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I am searching for a neat solution for an input voltage limiting circuit.

Situation:

I have a circuit with an input voltage range from 6 V to 20 V at 3 A peak (the load), however my source can be anything between 7 V and 27 V.

I am looking for a circuit in between that passes through any input voltage between 7 V and 20 V, and clamps any input voltage between 20 V and 27 V to the 20 V maximum of my load. (dissipating 7V*3A = 21 W)

limiting circuit

Desired behavior of the 'questionmark' - circuit: desired behavior

The solutions that already found:

  1. Using a resistor and 20V Zener diode --> Is not preferred because of the losses in the resistor which I don't want if my input voltage is below 20 V, and the losses in the Zener when my load current is near zero...
  2. Using a 6V LDO --> Does not satisfy the requirements, if I do have a 20V input available, I want to 'use' it.
  3. Using a 20V linear regulator --> Unspecified behavior when the input voltage is below 20V...
  4. Using a Zener with a Mosfet as in https://www.circuitlab.com/circuit/9sj5kb/zener-mosfet-regulator/ --> Simulation reveals that when the input voltage is below the zener voltage, the output is ~half the input voltage, desired behavior would be Vin = Vout.
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  • \$\begingroup\$ For the Zener/resistor combo, replacing the resistor with an incandescent lamp is a bit more efficient, since the filament resistance is low when cold, but rises when warm. Some auto headlights designed for ~5 A might be an improvement. \$\endgroup\$
    – DrMoishe Pippik
    Commented Aug 14, 2018 at 21:01
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    \$\begingroup\$ Here is a bit more sophisticated variant of your #4 \$\endgroup\$
    – Maple
    Commented Aug 14, 2018 at 21:07
  • \$\begingroup\$ An alternative which works for some cases is to simply shut down the circuit if excessive voltage appears, which avoids the complexity or dissipation. I have done that to deal with situations where the user can apply incorrect voltage. Of course it doesn't fill the bill if you actually need it to function with the higher voltage inputs. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Aug 14, 2018 at 21:18
  • \$\begingroup\$ @Maple That one actually does accomplish what I am looking for! Now to figure out what solution is cheaper / easier, the buck converter or the one from eetimes :) \$\endgroup\$
    – Arjo van der Ham
    Commented Aug 14, 2018 at 21:27
  • \$\begingroup\$ @SpehroPefhany I need it to function at the higher input voltages \$\endgroup\$
    – Arjo van der Ham
    Commented Aug 14, 2018 at 21:27

2 Answers 2

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Instead of dissipating 21W with a series pass element, you could use a buck regulator with the output set to 20V. For input voltages below 20V the regulator would be in dropout and pass the input voltage to the output. for input voltages above 20V it will behave normally as a step down regulator and keep the output at 20V. You need a buck regulator that can operate in drop out which requires that it can operate at 100% duty cycle.

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  • \$\begingroup\$ Interesting! I was hoping to avoid needing a switching regulator, to keep cost and complexity down (aim is < 1.50), but maybe I should investigate how expensive it would really be. \$\endgroup\$
    – Arjo van der Ham
    Commented Aug 14, 2018 at 21:07
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    \$\begingroup\$ I don't think it has to be complex (depending on what you find complex of course). Like the LM2596 could do it for instance. It's duty cycle can go upto 100%, and it only needs a few external components. It does have a dropout of ~1.3V, but at 3A, that's not too bad I guess. \$\endgroup\$
    – MartinF
    Commented Aug 14, 2018 at 21:19
  • \$\begingroup\$ I almost always opt for LTC, but if you looking for cheap then TI. \$\endgroup\$
    – EE_socal
    Commented Aug 14, 2018 at 21:35
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    \$\begingroup\$ Except the input voltage can drop as low as 7V. You would need a buck/boost regulator if the output was set to 20V. With a buck you could set the output to 6V. \$\endgroup\$
    – Misunderstood
    Commented Aug 15, 2018 at 14:41
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All linear solutions must dissipate 21W max when outputting 60W.

A variable Buck DC-DC regulator can be >=85% efficient but is more complex. Search online for such a solution like a motor speed controller and modify to limit range of control.

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