Analog/linear gate driver? - Page 1
Forum section: electronics / projects

author: Martinn
author_rank: Frequent Contributor
timestamp: January 13, 2025, 07:25:52 pm
content: Hi all,for my AC current source ideahttps://www.eevblog.com/forum/projects/ac-current-source-nmos-h-bridge/it still looks as if an analog N-MOSFET bridge is the easiest way to go. I also found that a few handfuls of ancient HEXFET devices are still available (check on Digikey), many even with DC SOA in the datasheet! Feels like christmas. I'd use four TO-247 ones, which would give plenty of current capability. However, they have input capacitances of up to 20 nF.How would I drive 40 nF with low enough phase lag to make closed loop stability not completely hopeless in the range up to 100 kHz to 1 MHz?Integrated gate drivers are digital only.The classical bipolar totem pole has nasty crossover distortion, maybe not the best option.A full blown class AB audio style driver?Are there any opamp buffers for high drive current or with high capacitive load capability (40 nF)? Preferably not with 1 GHz GBW to make life easier? Can't find anything decent right now.Thanks, Martin

author: jbb
author_rank: Super Contributor
timestamp: January 13, 2025, 07:59:38 pm
content: Well, if you want a linear power amplifier then you’ll need linear gate drivers. I expect it rapidly starts to look like a class AB audio amp. I’ve never used one but the LT1166 looks like an interesting part for such an amplifier.On a different track: have you considered using a power opamp? If a suitable one is available it could save you a lot of time. Texas Instruments makes some fairly chunky ones…

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2025, 08:23:39 pm
content: A linear amplifier needs more than just a gate driver. I would look at audio amplifier as a whole, not just as a gate driver. This can be MOSFET based, but also BJT based. With MOSFETs the stability of the standing current can be an important issue.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 13, 2025, 08:58:55 pm
content: Quote from: Martinn on January 13, 2025, 07:25:52 pmSounds like rapid feature creep.Your original spec in the other thread for motor drive was forQuoteThis is only for a one-off test, so just get a good Audio amplifier and a high amperage secondary stepdown transformer.It may be kg's but it is simple and cheap and predictable, fine for a quick test.

author: temperance
author_rank: Super Contributor
timestamp: January 13, 2025, 10:32:25 pm
content: Today I would build something like that with a class D amplifier.For an analog "audio" like amplifier I would stay away from MOSFET's. Many pro audio brands tried to use MOSFET's but many of them failed. Even well known pro audio brands.-Bias current instability.-MOSFET's are easy to use in parallel because they have a negative tempco until you really try that.-Excellent oscillators

author: Marco
author_rank: Super Contributor
timestamp: January 13, 2025, 10:39:15 pm
content: Quote from: Martinn on January 13, 2025, 07:25:52 pmIf you want something modern :https://www.mouser.com/pdfDocs/UnitedSiCAN0016-UnitedSiCJFETinActiveModeApplications.pdf

author: Circlotron
author_rank: Super Contributor
timestamp: January 14, 2025, 01:19:44 am
content: Quote from: temperance on January 13, 2025, 10:32:25 pmParalleled MOSFETS that arefully turned onwill somewhat share current because if one device hogs the current it will get hotter, causing its on resistance to increase, and so it backs off.But if MOSFETs are paralleled and used inlinear modethe hottest one will have its threshold voltagedecreasewith temperature and this causes it to conduct even more. The opposite of what you want.

