floating -12V rail on top of 30V rail for gate driver? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: June 19, 2025, 06:39:09 pm
content: Suppose I have a 30V rail that I want to switch some very large PFETs at a few 10s of kHz.What is the easiest way to get a floating -12V rail with respect to a 30V rail that is capable of supplying >10mA?I suspect maybe a TL431 might be able to be coaxed into negative supply rail duty, maybe with an output transistor. One snag is that it is subjected to short transients that can reach 60V.

author: shapirus
author_rank: Super Contributor
timestamp: June 19, 2025, 07:20:04 pm
content: Quote from: incf on June 19, 2025, 06:39:09 pmMaybe not even a TL431. A 12V zener should do, unless you need high precision (and you probably don't, as there's a wide range for the acceptable gate voltage).A naive attempt that seems to work in a simulator:edit: power-on state needs more consideration. As shown, OUT will be at ground potential on power-on before Q1 turns on. That'll fry the PFET if its gate is connected directly to OUT, so it needs some delay/gate protection etc.

author: langwadt
author_rank: Super Contributor
timestamp: June 19, 2025, 07:21:48 pm
content: zener diodewhy pfets?

author: incf
author_rank: Frequent Contributor
timestamp: June 19, 2025, 07:33:10 pm
content: Quote from: langwadt on June 19, 2025, 07:21:48 pmI guess I'm open to a charge pump and a N-FET.Is there a good way to do that with commodity parts that also tolerates static switching?

author: ajb
author_rank: Super Contributor
timestamp: June 19, 2025, 07:40:16 pm
content: How short are these transients, and do you need the device to continue operating through them? Basically any negative regulator topology will work here, except for that 60V transient requirement. Even the classic LM7912 will handle up to -35V. The most transient-immune solution would be an isolated DC-DC -- if you go that route you could put it on top of the supply and use NFETS instead, but your gate drive level shifting may become the limiting factor, unless you isolate that as well. That's very easy, although not the cheapest/lowest part count option.Quote from: shapirus on June 19, 2025, 07:20:04 pmLooks a bit upside-down. The 30V-12V output is being supplied via R2, at 10k that's only 3mA. The pass transistor needs to be on the bottom so that it can sink current into the output.

author: incf
author_rank: Frequent Contributor
timestamp: June 19, 2025, 07:56:58 pm
content: 1ms long transient, once per minuteit should continue operating without damage or interruption

author: langwadt
author_rank: Super Contributor
timestamp: June 19, 2025, 08:03:59 pm
content: Quote from: shapirus on June 19, 2025, 07:20:04 pmchange Q1 to a PNP and swap Q1/R2. R3 shouldn't be needed

author: Benta
author_rank: Super Contributor
timestamp: June 19, 2025, 08:07:53 pm
content: Just use a 79L12. Super-simple.

author: shapirus
author_rank: Super Contributor
timestamp: June 19, 2025, 08:12:17 pm
content: Quote from: ajb on June 19, 2025, 07:40:16 pmYes. In fact it needs a complementary push-pull stage to allow to both charge and discharge the PFET gate quickly. And twice that: first stage to buffer the zener, another to amplify that further and provide the necessary gate drive current.Something along the lines of:C1 is to mimic the gate capacitance of PFETs that we want to drive. The value is chosen ridiculously high to see the effects better.Simulates well, including power-on (shown at 2 ms):Power-on close-up, showing drive current via C1:Controlling the output to sit at 30V (PFETs off) or 30V - 12V (PFETs on) can be done by pulling the GATECTL node to the positive supply rail using, for example, a PNP transistor and pulling its base to ground via a resistor.This circuit is ugly. I don't like it. I'm sure there's a better way to do it.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 20, 2025, 12:32:35 am
content: Quote from: incf on June 19, 2025, 06:39:09 pmA pnp transistor and a zener is the simplest. That can sink the current peaks but have low average load.Quote from: incf on June 19, 2025, 07:33:10 pmPFETs are simpler, but above a certain current N FETS offer lower RDS.Since you likely already have a gate driver IC, just use another one (inverting version) as a high power schmitt inverter oscillator for the charge pump drive.eg Tokmas-UCC27517ADB is TSOT23-5, well spec'd

author: temperance
author_rank: Super Contributor
timestamp: June 20, 2025, 01:07:06 am
content: https://www.eevblog.com/forum/beginners/understanding-p-mosfet-drive/See reply 15 by Zero999Q3 is a constant current sink. The current trough Q3 is then translated into a voltage across R3. Q4 is a buffer to drive the high Gate Source capacitance.It's a classic high side gate drive. It performs excellent up to even a few 100 kHz. You can find some variants of this circuit with Q2 omitted on the web and on this forum. Q2 is only required for certain MOSFET types.Or something like this if you can use an N-ch MOSFET:https://www.infineon.com/dgdl/Infineon-1EDN6550B-DataSheet-v02_04-EN.pdf?fileId=8ac78c8c7dbca96b017dbd1ab4a7060a

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2025, 12:59:46 pm
content: Quote from: incf on June 19, 2025, 07:56:58 pmBootstrapping should work with an N-channel MOSFET. For a high duty cycle, use a high value, low leakage capacitor.Refer to the circuit I posted in the thread linked below:https://www.eevblog.com/forum/beginners/buck-converter-using-n-channel-mosfet-possible/msg5856799/#msg5856799

author: IconicPCB
author_rank: Super Contributor
timestamp: June 20, 2025, 01:44:52 pm
content: use a photo voltaic opto coupler.

author: temperance
author_rank: Super Contributor
timestamp: June 20, 2025, 02:47:30 pm
content: QuoteHow? I think those are too slow.

author: IconicPCB
author_rank: Super Contributor
timestamp: June 20, 2025, 09:00:30 pm
content: They might be quick enough for a few tens of KHz

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2025, 09:31:43 pm
content: Quote from: IconicPCB on June 20, 2025, 09:00:30 pmCan you please suggest a suitable part? The ones I’ve seen only output a tiny current which will result in slow switching, even for small, low current/voltage MOSFETs.

author: peter-h
author_rank: Super Contributor
timestamp: June 21, 2025, 07:27:55 am
content: If you have a 12V rail somewhere, then a capacitor+diode pump will give you -12V relative to any other rail.