I just took over a project from EX co-worker and not understand a circuit (Darlington circuit) designed to against load dump(5b) at automotive. in my previous experience, TVS usually is used for protecting load dump at automotive, not Darlington here is test requirement of load dump and the circuit. enter image description here enter image description here enter image description here

According to my ex co-worker explanation, the designed Darlington can suppress load dump to 33V that is safety voltage input buck DC/DC

Normal operation input voltage from Battery=24V at circuit (DC+) operation current measured from Battery=200mA

My question

  1. Is it correct at this Darlington application for load dump?
  2. If it could , how Darlington is activated when load dump is occurring ?
  • 1
    \$\begingroup\$ schematic, please \$\endgroup\$
    – Pete W
    Feb 17, 2021 at 5:49
  • \$\begingroup\$ the first designer might be going for an active filtering effect out of the darlington, and a limit on inrush rate, due to R16/C99. A more typical protection replacing the darlington subcircuit with a larger unidirectional TVS diode would not have this side effect. If you can, ask the original designer if that was the reason for the choice. \$\endgroup\$
    – Pete W
    Feb 17, 2021 at 13:35
  • \$\begingroup\$ What is connected to Q9-E ? 200mA load? \$\endgroup\$ Feb 18, 2021 at 2:43
  • \$\begingroup\$ @Pete W thanks and then the first designer has resigned for long time. i would like to understand why darlington was designed for load dump and try to replace it with TVS or something \$\endgroup\$
    – Ethan Ku
    Feb 18, 2021 at 3:06
  • \$\begingroup\$ @Tony Stewart Sunnyskyguy EE75 i updated the detail information \$\endgroup\$
    – Ethan Ku
    Feb 18, 2021 at 3:07

2 Answers 2


D2 is a 5kW MOV but the missing suffix defines the voltage and R236 @ 3R3 shunts the 1 Ohm impulse source and only reduces the voltage by about 25% due to 3R3/4R3 with the specs given.

D3 is just a 1A diode and Q9 tracks the DC+ with a three PN diode drop or 1.8V from the pullup of R16 to the Darlington to a 200mA load.

D8 is a 33V Zener with a knee resistance Zzt1= ~ 58 Ohms . If one estimates the hFE of Q6*Q9 to be 10,000, the emitter impedance reduces the Zener dynamic impedance by this amount or down to 6 mOhms.

Thus the Darlington, buffers the Zener voltage very well to < 32V as the MOV absorbs all the energy.


The Darlington pair in the circuit is always active, regardless of the spike on the supply rail: Q6 takes the base current through R16 (C99 along with R16 brings a crude soft start) and then provides its emitter current as a base current for Q9. This results in a voltage drop of 1.5~2 VDC (depending on the current draw) to the rest of the circuit.

I don't know the input voltage range of the circuit but I'm assuming that it's probably 9-16 VDC (i.e. UA-nom = 14VDC and UA-max = 16VDC). If so, according to ISO-16949 automotive electrical standard, the circuit will not see 18 VDC (temporarily extreme situation) during normal operation. Even if the circuit is designed for 18-32VDC (i.e. UA-nom = 28VDC and UA-max = 32VDC) D8 will never be in the Zener region during normal operation. But when the supply gets higher enough to make D8 in the Zener region (i.e. the load-dump state), the base voltage of Q1 will be ~33VDC and the rest of the circuit will see ~30-31 VDC for a short time. The protection shown in the circuit is a very dissipative method -- it's quite dissipative even for the normal operation.

When I was working in the automotive electronics industry, I used varistors along with resettable fuses for that purpose, even for UA=32VDC:


simulate this circuit – Schematic created using CircuitLab

RF (resettable fuse) should be selected according to the max operation current and max operating temperature (this can be as high as 85°C) as the trip current decreases as the temperature increases. Depending on the Class (i.e. test result - the circuit should operate as designed under Load Dump, or the circuit can stop temporarily during the Load Dump and should return to normal operation after the Load Dump, etc) the RF can be omitted.

  • 1
    \$\begingroup\$ The RF is mostly important for the usual failure mode of MOVs i.e. in short. Also it protects the wiring from whatever happens on the board. \$\endgroup\$ Feb 17, 2021 at 9:23
  • \$\begingroup\$ thanks of your answering @Rohat Kılıç . however , i did not understand totally as you mean " the base voltage of Q1 will be ~33VDC and the rest of the circuit will see ~30-31 VDC for a short time" \$\endgroup\$
    – Ethan Ku
    Feb 18, 2021 at 2:06
  • \$\begingroup\$ @EthanKu the drops are transistor BE junction drops. That's all. \$\endgroup\$ Feb 18, 2021 at 6:32

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