I need a gate driver, what's a good (ie no-fuss) one? - Page 1
Forum section: electronics / projects

author: knotlogic
author_rank: Regular Contributor
timestamp: March 07, 2017, 06:47:44 am
content: Like the topic says, I'm looking for a simple, easy to use, and gotcha-free driver for a MOSFET. The plan is to drive an N-channel FET from a microcontroller pin, the load being a resistive heater element. The heater will be driven either with about 12V or 24V DC, so that's available as a supply line to the gate driver.I've had a quick look and there's plenty of choices. The trouble is, there'splentyof choices. I'm not sure why I'd pick any one in particular over another, and I've not worked with any before, so I thought I would ask if there are any recommended devices to work with.Do I also need to match it closely to the FET I'm using?Thanks!

author: capt bullshot
author_rank: Super Contributor
timestamp: March 07, 2017, 07:33:56 am
content: As usual, it depends:e.g. on the switching frequency. Do you want to use a PWM to control the heater, or simply turn it on and off, say once per second?In the latter case, and if the current demand from the heater is low enough (say 1 ... 5 Amps), go for a logic level MOSFET which you can drive from a 3.3V MCU pin directly (add a small gate resistor anyways)

author: Phoenix
author_rank: Frequent Contributor
timestamp: March 07, 2017, 08:11:58 am
content: Quote from: knotlogic on March 07, 2017, 06:47:44 amWhile it doesn't need to be "closely matched" to the FET you're using, that detail is vital - as is other information. For what you're describing I'd look for one that takes a logic level input directly (there are other sorts that are just a pair of complementery BJTs for example).Then you want to consider switching frequency (delay, rise time, fall time), current drive capability, package/thermals, operating voltage (what gate voltage are you applying to the FET), independent source/sink output (probably not required). You probably don't want/need isolation if it's just a simple low side switch.E.g. (may not suit you but are examples of a couple of types)- FAN3122 is a powerful gate driver that takes a logic input- ZXGD3006 is complementary BJT totem pole so no voltage gain

author: ajb
author_rank: Super Contributor
timestamp: March 07, 2017, 12:47:12 pm
content: You didn't specify if this is for a low-side switch or a high-side switch, that will of course narrow the field.If the switching rate is low enough, slew rate becomes less of a concern and the only advantage to a gate driver is the higher static Vgs (->lower Rdson), so a couple of transistors and maybe a zener as a level shifter may be adequate.If you're going to be operating from 24V, you'll either need a <20V rail (typical max Vgs for many power FETs) or will have to find a driver that can operate at higher voltage without exceeding the FET's Vgsmax, which will limit your choices.If you've already picked a FET and know your switching frequency, you can use the FET parameters to calculate the required the required drive capability of the driver. This is also where you should do your thermal calculations, as switching frequency, transition time, load current, and power dissipation all go hand-in-hand.

author: Seekonk
author_rank: Super Contributor
timestamp: March 07, 2017, 01:30:18 pm
content: IR2101 are common and cheap. Not a lot of drive but sufficient for most applications. Prices for anything can vary greatly depending on source. I do heating with 50V and 10A at lower PWM frequencies and only use an opto isolator and 600 ohm pull down resistor. That is driving a couple high capacitance FET with no heating. The nice thing is with an opto you can place the FET right at the load and drive from 50 feet away. Using a FET driver can cause new problems if not careful.

author: ovnr
author_rank: Frequent Contributor
timestamp: March 07, 2017, 01:35:59 pm
content: Quote from: Phoenix on March 07, 2017, 08:11:58 amAnd just to make this clearer, because I didn't pay enough attention and got bit by it: The ZXGD300x series do not do level translation. So if you try to run them off of 12V with a 3.3V signal, they'll be entirely useless. They'llworkif you run them on 5V with a 5V micro, but it's mostly pointless.

