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I have recently discovered the exciting world of depletion mode mosfets. Curiously, even in this document, I found nowhere that a depletion mode mosfet could be used as a current limiting/inrush current limiting device. But this is strange, because what prevent us to use them like this:

enter image description here

or like this

enter image description here

(with a suitable heat sink of course) ?

To put flesh on bones, I have built a 800V, 10-20 mA PSU. I have a IXTY01N100D depletion mosfet, with 1000V breakdown voltage and 400mA current. It is not so easy to build a conventional inrush current limiter at this voltage. I wonder if I can use this transistor to limit the inrush current in the filtering output cap.

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  • \$\begingroup\$ See figure 6, 8 and 9. \$\endgroup\$ – Oskar Skog Dec 26 '19 at 10:28
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    \$\begingroup\$ I vaguely remember this application was mentioned in the AoE vol 3. I don't have it at hand to lookup now though. But you can definitely use depletion-mode MOSFETs as current-limiting devices. \$\endgroup\$ – anrieff Dec 26 '19 at 10:28
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It should work. It's a common use for depletion FEts and several examples in the document you linked to use them as current sources.

Respect the Safe Operating Area though:

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You can also make an AC current limiter with two depletion FETs:

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    \$\begingroup\$ Thank you for your answer, and for the ac current limiting circuit. Yes, it is well known that a depletion mosfet can be used as a current source, but it is not always immediate that 1+1 = 2, I mean, that a current source could be used as a current limiter. Even The Art of Electronics does not evoke depletion mosfets in the chapter related to current limiting. That is what I found strange. \$\endgroup\$ – MikeTeX Dec 26 '19 at 20:23

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