Need help understanding/calculating single pulse power for MOSFET - Page 1
Forum section: electronics / projects

author: jbrownshow
author_rank: Newbie
timestamp: June 28, 2020, 04:13:23 am
content: I'm working on a project where I need to quickly charge up a 1300uF capacitor to 100V from a battery. I'm using a low-side MOSFET that when switched ON, charges the battery through a 5.1 ohm resistor. I'm using LTSpice for simulations and can easily get the energy through the MOSFET. The problem is that I can't convert the waveform into a single square pulse to compare with the datasheets in the SOA and Thermal Transient Impedance plots. Can anyone help with this or have a better way of sizing a MOSFET?

author: ocset
author_rank: Super Contributor
timestamp: June 28, 2020, 11:46:35 am
content: which mosfet are you using?Loads of mosfets should be able to manage that.Theres very low V across the fet when its on, so the SOA chart wont be so useful as its more about operating in the linear region.A good low rdson fet would not have any problems. Max current is just some 19A.......even with a 10mR fet thats a instantaneous max power pulse of just 19^2 * 0.01 = 3.6watts

author: jbrownshow
author_rank: Newbie
timestamp: June 28, 2020, 01:18:53 pm
content: Thanks for the reply. I’ve selected a SQJA20EP for the time being but I would like to reduce its size. That’s why I’m trying to understand single pulse power limits better. I know for resistors you can convert an exponential decaying pulse like this to a square wave. Where, for a short pulse, P= Vpk^2*(t @ 37%Vpk)/(2R). But with the MOSFET it seems there would be a different formula because of the linear region. It would be great if I could covert the energy to a square power pulse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2020, 03:00:39 pm
content: That's 6.5J, so a lossy charge must dissipate 6.5J for a total of 13J consumed.You'll need a pretty big transistor to do that all at once in a hurry; or a slower (current limited) charge, giving time for the heat to spread out; or a saturating transistor with a ballast resistor that's similarly rated for the energy and power.The SQJA20EP is only rated 16mJ avalanche, so good luck doing it pulsed. That's fine; you're already looking at the resistor method. Then, how much does the transistor dissipate?If the Rds(on) is typ. 50mohm, then it will absorb about 1/100th the energy of a 5.1 ohm resistor, or 65mJ. Still above par.The time constant is 1.3mF * 5.1R = 6.6ms, so the most energy will be dropped in the first few ms, and essentially all of it after about 20ms.1ms SOA is almost 600W, or 600mJ total (single pulse, from 25C).But our pulse is longer, so the total allowed energy can only be higher still (10ms is like 180W, or 1.8J), and it's fine, even if it were a square pulse.This would seem to be fine.T_J initial is 25C in the SOA, and T_J final is 175C, the maximum rating. With the pulse delivering somewhere around 1/10 to 1/30 the allowable energy at these SOA curves, presumably the temperature rise is 1/10 to 1/30 of 150C, or 5-15C. So T_J max for your case should be less than 160C.I would recommend a comparator and latch, to watch for:- Overcurrent, in case the resistor fails shorted or something else nasty happens to the circuit- Charge (current * time), or capacitor voltage, in case the capacitor isn't charging (load short?)- Maybe temperature? It seems you have quite some margin here, so a simple limit at 150 or even 125C should be quite safe.It seems you could also perhaps use a smaller (cheaper?) transistor.Obviously, you need to run the same check for the resistor; it's dissipating 2kW peak and 6.4J total, so a modest power resistor will be needed. Wirewound resistors tend to be good at this, with vitreous enameled types preferred over molded types (including metal-case types which are sealed with plastic).Tim

author: jbrownshow
author_rank: Newbie
timestamp: June 28, 2020, 05:22:17 pm
content: Thank you Tim for your analysis. That definitely helps me understand how to relate my circuit to the datasheet. May I ask your opinion on the Vishay CRCW HP resistors for this application? I was thinking of using a CRCW08055R10FKEAHP for this application as opposed to a wire wound. What do you think?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2020, 06:36:17 pm
content: 0805, you're kidding right? If you're lucky it turns into a cute ball of slag.The energy goes into heating the component material. At the very least, you need a big enough component so that the energy, divided by heat capacity, divided by volume or mass, gives a reasonable temp rise.The heat also needs to get into the mass; film resistors don't do this very well, particularly at short time scales (1-10ms is transitional, not great, not terrible). A bulk, composition or metal (wirewound) type, does this well.Heat capacity is basically the same across materials.It's just a matter of having enough material, heated evenly enough.Page 5:https://www.vishay.com/docs/20043/crcwhpe3.pdfOnly the 2512 approaches 2kW, and that at 10us. At 1ms, you need 5-10 of them in parallel.There are a few SMT wirewound types that may still be compact, but something THT is the more likely option.If you absolutely must have a compact solution at any cost, consider a current limited switching regulator.Tim

author: jbrownshow
author_rank: Newbie
timestamp: June 28, 2020, 07:12:32 pm
content: Ok so what if I went with a WSC01\24R990FEA? It has a MIL rating of 5x power for 5 seconds for short overloads. At 0.5W x 5 x 5sec = 12.5 Joules rating. I would be about half of that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2020, 09:38:54 pm
content: Hmm, let's see:http://www.vishay.com/docs/30102/wscwsn.pdfNot bad for a 2012 (0805 in metric). You'll want to provide heatsinking copper on the pads to even approach that rating.No graph though. Let's follow the link:https://www.vishay.com/resistors/SMD-wirewound-pulse-capability-calculator/I get 0.128J at 100C max ambient, and a bit over 10ms.They also give this further information:https://www.vishay.com/docs/49076/_wirewound_resistors_pulse_handling_capabilities_vmn_pl0396_1604.pdfwhich supports what I've said (mainly in that it's material heat capacity that matters, and what the capacitor charging energy is). It does not give a conversion for exponential pulses of transitional duration, unfortunately.So, this was another 0805 part. Note it doesn't contain much more material than the previous choice...You can also choose a higher resistance, for less peak power and longer duration. You'll need quite a large value to use a resistor this small, giving a charging time of a few seconds minimum.Tim