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Im trying to figure out the current limitations of a mosfet(SISA18ADN) I'm planning to use to check how much headroom I have.

The datasheet states Continuous Drain Current that it can handle as 12.1Amps at Ta=70degreeC during 10seconds, and 38.3Amps at Tc=25C with no time specification.

But in the Safe Operating Area graph, apparently for DC it can handle less than 0.2A at 10V..? Am I getting something wrong when reading the graph or when reading the specs?

The Safe Operating Area graph is here:

http://imgur.com/530lyR8

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    \$\begingroup\$ 38.3 amps must mean that the device is fully turned on, and so it doesn't have much of a voltage drop. With a 10V drop, 0.2A means: 10 times 0.2 = 5W of dissipation. 38.3A could mean very little dissipation if the voltage drop is low. \$\endgroup\$
    – Kaz
    Feb 10, 2015 at 22:03
  • \$\begingroup\$ @Kaz 10 times 0.2 is 5? \$\endgroup\$
    – Spehro Pefhany
    Feb 10, 2015 at 22:05
  • \$\begingroup\$ Ahhh! Damn, I knew I must have been making a stupid mistake. So for a 10Vds at 0.2A, Rds would have to be 50R? which means a low Vgs, etc.. \$\endgroup\$
    – Wesley Lee
    Feb 10, 2015 at 22:07
  • \$\begingroup\$ @SpehroPefhany Err, flip that left to right. \$\endgroup\$
    – Kaz
    Feb 10, 2015 at 22:13

2 Answers 2

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The safe operating area is voltage versus current, whose product is power dissipation. The reason that the maximum current is low for, around 0.2A, versus 10V for continuous DC operation is that you have to look at the product of these values: 2W. Note that for a lower voltage drop, the device can handle a lot more current. Somewhere in the range of 0.1 and 0.2V across the channel of the device, it can handle 10A.

This 10A is the absolute limit for continuous operation: it is the highest amperage reached by the strictest dotted line (the DC line). Anything higher than 10A is outside of the DC safe area, achievable only by the time-limited safe areas. For 10 ms the device can go above 20A, but the voltage must be neither too high nor too low: it must be high enough that \$R_{\text DS(on)}\$ doesn't limit the current.

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The limitation is based on temperature rise in this case. Maximum thermal resistance (steady-state) junction-to-ambient is stated to be 81°C/W so at 25°C ambient, a power dissipation of 1.5W is the highest acceptable at an ambient temperature of 25°C (less when it's warmer) resulting in a junction temperature of 150°C (the absolute maximum rating).

enter image description here

This is a tiny part- it's best to have it fully turned on or fully turned off or it will do unpleasant things like unsoldering itself from the PCB.

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