author: Codebird
author_rank: Regular Contributor
timestamp: March 07, 2017, 01:59:20 pm
content: If you're doing this simple on-off, no high frequency/PWM stuff, you don't actually need a dedicated gate driver. A simple transistor and resistor, that's it.Gate drivers are for high frequency switching - ie, PWM. I've only done it once, and I used a TC4420. Simple, effective, dependable, but probably there are cheaper ones. I chose it because you can get it in DIP-8, and surface mount prototyping for a hobbyist is a very fiddly thing.In a piece of practical advice... put a low-value resistor on the gate pin. 47ohm is usually about right. Just to slow the rise/fall times a little, otherwise your gate is going to be ringing like a morris dancer on a trampoline and may go over max Vgs, blowing the MOSFET. Also, very few MOSFETs have a max Vgs over 20V, so if you're using a 24V supply you will need to bring that down to 10-15V first. You won't need much current, so any bog-standard linear reg will do for that. Or just a zener.

author: langwadt
author_rank: Super Contributor
timestamp: March 07, 2017, 08:29:24 pm
content: the really no fuss way it to use a smart switch/fet instead, comes in both high and low side, current limited, short circuit and over temperature protected protected

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2017, 10:02:35 pm
content: What's logic level, 3.3V or 5V?If 5V, use a protected switch, and don't worry about it.If less, use a level converter up to 5-15V first. This can be one or more transistors, or a logic gate.A full size gate driver is undesirable here: you don't need the speed to switch a heater, and you only invite problems by exciting all the parasitic AC parts of the circuit.Tim

author: langwadt
author_rank: Super Contributor
timestamp: March 07, 2017, 10:56:07 pm
content: Quote from: T3sl4co1l on March 07, 2017, 10:02:35 pmeven at 3.3V it shouldn't be too hard to find a smart switch

author: knotlogic
author_rank: Regular Contributor
timestamp: March 09, 2017, 09:12:02 am
content: Thanks for the input everyone. Let me see if I can reply to everything. Right now all the design parameters are a bit up in the air. I'm planning to use a ATmega644, this should provide me with a PWM signal of up to about 30 kHz with 8 bit resolution. The heater element is one of those "metal ceramic heaters" that are all over Amazon and Ebay and run off 12 or 24V. The one I have is rated 24V and measured at 4.9 Ohms, so say 5 A at rated voltage. I have a few different applications in mind, so my intent is to design something with some measure of flexibility.Now I realise I don't need to run the PWM at 30 kHz because of how slowly the temperature will change. I figure what I'll do is sample the temperature at 2 Hz (near or on the heater), and run the heater PWM faster than that. (Maybe one order of magnitude more?) With the maximum prescalar I can bring the speed down to around 30 Hz. I can also bump up the resolution (which brings down PWM frequency), but I don't know if that would be any use.I don't have to run at 24 V, and I suspect I have the headroom to run it lower, if max Vgs is a concern. I haven't picked a FET yet.I was thinking of using a gate driver + a decent FET to reduce switching losses. I've used an IRF7821 off a uC pin before (gate charge of 9.3 nC, input capacitance 1010 pF). The results... weren't great. Maybe I should have picked a different FET, but it did at least work. Also, I think the port pins on the ATmega644 apparently max out at 40 mA and I don't want to unnecessarily stress the part. Logic levels will be 5 V, unless I find that I need to move to 3.3 V.Plan is to use a N FET as a low side switch. Seems to me that's the easiest way to do it.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2017, 09:28:14 am
content: There is absolutely no need to try to reduce switching losses by using stronger gate drive, especially when you have a resistive heating element as the load, which guarantees maximum Id.Pick a FET which operates inside the SOA even if the switching took 1ms - this is extremely easy to analyze with resistive load, just look at the SOA curve at the maximum current.Possibly increase the resolution and run the PWM at a few Hz, even though 30Hz is surely not a problem whatsoever either.With 24V supply and a nice resistive load, a 35V rated MOSFET will do, especially because you switch slowly (little to no ringing and overshoot). If you end up using a 20V laptop supply etc., a 30V rated will do. You will find some that turn on at 5V just fine, except maybe at Id near the absolute maximum rating.Use series gate resistor to reduce the switching stress you are worried about. This will also prevent MCU IO voltage dips, ringing, and EMI. The value should be at least equal to the equivalent IO port resistance, which is typically somewhere around 20 ohms on these Atmel MCUs. A 47R will do fine.Absolutely no gate driver needed. Switching losses are minimal. Do the calculation to prove this to yourself!You can think about it this way if you want a quick&dirty reference point:Similar size FETs would be typically driven using gate drivers capable of 2A when used in a 250 kHz DC/DC converter, so that half of the total loss is switching loss. When you gate drive at 0.02A, you get the same switching loss percentage at 2.5 kHz. If you switch at 25Hz, instead, your switching loss would be 1/200th of the conduction loss.

