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In the datasheet, the transistor has two max current values, in the case of the IRF510

Continuous Drain Current = 5.6A
Pulsed Drain Current a IDM = 20A

with a pulse width of 20uS

Which is the assumed duty cycle for this values? Ts = 40uS and D = 50%?

Which current value should I consider if I want to work with a different duty cycle? 80% for example.

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    \$\begingroup\$ Note that those 'max current values' are absolute maxium ratings, not normal operating currents. The data sheet says: 'CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied.' \$\endgroup\$ May 25 '16 at 16:19
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    \$\begingroup\$ @BruceAbbott In this particular case the absolute maximum ratings are the same as the peak operating ratings (at least the parameters in the OP). \$\endgroup\$
    – user57709
    May 25 '16 at 16:31
  • \$\begingroup\$ @user57709 No, they are stress ratings, not operating ratings. Operating an IRF510 at 20A peak or 5.6A continuous will seriously impact its reliability. \$\endgroup\$ May 25 '16 at 17:09
  • \$\begingroup\$ @BruceAbbott The datasheet has the same values (for these two parameters) under "Absolute Maximum Ratings" and under "Specifications". There is usually a difference in these values, but not for this device. \$\endgroup\$
    – user57709
    May 25 '16 at 17:16
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    \$\begingroup\$ If its not explicit in the datasheet, then you should limit your duty cycle such that the average of the 'pulsed' current doesn't exceed the 'continuous' rating. \$\endgroup\$
    – brhans
    May 25 '16 at 17:20
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From the notes underneath that part of the datasheet:

Notes a. Repetitive rating; pulse width limited by maximum junction temperature (see fig. 11).

Fig. 11:

Fig. 11 - Maximum Effective Transient Thermal impedance, Junction-to-Case

From this, you can see that for 80% duty cycle (Duty factor, D = 0.8) we are beyond the scope of the graph (it only covers D <= 0.5). For that reason, you will need to work within the continuous rated current of the device.

For duty cycles <= 50%, you can read off the transient thermal impedance reduction for short pulses & low duty cycle, add in the impact of the case-to-ambient conditions and then ensure that the junction temperature stays below the absolute maximum rating for your power dissipation.

That said, I would just work well within the continuous current rating of the device and in fact overrate it by a significant margin to ensure that the circuit you design is robust in a wide variety of conditions.

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    \$\begingroup\$ There is a curve on this figure for different values of duty cycle. There isn't a curve for a value of 0.8, so one can't be assumed. This value would appear to be outside the scope of a repetitive pulse (in this context) and so the continuous ratings should be used (as you point out). \$\endgroup\$
    – user57709
    May 25 '16 at 16:34
  • \$\begingroup\$ You're quite right - I misread to 0-0.50 curve label. Will update the answer appropriately. \$\endgroup\$
    – stefandz
    May 25 '16 at 16:35
  • \$\begingroup\$ In addition 5.6A is the ideal value with Case temperature = 25C. Depending on the cooling conditions you need to further derate http://www.vishay.com/docs/91015/sihf510.pdf Fig 9 \$\endgroup\$
    – matzeri
    May 25 '16 at 20:21

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