# SMPS: use or not to use high freq. coef. multiplier for cap?

Discussion in 'The Projects Forum' started by techieguy, Oct 2, 2013.

1. ### techieguy Thread Starter New Member

Oct 2, 2013
3
0
Hello guys,

I'm looking for some help a couple days. I have the waveform attached, this is the ripple current (ac current) through a electrolytic cap. This cap in located on the output of a power factor correction circuit at the input stage (after the full wave rectifier bridge). There is a component frequency of 120Hz and switching noise of the PWM. The question is when choosing the cap, do I have to use frequency multiplier different that 1 to calculate the maximum ripple current?

Thank you for helping me.

Techie.

File size:
21.6 KB
Views:
25
2. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,799
The high frequency stuff will have a higher multiplier so if you are not up against the wall on cost or size I would just use the RMS of the waveform and the 120Hz number. If you need to squeeze the cost or size you would need to find the value at each of the 2 frequencies and use the 2 different multipliers.

3. ### techieguy Thread Starter New Member

Oct 2, 2013
3
0
Thank you ronv. This time I'm more worried with the reliability of the electrolytic cap. For high frequency the multiplier is about 1.5, for low frequency is 1. My questions was more to decide if this is a low/high frequency ripple. Did you see there a strong 120Hz harmonic in the signal? I think I will monitor its temperature as I don't have an equipment to analyze the harmonics.

Thanks,
techie.

4. ### ronv AAC Fanatic!

Nov 12, 2008
3,657
2,799
You could probably get pretty close by eye. Then measure. Here is the formula:

I= √(I1/C1)^2 + (I2/C2)^2

5. ### mik3 Senior Member

Feb 4, 2008
4,846
67
I think the 120Hz signal is just aliasing of the scope. Set the aquisition to peak instead of sample to verify.