Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > sci.electronics.design > #429670 > unrolled thread

Gate resistor for lateral power MOSFET?

Back to article view | Back to sci.electronics.design

Contents

  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

Page 1 of 2  [1] 2  Next page →

#429670 — Gate resistor for lateral power MOSFET?

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

[toc] | [next] | [standalone]

#429678

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.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

[toc] | [prev] | [next] | [standalone]

#429714

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

[toc] | [prev] | [next] | [standalone]

#429729

On Wed, 28 Sep 2016 22:22:08 -0000 (UTC), Frank Miles
<fpm@u.washington.edu> wrote:

>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

Wild guess, use a bead that's 30 ohms at 100 MHz.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

[toc] | [prev] | [next] | [standalone]

#429753

Frank Miles wrote...
>
> MOSFET "lore" (particularly from audio power land)
> says that you need fairly substantial series gate
> resistors to protect these FETs.  

 If the Vgs drive voltages go substantially beyond
 the listed gate-breakdown voltages, and cause a
 serious leakage event, series resistors won't help.

 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.


-- 
 Thanks,
    - Win

[toc] | [prev] | [next] | [standalone]

#429778

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.

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. 




....  Phil 

[toc] | [prev] | [next] | [standalone]

#429835

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

[toc] | [prev] | [next] | [standalone]

#429758

cassiope wrote:
>
> I'm designing a power amplifier using some Exicon lateral 
> MOSFETs as the final common-source stage.
>

** You mean connected as source followers ? 


> 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?
>

** Gate resistors of about 200ohms are necessary, but not sufficient to ensure freedom from parasitic oscillations. As you have realised, lateral mosfets have lots of power gain up into the VHF range and a little stray L or C will set easily them off. Wiring layout should be done with this in mind, keeping all tracks short as possible. You can avoid heatsink capacitance by isolating it from ground and connecting all the mosfet sources directly so it floats at output voltage.
 
An output stabilising network is essential, consisting of a inductor of about 5uH with RC Zobels on both ends to supply ground.

The operating bias current can play a role in HF stability, but try to keep it close to 100mA per device since this corresponds with the inflexion point in the tempco curve - where it goes from positive to negative. 


....  Phil 



 

[toc] | [prev] | [next] | [standalone]

#429763

Phil Allison wrote...
>
> As you have realised, lateral mosfets have lots of power
> gain up into the VHF range and a little stray L or C
> will set easily them off.  

 Ordinary VMOS power MOSFETs have *much* higher gm in the
 linear region than lateral MOSFETs, and techniques that
 work for them should be conservative for the laterals.


-- 
 Thanks,
    - Win

[toc] | [prev] | [next] | [standalone]

#429836

Winfield Hill wrote...
>
>Phil Allison wrote...
>>
>> As you have realised, lateral mosfets have lots of power
>> gain up into the VHF range and a little stray L or C
>> will set easily them off.  
>
> Ordinary VMOS power MOSFETs have *much* higher gm in the
> linear region than lateral MOSFETs, and techniques that
> work for them should be conservative for the laterals.

 I retract this statement, and bow to Phil's comments.


-- 
 Thanks,
    - Win

[toc] | [prev] | [next] | [standalone]

#429824

On Wed, 28 Sep 2016 19:52:00 -0700, Phil Allison wrote:

> cassiope wrote:
>>
>> I'm designing a power amplifier using some Exicon lateral MOSFETs as
>> the final common-source stage.
>>
>>
> ** You mean connected as source followers ?

Yes, quite so, I realized after I'd posted it that I'd mis-written, but
since no one had seemed to notice I didn't correct the error.

>> 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?
>>
>>
> ** Gate resistors of about 200ohms are necessary, but not sufficient to
> ensure freedom from parasitic oscillations. As you have realised,
> lateral mosfets have lots of power gain up into the VHF range and a
> little stray L or C will set easily them off. Wiring layout should be
> done with this in mind, keeping all tracks short as possible. You can
> avoid heatsink capacitance by isolating it from ground and connecting
> all the mosfet sources directly so it floats at output voltage.

