Frequency of ripple current in Electrolytic capacitor....and resultant dissipation

[spec]

Thanks, though if it was a constant current recharging every second, then that would be a high ESR, as the frequency of the current would be very low.

Ah now we are getting to the core of your concern.
A capacitor has ESR (effective series resistance) and SR (series resistance). SR is normally lower than ESR.

The dissipation with a constant current (Ik) and low dV/dt (rate of voltage change) will be Ik^2 * SR

Incidentally, as you no doubt know, the rate of voltage change is, dV/dt = C * I

Where

V: Voltage across the capacitor in Volts.
C: Capacitance in Farads.
I: Current flowing into the capacitor in Amps.

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[spec]

the cap gets charged up from 60v to 300v.......it then powers the xenon for about 1ms, and tumbles down to 60v again

All we need now is the actual capacitor value and preferably a data sheet or reference to one. Or is that information 'commercial in confidence'?

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[Flyback]

thanks, i cant hide this, as its common knowledge..the F&T range of xenon flash caps are the ones.
https://www.ftcap.de/fileadmin/user.../webboxen/Fotoblitz_kondensatoren/E_Types.pdf

360v, 1600uF are the ones

 

[Flyback]

But you will probably benefit from connecting a large polypropylene capacitor across the electrolytic capacitor.

thanks but we dont have room for a big one...do you reckon putting say a 1uF film or ceramic cap across the 'lytic cap will make the 'lytic cap run much cooler?

 

[spec]

thanks but we dont have room for a big one...do you reckon putting say a 1uF film or ceramic cap across the 'lytic cap will make the 'lytic cap run much cooler?

Quite possibly. But go for a polypropylene high current capacitor rather than polyester which have no where near the pulse performance. A ceramic capacitor would be good but would not gain that much.

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[spec]

thanks, i cant hide this, as its common knowledge..the F&T range of xenon flash caps are the ones.
https://www.ftcap.de/fileadmin/user.../webboxen/Fotoblitz_kondensatoren/E_Types.pdf

360v, 1600uF are the ones

Hi FB,

There is not much information on that data sheet- it is more like a product listing. Have you tried contacting the manufacturer for information on ESR etc.

At random this data sheet gives more information, but even so it is not comprehensive: https://www.mouser.com/ds/2/88/CGS-12194.pdf

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[spec]

Hi again FB,

As I now understand it, this is your system.
(1) A 1,600uF capacitor charges from 60V to 300V (dV of 240V) in 1 second
(2) A flash bulb triggers and conducts for 1 mili Sec and reduces the capacitor voltage to 60V
(3) The capacitor charges and so the cycle continues.

You have a requirement to minimize the loss in the capacitor and the only area that you have any control of is the charging of the capacitor. The flash discharge just happens in 1 mili Sec as far as you are concerned.

With the above in mind, the minimum dissipation in the capacitor and the least stress on the capacitor would be achieved by charging the capacitor with a constant current of 384 mili Amps, which will produce a linear voltage ramp across the capacitor from 60V to 300V in 1 Sec.

The formula for deriving the required constant charging current (Ik) is:

Ik = (C * dV)/t ...(f1)

Where,
Ik: Constant charge current in Amps
C: Capacitance in Farads
dV: Voltage difference in Volts
t: Time in seconds

Thus from (f1),

Ik = (1.6 * 10^-3 * 240)/1 = 384 mili Amps

Once the above is done, the only other way to further reduce the dissipation in the capacitor would be to extend the interval between firing the flash.

May I suggest that, to save time, you describe to the capacitor manufacturer the above and ask if their capacitor would be up to the job.

In terms of a practical implementation, a current limiting fly-back converter would not be that far off the ideal either.

I am sorry and glad to say that there has been an incorrect assumption about the waveform that a fly-back converter would produce when charging the capacitor. In fact, in one second, a 100K Hz fly-back converter will produce 100K minute voltage steps, that will be little removed from the ideal linear ramp generated by a pure constant current.

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