A 12V power supply is switching on externally connected 12V filament bulbs. The power supply's load switch implements an electronic resetable fuse (13A/τ=15ms) in case of an external short. In normal operation the load (bulbs) draw approximately 7A. To limit the bulbs turn on current, which is approximately 10 times higher than in normal operation an inrush current limiter is needed.

Coarse requirements of the limiter:

  • current limit ~10A
  • current limit active for approximately 1s, after that the limit shall be disabled. Otherwise the power supply internal short circuit protection wont trigger in case of a short
  • The current limiter must be somewhat independant from ambient temperature and Input voltage (ambient temperature range: -40°C to +70°C, Input voltage range: +9V to +16V).
  • Limiter shall be as small as possible to get mounted inline with the bulbs power cabling.

Proposed Implementation:

Considering the above i came up with two circuits which both implement the needed functions. The favoured circuit at this time is the one with less components due to the size requirement.

current limiter comparison schematic

current limiter comparison drain current

My questions at the moment are: does the limiter which has the lower component count has any downsides compared to the other one? Are there any obvious weaknesses in any of the two circuits which i did not think of (except that the PMOS chosen for the simulation is not suitable)?

  • 1
    \$\begingroup\$ Would a simple NTC suffice? JFET comes to mind too. \$\endgroup\$
    – winny
    Dec 7, 2018 at 8:38
  • \$\begingroup\$ Are you allowed to modify the powersupply, eg put a capacitor from the output to the feedback node? \$\endgroup\$
    – Jasen
    Dec 7, 2018 at 8:42
  • \$\begingroup\$ Sadly not, the power supply cant be touched. I looked for suitable NTCs too but could not find anything suitable, i think the requirements are just to restraining for using an NTC (10A limiting for 1s, after that no limiting, temp range -40 to 70°C). Also repetitive turn on actions in a short period of time will be an issue when using NTCs. \$\endgroup\$
    – Jogitech
    Dec 7, 2018 at 9:03


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