Unfortunately I have three channels of amplifiers, and the physical 
design (this is a "feature improved" replacement of a legacy device)
makes separate heat sinks kinda unlikely/difficult.  But now that I see
this I should revisit this, though it would take a huge package redesign
to accommodate separate heat-sinks for N and P devices and retain
compatibility with the existing units.  So far the heat sinks have
metal exposed to the outside world, which would have to change if not
grounded.

How did you determine the 200 ohms?
 
> An output stabilising network is essential, consisting of a inductor of
> about 5uH with RC Zobels on both ends to supply ground.

Ouch.  5uH has ~12ohms reactance at 400kHz.  I may need some inductance 
here, but hopefully less.  The usual Zobel impedances are also too low,
consuming far too much of the output at my much-higher-than-audio 
frequencies.

> The operating bias current can play a role in HF stability, but try to
> keep it close to 100mA per device since this corresponds with the
> inflexion point in the tempco curve - where it goes from positive to
> negative.
> 
> 
> ....  Phil

I noticed that.  So far bias stability hasn't seemed a problem, I've got
a thermal sense diode and running at 75mA quiescent.

Phil - thanks so much for your recommendations!  They have given me much
to think about.

    -F

[toc] | [prev] | [next] | [standalone]

#429862

cassiope wrote:
>
>
> >>
> >>
> > ** Gate resistors of about 200ohms are necessary, but not sufficient to
> > ensure freedom from parasitic oscillations. As you have realised,
> > lateral mosfets have lots of power gain up into the VHF range and a
> > little stray L or C will set easily them off. Wiring layout should be
> > done with this in mind, keeping all tracks short as possible. You can
> > avoid heatsink capacitance by isolating it from ground and connecting
> > all the mosfet sources directly so it floats at output voltage.
> 
>
> Unfortunately I have three channels of amplifiers, and the physical 
> design (this is a "feature improved" replacement of a legacy device)
> makes separate heat sinks kinda unlikely/difficult.  But now that I see
> this I should revisit this, though it would take a huge package redesign
> to accommodate separate heat-sinks for N and P devices and retain
> compatibility with the existing units.


** Lateral mosfets have all have the case connected to the source, so one hestsink can be used for all devices in a channel without any insulators.

I assume you are not using source ballast resistors as most designs leave them out. 

> So far the heat sinks have
> metal exposed to the outside world, which would have to change if not
> grounded.


** Another option is to ground the sources of all the mosfets, via a common heatsink that can serve more than one channel. This requires that the centre point of the PSU be available to serve as the output as in this Haffler schematic:

http://bmamps.com/Schematics/Hafler/Hafler_9300,_9500_Schematic.pdf


> 
> How did you determine the 200 ohms?
>  

 ** I didn't, but many other designers have settled on values of 220 ohms to 470 ohms per device for the job. Often N channel devices having a higher value than used for the P channel ones. 


> > An output stabilising network is essential, consisting of a inductor of
> > about 5uH with RC Zobels on both ends to supply ground.
> 
> Ouch.  5uH has ~12ohms reactance at 400kHz.  I may need some inductance 
> here, but hopefully less.  The usual Zobel impedances are also too low,
> consuming far too much of the output at my much-higher-than-audio 
> frequencies.
> 

** Yeah, that is a problem. 

 Any chance you could operate the mosfets outside the NFB loop with the gates linked via 220ohm resistors? Lateral mosfets start conducting at 0.4V so there would be a little crossover distortion and the output impedance would be around 1 ohm. 

