In an application simulation a fly-back diode 1N4001 current is peaking upto 3A and decays to zero in around 25ms as shown below:

enter image description here

The datasheet says the max forward current is 1A. But it also says 30A for 8.3 ms.

In my case it is a peak decaying from 3A to zero in 25ms.

Do I need to change this diode? What can be an alternative?

  • 4
    \$\begingroup\$ How often does this pulse occur? \$\endgroup\$
    – Dave Tweed
    Oct 24, 2018 at 15:28
  • 1
    \$\begingroup\$ Very very short leads will remove heat with less silicon-junction temperature rise. \$\endgroup\$ Oct 24, 2018 at 17:48

2 Answers 2


In the case of pulse loading to a component, it's the \$I^2t\$ that matters. This is a measure of the heat that's dumped essentially instantaneously into the device raising its temperature. 30A peak half sinewave pulse for 8.3mS is an \$I^2t\$ of about 3.7.

You can either integrate your graph properly, or do a simpler conservative upper bound and say that it's less than a full 3A flowing continuously for 25mS, which is an \$I^2t\$ of 0.225. This figure is so much less than 3.7 that's it's clear it will handle your proposed pulse with no trouble.

That rating is for a single pulse, with the diode cooling down between pulses. Is your flyback diode going to be hit repeatedly in a time less than the cooling down time? If so, you need to compute the average power that those pulses deliver, and make sure that's well below the continuous power rating.

  • 1
    \$\begingroup\$ No it will not be repetitive. Juts turn on and off application. \$\endgroup\$
    – pnatk
    Oct 24, 2018 at 13:51
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    \$\begingroup\$ @neil_uk: AFAIK, it's VIt that matters. I^2 t is correct for resistors, but only correct in a diode that's acting substantially like a resistor rather than showing a more or less constant voltage. \$\endgroup\$
    – TimWescott
    Oct 24, 2018 at 15:00
  • \$\begingroup\$ @panicattack is it theoretically possible that the device's power could be cycled rapidly several times in succession (bad power, idiot user rapidly flipping the switch, etc)? Such a scenario could result in these surges happening frequently enough to potentially present problems. Another consideration is heat dissipation -- is this going to be in a climate-controlled environment? Or is it going to be in a metal box mounted outside on a pole somewhere where it might bake in the sun? \$\endgroup\$
    – Doktor J
    Oct 24, 2018 at 18:38
  • \$\begingroup\$ @DoktorJ Outside in a plastic enclosure. Is there a better diode for this flyback purpose with better power capacity? \$\endgroup\$
    – pnatk
    Oct 25, 2018 at 0:24

The issue is how hot do various bits of the device get. Continuous current ratings are calculated based on the total thermal resistance of the device to ambient. For short, infrequent pulses the thermal capacity of the device matters. For short repeating pulses, the thermal capacity, the thermal time constants, and the dissipation to ambient of the device all matter. For complicated devices (or critical applications) there may be more than one time constant to worry about (i.e., a 1ms pulse of energy W may be OK because it can dissipate into surrounding silicon and packaging, but the same amount of energy in 1us may fry the junction or parts thereof, because the heat doesn't have time to move).

You can either try to find such data for the 1N4001 (and hope that it's the same from brand to brand), or you can figure that if it'll handle 30A for 8.3ms (250mJ, if you assume a 1V drop), it should have no problem with 3A for 25ms (75mJ).

If you study this data sheet from Diodes, Inc., you'll see that they have charts on voltage drop vs. current (which you need to determine power vs. current, which isn't really proportional to either I or I^2), current vs. ambient temperature which you can use to deduce the thermal resistance from case to ambient*, and curent vs. number of repetitive cycles at 60Hz, from which you can get a notion of the thermal time constant from the junction to ambient. From all of that, you can determine whether you're safe.

But to reiterate -- you should be fine. Look at Figure 3, and figure that your 25ms is less than two full cycles at 60Hz, and figure that they're allowing you more than 20A peak for a half sine-wave. The average current of a half sine wave is somewhere around 0.68 * peak (it's 2/pi or something -- I'd have to calculate it to remember), so that works out to 15A for 33ms. Even being really fast and loose with the numbers, it's obvious that you'll be fine with 3A for 25ms, unless your ambient temperature is way closer to 175C than it is to 65C (see their figure 1).

* Said thermal path is through the leads, see Note 3 to the maximum ratings table.


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