Electronics > Beginners
DC Electronic Load
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brybot:
Expanding on 'capt bullshot' post:

I also believe you're operating in saturation.

The IRF620 is rated for 5.2A continuous, at 25C, or 50W maximum power dissipation. When you're hitting 5A at 12V, that's exceeding the maximum rating of the device, which also assumes a really good heatsink.

I'm guessing your FET is heating waaaaay up. Probably over 100C, at which point the continuous current rating goes way down. And peak wattage goes down at 0.4W/C for this chip.

Check out Fig. 4 of the datasheet. Even if you can turn your FET on fully and achieve the 0.8ohm Rdson, the heat is going to push it up and limit your maximum current.

Fig. 1 shows (at 25C) that you need relatively large Vgs to avoid saturation in your application. The IRF620 is designed for fast switching and low on resistance, which probably means it's not ideal as a load, unless you put quite a few of them in parallel with one another.
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