Is FET internal diode rated to same voltage as  FET - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: January 07, 2023, 06:58:13 am
content: Hi,Is the diode in the IPTG111N20NM3 FET rated to 200V or maybe more?We are using this FET as a diode only.https://www.infineon.com/cms/en/product/power/mosfet/n-channel/iptg111n20nm3fd/

author: Psi
author_rank: Super Contributor
timestamp: January 07, 2023, 07:03:01 am
content: The diode has to be rated for the same voltage as the fet, or it would fail under normal fet operation.You cant exceed 200V, even if you're only using the diode. Something will reach break down and become a zener.However, the diode is not always rated for the same current as the fet. although, looking at that fet it seems it is the same current 108A at 25C.Not this is the current when forward biasing it, so 0.8V or whatever.Note, we call it a diode and show it as a separate diode symbol inside the fet symbol, but it's really the fet itself, It just acts like a diode and is convenient to show it separately.

author: Faringdon
author_rank: Super Contributor
timestamp: January 07, 2023, 08:05:50 am
content: Thanks, do you expect this 200V rated FET can handle leakage spikes up to 300V.?..the peaky thing is only some 100ns...but its every 3us...periodic

author: Psi
author_rank: Super Contributor
timestamp: January 07, 2023, 08:29:16 am
content: You wont get a voltage spike up to 300V because it will be clamped by the diodes avalanche break down (like a zener)Exactly what voltage this will happen is a bit unknown but they guarantee it wont happen until 200V. So it's probably a little bit higher to give it a safety margin.If you want to use the fet's diode in avalanche breakdown like this you have to be careful you don't damage it.Luckily your fet datasheet has a nice graph that shows the avalanche characteristics on page 9It's all about the energy you can dissipate safely, and since the voltage is somewhat capped around 200V its mostly about the current and the duration.So you need to know how much current your 100ns 300v spike will draw when clamped to 200V.Then you can check where it falls on the graph and determine how far inside the limits you are for the temp you expect the fet to run at.But looks like max 60A is worst case limit at 150C. Given that your duration is in ns and the graph is only really concerned with us level pulses

author: Marco
author_rank: Super Contributor
timestamp: January 07, 2023, 10:02:01 am
content: As I readitInfineon doesn't want to give repetitive avalanche ratings for Optimos because they aren't interested in optimizing for it whatsoever and it's pretty bad.I wouldn't count on it, pick something older and planar or purpose designed for repetitive avalanche.

author: Psi
author_rank: Super Contributor
timestamp: January 07, 2023, 11:34:03 am
content: Yep, good point, it's only got a single pulse rating. I missed that before.When it's not fully avalanche rated your in uncharted territory and you have make a call if you're far enough inside the single pulse spec to be safe. Given the energy you are dissipating.Or you use a different part.

author: Seekonk
author_rank: Super Contributor
timestamp: March 07, 2023, 09:31:38 pm
content: A while back I did some testing and the diode begins to conduct about 8-10% over the FET voltage rating.

author: David Hess
author_rank: Super Contributor
timestamp: March 08, 2023, 03:14:50 am
content: The body diode is the collector-base junction of the parasitic bipolar transistor structure and *is* the voltage rating of the MOSFET.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2023, 10:17:46 am
content: When you just need a diode, why not use a diode ?With such a relatively large FET there is quite some capacitance and this may make it have to reach 300 V for a short pulse.The FETs also have a limit on how fast the voltage should rise: the capacitive coupling from drain to gate may turn the FET on, unless the gate is hard pulled down.

author: David Hess
author_rank: Super Contributor
timestamp: March 08, 2023, 09:28:57 pm
content: Quote from: Kleinstein on March 08, 2023, 10:17:46 amIn the past big bipolar power transistors were sometimes used in place of diodes because they had higher current or power ratings and were available in bigger friendly packages. This applies even more so to power MOSFETs which are available with incredible current ratings.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2023, 09:52:23 pm
content: The current ratings of many modern FETs are kind of fake - in case someone manages to keep the case at 25 C. Often it is just a theoretical number calculated from thermal resistance and on resistance when still cold - so too high be design.The current rating for just the diode can often be lower, as the diode drop may be high than the FETs R_on drop, especially for lower voltage parts.It can still sometimes make sense to use a FET as a diode, but not that much when looking at very fast pulses.