What's the deal with MOSFET's extraordinarily high vds? - Page 1
Forum section: electronics / beginners

author: man1
author_rank: Contributor
timestamp: April 24, 2025, 09:29:17 am
content: I add 12v to my mosfet with a 50% duty cycle drive voltage. A constant current of 1a is added to the power circuit with a 2Ω load and measured my vds=1.2v does this make sense?

author: thm_w
author_rank: Super Contributor
timestamp: April 24, 2025, 11:48:19 pm
content: Vds of 1.2V would imply 0.6 ohm which seems too high. Datasheet gives 0.1 ohm at 20Vgs.Do you have oscilloscope captures of the gate voltage and Vds?

author: jwet
author_rank: Frequent Contributor
timestamp: April 25, 2025, 12:04:29 am
content: Your Vds sounds a bit high but you really haven't given enough details and I don't know what you're talking about adding a load of 2 ohm to a 50% drive. What is Vds and Vgs. If these parts are fully enhanced with 20v Vgs, they should have a Ron of about .1 ohm at 20 A. At 20 amps, Id, Vds would be about 2v- ie they would look like a .1 ohm resistor. The unique thing about them is that they can stand off 1200V.They are Silicon Carbide FET's. They excel at high voltage, high speed operation and high temps. They operate a lot like regular MOSFETs but really excel at high voltages and high temps like EV main drive electronics. If you're not switching a kilo volt, there are better parts for lower voltage applications.What are you driving the gate with? You say 50%, but is what is the frequency? What is Vgs and the drive level. These parts have a 4v max Vt and a 17 nC gate charge- that's fairly hefty. Most of the specs are at a 20v gate drive. (max is 25) To get to a saturated condition Vds > Vgs-Vt and the current follows a square law of Vgs-Vt, just like any MOSFET.I'll try to help but what you're doing isn't clear.

author: man1
author_rank: Contributor
timestamp: April 25, 2025, 01:32:06 am
content: Quote from: thm_w on April 24, 2025, 11:48:19 pmThis is what the oscilloscope shows

author: man1
author_rank: Contributor
timestamp: April 25, 2025, 01:40:37 am
content: Quote from: jwet on April 25, 2025, 12:04:29 amI'll re-describe my problem, I'm using an IR2127 driver chip to provide a mosfet with a drive voltage of 12v,50% duty cycle and a frequency of 5khz. The high level on my oscilloscope reaches about 12v (vgs), but shows a Vds value of about 1.2v. The main power circuit I supplied has a constant current of 1A and a load of 1 ohm, will my current added to the DS sides of the mosfet not reach 1A because of the duty cycle?

author: Konkedout
author_rank: Frequent Contributor
timestamp: April 25, 2025, 03:12:19 am
content: As JWET said, don't use a 1200V SiC FET to switch 5V or 12V. Get a 30V or 40V rated silicon MOSFET with Rds ON rated maybe 10 - 50 mOhms. This will perform better and be cheaper and smaller.

author: xavier60
author_rank: Super Contributor
timestamp: April 25, 2025, 09:38:19 am
content: Show everything in one diagram.

author: jwet
author_rank: Frequent Contributor
timestamp: April 25, 2025, 02:08:51 pm
content: With your details, the .6 ohms is likely correct. You're not really in saturation. Can you adjust your driver vcc to 20v. This will get you to a .1 ohm Ron.

author: thm_w
author_rank: Super Contributor
timestamp: April 25, 2025, 09:21:08 pm
content: Quote from: man1 on April 25, 2025, 01:32:06 amThat doesn't look like 0 to 12V, it looks like you are driving the gate with 2 to 10V.As others asked, what is VIN+ maximum voltage? If its not 500V+ then yeah, using another MOSFET type is better, with a lower voltage rating.