Sizing Resistor Wattage for One Short Pulse

Artist · 2015-07-13T04:50:30+00:00

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 -- To email me directly remove sj. in my email address.

Jul 13, 2015 67 Replies

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tabbypurr 11 years ago

Max clamping voltage 19.9v. So crowbar isn't 32v. Whether you need to protect the caps against a fault rise in V_out I don't know. Do you?

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bloggs.fredbloggs.fred 11 years ago

The power supply is actually 13.6V unless you special order, so that TVS with a minimum breakdown of 13.3V is another bad choice. The power supply will blow that little TVS right off the board.

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Harry D 11 years ago

How is the energy stored in the capacitor, and wattage necessary to put the energy there within the charge time, relevant here?

Stephen,

The energy stored in the cap equals the energy dissipated in the resistor in series with the cap. Most of that energy is dissipated in R*C so now you must find peak power in the resistor. so Ppk = Edr/RC = C*V^2/2*RC = V^2/2*R +/-10%

Now, prove me wrong.

Cheers, Harry

Ok, ok, So some smart math jockey out there should be able to come up with this much needed exact equation of Peak Resistor Power as a function of V, C, R to charge a cap. No integrals please.

Cheers, Harry

Since no one has stated the exact equation for peak power dissipated (1 cycle) in a resistor charging a cap, I state the following; Pd = V^2/2R and T=R*C. The resistor must withstand 2*Pd watts initially (T0) and Pd watts for T seconds. Lots of SMD in thick film will meet these requirements.

Cheers, Harry

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Blarp 11 years ago

Just a silly solution:

Ye olde incandescent lamps make nice robust dump resistors. A 20W 24V halogen bulb might work. Not really SMD though. But optical signalling for free :) OT: PTC characteristic also makes for a very crude current limiting effect

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tabbypurr 11 years ago

I've used bulbs as current limiters, but the very wide resistance swing is a pita. Pick one that doesn't light up, then it lasts and doesn't look weird.

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Stephen 11 years ago

The power supplies were ordered with the 12V option. The power is 12V.

To reply directly remove sj. from the domain name. This is a spam jammer.

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Stephen 11 years ago

I have it from the manufacturer's tech support that there is a current limiter that will not exceed rated current. It essentially becomes a current source when when the limit is reached. The supply has a surge capability up to 25A for ten minutes. After that it reduces the limit to

20A.

So I do not need inrush control with this supply. But I want this board to work with any supply, and cover the possibility that all three boards, and possibly more of them, will work with a single supply of much higher current rating. So I am maintaining the inrush control. Instead of a resistor I am using a pair of current regulating diodes to charge the capacitors at a controlled rate. These are CLD20B-TP.

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I have also added a fuse. It is of the automotive ATC type. It is placed between the power connector and the TVS diode.

To reply directly remove sj. from the domain name. This is a spam jammer.

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tabbypurr 11 years ago

At 20mA I'm not seeing the point. You could charge the caps at 10 or more times that with a simple less than 1 cent resistor.

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