[ARC5] Prelim comments on a DC - DC B+ converter--capacitor ripple currents

[Scott Robinson]

Folks,

Bob is absolutely right about the importance of ripple current ratings 
in electrolytic capacitors. If you can find a manufacturer's part number 
on the cap (example: Nichicon UVR2W220MHD) you can go to the 
manufacturer's site and look up the ripple current rating. For the 
example given (22 uF 450V) it's 165 mA at 120 Hz and 1.6 times that or 
264 mA at anything above 10 KHz.  In a basic full wave rectifier, the 
ripple current is generally about 2-3 times the DC load. Depending on 
the configuratoin of hte SMPS, the ratio of ripple current to DC load 
can be higher; flybacks are the worst in this respect, and beingcheap, 
are very common.

I use Nichicon as an example, because 1) their parts are very good, and 
2) they are representative of the general run of such parts The UVR 
series is a standard kind of part.

The ripple current is a thermal rating; the heat is generated by the 
ripple current heating the equivalent series resistance of the cap. Keep 
things cool and ventilated and it's all better; thus, shielding done 
with perforated metal r copper window screen is better than a solid box.

When you see several caps in parallel at the output of a SMPS, it's 
usually not for lower ripple but for increased ripple current capability.

As a parting shot, I'll warn against using modern 10 uF 450V 
electrolytics as the input filter caps  on a tube radio. These parts are 
very small, and can't dissipate much heat. The first cap after the 
rectifier gets a lot of ripple current (say, 2.5 x 90 mA DC drain) but 
the cap is rated at only 90 mA. Larger caps (more capacitance) beat up 
the rectifier tube with higher peak currents, so that isn't good. What I 
do is buy 10 uF 600V mylars at about $5 each. Example: Kemet 
R463W510050M1K , available from Mouser for $4.79

FWIW,

Scott Robinson

On 4/18/18 6:34 AM, Bob kb8tq wrote:
> Hi
> 
> Check the capacitors on the board. They (obviously) have a voltage rating. They also
> these days have a current rating. In high frequency switcher duty, current in the caps
> can be a fairly big deal …. finding that information may not be easy at all. In the case
> of voltage, I’d be a lot happier at 80% of the marked numbers than I would be at a
> higher level.
> 
> Bob
>