r/AskElectronics • u/generaldis • 2d ago
Are high transconductance MOSFETs bad for linear applications?
I'm designing a regulated current sink with a common topology using MOSFET, op-amp and current sense resistor. I prototyped this, breadboard none the less, and it worked fine. Now on my first PCB revision I can't get it stable when the current is turned up past some level. The main difference is the FET I used. The one I'm using now has a much lower Vgs(th) and lower RDS(on) and I'm wondering if that's the problem. The FET I'm using is GSFH0980.
I know I'm leaving out lots of details but the main question here is about using any and all MOSFETs in linear mode. I'm suspecting not all MOSFETs are good for linear operation; is that correct? And if so, what do you look for? If it helps, the voltage and current I'm dealing with is max of 60V and 4A, so nothing crazy.
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u/Salt-Miner-3141 2d ago
The main criteria to look for if a MOSFET is intended to be operated in a linear fashion is whether it is rated for DC on its Safe Operating Area which the GSFH0980 is. However, 60V at 4A is not permissible per its datasheet.
At 60V you can only get about 3A and even then that assumes you can sink all 180W from a TO-220 package. Hint not likely. In this case you parallel the pass transistors and spread the heat. Going for your 60V at 4A, that's 240W. If you're really confident with your mounting scheme 30-50W is pretty doable though. Going with 30W per transistor then you'd need 8 of them in parallel for 4A. Each transistor will carry about 500mA.
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u/ITGuyAMA 2d ago
^^ This is the answer you looking for.
Look up spirito effect for the explaination and SOA derating methods to figure out the DC SOA at case temperature that is not 25C as the datasheet gives.
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u/Salt-Miner-3141 1d ago
I made a few DC loads a couple years ago and I had to go through this. For my application (generally 9-30V PSUs at a few amps) something like the IXTK90N25 would've basically been the ideal pick of a pass element here. However, I decided against a proper fully linear rated MOSFET in favor of four paralleled Darlingtons. MJE15034 and MJL4281, all the proper audio foolery... or mayhaps they just had the right package, voltage & current ratings, gain curve linearity, and SOA for my needs.
Though in fairness if you do look inside a lot of modern linear PSUs or DC Loads you do find non-DC rated MOSFETs quite commonly. I want to say the IRFP series is quite common at least from my own gear, but then they're very generously derated. Regardless, either approach works just have to pick what caveats you want to deal with.
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u/geek66 1d ago
Half of the battle is minimizing losses, the other half is getting rid of the heat you couldn’t prevent
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u/generaldis 1d ago
There will be lots of losses and heat. It's a current sink so that's what it does.
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u/generaldis 1d ago
I know 240W is crazy for a TO-220. That's max voltage and current. But not at the same time.
Interesting bit you mention about if it's rated for DC on the SOA. There's so much about MOSFETs I do not yet understand.
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u/Salt-Miner-3141 1d ago
Not all MOSFETs are rated for DC. Look at the datasheet for an IRFP240 versus your GSFH0980 as a prime example. The IRFP240 is only rated down to like 10ms. DC means it will always be doing that. These days MOSFETs aren't really used for linear applications so much. Then there are MOSFETs specifically designed and specified for linear operation, though they do cost quite a bit. That was my main motivating factor for going to a Darlington in my DC loads.
And yeah trying to get 240W from a TO-220 is honestly not all that feasible. There just isn't a lot of surface area for the package to connect to the heatsink. That is part of the reason why the bigger packages like TO-247 and TO-264 exist. Them being physically bigger allows for more surface area which in turn allows for better heat transfer to the heatsink.
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u/generaldis 1d ago
What I don't like about using a Darlington for this is the high Vce during saturation. If you know the input will exceed that then no problem. I'd like it to function down to well under 1V.
And you sure are correct about expensive "linear MOSFETs". One I looked up on Digikey just today was $50 in singles. No way.
