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

Reply to
Frank Miles
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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  
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Reply to
John Larkin

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 to
Frank Miles

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

--
John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

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 to
Winfield Hill

** You mean connected as source followers ?

** Gate resistors of about 200ohms are necessary, but not sufficient to ens ure freedom from parasitic oscillations. As you have realised, lateral mosf ets 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 abo ut 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 poi nt in the tempco curve - where it goes from positive to negative.

.... Phil

Reply to
Phil Allison

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 to
Winfield Hill

** What you "believe" is hardly relevant to anyone but you.

I hope you agree changing the gate resistor value downwards lessens the sta bility 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 yo u 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 worl d of live music sound reinforcement and quickly destroys the 100nF film cap acitors 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 ti mes they only appear superimposed on one half wave when a 4ohm load is driv en to near full output.

BTW: to evaluate how prone a certain Perreaux mosfet model was to supersoni c 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 to
Phil Allison

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.

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?

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.

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

Reply to
Frank Miles

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!

--
 Thanks, 
    - Win
Reply to
Winfield Hill

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

--
 Thanks, 
    - Win
Reply to
Winfield Hill

** 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.

** 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:

formatting link

** 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.
** 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

Reply to
Phil Allison

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

Reply to
boB

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.

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.

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.

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.

Reply to
Frank Miles
[snip]

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

Reply to
Frank Miles

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.

Reply to
George Herold
[snip]

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

Reply to
Frank Miles

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
Reply to
Winfield Hill

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.
Reply to
Dave Platt

** Using a diode like that could result in over compensation.

Lose the 0.22ohms, especially if they are WW types.

** The main DC supply needs to be floating but drive can be from an op-amp running on +/-15V rails with centre grounded. Note how there are no load isolating components - not even an RC zobel.

.... Phil

Reply to
Phil Allison

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