....   Phil 

[toc] | [prev] | [next] | [standalone]

#429879

On Thu, 29 Sep 2016 18:52:53 -0700 (PDT), Phil Allison
<pallison49@gmail.com> wrote:

>cassiope wrote:
>>
>>
>> >>
>> >>
>> > ** Gate resistors of about 200ohms are necessary, but not sufficient to
>> > ensure freedom from parasitic oscillations. As you have realised,
>> > lateral mosfets have lots of power gain up into the VHF range and a
>> > little stray L or C will set easily them off. Wiring layout should be
>> > done with this in mind, keeping all tracks short as possible. You can
>> > avoid heatsink capacitance by isolating it from ground and connecting
>> > all the mosfet sources directly so it floats at output voltage.
>> 
>>
>> Unfortunately I have three channels of amplifiers, and the physical 
>> design (this is a "feature improved" replacement of a legacy device)
>> makes separate heat sinks kinda unlikely/difficult.  But now that I see
>> this I should revisit this, though it would take a huge package redesign
>> to accommodate separate heat-sinks for N and P devices and retain
>> compatibility with the existing units.
>
>
>** Lateral mosfets have all have the case connected to the source, so one hestsink can be used for all devices in a channel without any insulators.
>
>I assume you are not using source ballast resistors as most designs leave them out. 
>
>> So far the heat sinks have
>> metal exposed to the outside world, which would have to change if not
>> grounded.
>
>
>** Another option is to ground the sources of all the mosfets, via a common heatsink that can serve more than one channel. This requires that the centre point of the PSU be available to serve as the output as in this Haffler schematic:
>
>http://bmamps.com/Schematics/Hafler/Hafler_9300,_9500_Schematic.pdf
>
>
>> 
>> How did you determine the 200 ohms?
>>  
>
> ** I didn't, but many other designers have settled on values of 220 ohms to 470 ohms per device for the job. Often N channel devices having a higher value than used for the P channel ones. 
>
>
>> > An output stabilising network is essential, consisting of a inductor of
>> > about 5uH with RC Zobels on both ends to supply ground.
>> 
>> Ouch.  5uH has ~12ohms reactance at 400kHz.  I may need some inductance 
>> here, but hopefully less.  The usual Zobel impedances are also too low,
>> consuming far too much of the output at my much-higher-than-audio 
>> frequencies.
>> 
>
>** Yeah, that is a problem. 
>
> Any chance you could operate the mosfets outside the NFB loop with the gates linked via 220ohm resistors? Lateral mosfets start conducting at 0.4V so there would be a little crossover distortion and the output impedance would be around 1 ohm. 
>
>....   Phil 
>


I would imagine that the gate resistor could be very high in value, at
least until it gets in the way of the desired slew rate or high end
frequency response of the amplifier ?

boB

[toc] | [prev] | [next] | [standalone]

#429930

On Fri, 30 Sep 2016 00:54:45 -0700, boB wrote:

[snip]
> 
> I would imagine that the gate resistor could be very high in value, at
> least until it gets in the way of the desired slew rate or high end
> frequency response of the amplifier ?
> 
> boB

The input capacitances are so high that I'm observing ringing step
responses with "higher" (300ohm) gate resistors with no load; and more
problems with reactive loads.  Thus my effort to better understand
what's going on...

   -F

[toc] | [prev] | [next] | [standalone]

#429928

On Thu, 29 Sep 2016 18:52:53 -0700, Phil Allison wrote:

> cassiope wrote:
>>
>>
>> >>
>> >>
>> > ** Gate resistors of about 200ohms are necessary, but not sufficient to
>> > ensure freedom from parasitic oscillations. As you have realised,
>> > lateral mosfets have lots of power gain up into the VHF range and a
>> > little stray L or C will set easily them off. Wiring layout should be
>> > done with this in mind, keeping all tracks short as possible. You can
>> > avoid heatsink capacitance by isolating it from ground and connecting
>> > all the mosfet sources directly so it floats at output voltage.
>> 
>>
>> Unfortunately I have three channels of amplifiers, and the physical 
>> design (this is a "feature improved" replacement of a legacy device)
>> makes separate heat sinks kinda unlikely/difficult.  But now that I see
>> this I should revisit this, though it would take a huge package redesign
>> to accommodate separate heat-sinks for N and P devices and retain
>> compatibility with the existing units.
> 
> 
> ** Lateral mosfets have all have the case connected to the source, so one hestsink can be used for all devices in a channel without any insulators.
> 
> I assume you are not using source ballast resistors as most designs leave them out. 

