Maximum practical mosfet gate voltage - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 06:05:30 pm
content: So if a mosfet has an absolute maximum gate voltage of 20V am I OK to pulse it at 15V ?, most mosfets state their Rdson at 10V but I'd like to guarantee fastest gate charge and going from 10V to 15V effectively makes it 5 times faster as the time to reach 2/3V in an RC circuit is R*C seconds whereas to reach full supply voltage it's 5RC seconds

author: capt bullshot
author_rank: Super Contributor
timestamp: November 12, 2017, 06:11:05 pm
content: As long as you stay below abs max (or operating max if specified), it'll be OK.Charging a MOSFETs gate isn't a simple R/C function, as there is the miller capacitance from drain to gate, coupling back an amount of charge into the gate. So don't expect your MOSFET switching that much faster with higher drive voltage. In a power MOSFET application, the driver's strength vs. gate charge gives the switching speed.Edit: 15V is a commonly used gate drive voltage for MOSFETs.

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 06:15:29 pm
content: OK, well I have a nice strong driver that can do 12A peak but even with the drain to gate capacitance we are still talking about charging / discharging so with a higher drive voltage I'd expect to have some edge. I'm not dependent on it. i have a 15V regulator in my design that I hope to develop for 12 or 24V, so for 12V it will mostly just pass power and supply at least 10V to the gate driver unless for some reason the voltage rises significantly, at 24V (20-32V) it will effectively stop the gate driver and gates getting damaged.

author: Zero999
author_rank: Super Contributor
timestamp: November 12, 2017, 07:34:11 pm
content: Yes 15V should be safe but don't expect any significant increase in speed. The greatest effective capacitance of a MOSFET's gate-source junction, is seen from point, when the MOSFET is starting to turn on and conduct current, which occurs as the gate threshold voltage is exceeded and when the drain-source voltage falls below the gate threshold voltage. At that point in time, the gate capacitance is much higher, than that specified on the data sheet. Use the gate charge specification to determine how much current needs to flow in/out of the gate, within a certain time, to achieve the required switching speed.

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 07:39:13 pm
content: You mean because the capacitance from the gate to the drain will now be in parallel with the capacitance from the gate to the source this goes up greatly as the mosfet turns on more. Hm again if this is reduced down to an RC situation more volts should charge faster to a same voltage? I obviously need to achieve 10V for the stated on resistance and if to get the mosfet turned on i have all of this capacitance to charge, charging to 10V with 10V will take 5RC, charging to 10V with 15V will take RC, 1/5 the time, obviously the gate driving "gear" may not act like a perfect resistor.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 12, 2017, 08:35:03 pm
content: Depending on the MOSFET, but assuming some kind of non-logic-level power device, the biggest "charging" event happens at about Vgs=3V. This, and slightly after this, is where most of the losses are. Then the charging gets quick again, and losses also start dropping.So, the difference between 10V and 15V in relative charging speed is more like the difference between 10-3=7V and 15-3=12V, or about 1.7x, not 5x. And, of course, because there is reserve voltage left at this point, you could achieve the same improvement just by using a smaller gate resistor.Most FETs (except some very large high voltage ones at very high drain currents) are in relatively low-dropout mode starting at around Vgs=6V; after that, it's mostly a slight Rds(on) optimization, which doesn't need to happen quickly. Charging after the threshold happens relatively quickly anyway since the effective capacitance seems to drop back.Just googling "mosfet gate oscilloscope" gives this nice image which illustrates the gate charging:http://sphinx.mythic-beasts.com/~markt/MOSFET-9V.jpg. Note that this particular FET is "stuck" charging slowly between 2.0 and 2.5V, so the Vgs(th) is slightly below 2.0V for this part.So yes, you get some reduction in switching losses by going from 10V to 15V, but not necessarily as much as you may think. For even more improvement, you need to implement the negative voltage supply for the gate driver to turn off quickly, too. For power IGBT bricks, I sometimes used +17/-5V as a rather "balanced" system, still fitting inside many gate driver voltage range. Happened to have some +/-5V and 12V isolated DC/DC modules, easily generating this voltage set when put in series.To answer your question, if you actually can guarantee that your gate is driven to 15V at all times, no problem whatsoever and you even have quite some safety margin there.But as you are switching quickly (that was the point anyway), beware of possible gate oscillations and overshoot. 5V is good margin, but substantially less than 10V is.Also look at the gate driver output stage dropout when choosing the voltage. When learning to design IGBT gate drives for EV traction, I often found application examples utilizing kind of darlington stage in the gate drivers dropping around 1.5V, powered from 15.0V, only giving 13.5V to the gate actually, barely enough to safely turn the devices on, with a lot room for improvement by just going a bit higher in voltage (hence the 17V supply mentioned earlier). These solutions were often analyzed without taking the dropout into account!

