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I was trying to clamp voltage down across a 2.7 V supercapacitor and by measuring what is going on was reminded that Zener diodes have both a very soft "knee" region and a leakage current I can't ignore (several mA for the ones I've used). Is there a type of Zener diode which would have a sharper knee and next-to-no leakage current?

Alternatively, what else could be used instead of a Zener diode? What leakage could be expected from an LDO regulator?

Well... garbage in, garbage out :) I was using the name "ideal diode" in the sense of such circuits (made from FETs) I've seen described, but after reading more about them, they are probably not what I can use here.

I'd value more something which would have next to no leakage current (single-digit µA range) than a sharp knee (where I basically need to clamp a power source which could in theory spike up to 5 V down to the 2.7 V the supercapacitors can handle).

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    \$\begingroup\$ They are in your "get free lunch" electronics parts shops right next to ideal 0ΩRDSon FET section under the zero voltage drop schrottky diodes. \$\endgroup\$
    – PlasmaHH
    Commented May 15, 2017 at 13:47
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    \$\begingroup\$ What's wrong with a normal shunt regulator + external transistor? How sharp knee do you need? Can you feed the bias from elsewhere? \$\endgroup\$
    – winny
    Commented May 15, 2017 at 13:49
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    \$\begingroup\$ By definition, there is no such thing as an ideal anything, I'm afraid. All components are a trade-off of pros and cons. \$\endgroup\$
    – TonyM
    Commented May 15, 2017 at 14:23
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    \$\begingroup\$ My ideal wife sells them, along with ideal transformers and ideal motors. \$\endgroup\$
    – Trevor_G
    Commented May 15, 2017 at 14:37
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    \$\begingroup\$ The sharp knee zener is called a shunt-regulator or shunt-reference, its a complete IC. Zero-leakage current is a tough one. \$\endgroup\$
    – sstobbe
    Commented May 15, 2017 at 14:43

2 Answers 2

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What you'd probably want is a "crowbar circuit", ie. a circuit that uses e.g. a MOSFET to short the supercap if voltage crosses a certain threshold.

An example of that can be found in the Ettus USRP N200 / N210 Schematic file on page 12:

crowbar circuit

You could implement that thresholding using a Zener diode with a significant series resistor.

Another example of a crowbar overvoltage protection approach can be found on wikibooks.

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    \$\begingroup\$ don't forget to fuse input in case of fault \$\endgroup\$
    – D.A.S.
    Commented May 15, 2017 at 16:12
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    \$\begingroup\$ This isn't a crowbar circuit; a crowbar shorts the input to ground on an overvoltage condition, with the intention of blowing a fuse. This circuit merely cuts the input on overvoltage, it doesn't short to ground. \$\endgroup\$
    – marcelm
    Commented May 15, 2017 at 16:19
  • \$\begingroup\$ of course, I said in case of fault \$\endgroup\$
    – D.A.S.
    Commented May 15, 2017 at 18:26
  • \$\begingroup\$ @TonyStewart.EEsince'75 My comment was in response to the answer, not to your comment ;) \$\endgroup\$
    – marcelm
    Commented May 15, 2017 at 18:30
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    \$\begingroup\$ correct it is a more complex OVP, usually delegated to the regulator and OVP done by foolproof crowbars with current limit to avoid explosions.. even Tesla uses microfuses on LiPo's at Both ends inside terminal. simple OCP with redundancy necessary for internal faults. \$\endgroup\$
    – D.A.S.
    Commented May 15, 2017 at 18:32
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With the exception of "cryonic" superconductors, no electronic component has near zero series resistance.

However with comparators having gains of 106 or more, you can reduce the impedance by negative feedback gain, but still current limited by the series switch RdsOn.

All low voltage zener and LED diodes have similar Rs or ESR "bulk series resistance" that affects the knee, but remember this: The higher the current rating of the diode, the lower its ESR when saturated, or Rs or Rz or Zt as they call it for Zener diodes.

  • Rated max Pd = 1/ESR +/- xx % (Stewart's ESR Theorem)

  • It is exactly the same for 3 V LED diodes which you can use as. Zener diodes except you do not regulate by dumping the maximum load into the Zener diode; rather you regulate the series PWM with an inductor current and pulse and compare that voltage and current with a stable current into a low-power Zener diode, programmable Zener diode, or an LED voltage.

So in conclusion, the regulation of the voltage is due to the loop gain and fixed voltage reference (LDO or band gap or LED) and controlled series R or PWM regulated Buck current charger with a voltage limit.

In other words, think of it like a small lithium polymer battery SMPS battery charger with a different profile of Vmax and Imax. See? In fact a lithium polymer battery has more than 100k farads, but due to double electric charge layer has short term memory from 4.2 V to 3.7 to 3.6 V. Which I have explained in other questions.

An OVP crowbar must have a series fuse or polyfuse in case of series regulator failure!!

You would expect a TVS diode to have an extremely sharp knee (read low ESR). This one is rated at 0.07 ohm for ESR (see Rdyn). According to my formula the package maximum rating with a heat sink should be 14 watts at 85 °C (read ESR =1/Pd max rated continuous at 85 °C with a heat sink.

But then a TVS diode is not intended to use as a zener but in pulse mode , you could. They do not give a Pd rating, but if you see the size, you will see it is close with a large metal plane. Ppk = 800 W

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