Are logic level gate mosfet slower than regular mosfet? - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: October 29, 2018, 05:27:28 am
content: 1. Are logic level gate mosfet only used for static applications? Can they be used for medium frequency 10khz, switching?2. Are they slow in turn on? Will they heat more that the typcal mosfet + gate driver combination?I'm looking at switching 10's of amps at a frequency between 5 to 15khz. voltages would be around 10 to 20V.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2018, 06:06:01 am
content: 1. No, they're used all the way up into the MHz. Design accordingly though.2. In reference to 10V drive, turn-on is faster, but turn-off is slower (can you reason out why?).Also, low voltage (Vdss < 30V) parts have lower Vgs(max) and higher gain, allowing quite good performance at Vgs(on) = 5V), even Vgs(on) as low as 1.8V for some battery management transistors (which are available with Vdss ~ 6V and Rds(on) < 1mohm!). If you have the choice between a higher or lower voltage part (say with a topology tradeoff, half bridge vs. push-pull?), it can be better to go with the lower voltage -- higher performance -- part.Tim

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: October 29, 2018, 07:05:34 am
content: Quote from: T3sl4co1l on October 29, 2018, 06:06:01 amThanks, I will think about the slow turn off issue, cannot say why it turns of slowly currentlyWill the gate present a significant load on the driving pin, lets say a microcontroller I/o pin |@ 10khz frequency.what kind of drive requirements is one looking at?I see that mouser as a Vgs(th) column in the selection table no Vgs(on) so I would have to click on each entry to find the Vgs(on)?

author: Ian.M
author_rank: Super Contributor
timestamp: October 29, 2018, 07:26:26 am
content: As a rough rule of thumb, look for max Vgs(th) less than half your logic '1' level.The gate looks like a *LARGE* capacitive load, so whether or not your MCU will be happy driving it direct depends on its output characteristics and tolerance for brief overloads. At 10KHz you'll *probably* get away with it unless your MCU has really wimpy pin drivers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2018, 09:56:35 am
content: For a transistor of that size, I would recommend a driver, not a bare MCU pin. A 74HC3G04 (all sections wired in parallel) would be good enough, or a proper driver of any various sort (discrete or integrated, however you like).If you go discrete, I'd kind of hesitate against using a complementary emitter follower output stage, because of the 0.7V offset. It's usually okay for logic level gates still, but there may be better ways. Unfortunately that rules out most simple methods (at least that I know of, and can think of), so I'd stick with the parallel logic gates or integrated driver.Note that Rds(on) is always better at higher Vgs(on), with a lot of 5V "logic level" spec'd parts being headline spec'd at 10V. If you have the higher voltage available, it's worth borrowing.Tim

author: Ian.M
author_rank: Super Contributor
timestamp: October 29, 2018, 10:26:20 am
content: Sorry, I didn't notice "... switching10'sof amps ...".As Tim points out, that really needs a gate driver because a large power MOSFET will have much higher total gate charge so if you direct drive it the MCU pin will be overloaded for longer, + the MOSFET will spend more time in its linear region so its dissipation will probably be excessive.It *may* be possible to get away with a discrete push-pull complimentary emitter follower gate driver if you add a resistor of a few hundred ohms from the bases to the emitters, so the transistors provide nearly all of the drive while slewing through the plateau of the gate charge vs Vgs characteristic, but the last 0.7V to get right to either rail comes from the MCU output via the resistor. You'll have to take a close look at the Rds_on vs Vgs characteristic and check how long that last 0.7V takes at turnon to see if there will be significant extra losses due to lack of hard rail-to-rail drive.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: October 29, 2018, 05:55:13 pm
content: If you have to use a driver anyway, is there any point in using a MOSFET with a logic-level gate?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2018, 06:47:16 pm
content: It's probably not worth going out of your way to make a 9-15V supply, if you don't have it.Although it's common enough these days that you don't even have a 5V logic supply, only 3.3V. That's even harder to satisfy (but not impossible: there are some Infineon protected MOSFETs spec'd for 3.3V operation!), and you'll have to go out of your way to make a suitable supply anyway and might as well go for the higher value. (Although a simple charge pump doubler to 6.6V is not unreasonable.)Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2018, 07:24:34 pm
content: Do logic level power MOSFETs tend to have higher gate charge requirements? It seems that way every time I consider using them but I have not done a detailed analysis.There might be a suitable but tricky common emitter totem-pole output stage suitable for driving a logic level power MOSFET from only a logic supply but if you go to that much effort, it is easier and probably better to use an integrated driver like T3sl4co1l suggests.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2018, 07:59:51 pm
content: Parasitic turn-on is another thing that comes to mind. Had an application with a P-ch FET with high switching loss, ended up driving it from a DC restorer referenced to the 3.3V logic supply (12V gate driver), so it gets +4/-8V drive, enough to prevent turn on, and also keeping the transition speeds similar.Tim

