Fast Capacitor Discharge

[GaryA]

I have read that if the value of the capacitors is too big the diodes will blow when the output of a voltage doubler circuit is shorted. It goes without saying that the charge voltage of the capacitor cant be greater than the breakdown voltage of the diode, but the bigger the capacitance the more energy there is to discharge. The maximum current rating of a diode can be looked up, but how can you calculate the maximum value of capacitance without damaging the diode?

 

[Optikon]

I have read that if the value of the capacitors is too big the diodes will blow when the output of a voltage doubler circuit is shorted. It goes without saying that the charge voltage of the capacitor cant be greater than the breakdown voltage of the diode, but the bigger the capacitance the more energy there is to discharge. The maximum current rating of a diode can be looked up, but how can you calculate the maximum value of capacitance without damaging the diode?

Calculate the peak current in the discharge - that's what the diode sees and that is what might be troublesome. Diodes can usually withstand much higher than rated currents for a sufficiently short time - see surge current rating in the diode specs. The peak current will heavily depend on the rate of discharge as well as series path resistance (both intentional & parasitic).

 

[Nigel Goodwin]

Bear in mind though, to get the high peak currents you need large capacitors, and you only use large capacitors if you need large currents - in which case the diodes will be suitably rated for those currents. So generally it shouldn't be a problem, unless you're using very over-sized capacitors, and pushing your diodes rather hard.

 

[GaryA]

Calculate the peak current in the discharge - that's what the diode sees and that is what might be troublesome. Diodes can usually withstand much higher than rated currents for a sufficiently short time - see surge current rating in the diode specs. The peak current will heavily depend on the rate of discharge as well as series path resistance (both intentional & parasitic).

I think this means the real question I am asking is how long can I overstress the diodes for before they give up? I say this because the peak current would stay the same if I increase capacitance, but the discharge would take longer. I have looked at diode data sheets and surge currents are stated at number of cycles at a set frequency (normally 60Hz). A sudden discharge would be like a one off event, so how can I use this information?

 

[Hero999]

What diodes are you using?

Information about the maximum surge current of the 1N4001 can be found on the dataheet which can be found from Google.