MOSFET BVDSS and adding TVS diodes - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: November 25, 2022, 02:25:00 am
content: I have a circuit that is working fine to switch an inductive load and it has all the TVS it needs to clamp that inductive load and protect those mosfets. But currently that circuit needs a mech relay to protect itself from reverse battery and provide a master on/off control.I really want to replace that mech relay with 2 more banks of N-FETS in the ground line. (one bank for the rev pol and one for on/off).My question is about the need, or lack thereof, to have TVS protection for BVDSS in this case.in DC load on/off control when do you make the call that your mosfets are sufficiently large enough that they can handle any typical spikes themselves and don't need an external TVS?I know it's not a question that can be definitively answered except with math. I'm more looking for rule of thumb and what people thoughts are about it. (I really don't have PCB room to add TVS diodes for this and am hoping it's not needed.)More background..This system is powered by a 12V lead acid. It is automotive in a very lose sense but this 12V battery is not the cars main battery and my system only has a low current 'charge path' from the main car battery.The fets (TPH1R204PL1) are 150A 40V with 3 in parallel so can handle 450A or 1500A for a short time.(Single-pulse avalanche energy 125mJ 120A per fet, and there's 3)For the FETs that will perform rev polarity protection they are going to be active 24/7 so I see no need for any TVS protection on them.For the on/off FET bank I'm wondering if I can rely on the FETs to protect themselves from any spikes over 40V.Since..A) They are pretty high current fets with a fair amount of avalanche current.B) They are somewhat decoupled from the main automotive power, so any spikes from there should be minimal or low energy.C) All spikes from my own inductive load switching have already been taken care of.D) They will never disconnect an active high current load (They replace a relay and the system is already designed to never turn the relay on/off when high current is flowing)Basically my question is, at what point do you have so much mosfet avalanche current rating that it's basically acting like a big enough TVS itself.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2022, 04:29:11 am
content: Right, switching can be slow so it's not a big deal. That leaves automotive transients including EFT, those various contact-switching surges, and load dump. How many of them do you want to support? The faster and higher impedance ones will easily be met with filtering and a small TVS (maybe even relying on MOSFET avalanche alone, though I wouldn't recommend it), and that basically leaves load dump.Do check that it responds quickly enough to handle reversal; a mechanical connection is quite fast (nanoseconds). Likely both NMOS will be off initially (assuming time given between turning off in the forward direction, not a sudden reversal from normal operation*) so it's just down to drain capacitance, which should charge fairly quickly, and maybe provide some damping or clamping to deal with possible ringing.*Wait, is there a transient that does that, full reversal from normal operation? I forget... That would be the limiting case if so -- whatever rise time is specified.Tim

author: thm_w
author_rank: Super Contributor
timestamp: November 25, 2022, 11:12:28 pm
content: If you don't see any reason a high voltage spike would be generated there, then I don't see why you'd need a TVS.Be sure that Vgs is not exceeded in any cases though (+/-20V for this part). Depending on your circuit a TVS may be needed there. The headroom for Vgs is often very low.Also try to consider any situations where the circuit doesn't act as you intended. What if the main supply is disconnected, and the circuitry being powered still has stored voltage, would that cause any issues? etc.

author: Psi
author_rank: Super Contributor
timestamp: November 26, 2022, 01:21:46 am
content: Quote from: thm_w on November 25, 2022, 11:12:28 pmyep, I have a tiny TVS on Vgs already. It's Vds that is hard to protect without a huge TVS.

author: Psi
author_rank: Super Contributor
timestamp: November 26, 2022, 01:25:07 am
content: Quote from: T3sl4co1l on November 25, 2022, 04:29:11 amNa, reverse battery protection just needs to handle someone connecting it backwards from an off state.There's no reversal while active.