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 05:45:19 am
content: If it's any help, attached is a list of low threshold NMOSFETs that I compiled for a similar application. All the MOSFETs will turn on with 3.3V on their gates.The actual threshold voltage for each NMOSFET is shown as are the other major parameters.The quality of the jpg image is not too good due to the EEV limit of 100kB file sizeBy the way, it is generally a good idea to connect a 100 Ohm resistor directly to the gate of a MOSFET to act as a gate stopper.And, as has been said, due to the colossal effective gate capacitance of low gate threshold MOSFETs, it would be advisable to use a gate driver chip.

author: Ian.M
author_rank: Super Contributor
timestamp: November 01, 2018, 05:57:03 am
content: Please repost your list as a text file, csv file, word doc or spreadsheet (basically anything *EXCEPT* image files). Zip it to avoid trouble with the forum's permitted attachment types.

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 06:11:59 am
content: Hi Ian,I do not have the file in text. It is actually a screen shot of a Windows 10 file directory.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 01, 2018, 06:16:04 am
content: cmd into the folder, type DIR and copy the text??Tim

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 06:19:59 am
content: Brilliant

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:42:36 am
content: Quote from: blueskull on November 01, 2018, 06:23:04 amThanks: and I have been struggling to keep files below 100kB

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:44:54 am
content: Quote from: T3sl4co1l on November 01, 2018, 06:16:04 amThank you Tim.File now in .doc format

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:47:05 am
content: Quote from: Ian.M on November 01, 2018, 05:57:03 amThanks for advice Ian

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 08:02:52 am
content: Back on piste!The LT1157 may be of interest. It is a gate driver with an-onboard charge pump to generate a gate drive voltage suitable for standard MOSFETs.As far as I can tell, the charge pump runs all the time, and is not dependent on input signal transitions, so the LT1157 is suitable for "DC" switching.https://www.analog.com/media/en/technical-documentation/data-sheets/lt1157.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: November 01, 2018, 08:48:44 am
content: Interesting chip, but its strictly for low speed switching. Even 1KHz PWM would be too fast for it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 01, 2018, 09:01:22 am
content: doc format? But... that's almost as bad, what's wrong with plain text?Here's plain text, cleaned up a bit; view with tab size 4. I'm not sure if that imports into, say, Excel cleanly.Tim

author: exe
author_rank: Unknown
timestamp: November 01, 2018, 09:13:58 am
content: Quote from: spec on November 01, 2018, 05:45:19 amDo you mean in series with the gate? To me value seems to be too high, I saw much lower values (used to slow down switching to reduce noise). BTW, what is "gate stopper"?

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 09:48:35 am
content: Quote from: exe on November 01, 2018, 09:13:58 amYes, in series with the gate and mounted directly to the gate terminal. In fact, a resistor on the gate serves quite a few functions, one of which is to act as a gate stopper which is akin to the grid stopper used on thermionic valves.MOSFETs have a very high frequency response, but unfortunately they also have amazingly high parasitic capacitances. The capacitance from the drain to the gate Cdg often being the most troublesome. The upshot of all this is that MOSFETs have a tendency to ring and even oscillate. A gate stopper reduces the gain of the MOSFET at high frequencies and thus makes them more stable. Of course, good grounding and short wires and decoupling also help.But the gate resistor has another function too- it is used to shape the gate drive waveform. There is a lot going on when the gate of a MOSFET is being driven by a reasonably fast waveform. For example, the already large Cdg appears at the gate as a virtual capacitor of ACdg (A= MOSFET voltage gain) due to the Miller effect. And if that were not enough, the capacitance values vary throughout the voltage excursion of the input waveform and output waveform.As to the value of the gate resistor, it takes a bit of working out, but as a rule of thumb 50 Oms to 100 Ohms seems to be a good compromise. But you will see much lower values in high speed, high power applications.Gate pulse shaping is not really that important if you are just turning a LED on and off now and then. But, in more critical applications the gate resistor can help protect the MOSFET from damage. Imagine MOSFETs being used in a welder or an inverter.While on the subject, just a word about gate drive voltage and current. In general, the higher the gate drive voltage the lower the drain source resistance (RDS), which means that your circuit is more efficient, but it also means that the MOSFET is dissipating less power and thus is under less stress.On to the gate drive current. It seems strange that a device with an input resistance of megaohms should require any base current, and at DC that is true. But as the frequency increases, more and more the effects of the MOSFET's parasitic capacitances come into play and the more input/output current is required. Many people are amazed to learn that gate drivers can source and sink from 100ma to 7A.I have been rabbiting more than intended, so i will stop now, But just to say that this is a complex subject and I have only skimmed the surface.

author: exe
author_rank: Unknown
timestamp: November 01, 2018, 10:07:12 am
content: BTW, while higher gate voltage means lower Rds(on), I found there is a diminishing return. E.g., a 10V drive was barely better than 7V drive. So, it may not make much sense for charge pumps, etc for a logic-level fet. I suggest do some measurements (but leave some voltage headroom for process variation, temperature effects, etc).Also datasheets often provide guaranteed Rds(on) at specific voltages, the actual resistance is even smaller (but do not rely on this in mass production).

