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Re: Gate resistor for lateral power MOSFET?

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Phil Allison wrote...
>
>Winfield Hill wrote:
>
>>  The only reason for a series resistor, as others
>>  have stated, is to dampen RF oscillation.  I'd not
>>  expect a problem, given the low gm of lateral parts,
>>  but I've seen ferrite beads on the gates of these
>>  parts in Hafler's MOSFET amplifiers.  Nonetheless,
>>  they also placed 470-ohm resistors in series with
>>  the gates, according to manual schematics.  But I
>>  do not believe such large values are necessary.
>
> ** What you "believe" is hardly relevant to anyone but you.
>
>I hope you agree changing the gate resistor value downwards
> lessens the stability margin, rather than improves it - so
> is not very likely to help the OP.

 Normally adding extra poles is worse for stability.
 But when I read your horror stories below, I see
 that my experience is certainly not relevant here!

> As someone who has spend a LOT of time puzzling over stability
> problems in power amplifiers using Hitachi and Semelab lateral
> mosfets, I can assure you none of them have much stability
> to spare.
>
> Any of them WILL develop parasitic oscillations in the range
> of 2 to 10MHz, with resistive or even no load connected, when
> any of the zobel components become damaged.  Supersonic
> oscillation is a common event in the cruel world of live
> music sound reinforcement and quickly destroys the 100nF film
> capacitors and 4.7 ohm resistors typically employed - often
> invisibly.
>
> If the circuit uses 1 watt film resistors here, upgrading them
> to 5 watt WW types is very likely to induce parasitics - due
> to the inductance of such components. Adjusting the idle bias
> to a lower value does the same in some models - which can then
> be cured by adding another RC zobel from the common
> source point to supply ground.
>
> Sometimes the oscillations are continuous at a low level while
> and other times they only appear superimposed on one half wave
> when a 4ohm load is driven to near full output.
>
> BTW: to evaluate how prone a certain Perreaux mosfet model
> was to supersonic oscillation, I EXTERNALLY tried various
> low value capacitors from output to input and carefully
> advanced the volume.  Full power oscillation at 60kHz or
> higher was the result every time until the value used
> was below 22pF.


-- 
 Thanks,
    - Win

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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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