -F
> 
> Mosfets sometimes like to RF oscillate, especially when used in linear
> applications, especially with the source not grounded. A gate
> resistor, 10s of ohms, will usually kill that. I know of no other
> "protection" function of a resistor, but maybe there could be a gate
> zener or something that needs protection.
> 
> I'd add a resistor to the layout and stuff it with some low value that
> doesn't compromise your loop. If you see oscillation (which would be
> 10s or maybe 100s of MHz) you could increase the resistance or try a
> ferrite bead or something.
> 
> Laterals have relatively low Gm, don't they? That might reduce the
> tendency to oscillate.

Thanks, John.  I haven't observed any misbehavior that I would want
to fix with a gate resistor - just trying to prevent difficulties
in circumstances that I failed to foresee.

I like the ferrite idea (assuming that the mechanism is the unintended
oscillator).  I'll look for a part that's good and lossy above the
band that the loop has to operate over, lower resistance below.

   -F

Thread

Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-28 17:46 +0000
  Re: Gate resistor for lateral power MOSFET? John Larkin <jjlarkin@highlandtechnology.com> - 2016-09-28 11:35 -0700
    Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-28 22:22 +0000
      Re: Gate resistor for lateral power MOSFET? John Larkin <jjlarkin@highlandtechnology.com> - 2016-09-28 17:20 -0700
  Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-28 19:16 -0700
    Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-28 23:07 -0700
      Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-29 12:51 -0700
  Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-28 19:52 -0700
    Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-28 20:13 -0700
      Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-29 12:52 -0700
    Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-29 16:41 +0000
      Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-29 18:52 -0700
        Re: Gate resistor for lateral power MOSFET? boB <boB@theresnoplacelikehome.com> - 2016-09-30 00:54 -0700
          Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-30 15:57 +0000
        Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-30 15:53 +0000
          Re: Gate resistor for lateral power MOSFET? George Herold <gherold@teachspin.com> - 2016-09-30 09:19 -0700
          Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-30 20:04 +0000
            Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-30 14:07 -0700
              Re: Gate resistor for lateral power MOSFET? dplatt@coop.radagast.org (Dave Platt) - 2016-09-30 14:52 -0700
              Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-01 17:29 +0000
                Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-10-01 12:19 -0700
                Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-02 02:59 +0000
                Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-10-01 21:10 -0700
                Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-03 00:20 +0000
                Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-10-02 09:43 -0700
                Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-03 00:33 +0000
          Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-30 22:40 -0700

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