author: BrianHG
author_rank: Super Contributor
timestamp: March 09, 2017, 09:38:15 am
content: Any 8 pin IC gate driver with logic-level input will work.I use these in some of my projects:http://www.digikey.com/product-detail/en/microchip-technology/TC1427CPA/TC1427CPA-ND/267372Note that this is a dual driver, 2 logic inputs and 2 gate drive outputs.Non inverting.Up to 16v volt support.Input logic 1 at 3v max.There are many others out there like: (These guys logic 1 on at 2.4v & 18v support)Singlehttp://www.digikey.com/product-detail/en/microchip-technology/MCP1407-E-P/MCP1407-E-P-ND/1228640Dualhttp://www.digikey.com/product-detail/en/microchip-technology/MIC4427YN/576-1205-ND/771674

author: Codebird
author_rank: Regular Contributor
timestamp: March 09, 2017, 09:55:14 am
content: There are N-channel power MOSFETs with a threshhold low enough to be driven off of 5V logic directly, no driver needed - just a 47ohm gate resistor. Obviously you won't get high frequency switching that way, but you don't need more than 100Hz or so at the very most. That's the simplist and easiest way. As per Siwastaja's suggestion. That's what I'd do. Using a gate drive IC is an option, but it's really overkill. There's no need for such expense.If you do decide to use 3.3V logic, then you'll need a gate drive - and the easiest way to do that is a little MOSFET like a BS170 or a 2N7000 with a 10K pull-up resistor. Two components, both super-cheap and readily available. Add a second 10K resistor in divider configuration if you're using a 24V supply.

author: knotlogic
author_rank: Regular Contributor
timestamp: March 14, 2017, 02:39:39 am
content: Quote from: Siwastaja on March 09, 2017, 09:28:14 amThanks guys. I'll try driving the FET from the uC directly. Figure I'll gang 3 I/O pins together and drive them through a single 47r resistor. Going this route I also won't have to worry about max Vgs.Anything I should look out for on the FET side in terms of gate charge and capacitance? I've done a quick look and there are devices with Vth of down to 2 V with Rdson under 50 miliOhm. One of those should do nicely, but gate charge and capacitance goes up with decreasing Rdson.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 14, 2017, 09:14:04 am
content: Quote from: knotlogic on March 14, 2017, 02:39:39 amNooo. Don't parallel the output pins just like that. It's a huge SW development risk, where a single bug can blow the MCU instantly. Even if you managed to switch them at exactly the same time in your code, you are still probably causing shootthrough issues. The CMOS structure is designed to be "in sync" to provide minimal shootthrough dynamic consumption only within one IO cell - paralleling multiple won't work, except by luck.And if you are going to use 47R, one single output pin provides enough current anyway. If you want to have equivalent drive current to 3 parallel pins + 47R, just use one pin and 33R instead.QuoteVth is irrelevant. It correlates, yes, but it doesn't tell you the number you need to know. You need to look at the curves to see the actual Vgs that actually turns the FET fully on (or, at least to Rdson small enough for you). Suitable FETs often specify Rds(on) at Id near maximum rating at Vgs=4.5V on their front page.