author: CaptDon
author_rank: Super Contributor
timestamp: January 14, 2025, 02:49:09 pm
content: If you want linear output from an H bridge you don't need linear gate drivers!! That would be 100% counter intuitive to even using an H bridge. You need to convert the clock frequency to a smooth essentially 'variable bipolar d.c. output'. This will require the choke / capacitor low pass filter which could also be thought of as the 'd.c. component' restorer. It creates the d.c. value of the duty cycle of the PWM modulated switching or 'clock' frequency. There are many high current gate driver packages readily available. For a really robust H bridge drive circuit do not use topology that relies on 'charge pumping' to obtain gate drive on the high side!!! You can run into the high side running out of gate drive current/voltage! Two isolated 15vdc supplies for the high side is a better bet but requires more parts. Those two supplies usually come from a three winding toroid (one primary, two secondaries) which is driven with perhaps 10KHz on the primary and the secondaries are rectified and used for the high side isolated gate driver supplies. (you can wind these yourself, at 10KHz you don't need a lot of turns on the toroid core. In many really brute force H bridge motor drivers they don't even use the low pass filter method. The motor itself becomes the 'series choke'. Although you can measure the EMI generated a hundred feet away!! Not sure exactly want you want but look at the Peavey DECA 700 / 724 / 1200 series of audio amplifier schematics available online. They can generate 100's of watts of clean audio over 20KHz with a clock of 500KHz. The output topology could work all the way down to d.c. but that is disallowed and sensed as an 'excess d.c. offset failure' and shuts the module down to protect the speakers. The 724 and 1200 were very robust, the 700 was mostly 'busted' due to early design flaws that were later corrected. Those are stereo amplifiers. Each channel is actually a 'half bridge' when in stereo mode. When switched to mono mode where you drive one channel's input and the other channel is driven with an inverted signal you then take the output across the two half bridges basically forming a full H bridge. Within reason I have used these amplifiers to drive induction motors up to about 1/10th horsepower.

author: temperance
author_rank: Super Contributor
timestamp: January 14, 2025, 03:24:39 pm
content: Quote from: Circlotron on January 14, 2025, 01:19:44 amThis can be compensated for with source resistors just like you would do with transistors. At least, that is what some old app notes discuss this at length. (I don't have any of those anymore) Some pro audio companies did that and failed. As a result you could adjust the bias current and the bias would drift away into thermal runaway or fall back to almost zero. I've seen many of those going to the scrap yard because you couldn't make them work properly even with Vgs-th matched MOSFET's they would sooner or later go up in smoke.That being said I would go for a class D amplifier for this application. The required bandwidth is low so closing the loop around the output filter isn't difficult. And if must be linear at all cost i would use good old power transistors. Plenty of those are available like the 2SC5200 and it's counterpart.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2025, 04:10:00 pm
content: The source resistors when using parallel MOSFETs in linear mode need to be relatively large, as the TC in the gate voltage is usually larger the the -2 mV/K of BJTs.Some circuits use active balancint with additional BJTs to control the DC balance. Electronic loads often use a seprate control loop (OP-amp) per FET. This also reduces the capacitance seen per OP-amp.

author: Martinn
author_rank: Frequent Contributor
timestamp: January 14, 2025, 07:58:46 pm
content: Quote from: PCB.Wiz on January 13, 2025, 08:58:55 pmYou mean because of the frequency? No, I am more concerned about closed loop phase margin. Although this is a zero order plant without inherent delay, much better than a first order (motor current control, SMPS current mode) or second order (position control or SMPS voltage mode) where you lose 90 or 180° phase (out of 180 max) by the single/double integration already. But still each stage adds up so in the end you run out of phase margin and have an oscillator.Quote from: PCB.Wiz on January 13, 2025, 08:58:55 pmMaybe... does anyone know for what frequency range mains toroids are useful? They would have the benefit of easy access (unless potted) for a few turns secondary, while regular (like E core) output transformer would have to be custom made.

author: Martinn
author_rank: Frequent Contributor
timestamp: January 14, 2025, 08:02:49 pm
content: Quote from: jbb on January 13, 2025, 07:59:38 pmLT1166: Nice! Never heard of that (although a few mentions on diyaudio, so not totally unknown it seems). Strangely it does not show up on the ADI page unless you explicitly search for it.Audio amp: Might be the simplest option. OPA541? 10 amps, various modules on Alihttps://www.aliexpress.com/item/1005004573743361.html(could they be fakes) for easy breadboardingWould need to run at 10 V or so, waste of energy, but why not.