Actually I'd put some in :( figuring that they'd be convenient to measure
the quiescent current.  Before I fixed my bias generator (using the
thermal sense diode attached to one of the MOSFETS), the current wasn't
as stable as I wanted so I bumped these to 0.22ohms.

>> So far the heat sinks have
>> metal exposed to the outside world, which would have to change if not
>> grounded.
> 
> 
> ** Another option is to ground the sources of all the mosfets, via a common heatsink that can serve more than one channel. This requires that the centre point of the PSU be available to serve as the output as in this Haffler schematic:
> 
> http://bmamps.com/Schematics/Hafler/Hafler_9300,_9500_Schematic.pdf

I'd seen this design earlier - cute!  IIRC it requires (one? two?) separate 
floating power supplies for each channel, unfortunately, which makes the cure
probably worse than the disease in my situation.

>> How did you determine the 200 ohms?
>>  
> 
>  ** I didn't, but many other designers have settled on values of 220 ohms to 470 ohms per device for the job. Often N channel devices having a higher value than used for the P channel ones. 

Certainly the N channel device has less capacitance so the disparity makes
sense.  I'd sure like to understand the need for these resistors better than
the WAG about HF oscillations.

>> > An output stabilising network is essential, consisting of a inductor of
>> > about 5uH with RC Zobels on both ends to supply ground.
>> 
>> Ouch.  5uH has ~12ohms reactance at 400kHz.  I may need some inductance 
>> here, but hopefully less.  The usual Zobel impedances are also too low,
>> consuming far too much of the output at my much-higher-than-audio 
>> frequencies.
>> 
> 
> ** Yeah, that is a problem. 
> 
>  Any chance you could operate the mosfets outside the NFB loop with the gates linked via 220ohm resistors? Lateral mosfets start conducting at 0.4V so there would be a little crossover distortion and the output impedance would be around 1 ohm. 
> 
> ....   Phil

That's something I'm hoping to explore today.  The feedback network is
a standard pair of parallel RC (identical t.c.s); so I'm going to try
connecting the output-side resistor to the "real" output, and the 
capacitor to the output of the voltage gain stage.  The breakpoint is
around 1MHz, so distortion may be tolerable.

[toc] | [prev] | [next] | [standalone]

#429935

On Friday, September 30, 2016 at 11:53:51 AM UTC-4, cassiope wrote:
> On Thu, 29 Sep 2016 18:52:53 -0700, Phil Allison wrote:
> 
> > cassiope wrote:
> >>
> >>
> >> >>
> >> >>
> >> > ** Gate resistors of about 200ohms are necessary, but not sufficient to
> >> > ensure freedom from parasitic oscillations. As you have realised,
> >> > lateral mosfets have lots of power gain up into the VHF range and a
> >> > little stray L or C will set easily them off. Wiring layout should be
> >> > done with this in mind, keeping all tracks short as possible. You can
> >> > avoid heatsink capacitance by isolating it from ground and connecting
> >> > all the mosfet sources directly so it floats at output voltage.
> >> 
> >>
> >> Unfortunately I have three channels of amplifiers, and the physical 
> >> design (this is a "feature improved" replacement of a legacy device)
> >> makes separate heat sinks kinda unlikely/difficult.  But now that I see
> >> this I should revisit this, though it would take a huge package redesign
> >> to accommodate separate heat-sinks for N and P devices and retain
> >> compatibility with the existing units.
> > 
> > 
> > ** Lateral mosfets have all have the case connected to the source, so one hestsink can be used for all devices in a channel without any insulators.
> > 
> > I assume you are not using source ballast resistors as most designs leave them out. 
> 
> Actually I'd put some in :( figuring that they'd be convenient to measure
> the quiescent current.  Before I fixed my bias generator (using the
> thermal sense diode attached to one of the MOSFETS), the current wasn't
> as stable as I wanted so I bumped these to 0.22ohms.
> 
> >> So far the heat sinks have
> >> metal exposed to the outside world, which would have to change if not
> >> grounded.
> > 
> > 
> > ** Another option is to ground the sources of all the mosfets, via a common heatsink that can serve more than one channel. This requires that the centre point of the PSU be available to serve as the output as in this Haffler schematic:
> > 
> > http://bmamps.com/Schematics/Hafler/Hafler_9300,_9500_Schematic.pdf
> 
> I'd seen this design earlier - cute!  IIRC it requires (one? two?) separate 
> floating power supplies for each channel, unfortunately, which makes the cure
> probably worse than the disease in my situation.
> 
> >> How did you determine the 200 ohms?
> >>  
> > 
> >  ** I didn't, but many other designers have settled on values of 220 ohms to 470 ohms per device for the job. Often N channel devices having a higher value than used for the P channel ones. 
> 
> Certainly the N channel device has less capacitance so the disparity makes
> sense.  I'd sure like to understand the need for these resistors better than
> the WAG about HF oscillations.