author: knotlogic
author_rank: Regular Contributor
timestamp: March 14, 2017, 02:17:51 pm
content: Quote from: Siwastaja on March 14, 2017, 09:14:04 amUh.... I didn't think there would be any significant timing difference between cells (the SW bit didn't really bother me). Thing is, I'm plan to run the uC at 5 V. With a 47r (neglecting internal resistance and whatnot) that works out to just over 100 mA. Each pin is rated up to a max of 40 mA IIRC, and that's into what's effectively a capacitor to ground. Wouldn't that be bad anyway?Quote from: Siwastaja on March 14, 2017, 09:14:04 amMy bad. The Vth is around 2 V, but I should have clarified that they do spec the Rds(on) at 4.5 V and 10 V. The 50 mOhm figure is max resistance at 4.5 V.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 14, 2017, 03:48:56 pm
content: Have a read about CMOS logic. Shootthrough power consumption is a real issue when designing those chips. So the NMOS/PMOS are carefully matched.Yes, I'm sure it will work, the Rds(on) is big enough (around 20 ohms) so that it limits the shootthrough current. You are probably going to have orders of magnitude more shootthrough than during normal, intended operation, but it's most likely still ok.But, why do it intentionally in a "dangerous" way when it doesn't help you?If you really want to parallel these pins and add 47R of resistance, it's better to divide it to 3x15ohm - so that the shootthrough current is limited by extra 30 ohm.On the other hand, driving a capacitive load such as MOSFET gate using 47R series resistor is not an issue - the average current is nearly zero (because you design it for negligible switching losses anyway). The theoretical peak of 5V/(47R+Rio) = about 70-80mA is extremely short, and made even shorter by the nonlinear nature of gate capacitance.IO pinsaredesigned to drive capacitive loads; after all, normal logic loads are capacitive. Your FET is only a bit more capacitive than more "usual" loads, which is why terminating it with a series resistor makes really sense from the AC viewpoint.So it's very likely that the remedy (of paralleling pins) is far, far worse than the actual "problem" (very short, very low-duty pulses exceeding DC maximum limit by only 2x) here. And if you are uncertain, make the R 120 ohms. You can still do great with negligible switching losses. This would guarantee that you are always inside the SOA curve of the IO pin. It would be nice if this curve was available.

author: knotlogic
author_rank: Regular Contributor
timestamp: March 20, 2017, 06:33:54 am
content: OK, I'll give it a shot with a single pin and a 47r resistor.It's going to take me a while to get it all together, and I'll try to scope out the gate and drain and see what the switching times are like.

author: Psi
author_rank: Super Contributor
timestamp: March 20, 2017, 06:46:49 am
content: You can opto lsolaters with push pull 2A output drivers. They are easy to use and work well for gate drive

author: eneuro
author_rank: Super Contributor
timestamp: March 22, 2017, 09:42:03 pm
content: Quote from: Psi on March 20, 2017, 06:46:49 amBack in the futurehttp://www.tme.eu/en/Document/4f04bbb9052db9655dbc35b071442386/TLP250(F).pdfWhy this optoisolated gate driver (TLP250F) is marked not recomended for new designs? Reason is THT version while I can not find any SMD ones?Not great switching frequency but probably not so bad for heater and can help not to kill uCUpdate: This oneTLP350Flooks much better than TLP250F