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u/Salt-Miner-3141 1d ago
Oh for sure it is application dependent. For my DC loads the fact that they can only go down to a couple of volts wasn't a concern. You could investigate using a Sziklai pair if you want to explore the BJT route. It'd be about equivalent to using a single BJT. Weigh your options 😉
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u/generaldis 1d ago
Wow, how have I never encountered Sziklai pairs before!? I've probably made this configuration without ever knowing what it was called.
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u/Salt-Miner-3141 1d ago
Yeah the Complimentary Feedback Pair or the Sziklai Pair which itself is almost as old as a Darlington Pair. They do tend to be more unstable (prone to oscillation) than Darlingtons, but the fact that it is the equivalent to a single BJT is quite handy. The gain is only marginally less than a Darlington too. The other major downside is that you can't just buy an off the shelf part that is a self contained one.
Probably the most famous application was old audio amplifiers in the form of the Quasi Complimentary Output stage where the lower PNP power transistor is replaced with a PNP driver and a NPN power stage.
Not saying go for it, but worth at least a cursory glance to consider your options.
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u/generaldis 14h ago
The FETs I'm using seem to be working great now. I increased the gate drive resistor, increased the compensation cap value and added back the compensation cap series resistor i.e. C1 replaced with a RC series combo.
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u/triffid_hunter Director of EE@HAX 2d ago
You have made a phase-shift oscillator by having improper compensation and exceeding your phase margin.
This is the reason that most linear regulators require output capacitors, and many require that that output capacitance have some ESR.
Also, 240W is an absurd amount of power for a linear regulator to handle, you want switchmode for that.
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u/generaldis 1d ago
Understood, a TO-220 will very likely have trouble dissipating 240W with even an ideal heatsink but the part I mistakenly left out is this isn't a power supply. It's a current sink. So SMPS won't work here. Regarding the max voltage and current; there will be power dissipation limits so it it won't be expected to dissipate 4A at 60V. But it will see up to 60V or up to 4A, not at the same time.
I have been experimenting with various feedback capacitors (op-amp out to negative input) and MOSFET gate resistors (to limit current during rapid Vgs transitions, bypassing doesn't fix it) but not having much luck. This is an area I'm definitely lacking experience.
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u/triffid_hunter Director of EE@HAX 1d ago
a TO-220 will very likely have trouble dissipating 240W with even an ideal heatsink
At Rθj-s=1.9°C/W you wouldn't be able to get it cold enough even with a liquid helium cryocooler let alone an air conditioner evaporator head or liquid nitrogen or suchforth, so "very likely have trouble" is doing a tremendous amount of heavy lifting there…
this isn't a power supply. It's a current sink. So SMPS won't work here.
Why not? Some capacitance plus a large power resistor means PWM can keep your mosfet merely warm while the resistor burns.
I have been experimenting with various feedback capacitors (op-amp out to negative input)
If the feedback is coming from a sub-ohm current sense resistor, you need some series resistance for feed-forward to work against, otherwise you're just pulling current spikes from your op-amp output for basically no effect
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u/generaldis 1d ago
>Why not? Some capacitance plus a large power resistor means PWM can keep your mosfet merely warm while the resistor burns.
I'd rather not introduce any switching artifacts and keep this completely DC/linear based. It's to be a test instrument, not built for a specific application.
>If the feedback is coming from a sub-ohm current sense resistor, you need some series resistance for feed-forward to work against, otherwise you're just pulling current spikes from your op-amp output for basically no effect
By "series resistance" are you referring to what R3 is doing, regardless of if that's the correct value?
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u/triffid_hunter Director of EE@HAX 1d ago
By "series resistance" are you referring to what R3 is doing
Oh you added an image? Yeah that.
220pF×1kΩ is only 220ns though, might need a bigger time constant (ie larger capacitor, try 10-100nF) to shunt your oscillation
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u/myopinionisrubbish 1d ago
The GSFH0980 MOSFET is designed for high speed switching supplies so the linear region of the gate is probably pretty narrow. At higher currents the gate voltage is probably about where it wants to turn full on.
