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I want to know what Pulsed Drain Current has to do with discharging energy storage devices. What does a mosfet need to have in order to be capable to discharge a cap? How do I determine a rate of discharge which the fet can handle repeatedly discharging a cap? What particular items are key in mosfet datasheet to this subject matter?

Please note - this is a mosfet specific question. This is about mosfet calculations and selection, not about the circuits involved in discharging into the mosfet, I am only interested in a capacitor discharge situation because inductors are often too common.

user33915
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    Present a datasheet for one part that looks OK and explain why you think it does, and how you would heatsink it, for review. (Also, 180/0.37 > 200. Any comment on that? And what's the repetition rate?) –  May 14 '18 at 09:09
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    Draw what you think will be the possible circuit and show all voltage sources. Do not omit anything that might be significant to calculating the peak power dissipation in the "mosfet". – Andy aka May 14 '18 at 09:11
  • This is only cursory question, I threw values and parameters from a project I have. But I am more interested in how to actually do proper calculation and part selection, therefore I dont see any need to provide circuitry or any fets which I found. This is a mosfet specific question, not a circuitry question. – user33915 May 14 '18 at 09:45
  • Do you need to interrupt the discharge (at 200 A you'll get a large inductive voltage spike from doing that) by turning the MOSFET off while current still flows? If yes, you'll also need a freewheeling diode to provide another path for the current sourced by the stray inductance of the load, to clamp the transient. Is the load highly inductive (e.g. coilgun winding)? If yes, you'll probably want a H-bridge so that you can reverse the voltage and recover the remaining energy from the inductance. What are you trying to do? – jms May 14 '18 at 09:46
  • @jms Im trying to figure out how to do calculations (which hopefully wont be overly complex) and proper part fet selection. This is not a circuit question it is a mosfet specific question. – user33915 May 14 '18 at 09:48
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    I just suspect that there are other challenges besides selecting the right MOSFETs which you aren't aware of (on the basis that you have to ask in the first place). If this is purely a "find a good MOSFET for me" question, it'll get closed as off topic (see "shopping questions" in the help center). – jms May 14 '18 at 09:59
  • This SE is a good explanation of pulsed drain current: https://electronics.stackexchange.com/questions/137513/why-pulsed-drain-current-is-higher-than-continuous-drain-current-in-mosfets – AlmostDone May 14 '18 at 11:59

2 Answers2

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There are two parameters in particular to look at. You have to make sure you don't exceed the maximum current, and the fast energy dump can't heat the die too much.

The first is obvious. Take the maximum capacitor voltage, divide it by the maximum allowed FET current, and that is the total series resistance needed. In theory, the cap's ESR and the FET's on-resistance contribute to the total series resistance. However, those two are usually specified as maximums that will not be exceeded, so you can't count on minimum values. The safe thing to do is to size the resistor as if it were there only series resistance.

Once you know the current is limited, you have to make sure the FET doesn't get too hot. Here you probably need to do some guessing and derating. The datasheet will tell you the maximum continuous dissipation the FET can handle for whatever conditions of cooling and heat-sinking you set up. However, it doesn't usually tell you how much energy it takes just to get to a particular temperature. You can exceed maximum continuous dissipation for a short term when starting from cold, but it's often not specified how much more dissipation for how long. Use common sense and be conservative.

One strategy is to have the resistor dissipate most of the cap's energy. In this case you use the FET's maximum guaranteed on resistance. The fraction of the total energy dissipated by each series element is its resistance divided by the total. For example, if the deliberate resistance is 500 mΩ and the FET's maximum on resistance is 25 mΩ, then the FET is guaranteed to not dissipate more than (25 mΩ)/(525 mΩ) = 4.8% if the cap's energy.

A larger series resistance slows the cap discharge time, and shifts more of the total dissipation to the resistor.

Olin Lathrop
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  • Would IGBT's the next best choice next to an SCR for discharging caps? Because what I got from the answers is MOSFETs have many consideration to take into account unlike SCRs when it comes to high energy conduction like a cap discharge. – user33915 May 14 '18 at 20:17
  • @user: There are many options. I answered what you asked about. The best overall strategy depends on quite a number of particulars you haven't told us, like what voltage? What current? How large a cap? How long can it take? – Olin Lathrop May 14 '18 at 20:44
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If you're discharging the caps completely, an SCR will be a better choice. For MOSFETs there is a lot more to worry about.

The pulsed current rating is one of those things manufacturers can "fudge" a little. There is usually a chart further down in the data sheet that shows just how long the FET can handle that kind of current.

To find out just how long the FET will conduct you can use the RDSon and the capacitance to find your time constant. Multiply that time constant by 5 or 6 to find the time it takes to fully discharge.

The next calculation will be to see how much power is going through the FET. V^2/R will give you the power dissipated at first. Since RDSon is usually in the mOhm range, your dissipated power is going to go up incredibly fast with voltage.

In summation, you're looking for the amount of power that the FET dissipates and whether or not it can handle that amount of power for the given amount of time.

Let me know if I can elaborate on anything more.

user187594
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  • Also, be aware of SCR spreading velocity ( lateral turn on velocity) – Marla May 14 '18 at 15:39
  • I guess MOSFETs are problematic when it comes to high energy conduction. I thought they were fine. – user33915 May 14 '18 at 20:15
  • Like Olin has said, it depends on how fast you need things discharged. You can use a mosfet it's just that your power requirements go up the cost of your fet is going to go up a lot as well. I have a circuit I'm using to discharge a cap at several hundred volts but it goes through a large resistor as well. This takes less than a second but depending on your application that may be too long. – user187594 May 15 '18 at 15:54