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 10:09:52 am
content: Quote from: Ian.M on November 01, 2018, 08:48:44 amYes, quite true- I just mentioned it as an example. There are no doubt better standard MOSFET drivers.

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 10:12:29 am
content: Quote from: T3sl4co1l on November 01, 2018, 09:01:22 amDon't be too hard on me- I'm just a simple person

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 10:26:39 am
content: Quote from: exe on November 01, 2018, 10:07:12 amTrue, and temperature has a big impact on Rds.The charge pump gate driver was suggested as a way to use standard MOSFETs, which are more common, cheaper, and tend to have lower parasitic capacitances.You mention the law of diminishing returns in regard to drive voltage/Rds. As you imply, the secret is to establish the sweet spot in a design

author: exe
author_rank: Unknown
timestamp: November 01, 2018, 10:32:29 am
content: There are also mosfets with integrated drivers. E.g., this one:https://www.onsemi.com/pub/Collateral/NCP5369-D.PDF(just first one from Google).There are devices with integrated charge pump, over-current, over-voltage (not sure what this exactly means, check datasheet if important) and thermal protection.Other devices are below. Just google "smart fet", "protected fet", etc. At least some of them declare switching speed up to 1MHz.-https://www.infineon.com/dgdl/Infineon-ApplicationNote_v12-AN-v01_00-EN.pdf?fileId=5546d4625b62cd8a015bc8c8e5ae31d1-https://eu.mouser.com/new/diodes-inc/diodes-zxms-mosfets/

author: exe
author_rank: Unknown
timestamp: November 01, 2018, 10:43:21 am
content: Quote from: spec on November 01, 2018, 10:26:39 amYeah, but for a one-off device I'd suggest save time and just get the best part. Sort of overkill, but saves a lot of time... and often board space.I have CSD17313Q2T (that I'm yet to try), but even at 3V drive it gives less than 50mOhm over entire temperature range (hope it's not a "typical performance") and has very good performance in terms of Rds(on) vs gate charge.There are inexpensive parts with threshold of less than 1V in sot-23 package with quite low Rds(on). Like AO3414. I bought it because it's cheap (yet to try it, don't have a lab at the moment). Just threshold voltage can be as low as 0.4V, so be sure driver can pull that low.

author: Ian.M
author_rank: Super Contributor
timestamp: November 01, 2018, 11:14:05 am
content: Quote from: T3sl4co1l on November 01, 2018, 09:01:22 amIt doesn't import cleanly. I've gone back to Spec's list in the .doc, used '_' as the separator then cleaned up out of order columns to get a CSV file that should import OK to any spreadsheet.

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 11:53:14 am
content: Quote from: exe on November 01, 2018, 10:43:21 amHmm, sounds like you have some interesting parts and plans. I would like to see any circuits that you design. I'm in the same boat as you- no lab facilities at the moment as I'm living in one house, temporarily, while refurbishing another house which does have a large garage and potential for a lab, but I need approval from the domestic authority for that.

author: exe
author_rank: Unknown
timestamp: November 01, 2018, 03:09:54 pm
content: Quote from: spec on November 01, 2018, 11:53:14 amI'm building a four-channel linear lab supply heavily inspired by this one:https://www.eevblog.com/forum/beginners/how-does-blackdog_s-psu-work/. Basically, I'm taking that design and adding the following features:1) Digital control2) Current sinking (but not much, more like 100-500mA)3) Dual current range (via a switchable shunt)4) 4 isolated channels5) graphical display. May be touch control.6) remote control (probably add this later)7) very compact, half-rack widthswitchable output capacitance. This can be a unique feature, haven't seen this before.9) probably two channels will be "beefy" (15V 3A), and two will be will low range (both current and voltage), but more precise and with small output capacitor (like 6V, 600mA).So far I built 6 and half prototypes of power part, now messing up with software part. Since I'm still evaluating different modules, there is no finished schematic yet (don't want to show discarded designs). Also I don't think I'll be able to implement all the features. Current goal is to finish schematic, make one channel evaluate it, learn from mistakes, and will see. Probably, I'll have to give up on some ideas.

author: spec
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:01:35 pm
content: Wow, quite a project. As I said, keep us informed about progress.