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I find this voltage limiter circuit very simple and useful:

schematic

simulate this circuit – Schematic created using CircuitLab

To my surprise, neither Mouser nor Digikey have P-Channel Depletion Mode MOSFETs available anymore.

My question: What is the "industry standard" / commonly / often used replacement of this circuit?

My use cases is to limit rectified transformer voltage to a maximum, so that a linear voltage regulator does not see destructive voltage but bulk capacitor charging current still is mostly unimpeded up to the limit voltage.

Notes: D2 could be replaced by a short to ground and is only there if Vlimit exceeds Vgs of the depletion mode MOSFET. I have used this circuit without D2, but the above mentioned use case requires Vlimt to be a bit below 80 V.

HannesW
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    The natural replacement for this circuit is a voltage regulator. – Andy aka Feb 05 '23 at 11:31
  • Is the question *what is a simple voltage regulator for voltages outside the range of integrated ones?* Note that voltage limits more often are on \$V_{in} - V_{out}\$  rather than on \$V_{in}\$ or \$V_{out}\$. – greybeard Feb 05 '23 at 11:37
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    Maybe I misunderstand it. But doesn't it need an N-channel depletion mode FET to work as drawn? Those are available – tobalt Feb 05 '23 at 11:37
  • Use a variation of the circuit that turns an enhancement mode mosfet off when the zener conducts. – Kartman Feb 05 '23 at 12:00
  • @Andyaka This circuit would follow Vin quite precisely when below Vlimit. I do not know if a LinReg would behave the same given dropout? – HannesW Feb 05 '23 at 14:23
  • @greybeard The characteristics of a voltage regulator may be different, specifically the dropout? – HannesW Feb 05 '23 at 14:23
  • @tobalt My understanding is that an n-channel dep. mode turns off when VGS is positive? An answer pointed out JFETs can do this. – HannesW Feb 05 '23 at 14:24
  • @Kartman Yep, that thought did occur :D. As a JFET can do this (happy that I asked and this was brought up) I can keep it simple. – HannesW Feb 05 '23 at 14:26
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    @HannesW a p channel FET turns **on more** when lowering gate voltage. a P JFET will **not** work in this circuit..Such an answer is wrong. N channel depl. FET will turn off when gate is pulled negative. – tobalt Feb 05 '23 at 16:08
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    @tobalt You are correct, the answer has been updated and accepted. – HannesW Feb 05 '23 at 16:15

1 Answers1

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To my surprise, neither Mouser nor Digikey have P-Channel Depletion Mode MOSFETs available anymore.

There never were. Depletion mode MOSFETs have always been only N-Channel, as far as I can remember. (And I am older than MOSFETs.)

Do not despair though.

  1. Your circuit should use an N-channel depletion mode MOSFET, not P-channel
  2. You can also use a N-JFET instead
  3. Forget making your own voltage reference from scratch and instead use a shunt voltage regulator IC: precise, simple, guaranteed to work
  4. Step back and tell us what the ultimate problem you wish to solve is (X-Y problem) and we may have a different and better approach to solving it.
Davide Andrea
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    Thank you very much, I must have used a JFET then... Do you mean a N-JFET as the gate is negative to both Drain and Source? My problem is to create 48 V phantom power from rectified 56VAC, blowing the first of 3 series LT3011xMSE. This is a hobby project, limited time and its success is not critical. One reason for me to have used this circuit previously at work was that the FET I found could deal with quite high voltages and give me confidence that the voltage below V limit is virtually the same as Vin, for an ADC to measure. I would have had to qualify each LinReg for this. – HannesW Feb 05 '23 at 14:07
  • "Do you mean a N-JFET ". No I do mean a P-JFET. – Davide Andrea Feb 05 '23 at 14:20
  • "My problem is ..." Start a new question, show the schematic, explain the problem. Someone will suggest a more practical, effective, and simpler solution. – Davide Andrea Feb 05 '23 at 14:22
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    "No I do mean a P-JFET" Pardon the uncertainty, but looking at p-JFET datasheets Vgs seems to be positive, while in N-JFETs Vgs seem to be negative for turning the device off. Since a JFET is a specially constructed diode with the symbols arrow pointing in the forward direction of the diode, a P-JFET would short Vin to Q1 via its body diode? – HannesW Feb 05 '23 at 14:47
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    Oh, now I see the problem: your circuit uses a P-channel depletion MOSFET, but it should use an N-channel. I took you at your word, so I replaced a P-channel depletion MOSFET with a P-channel JFET. By noticing that the polarity of a P-channel JFET is incorrect, you unintentionally pointed out to me that the P-channel depletion MOSFET is incorrect in your circuit. – Davide Andrea Feb 05 '23 at 14:52
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    Could you, or may I change the P-JFET to N-JFET in your Post so that I could accept your answer? – HannesW Feb 05 '23 at 15:07
  • Very kind of you. I have edited my answer, as you suggested. Thank you. – Davide Andrea Feb 05 '23 at 15:13