I know little about these issues, but would it make any sense to try a big 
ferrite bead?  Maybe with a little R too.    

George H. 
> 
> >> > An output stabilising network is essential, consisting of a inductor of
> >> > about 5uH with RC Zobels on both ends to supply ground.
> >> 
> >> Ouch.  5uH has ~12ohms reactance at 400kHz.  I may need some inductance 
> >> here, but hopefully less.  The usual Zobel impedances are also too low,
> >> consuming far too much of the output at my much-higher-than-audio 
> >> frequencies.
> >> 
> > 
> > ** Yeah, that is a problem. 
> > 
> >  Any chance you could operate the mosfets outside the NFB loop with the gates linked via 220ohm resistors? Lateral mosfets start conducting at 0.4V so there would be a little crossover distortion and the output impedance would be around 1 ohm. 
> > 
> > ....   Phil
> 
> That's something I'm hoping to explore today.  The feedback network is
> a standard pair of parallel RC (identical t.c.s); so I'm going to try
> connecting the output-side resistor to the "real" output, and the 
> capacitor to the output of the voltage gain stage.  The breakpoint is
> around 1MHz, so distortion may be tolerable.

[toc] | [prev] | [next] | [standalone]

#429965

On Fri, 30 Sep 2016 15:53:36 +0000, Frank Miles wrote:

[snip]

> The feedback network is
> a standard pair of parallel RC (identical t.c.s); so I'm going to try
> connecting the output-side resistor to the "real" output, and the 
> capacitor to the output of the voltage gain stage.  The breakpoint is
> around 1MHz, so distortion may be tolerable.

Early testing looks very good!  Stability seems fine with a few different
loads, distortion (at least as viewed on the 'scope) is negligible, and
BW and slew rates are at least twice as fast as I need.  This is with gate
resistors around 240 ohms (no beads, at least not yet).

It would still be nice to have a better understanding of why these
resistors are needed.  For now I'll content myself with a fuller testing
with more reactive loads.  Hopefully that will shake out any weaknesses.

Thanks Phil and everyone else that contributed to the discussion!
   -F

[toc] | [prev] | [next] | [standalone]

#429974

Frank Miles wrote...
> Frank Miles wrote:
>>   [ snip ]
>> The breakpoint is around 1MHz, so distortion may be tolerable.
>
>Early testing looks very good!  Stability seems fine with a few different
>loads, distortion (at least as viewed on the 'scope) is negligible, and
>BW and slew rates are at least twice as fast as I need.  This is with gate
>resistors around 240 ohms (no beads, at least not yet).
>
>It would still be nice to have a better understanding of why these
>resistors are needed.  For now I'll content myself with a fuller testing
>with more reactive loads.  Hopefully that will shake out any weaknesses.
>
>Thanks Phil and everyone else that contributed to the discussion!
>   -F

 I wasn't in the discussion, but would like to point
 out / remind anyone interested, of my 200 W amplifier
 project of last fall.  This has a 1000 V/us slew rate
 and a DC to 10 MHz response (-3dB rolloff frequency).
 But ahem, it doesn't use lateral MOSFETs.


