I am designing an inrush control circuit to prevent a pair of electrolytic bypass capacitors exceeding their rated ripple current when power is suddenly applied. When the capacitor is fully charged it will be grounded by a MOSFET. In parallel with the MOSFET there is a current limiting resistor.
I do not know what the rise time of the power supply will be. I do know I must protect against its being plugged in while turned on. So I must assume the voltage change across the capacitors is instantaneous.
The resistor value is determined by the capacitor leakage current, and maximum ripple current. Once the capacitor is fully charged its leakage current will drop voltage across the resistor. Then when the MOSFET is turned on, current will be limited only by the capacitor's ESR. So the upper limit of the resistor is the value that will cause no more than the rated ripple current to flow when the MOSFET is turned on. When the MOSFET turns on the voltage that was across the resistor will appear across the capactor's ESR value. So the upper limit is:
R = Irip * Resr / Ileak
For a safety margin it will actually be no more than half this value.
The amount of peak wattage will be high, roughly 30 Watts, but it will be a very short pulse that happens once. I know that due to the shortness of this dissipation, which is over well before thermal equilibrium is reached, it is not necessary to have a large and expensive 30 Watt resistor there. But over what amount of time should this wattage be averaged to determine the resistor's wattage rating? I know that heat capacity figures into it, but that data is not available to me.
I figure the resistor to be about 5 ohms. It will be a surface mount one.
The capacitors are a parallel pair of 16SVPF1000M
Resr 12mOhm Irip 5.4A Value 1.00mF Tolerance 20% Ileak 3.20mA
The MOSFET is a CSD17312Q5