Surge Current

crutschow

Joined Mar 14, 2008
27,241
The single cycle surge current generally has no limits other that that determined by the ESR. There is a limit for ripple current which is given in the data sheet.
 

studiot

Joined Nov 9, 2007
4,998
Surge currents of all types are normally empirically determined and rated by the manufacturers after extensive testing.
 

Thread Starter

gusmas

Joined Sep 27, 2008
239
Well in my case the surge current is not provided... and i need to know the max current the cap can handle due to transients at startup. any ideas
 

studiot

Joined Nov 9, 2007
4,998
Well you will just have to do what they do in Texas.

Some experiments on a bunch of samples.

Of course you can increase the longevity of the samples by starting with a limiting resistor/choke and slowly reducing its value and noting the effect.
 

crutschow

Joined Mar 14, 2008
27,241
Surge currents of all types are normally empirically determined and rated by the manufacturers after extensive testing.
Capacitors are usually immune to single-cycle surge currents. The one exception I know of is solid tantalum capacitors which may have to have a small resistor added in series to limit surge current.
 

richard.cs

Joined Mar 3, 2012
162
It's not an issue for most applications but high voltage pulse discharge damages capacitors of rolled foil construction. The roll acts as a transmission line to the high frequency pulse and a high voltage is developed at the ends of the roll which behave as open-circuit stubs, this then punches through the insulation. So if you build a can-crusher with microwave oven capacitors expect problems.
 

#12

Joined Nov 30, 2010
18,222
Choosing not to give us any information about the amount of voltage, current, time, application, or frequency only gets a lot of guesses. If that's what you want, it's OK, but we always worry that you might want a good answer that is applicable to the situation.
 

richard.cs

Joined Mar 3, 2012
162
Not really related to the origonal question but one of the interesting things about the datasheet given is the graph on page 10 which implies you can get a 5000 hour life at 3.5x the rated ripple current so long as you keep the ambient temperature down. It makes sense but it's not something I'd ever considered.
 

studiot

Joined Nov 9, 2007
4,998
Yes, heat is the enemy of most (if not all) components and capacitors are no exception.
So if you can control internally generated heat and the external environment you are on to a winner.
 

Thread Starter

gusmas

Joined Sep 27, 2008
239
Choosing not to give us any information about the amount of voltage, current, time, application, or frequency only gets a lot of guesses. If that's what you want, it's OK, but we always worry that you might want a good answer that is applicable to the situation.
I apologize:

Steady State voltage: 50V


The current you can see in the attachment :).
 

Attachments

studiot

Joined Nov 9, 2007
4,998
If your capacitor is part of a capacitor bank, there is a great deal of manufacturers published information about sizing and rating of components, available via Google.
 

JMac3108

Joined Aug 16, 2010
348
Richard,

Actually that is not unreasonable and is often taken into account when specifying choosing a capacitor for a power application. Remember, the reason for having a ripple current limit is power dissipation in the capacitor's ESR. If the ambient temperature is lower, then you can dissipate more power before having a thermal problem, thus you can allow more ripple current.
 

Thread Starter

gusmas

Joined Sep 27, 2008
239
Richard,

Actually that is not unreasonable and is often taken into account when specifying choosing a capacitor for a power application. Remember, the reason for having a ripple current limit is power dissipation in the capacitor's ESR. If the ambient temperature is lower, then you can dissipate more power before having a thermal problem, thus you can allow more ripple current.
ye i get that.. thing is the ripple current from the datasheet for my output cap is 36A and from simulation it can be seen that the transient hits around 300A.
 

The Electrician

Joined Oct 9, 2007
2,834
ye i get that.. thing is the ripple current from the datasheet for my output cap is 36A and from simulation it can be seen that the transient hits around 300A.
Does this only happen when you first power up the converter? How many times a day will this happen?

What does your simulation show as the peak current if you add 10 mΩ in series with your capacitor? 20 mΩ?
 
Last edited:
Top