Ripple current rating of SMPS input electrolytic caps. 3

[Fluorescence]

Hello,

I am trying to work out the RMS ripple current in the DC Electrolytic Caps that occur just after the diode bridge rectifier in my 10W offline SMPS.

I apologise for the lengthiness of this post.

It is a one transistor forward converter (Mains is 240V, 50Hz).

Here is the representation of these caps…

C BUS CAPS:

The capacitance has been kept low so as to improve the diode bridge conduction time and hence improve the power factor.

The capacitance is only 1.47Uf (with total ESR of 180R).
(Jamicon TK electrolytic caps)

Here is the voltage waveform on these caps….

:CAPACITOR'S VOLTAGE WAVEFORM:

Is it right that……

1. During ‘A’ the caps alone supply current to the SMPS ?

2. During ‘C’ the mains alone supplies the SMPS current since the caps are being charged up ?

3. During ‘D’ both the mains and the caps supply current to the SMPS, but since the caps have an ESR of 180R, the mains will supply the majority of the current during this period ‘D’ ?


Is this correct for how the current in the capacitors would look like ?….......

:CAPACITOR CURRENT WAVEFORM:

If it is correct, how can I calculate the RMS value of this current ?….i.e. the ripple current in these caps ?


I am thinking that over several cycles, the RMS current going into the caps must equal the RMS current coming out of the caps (?)

Therefore, if I simply calculate the rms value of the current going into the caps (the charging current in period ‘C’), then double this value…….does that then give the total RMS ripple current experienced by the caps ?

Most grateful for your reading and any thoughts at all greatly appreciated.

 

[Skogsgurra]

I think that the Linear Technology Spice simulator could help you out here.

One thing, the constant RMS is not true. It is charge that is same during charge and discharge. So use As+ = As- instead.

But, as I said, the LT Spice is just right for you. And free! Good luck - like your approach.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricuwe]

You will not be able to find a suitable electrolytics capacitor that provides acceptable lifetime. On typical feature of electrolytics is quite low RMS-current per µF and in many applications you have to accept oversizing the capacitance.

 

[ScottyUK]

You might find this tech note of use:

http://www.evoxrifa.com/electrolytic_cat/electrolytic_appguide.pdf

Rifa have a few other tech notes which might be useful to you.

http://www.evoxrifa.com

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If we learn from our mistakes I'm getting a great education!

 

[Fluorescence]

Thankyou to all,

I did a mock-up of my circuit with LT Spice (using LT1241 instead) and the RMS capacitor current came out as 74mA. I am taking it that this is the RMS ripple current.

That's a great program and i am going to see how to just calculate the RMS over one mains cycle instead of several as it took 12 minutes to calculate outputs over the first 60ms after "switch-on".

Do you know if there's a support website for this LT Spice?