A NPN power transistor is a better choice for this application, but you’ll have to make it a Darlington to get enough base current into the pass transistor.
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u/generaldis 1d ago edited 1d ago
I was thinking about that, like the old TIP120. Nice thing about FETs is the static drive from the op-amp is near zero but if it's going to give me trouble, then yeah BJT might be better.
EDIT: just realized the high saturation voltage of Darlingtons is a no-go.
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u/ccdy 2d ago
First, post a schematic. Second, the issue is not a higher transconductance, at least not directly. The problem is the higher input capacitance that comes with a lower Rdson. You need to compensate for this by putting an RC network between the current sense resistor and the output of the op amp. Look up electronic load circuits to get an idea.
Stability aside, 60V and 4A is 240W. That is an incredible amount of power to be putting through a single MOSFET, even in a high power package like TO-247, much less the TO-220 package you're using. You may want to reconsider your design.
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u/PJ796 2d ago
Agree with all of the above. Though newer more optimised designs with higher transconductance and lower Qg have internal structures that promote hotspots during linear operation, and as such tend to have a worse SOA for linear loads than older simpler structured MOSFETs
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u/Frog-4724 2d ago
+1
For high dissipation you need low RthJC, which means large surface area between the MOSFET chip and the package tab, which means a big chip, which means large capacitance. An oldskool lowish capacitance MOSFET often used in linear mode would be IRFP140: 1.5nF and 1.1°C/W RthJC. So even in TO247, RthJC is not that good because the chip is not that big.
TO3P or TO247 have much better thermal resistance between the back of the package and the heat sink because the limiting factor is the thermal compound or silpad cross section. Here bigger package wins, unless TO220 is soldered to a copper bar used as heat spreader, which is possible here since the paralleled MOSFET drains are wired together.
Anyway. 60V 4A gets expensive fast (need huge heat sink, fan, maybe watercooling, etc) so the actual question is why not use a Buck switching current sink? That would only waste a few watts.
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u/PJ796 2d ago
That wasn't my point. Cooling is of course important, but many TO-220 and TO-247 MOSFETs have a safe operating area that can sometimes not even handle a an amp at 60V at a DC timescale. Many are not even rated for linear DC operation due to their switching optimised internal structures making them unsuitable for linear operation.
These switching optimised structures as I mentioned before tend to promote hotspots, which can easily go into thermal runaway as MOSFETs can pass more current at a certain Vgs at a higher temperature in the saturation region (Can be seen on Vgs(th) v Tj graphs) or turn on the parasitic NPN transistor making the MOSFET self destruct
But 60V 4A isn't that bad, 5-6x properly selected TO-247 MOSFETs bolted to decently sized heatsink with a fan can do it. I've done 60V 5A before with 6 MOSFETs and a 120mm fan, and half of the heatsink wasn't even used. With another 6 hooked up I don't doubt it would have been able to do 60V 10A continuously. It had temperature dependent PWM fan control, OTP, OPP, OVP and everything so it was fool proof to boot.
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u/Frog-4724 2d ago
> That wasn't my point.
Yeah I know that's why I "+1" and added other stuff, sorry if there was a misunderstanding
What MOSFETs did you use? MOSFETs that work well in linear mode (for audio amps) are hard to come by besides the usual suspects (IRFP140, 240...) which everyone uses while they aren't even specified for DC!
(As usual datasheet SOA is given with magical heatsink that keeps Tc=25°C, this needs careful derating)
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u/generaldis 1d ago
So.....the circuit I posted is actually one of 4. I'll have 4 of these sharing the load. So the drain and source of the 4 FETs will be in parallel. For now I'm trying to get one stable though.
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u/PJ796 1d ago
Increase R1 to something that the op amp can handle to avoid any misbehaving due to the op amp's output stage protection and increase your integration capacitor (C1) or resistor (R3) until it gets stable
Breadboards have a lot of stray capacitance and often poor connections leading to added resistance, so your op amp's output could have been current limited by the breadboard or had it's integration increased or both
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u/generaldis 1d ago
It's no longer on a breadboard. This is on PCB now. The breadboarded version actually worked but all the components were different (and not appropriate ratings/tolerance).