-- 
 Thanks,
    - Win

[toc] | [prev] | [next] | [standalone]

#429978

>Early testing looks very good!  Stability seems fine with a few different
>loads, distortion (at least as viewed on the 'scope) is negligible, and
>BW and slew rates are at least twice as fast as I need.  This is with gate
>resistors around 240 ohms (no beads, at least not yet).
>
>It would still be nice to have a better understanding of why these
>resistors are needed.  For now I'll content myself with a fuller testing
>with more reactive loads.  Hopefully that will shake out any weaknesses.

My recollection/belief is that you're dealing with the equivalent of
the "grid stopper" resistors that were used with many vacuum-tube
amplifier circuits.

MOSFETs, like tubes, have a significant amount of capacitance at their
gates/grids (both gate-to-source, and the Miller capacitance).  The
leads/traces connected to the gates have a non-zero inductance... so
you've got a series-resonant circuit, with the gate right in the
middle of it (maximum-voltage-excursion point).  This can be a recipe
for instability, up to and including parasitic oscillation at the
resonant frequency.  A term I recall from the vacuum-tube days was
"snivets" (these were thin vertical lines on a TV screen, caused by
parasitic oscillation of this general sort in the output tube).

Putting a stopper resistor in series with the gate or grid (ideally,
close to the device) both rolls off the bandwidth (resistor R
interacting with gate C) and kills the Q of the L/C resonant circuit.
It quiets the shrieking and screaming no end and can help keep the
Magic Blue Smoke where it belongs :-)

Anyhow, that's my (possibly-faulty) recollection and analysis.

I had to deal with a somewhat-related problem some years ago,
trouble-shooting a simple twin-audio-tone oscillator designed for
doing IM analysis of single-sideband ham transceivers.  Very simple
circuit, which a guy built based on an article in QST - two twin-T
audio oscillators using 2N2222 transistors.  It did oscillate, but the
frequencies and amplitudes were unstable and it'd misbehave if you
brought your fingers near the circuit.  The owner couldn't figure
out the cause, and gave me the box as a "use this for parts if you
like" gift.

A bit of poking around with a spectrum analyzer showed that this
"audio" oscillator was breaking into RF parasitic oscillation at
upwards of 100 MHz!

Sticking a ferrite bead around the base and collector leads of each
2N2222 killed the Q of the resonances and fixed the problem.  A
resistor of a few tens of ohms in the base would probably have done
just as well.

[toc] | [prev] | [next] | [standalone]

#430061

On Fri, 30 Sep 2016 14:07:16 -0700, Winfield Hill wrote:

> Frank Miles wrote...
>> Frank Miles wrote:
>>>   [ snip ]
>>> The breakpoint is around 1MHz, so distortion may be tolerable.
>>
>>Early testing looks very good!  Stability seems fine with a few
>>different loads, distortion (at least as viewed on the 'scope) is
>>negligible, and BW and slew rates are at least twice as fast as I need. 
>>This is with gate resistors around 240 ohms (no beads, at least not
>>yet).
>>
>>It would still be nice to have a better understanding of why these
>>resistors are needed.  For now I'll content myself with a fuller testing
>>with more reactive loads.  Hopefully that will shake out any weaknesses.
>>
>>Thanks Phil and everyone else that contributed to the discussion!
>>   -F
> 
>  I wasn't in the discussion, but would like to point out / remind anyone
>  interested, of my 200 W amplifier project of last fall.  This has a
>  1000 V/us slew rate and a DC to 10 MHz response (-3dB rolloff
>  frequency).
>  But ahem, it doesn't use lateral MOSFETs.

I'm sorry, Win - I didn't recall that.  I have no vested interest in the
lateral MOSFETS.  Is there a web link or at least a title that I might
search for?
  Thanks!

[toc] | [prev] | [next] | [standalone]

Page 1 of 2  [1] 2  Next page →

Back to top | Article view | sci.electronics.design

csiph-web