Understanding P Mosfet drive - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: March 25, 2019, 11:55:30 am
content: I need to switch a grounded load using a high side P Channel mosfet similar the image attached.The supply to mosfet source would be around 50Vdc to a maximum of 100VdcThe mosfet is the traditional IRF9540.My first thought that mosfet driver like the the TC4429 would be suitable for this however it soon dawned on me that1. the TC4429 max supply voltage is 18VDC.2. The Mosfet Vgs max is +/-20V.So even if I ground the gate to turn it on the Vgs rating is likely to be exceeded.A zener might probably mitigate that but it would affect the turn on time.The turn on times I require are under 10uS.What would be the correct way to switch on the P channel mosfet?

author: SteveyG
author_rank: Unknown
timestamp: March 25, 2019, 12:02:10 pm
content: What's your Vdd voltage?

author: andybarrett1
author_rank: Regular Contributor
timestamp: March 25, 2019, 12:08:57 pm
content: I asked similar question few days back.... Lots of useful pointers here :-https://www.eevblog.com/forum/beginners/mosfet-175797/msg2283114/#msg2283114Thank you allAndy

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: March 25, 2019, 12:11:21 pm
content: Quote from: SteveyG on March 25, 2019, 12:02:10 pm50 to 100V as given.

author: SteveyG
author_rank: Unknown
timestamp: March 25, 2019, 12:19:33 pm
content: Is your micro going to sit referenced to the 50 to 100V or were you planning a common ground?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: March 25, 2019, 12:22:18 pm
content: Quote from: SteveyG on March 25, 2019, 12:19:33 pmIts going to be a common ground. Its would be wired up as micro to driver to mosfet.The image attached above doesn't show the driver.

author: SteveyG
author_rank: Unknown
timestamp: March 25, 2019, 12:25:59 pm
content: You'll need some means of voltage translation to do that high side switching. Do you actually need the common ground? The easiest method would be to have the micro referenced to the high side so you don't see massive Vgs swings.With the current schematic, you'll destroy your micro and won't be able to turn off the MOSFET

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: March 25, 2019, 01:09:52 pm
content: Quote from: SteveyG on March 25, 2019, 12:25:59 pmThere might be additional sense circuits like the load current sense so a common ground would be useful.

author: Ian.M
author_rank: Super Contributor
timestamp: March 25, 2019, 01:20:14 pm
content: Integrated 100V high side PMOS drivers are rare like hens teeth.Attached is a sim of a discrete solution. The ratio of R1 to R2 sets the gate drive voltage. C1 is a speedup capacitor - pick it to get fastest drive without excessive overshoot with no MOSFET connected.N.B. you'll probably want to use different transistors for the driver and level shifter. Zetex have some nice super beta HV ones.

author: Psi
author_rank: Super Contributor
timestamp: March 25, 2019, 01:46:01 pm
content: What kind of current do you need?If it's low (under ~100mA) then optocouplers exists in your 10us specs at 100V.Using something with isolated control gets around the interface problems of a fet and its gate voltage limits

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2019, 02:29:21 pm
content: https://www.eevblog.com/forum/beginners/mosfet-gate-breakdown-survivable/msg1826480/#msg1826480

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: March 25, 2019, 04:36:18 pm
content: Quote from: Psi on March 25, 2019, 01:46:01 pmLoad current upto 5A.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: March 25, 2019, 04:38:23 pm
content: @Zero999 and @Ian.MThanks for the awesome circuits and sim's.Are discretes the only way to go here? I mean aren't there any single chip driver for the P-channel fets?

author: Peabody
author_rank: Super Contributor
timestamp: March 25, 2019, 05:18:58 pm
content: Zero999, I'm tryng to understand your circuit. If V2 is 5V, what are the collector and emitter voltages on Q3?

author: magic
author_rank: Super Contributor
timestamp: March 25, 2019, 06:50:59 pm
content: Zero999's circuit is roughly what I wanted to suggest.With the Q4/Q2 totem pole, I think it might even be feasible to increase R2 and R3 10x to reduce power dissipation on Q3 which is currently 1W at 100V.Quote from: Peabody on March 25, 2019, 05:18:58 pmThat's a standard BJT current source. B-E junction is basically a diode, so driving the base at 5V pulls the emitter up to about 4.3V. Then there is ~11mA current through R2. Due to the current gain of the BJT, only a small bit actually comes from the base and majority from the collector. Collector voltage dynamically adjusts to maintain that 11mA, here it will be 11V below the top rail.

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2019, 10:25:59 pm
content: Quote from: magic on March 25, 2019, 06:50:59 pmYes, the resistor values can be increased to save power, but bear in mind it will start to slow it down, especially if you go too high. A low value capacitor in parallel with R2, say 47pF, can also help with the turn-on time. However it won't improve the off time. To help with that, more current gain is required. A Sziklai pair can be used for the upper transistor.

author: langwadt
author_rank: Super Contributor
timestamp: March 25, 2019, 11:02:44 pm
content: Quote from: ZeroResistance on March 25, 2019, 04:38:23 pmhttps://www.mouser.dk/datasheet/2/609/LTC7001-1504282.pdf

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2019, 09:46:25 am
content: Quote from: langwadt on March 25, 2019, 11:02:44 pmThat's an N-MOSFET driver, not P-channel, but is a good idea and can do 100% duty cycle.If the duty cycle is always under 100%, then a much cheaper MOSFET drive IC can be used. They're normally sold as half bridge drivers, but if only the high side is needed, the low side can be unused.https://www.mouser.com/datasheet/2/268/mic4103-4-778767.pdfhttps://www.onsemi.com/pub/Collateral/NCV51511-D.PDFhttp://wiki.icub.org/misc/datasheets/LM5100A.pdfhttps://www.maximintegrated.com/en/products/power/mosfet-drivers-controllers/MAX5063A.html