author: Simon
author_rank: Unknown
timestamp: November 12, 2017, 08:42:20 pm
content: The gate driver has mosfet oputputs with a 1mR resistance, I am using gate resistors of 1R right next to gates also as I have 4 mosfets in parallel, so I am thinking that having put resistance in series intentionally increasing voltage would not hurt to maintain stability and keep charging quick. Yes I am using a 15V regulator (7815 or more modern derivative) so it is guaranteed to stay at 15V.What actually causes that flattening out of the voltage rise? is it the sudden increase in capacitance ? that is one reason why i wanted to allow more voltage to overdrive through any resistance and help (ok not sure) potential delays.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2017, 12:53:14 am
content: Quote from: Simon on November 12, 2017, 08:42:20 pmReally?Sounds pretty slow then...QuoteJust how fast are you looking to go, here?FYI, if you increase the on-voltage in a search for speed, you should decrease the off-voltage as well. +15/-5V is very typical for IGBT driving.What does your layout look like? Is it tight enough to handle such fast switching speed?QuoteCdg. Note that Cdg is very large at small drain voltage, so most of the Miller step is spent at low drain voltages, where loss is small.This isn't so exaggerated on lower voltage devices (what I assume you're talking about), but is considerable for high voltage devices. This is a 650V part, resistive load, 1mA gate drive. The weak drive slows down the turn-on transient so it is fully visible.(The red curve overlay is comparison with the SPICE model I created for it.) See how the voltage drops 90% of the way before it slows down? Cdg increases by about 100 times in a short span!Tim

author: Simon
author_rank: Unknown
timestamp: November 13, 2017, 07:37:12 am
content: Yes that is why I am putting a resistor in series with the gate and it will go close to the gate to stop the ringing.

author: Simon
author_rank: Unknown
timestamp: November 13, 2017, 07:38:57 am
content: Quote from: T3sl4co1l on November 13, 2017, 12:53:14 amIt was one of the heavier duty ones I could find:https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwj7wu_ghbvXAhUB_aQKHQOhAHcQFgguMAA&url=http%3A%2F%2Fww1.microchip.com%2Fdownloads%2Fen%2FDeviceDoc%2F20001987C.pdf&usg=AOvVaw0ulT-DYgi_OoSRclTSnWKz

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2017, 08:09:04 am
content: Oh, 1.0 ohm (give or take).Tim

author: Simon
author_rank: Unknown
timestamp: November 13, 2017, 08:21:24 am
content: oh yes, why did I think it was 1mR, doh. Is it any good? I know microchip have a reputation for making cheap rubbish with wonderful caveats.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2017, 09:06:58 am
content: TC4xx(x) gate drivers are well known. Not aware of any quirks.How about some more questions-Quote from: T3sl4co1l on November 13, 2017, 12:53:14 amTim

author: Simon
author_rank: Unknown
timestamp: November 13, 2017, 09:11:39 am
content: The layout is yet to be done, I intend to make the driver to gate paths short. I'm switching at 1-2 KHz

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2017, 09:24:21 am
content: No not fundamental frequency, transition rate. 12A gate driver implies you're targeting the low nanoseconds here, which will require a layout that may not be physically possible.Tim

author: Simon
author_rank: Unknown
timestamp: November 13, 2017, 10:19:44 am
content: Oh well i just wanted something tough and I'm using it to drive 4 mosfets so I've certainly maxed out the capacitance it can drive easily. No I don't need high speed but would rather err on the side of caution and use gate resistors to slow the transition down if needs be.