Calculating input ripple voltage to SMPS

[king.oslo]

Hello everyone

I thought I would make a SMPS. After rectification of the AC voltage, I need to work out how much capacitance I must add.

I found this really nice formula which calculates the capacitance required to reduce to 10% ripple output. The website says this is a typical requirement:

Code: [Select]

C = (5 × I_out) / (V_out × f)

// source: [url=http://www.kpsec.freeuk.com/powersup.htm]www.kpsec.freeuk.com/powersup.htm[/url]

1. That is great, but I cannot use it directly to get values for 5% or 20% ripple. Which formula should I use to find this?
2. When looking at the datasheet of switching controllers; what is maximum input ripple usually called? I can only find V_in: 2V-26V.

Thank you.

Kind regards,
Marius

[amspire]

Lets go to basics. Worst case, the capacitor is charging for almost zero time, and discharging for the rest of the time

The half cycle period of the mains  is  1/2 x 1 / f.

I = C x dV/dT  (This is the standard formula relating current to rate of change of voltage for a capacitor)

So worse case, the capacitor will discharge by  dV = (I / C) x dT = I /( C * 2 * f) if you are using a full wave rectifier.

Rewriting this, C = I / (dV * 2 * f)

dV = V_out x R_f (where R_f = the ripple factor.  10% ripple equals a ripple factor of 0.1)

so C = I / ( V_out * R_f * 2 * f )

Put in 0.1 for R_f and it matches the equation you found which is good.

Richard

[amspire]

In terms of the datasheet, the amount of ripple will be determined by two things.

The minimum voltage in has to be above the minimum voltage the regulator needs to operate. So if you had a 12V buck regulator, then the regulator and switching device voltage drops may mean you want at very minimum 13V in.

The second thing is there is probably an input rejection ratio spec'ed for the regulator. If it was 40dB, and you wanted the output ripple due to the input ripple to be less then 10mV, then you need the input ripple to be less then 1V p-p.

Richard.

[Psi]

You can also use something like LTspice to simulate it and see what the output looks like.

It can be a bit confusing using spice at first because there are quite a few options.
But once you have your circuit working you can try different value caps with different esr and see what sort of ripple you'll get.