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
Gate resistor for lateral power MOSFET?
Started by ●September 28, 2016
Reply by ●September 28, 20162016-09-28
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! > -FMosfets 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
Reply by ●September 28, 20162016-09-28
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
Reply by ●September 28, 20162016-09-28
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. > > -FWild 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
Reply by ●September 28, 20162016-09-28
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
Reply by ●September 28, 20162016-09-28
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
Reply by ●September 29, 20162016-09-29
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
Reply by ●September 29, 20162016-09-29
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
Reply by ●September 29, 20162016-09-29
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. > > > .... PhilI 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
Reply by ●September 29, 20162016-09-29
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
