Solved: Driving a P-FET @ 100kHz in a +60Vin, -30Vout inverter? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: March 07, 2025, 12:28:02 am
content: I have a commercial inverter design that I'm looking to adapt to a higher input voltage.It uses one of the common FANxxxx drivers that has a N and a P FET output.Those drivers are limited to 27V and are quite weak - even paralleling several of them results in appreciable switching losses with a "beefy" PFET.As far as I can tell, I would struggle to find a suitable gate driver for my application that has more than one supplier.I suspect I may have to design a discrete gate driver.I would like input on driving PFET in this application. Perhaps I am mistaken about component availability.The parameters of my application are:Vi=60Vmax absolute, min ~15VVo=-30Vmax absolutePo= ~25WFsw=100kHz +/- 5%Qg >300nC (TBD)edit: added minimum input voltage - it's a wide range converter

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 07, 2025, 08:22:13 am
content: Quote from: incf on March 07, 2025, 12:28:02 amThey are rare and getting rarer.An alternative could be a normal half bridge driver, and use a second SOT25 fast inverting, low cost, gate driver like a 27517 to flip the high side for PMOS.The dead band handling is then correct.

author: incf
author_rank: Frequent Contributor
timestamp: March 07, 2025, 11:51:36 am
content: Quote from: PCB.Wiz on March 07, 2025, 08:22:13 amWhy are gate drivers getting rarer?Quote from: PCB.Wiz on March 07, 2025, 08:22:13 amThat gate driver appears to tolerate ~18 volts. Reading between the lines, are you suggesting that I build a floating power rail that sits roughly 15 volts below the 60V power rail?For some reason up to this point I've found the idea of a floating rail intimidating, even though it's probably only a few milliamps (if enough decoupling capacitance is used). Now that I think about it, a "negative output" adjustable linear regulator could probably do it with ease and without much risk of surprises.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 07, 2025, 12:29:31 pm
content: Quote from: incf on March 07, 2025, 11:51:36 amProbably because people weren't buying enough of them, and are moving towards using N FETs only for half bridges. If you're going to use a dedicated gate driver chip, it's the sensible choice in the majority of cases.In the case of a buck-boost converter (not a half bridge), a dedicated P FET driver is still useful, but it's becoming quite rare to actually see buck-boost converters (maybe because of the lack of suitable gate drivers and PWM controllers).Is there any reason you can't just use a flyback or Cuk converter instead? You'd probably get similar performance, and not have to worry about high side gate driver at all.

author: incf
author_rank: Frequent Contributor
timestamp: March 07, 2025, 01:06:21 pm
content: Quote from: mtwieg on March 07, 2025, 12:29:31 pmYou are right. We avoided flyback because we couldn't find any off the shelf magnetics from multiple suppliers that fit the bill.Dang, I wish I'd thought to switch over to a Cuk converter - I only considered the "big 3" topologies. It looks perfect

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 07, 2025, 07:22:42 pm
content: Quote from: incf on March 07, 2025, 11:51:36 amI was meaning the PMOS+NMOS drivers, as new designs tend to not use them.There are shiploads of NMOS drivers, half bridge with dead band controls.Problem is the top drive is upside down, so it just needs a small inverter.Quote from: PCB.Wiz on March 07, 2025, 08:22:13 amYes, if you want to get "beefy" drive, you will need a local PMOS rail, but because it is below the main supply, it can be quite simple.The drivers have UVLO, so a Zener+pnp could even work. You do not need the bootstrap cap then.I have seen old circuits that couple a CAP to a Zener on an upper PFET as a simple level shifter, but that has larger current loops and main power ramp wants to turn on the PMOS.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 07, 2025, 08:06:05 pm
content: Quote from: incf on March 07, 2025, 01:06:21 pmFor your case where you don't need isolation and your Vin and Vout of similar magnitude (like within a factor of four), you shouldn't be looking at general flyback transformers which are designed with isolation and large turns ratios in mind. I would instead look for a "coupled inductor" or "inductor array," which has two or more identical windings. Something like this should work for you:https://www.digikey.com/en/products/detail/eaton-electronics-division/DRQ125-220-R/667240Typically these use bifilar windings so the coupling coefficient is very high.QuoteCuk is quite esoteric and it's pretty rare to see in the wild. Nice if you want low conducted EMI, but can be tricky to stabilize.

author: incf
author_rank: Frequent Contributor
timestamp: March 07, 2025, 09:05:03 pm
content: Quote from: PCB.Wiz on March 07, 2025, 07:22:42 pmI interpreted as PMOS drivers in general. (They certainly seemed rare when I looked for "jellybean"/commodity drivers)Quote from: mtwieg on March 07, 2025, 08:06:05 pmI didn't even consider those. (Also, I'm pretty sure that specific inductor is way undersized for 25W and was just meant as an example)That would make using a low side switch extremely easy.I also see that those devices more mostly available in lower inductances - I guess, I'm also somewhat weary of higher frequencies. (But, I probably shouldn't be)Quote from: mtwieg on March 07, 2025, 08:06:05 pmIt's a current mode / constant current application (charging banks of batteries) - I think it's easier to be stable in that application than compared to a fixed voltage rail.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 07, 2025, 10:55:20 pm
content: Quote from: incf on March 07, 2025, 09:05:03 pmOh yeah, for your specs you'd want something much bigger, maybe this:https://www.coilcraft.com/en-us/products/power/coupled-inductors/1-1-shielded-coupled/msd/msd1514/?skuId=13153Not sure if coilcraft offers core loss estimates for these parts though.Quote100-200kHz for a small flyback is no big deal, though you'll need to be careful selecting the catch diode, and maybe use an RC snubber.QuoteWell for charging batteries you probably don't have to care about high control bandwidth, so that should make a Cuk easier.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 07, 2025, 11:22:55 pm
content: Quote from: incf on March 07, 2025, 09:05:03 pmSingle gate drivers with symmetric drive do not really care if they are connected to NMOS or PMOS.They are just fat buffers.It is when you hit dual drivers like half bridge with cross coupling like dead-band features, that the upper side drive is upside down for a PMOS part.

author: Wolfram
author_rank: Frequent Contributor
timestamp: March 09, 2025, 12:51:01 pm
content: What PMOS part did you have in mind? 300 nC Qg for a part used in a 25 W converter sounds very high.

author: incf
author_rank: Frequent Contributor
timestamp: March 09, 2025, 01:35:26 pm
content: Just a rough estimate, a low Rds-on 80V PFET that we use has a Qg-max of 280nC. I figured a 100+ V FET might be similar.