Fast Mosfet Surge - Page 1
Forum section: electronics / projects

author: sheitbrink
author_rank: Newbie
timestamp: June 30, 2017, 04:47:18 pm
content: Hi,I guess I have the question: is this a good idea. I'm working on a circuit that might, I think, get exposed to really fast voltage transition. If we use a good ceramic capacitors, think we could get some really large, but very short transients.I guess the question is: do you guys/gals think that such a short transient could damage the MOSFETs? Or is their something I'm not modeling that will limit the in-rush current into these ceramic caps?Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2017, 07:56:59 pm
content: What are you plugging it into? Is this a hot-plugging situation? Or is this for transients, from various sources (usually surges due to interruption of other loads on the same supply, and fast transients coupled in from nearby wiring in a wiring harness, conduit or raceway environment)?What is the load? Is it just a capacitor and a resistor representing a modest DC load? Or might it ever be a low impedance (a very large capacitor, a motor, or a short circuit, accidental or otherwise)?Based on these answers, you can design:1. The source. These transients all have different impedances and magnitudes. You certainly cannot capture the impedance by plopping an ideal SPICE source on the schematic.* Usually, an RLC circuit, with an initial condition, and a switch to dump it (use a SPICE ideal switch, triggered by a VPULSE), does a very good job of modeling real generators. Of course... you need the relevant standards to construct the models from.*LTSpice hides parasitic components inside the symbol. If you've set these, please show them on the schematic, otherwise I'm going to assume what you've modeled is what you've drawn...2. The input impedance.If you need RFI immunity, add filters: ferrite beads, inductors and capacitors.If you need fast transient and ESD immunity, filtering helps some, but the pulses are usually too large (kV!) to filter effectively (ferrite beads saturate, common mode chokes spark over). Shielding and clamping (TVS diodes), with filtering to clean up what's left (which is now a much more manageable, say, 20-50V hump), is the way to go.If you need surge immunity (anything from automotive transients -- which really aren't all that bad -- to IEC 61000-4-5 mains surge, and up to lightning and nuclear EMP!), filtering is again impractical, not just from the amplitude (~100s V for automotive, kV for mains, etc.), but also because the pulse is pretty slow: risetime in the microseconds, pulse width in the microseconds to low milliseconds. This is high energy (joules at a time), best clamped with a MOV. MOVs aren't very good clamps (the peak voltage drop may be 2-3 times the rated voltage), so you still have cleanup work to do afterwards (some series resistance, and more clamps?) before it's safe for your circuit. In some cases, it may be reasonable to let your circuit "ride through" the transient. Which brings me to...3. The load. Does the load need those capacitors? Can you afford to add some ESR or ESL? Can they be charged with a current-limiting circuit instead?Automotive load dump is a very slow transient (~ms rise, 100s ms width), but it contains a huge amount of energy (100s J), enough to destroy MOVs even. It's not too much extra voltage, though: it's quite feasible to design a circuit to ride through it. Example: put an LDO in series with the input, set for an output voltage much higher than nominal. Say, for a 24V circuit, you "regulate" it at 30 or 36V. Normally, the regulator is in drop-out: it's saturated, not regulating at all. Its voltage drop is minimal, so it doesn't cost you much efficiency. But if the input swells well beyond 30V, the output voltage remains limited right at 30V (or whatever it's set to). Rather than bearing the brunt of the (perhaps 100V and 100A) pulse, the LDO only draws whatever current your load draws: it still dissipates a lot of power when the voltage is high (70V times load current), but much less than a shunt (MOV or TVS) device.As a bonus, an LDO probably has current limiting and protection features, so your inrush and overtemp problems are solved at the same time!A comment regarding speed: note that fast transients will pop right through a power transistor, even when turned off. They're easily coupled through by capacitance alone. Surge transients may pass through a lazy polarity protection circuit, but can be interrupted by faster or active circuitry just fine. (The usual reverse polarity protection circuit has a nice and slow resistor charging the gate, which would let through surges. However, an active driver could control the transistor during the pulse.) Load dump is slow enough to look like "DC" to most circuits (even slow resistor-drive circuits).Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 01, 2017, 11:13:12 pm
content: irf7324 is speced to 71A pulsed/surge current. your graph showing near 200A, it will certainly get damaged in quick time...

author: sheitbrink
author_rank: Newbie
timestamp: July 03, 2017, 04:37:33 pm
content: HiThanks for the help you guys. You've really given be something to think about.For your information, this circuit will be used in a kind of an automotive situation. Although it will probably be plugged into a few circuits made by mechanical engineers so, you know, being pretty over build doesn't hurt.The output of this sub-circuit would be a switching buck converter which would power a 1 to 2 watt embedded system. A 10uF or cap very close to SMPS input is a really good idea just for stability reasons. I would need some filtering anyhow just to keep the FCC from knocking at our door. So I suppose beefing up this filtering a bit to keep the transient well bellow 71A would be a good idea.As far as the spice circuit goes. I have added a few tens of mOhms to all the caps just to get the thing in the neighborhood of reality. Although I need to look at the specific caps, and add all the real parasitics in.

author: dmills
author_rank: Super Contributor
timestamp: July 03, 2017, 05:30:41 pm
content: Do NOT forget cable parasitics, parasitic L in particular can be a nasty surprise when suddenly connected between a stiff voltage source and a high value ceramic cap, you can easily see it ring up to two or three times the steady state value, and it is quite capable of taking out an integrated buck converter for example (Who, Me? Never!).If you are not planning on drawing much power then series resistance is actually not a horrible thing, even an ohm or so can help a lot.Regards, Dan.