I've got a circuit that's having problems at startup with inrush current, leading to large spikes on VCC (parasitic inductive of the power supply / leads etc) that sometimes results in smoke coming from chips in my circuit. It's powered with ~12-16V and drives a 5v circuit that draws 40-80mA @ 5v.
The excessively large 82uF input capacitor is an electrolytic, it's there so transient disruptions to VCC won't reset the microprocessor, and the 10uF input capacitor is a ceramic SMD component - as suggested by the DC-DC converter IC datasheet.
I'd like to include a fuse, so failure of this circuit won't effect any other consumers connected to VCC (and fire and heat coming from this circuit is not acceptable under any circumstances) - but this has not worked out so far due to the inrush current.
I'm considering adding a 25 ohm input resistor in series with VCC (ie before fuse) to limit the circuits current. Under worst case normal operating conditions, this would have a drop of 2v, dissipating 0.16 watts. At startup, it would limit inrush current to 0.64A - which would also allow me to use my 1A fuse as well as avoiding any inductance in the supply leading to deadly voltage spikes.
Does this sound like rational circuit design, and an acceptable solution to the problem?
I intend to use 0603 SMD components for this input resistor, and fuse - or should I size them up?
To demonstrate the source of my pain, I setup this test circuit and apologetically powered it with 16v by touching power to the wires. In the scope trace, you can see a spike in voltage up to 19v coming from my (cheap) bench PSU.
If I add a bunch more parasitic inductance to the power supply to simulate an even worse supply, these spikes quickly become deadly. I have the scope connected across the load resistor, and the inductor inline with the supply leads.
This is definitely worst case scenario, and the offender is my rubbish bench PSU - but one of my design requirements is for my circuit to be near bulletproof to external factors.