author: Martinn
author_rank: Frequent Contributor
timestamp: January 14, 2025, 08:15:44 pm
content: Quote from: CaptDon on January 14, 2025, 02:49:09 pmI am fully aware of how motor current control works (I designed a few controllers myself).The general topic here is switching vs. linear. Obviously the modern approach would be switching. I am not conviced that you would get good performance of it, for a number of reasons:Residual current ripple limits useful dynamic range and introduces "noise"Either the choke gets huge (for like 50 A) or the switching frequency high (don't want to mess with GaN or such), also hard switchingFrequency range limited by possible switching frequencyI am also not sure if "audio" amplifiers are a good fit. 20 A into 8 ohms is 3200 W - this would be a monstrous amplifier. Below 10 A probably yes, but then it would be more straightforward to just use an OPA541.The general idea would be: How do you drive very high currents into a very low impedance (think of a 100 A shunt) load efficiently? I don't see "motor current control" (hard switching H bridge) or audio class D (or analog) as good fit, as they are made for (much) higher voltages and lower current.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2025, 09:06:26 pm
content: If this is just for high current at low voltage, one can use a transformer. The point than is having an amplifier that can tolerate a transformer as load.Audio amplifiers are not all 8 ohms. For moderate high power in cars they also use 2 Ohm and changing a circuit for lower impedance is not that difficult.For the very high currents a current transformer may anyway be more accurate than just a shunt.

author: CaptDon
author_rank: Super Contributor
timestamp: January 14, 2025, 10:04:41 pm
content: Well, we use H bridges to drive traction motors of 400+ horsepower and we can get regenerative braking also with the same inverter and a load resistor on the link voltage. I don't understand why your requirement is so complicated? We had a Fuji 'chip shooter' SMD placing machine (sadly I forget the model, maybe 724) with a servo drive that shuttled probably over 1000 pounds when fully loaded. It was lightning fast and if the safety light curtain was disabled could snap a pine 2X6 like a toothpick. Using class D switching H bridges with a clock of 500KHz will surely be capable of sinewave response to 20KHz and probably nearly squarewave up to 20KHz also. Our gate drivers were pre-packaged units which used 25KHz drive into a small transformer that was rectified and used by very high current totem pole devices to provide the gate drive. The signal input to the gate drivers was a PWM signal delivered via fiber optics. Our link voltage was 1400vdc. One 400HP inverter stack had six gate driver packages and six huge IGBT devices forming three half bridges driving a three phase induction motor. Are there not any off-the-shelf solutions to your requirements? I haven't seen any servo system with a phase change speed over a couple of kilohertz, hence the audible whine. Compensation for inertia and coil current lag is mostly software tuning. The drive voltage is exceedingly high at first to get the current to ramp up quickly, then drops back to a safe 'hold' level until the next phase to phase change. Building the H bridges up to 1400 volts and up to 600 amps has off-the-shelf solutions!! Tuning it is a P.I.T.A.!!!

author: Martinn
author_rank: Frequent Contributor
timestamp: January 15, 2025, 05:35:02 pm
content: Quote from: CaptDon on January 14, 2025, 10:04:41 pmOne reason is I care about the current and in a servo drive you do not.Once I designed a high performance motion controller in an FPGA and for fun simulated what influence it would have to reduce the current ADC resolution (from 12 bit IIRC) to maybe 3 bits. Result: None. You could even have bang-bang on/off (if that would be possible with given motor inductance) and still have closed loop position control in the submicron range. The reason for that is that a servo system is a double integrator: Current gives torque or acceleration, integrated once velocity, integrated another time position.That is a second order plant or a 40 dB/decade lowpass filter. For that reason, current fidelity is completely irrelevant, as long as it averages out to the correct value. More or less this is similar to delta-sigma conversion.Same with class D audio: The amplifier generates a voltage with lots of ripple. You don't hear voltage on the speakers. One integration gives current in the coil, which you don't hear either. Next integration gives velocity, still inaudible. One more finally gives membrane displacement, giving pressure variations that travel to your ear. So that's a 3rd order or 60 dB/decade filter. I briefly wondered how you can state THD in a class D amplifier datasheet, but it turns out that they require a beefy "measurement filter" to remove the PWM.With high enough PWM resolution and low drive voltage a H bridge might work - maybe. But I'm not sure how switching losses of a hard-switched 20-50 A bridge at 200-500 kHz would be. Audio class D in that range seem unsuitable, as they require a pretty high (20-50 V) power stage voltage.

author: temperance
author_rank: Super Contributor
timestamp: January 16, 2025, 10:49:51 am
content: An LC circuit driven at resonance and a current sense resistor as reference might work. (with a few turns more on the current sense transformer)