hold up time or storage time for the electrolytic capaictor

[srevish]

Dear Sir,

In my SMPS I am using 470uF, 35V capacitor, the output voltage is 24Vdc.

This 24Vdc output is given to the load of 2.5W through this capacitor.

Input voltage of this SMPS is 72Vdc.

I want to know after switching off this SMPS, what will be the time taken by this electrolytic capacitor of discharge its energy.

Is there is any formula to calculate it.

 

[ravindragudi]

The time constant of the RC circuit will decide how long the capacitor can supply the current to the load. As you are saying a 2.5W load with 24V operation, the resistive load for this would be (24^2)/ 2.5 = 230.4 ohm. Now with the capacitor of 470uF and 230.4 ohm load, one time constant will be RC = 108 msec. The capacitor will get fully discharged in 5 time constants = 541 msec.

As the capacitor discharges providing the load current, the voltage across the capacitor also drops exponentially. The instantaneous voltage can be calculated by the formula: Vb X exp(- t/RC). Where Vb will be 24V initial voltage, t is time instance, R is load resistance and C is capacitance.

For example the instantaneous voltages at 10msec, 15 msec, 100 mesc and 150 msec will be 21.9V, 15.1V, 9.53 and 6V.

 

[srevish]

The time constant of the RC circuit will decide how long the capacitor can supply the current to the load. As you are saying a 2.5W load with 24V operation, the resistive load for this would be (24^2)/ 2.5 = 230.4 ohm. Now with the capacitor of 470uF and 230.4 ohm load, one time constant will be RC = 108 msec. The capacitor will get fully discharged in 5 time constants = 541 msec.

As the capacitor discharges providing the load current, the voltage across the capacitor also drops exponentially. The instantaneous voltage can be calculated by the formula: Vb X exp(- t/RC). Where Vb will be 24V initial voltage, t is time instance, R is load resistance and C is capacitance.

For example the instantaneous voltages at 10msec, 15 msec, 100 mesc and 150 msec will be 21.9V, 15.1V, 9.53 and 6V.

Thanks for your reply,

Also please I have checked this link

http://www.aimtec.com/site/Aimtec/f...tionNotes/increasing hold up time in smps.pdf