I need an input protection for a DC-DC converter in a 24V automotive system environment. The DC-DC is capable to accept 50V input but I need some sort of protection against load dump pulses which can go over 100V for 50-100mS. So I need to clamp the voltage to <50V for a short period. I can use in the circuit below a 47V zener or a TVS. Is there any problem doing it with a N Channel MOSFET? I made no test yet of the overvoltage but one strange thing I see is that there is a constant 4V drop between the input and the output over the nominal 18-32V working range. Am I doing something wrong?

circuit with 31V TVS

  • \$\begingroup\$ N-ch in high-side needs about 4V higher voltage on gate than Vin to get zero drop. You have to use a P-ch instead. \$\endgroup\$ Commented May 15 at 15:15
  • \$\begingroup\$ @MichalPodmanický But then it would turn "on" more, not "off". \$\endgroup\$ Commented May 15 at 15:16
  • \$\begingroup\$ What current draw is this for? \$\endgroup\$ Commented May 15 at 15:17
  • \$\begingroup\$ About 1A @Tim Williams \$\endgroup\$
    – Emanuel
    Commented May 15 at 15:24
  • \$\begingroup\$ @Michal Podmanicky I have no problem with the 4V drop except for the unnecessary heat during normal use \$\endgroup\$
    – Emanuel
    Commented May 15 at 15:28

2 Answers 2


Simple fix: use a depletion MOSFET. This has negative Vgs(th), so that normally the source is pulled above the gate by a few volts, when wired as a source follower (as here). When the zener is not conducting, the gate is pulled up to drain, and the transistor saturates with low dropout*. As input rises, the zener begins to conduct, and output voltage is limited.

A zener from gate to source may be prudent, to cover for cases where the input is suddenly pulled down, faster than the gate node would otherwise fall, and thus cause Vgs overvoltage.

*As always, to be clear: I mean voltage saturation here. I would like "current saturation" or "FET saturation" to simply be forgotten, they aren't consistent with any other use of the term; but until such time, I will continue adding this note.

Depletion MOSFETs aren't available in a huge variety of ratings, but if your load is small (say, 10mA?), a DPAK size device will easily handle this. Namely, I did an application for a small control unit in automotive setting, some years ago; with a DN2625K4-G and a 43V zener, and a maximum current draw of something like 10 or 20mA at 40V, it handled not only the worst load dump specified, but indeed continuous operation at input of 100V or more.

For higher current loads, transistor size and SOA quickly become a concern, not to mention the cost and limited selection of depletion types. A turn-off scheme quickly becomes desirable, whether by discrete circuitry (a few transistors, a comparator, etc.?), or integrated means (e.g. LT4363 "surge stopper").

If the circuit must remain operational through the load dump pulse, you could do worse than adding a whole switching pre-regulator (but, make sure it is capable of 100% duty cycle for low dropout in normal operation). Options won't be cheap in this case, but a switching pre-regulator is one of the more appealing ones.

  • \$\begingroup\$ Thanks for the explanation. I worked with the LT4356 surge protector a lot, also with the LT4363. But I try to avoid this circuit in a new project. In fact the only reason I need this protection is because I have a height limitation. The DC-DC is capable to work with Vin 100V but I need the EMI filtering and 100V aluminum capacitors are way above the 6.5mm height limitation I have. So I though about a simple circuit necessary for a very short period of 50-100mS \$\endgroup\$
    – Emanuel
    Commented May 15 at 15:47
  • \$\begingroup\$ Heh, yeah, the required transistor volume likely uses up what space you were hoping to save. Maybe it still fits if you have the area (just not the height) and a D3PAK or TO-247 is available? Also, you may have some luck with ceramic caps in leadframes; usually not very cheap, but if these are your options, maybe it's worthwhile. \$\endgroup\$ Commented May 15 at 16:07
  • \$\begingroup\$ If I try to think deeper, as you mentioned the LT43xx surge protectors, they work with MOSFETS with large SOA and practically the MOSFET takes the voltage difference during a load dump pulse and develops a very high power which is time limited by the chip, after which even the surge stopper cuts the output. My problem is during normal use because of the approx. 4V voltage drop which causes the MOSFET to be hot constantly. What if I use a transistor instead? I will probably need a darlington to keep the zener&base current low but I reduce the voltage drop to 1.2-1.4V. @tim williams \$\endgroup\$
    – Emanuel
    Commented May 18 at 18:01
  • \$\begingroup\$ They use a charge pump to keep Vgs(on) high. Perhaps you could design a similar solution? \$\endgroup\$ Commented May 18 at 19:36

Simple with P-ch mosfet and optocoupler like PC817:

To determine exact Vout you have add about 2V Led drop to 47V zener voltage (49V together), so you may need to use a smaller zener to not be very close to 50V limit. enter image description here

  • \$\begingroup\$ While load dump has a relatively slow risetime (~ ms), there are other automotive transients with us type risetimes; you might use a TSV also. If you intend to keep operating during the load dump, be aware that the 1A * 40 V == 40 W dissipated during this time (~ 100 ms total duration of load dump) may overheat the pass FET. \$\endgroup\$
    – jp314
    Commented May 15 at 15:51
  • \$\begingroup\$ @jp314 There is a TVS on the input not shown in the circuit above for the other short or low energy high voltage spikes or ESD. \$\endgroup\$
    – Emanuel
    Commented May 15 at 15:54

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