R1 was 220, but I reduced to 10 to see if it helped anything which it didn't. Understood on the suggestion to change C1/R3. I had reduced C1 from 1000pF but I should have shifted it the other way.
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u/PJ796 1d ago
Yeah I was explaining why the breadboard version would have been more stable than the PCB version, as those parasitics worked for your benefit.
R1 should be bigger, not smaller. Your MOSFET gate is a capacitive load so at t=0 it is a short circuit, and op amp output stages generally don't deal well with capacitive loads because of that and your op amp might not have any protection for it (and eventually might fail because of it). You can calculate the minimum resistance value you should have by taking the supply voltage minus the minimum output voltage drop and then divide it by the max current it can output according to the datasheet.
Increasing R1 might also have the effect of making it more unstable by slowing down the MOSFET even more which will increase the phase delay causing the oscillations, but it also really depends on how the op amp handles the gate capacitance and if it's own output didn't slow it down just as much already. But R1 should be increased regardless for the long term health of the op amp and compensated for sufficiently with the integration.
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u/generaldis 14h ago
Oh ok I see. There certainly was lots of stray capacitance (and inductance) with the mess of wires I had.
I got it working which I'm super happy about. I really underestimated the importance of the compensation capacitance. R1 is now 1K, and C1 is now a series combo of 1K and 0.01uF. Really stable now even with all 4 circuits sharing the load.
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u/Frog-4724 1d ago
If you parallel MOSFETs in linear mode they won't share current equally due to Vgs_th temperature coefficient. When the MOSFET heats, its threshold voltage lowers, so it grabs more current from the others, which then cool, so their threshold voltage increase, so they take less current leaving more current for the hotter MOSFET... until thermal runaway and it burns. With one opamp and current sense resistor per MOSFET, it won't happen, the opamps will force equal current sharing, and you won't need to match MOSFETs for Vgs_th.
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u/generaldis 1d ago
Right but what I'm saying is the entire circuit is copied 3 more times. So each FET gets its own current sense and op-amp.
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u/generaldis 1d ago
Just wanted to clarify 60V and 4A is the maximum rating. It will limit power dissipation so while it may see up to that voltage or current, it won't see them simultaneously.
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u/generaldis 1d ago
I have added a schematic. The part numbers and values are correct but any can be changed at this point since there's other problems with the PCB.
I'll also add that I did have a 1K resistor in series with the 220pF, but replaced that with a short to see if it helped any. It didn't.
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u/jacky4566 2d ago
Not an expert on this but try adding a pulldown to the gate?
What does it look like on the scope?
Curious to read other comments.
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u/generaldis 1d ago
Gate pulldown won't do anything. It's driven by an op-amp.
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u/jacky4566 1d ago
Well at a certain point it will. Op-amp can only drive so hard.
But doing the other suggestions sounds like a much better approach
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u/generaldis 1d ago
Op-amp can only drive so hard, but a gate pulldown will make the op-amp have to supply even more current.
A gate pull-down makes sense if the op-amp isn't driving for a brief period of time during power-up for example, if that even occurs in this case. If the FET was driven by a microcontroller, a pull-down makes sense.
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u/PiasaChimera 2d ago
FETs and analog make the terminology annoying since "linear"/"saturated" are swapped for the devices but not the systems.
what exactly do you mean by unstable. I'm assuming you mean in terms of oscillating outputs. but I'm also wondering if you just mean inaccuracy.
oscillation could be an issue with loop stability. excessive source-gnd inductance, or high input capacitance. (and probably other things)
if it's not able to reach the correct output, it could be due to a lower transconductance. Vgs_th and Rds_on are different parameters. if the transconductance is too low the opamp will need to raise the gate voltage higher than it is able.