author: ohdsp
author_rank: Contributor
timestamp: March 22, 2017, 10:08:22 pm
content: Optoisolators can make life easy and they also offer isolation which can be useful in a lot of applications. The main issues if you want to drive at a higher than DC frequency is the current you need to charge and discarge the input/gate capacitance at the frequency you have chosen. At a "DC"/low frequency (anything upto a few hundred Hz) shouldn't give you any issues. Otherwise you need to do some calculations based on the MOSFET you choose and it's gate/input capacitance.With an opto-isolatior the micro can drive it's low current into a LED, presume upto 20mA which is the max of a lot of microcontroller output pins; but you should check the microcontroller specifiction. The MOSFET side of the opto-coupler can be independant of the microcontroller and run at the maximum current of the optocoupler. And while the optocouple might be specified at a maximum of 50mA continous current, if your maximum duty cycle is 50% you might be able to use 100mA peak current (but again you need to do your research/check the datasheets).Also some of the opto-isolators have pretty good bandwidth for many applications (like feedback circuits in many SMPS supplies). Something like FOD817 is pretty cheap and used a lot.This would probably be my choosen method of interfacing 2 circuits of different voltages these days, pretty cheap, reliable, and already used in many applications.

author: LukeW
author_rank: Frequent Contributor
timestamp: March 23, 2017, 06:36:11 pm
content: I like the MCP1402 as a general purpose fast gate driver where you want to kick up the gate voltage a bit too.

author: eneuro
author_rank: Super Contributor
timestamp: March 23, 2017, 06:37:02 pm
content: Quote from: ohdsp on March 22, 2017, 10:08:22 pmIt has similar rise & fall times to this PC817 so cuttoff frequency only around 80kHz and it is not optoisolated gate driver so too many additional elements needed to drive mosfet gate reliable, so saving cents on thing like this might cost someone killed mosfetshttp://www.tme.eu/en/Document/7534c9f89aa4b1eba4ec90182e378328/PC817_2_3_47.pdfIn my recent BLDC motor driver with synchronous rectification I use fast 6N137/HCPL-2200 with TC4426 3A as optoisolated gate driver

author: Codebird
author_rank: Regular Contributor
timestamp: March 24, 2017, 10:02:07 am
content: Why are we worrying about switching frequency? It's for a heater. You could PWM the thing at 1Hz just fine. There is just no need for a high-frequency driver here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2017, 01:15:41 pm
content: Not necessarily. A rate that low will physically "click" from thermal expansion alone; even in the 100s of Hz to low kHz, might be insufficient because of input ripple or power factor reasons.Ripple is undesirable in many places, even if you have no obligation for high power factor. For example, a space heater running at full power prevents you from using any other large load on that circuit. Got shitty house wiring, and you're trying to stay warm while brewing your morning coffee? Yeah nope, gotta go into the cold basement and reset the breaker again! Fffffuuuuuuuu!Even at DC, power factor matters. The same argument might reappear, but for automotive circuits, say.Or because of the effect of transient load on the supply. An unregulated supply will droop when loaded heavily, which might be undesirable for other loads. A poorly compensated switching supply may fight such a load, and undershoot and overshoot. An automotive supply might have a long time constant, say from the alternator; particularly if the battery is old (high ESR).Tim

author: knotlogic
author_rank: Regular Contributor
timestamp: April 10, 2017, 03:54:50 am
content: So one of the potential applications for this is no longer simply potential... Leading me to resurrect this thread with another question:If I want to *minimise* any generated EM/RF noise, and have a long lead supplying power to the heater block, do I want to go to a higher switching frequency? Would that (plus just enough capacitance at the device end) lower the current pulses through the power lead? (Plus, everything Tim just said.)The application for this is for use in a MRI experiment. Not in or close to the the bore of the magnet itself, but in the same room. For those unfamiliar with the topic, RF is a big deal since the system is a RF transceiver operating at ~120 MHz. A big enough deal that rooms for MR systems are Faraday cages.

author: danmcb
author_rank: Regular Contributor
timestamp: April 10, 2017, 08:24:10 am
content: right. You could probably drive the FET straight from the pin via a resistor to limit current, depending on the threshold and the voltage of the controller. The gate looks more or less like a very high resistance (10s or 100s of MOhm) with a capacitance to ground. Check the datasheet for the capacitance - typically a few hundred pF to 1 or 2 nF.A gate driver circuit is usually only needed when you want to drive this load at HF, with a controlled rise time - as in a SMPS.