lpfpower supply
I am designing a small DAC and I want to clean the power supply (its noisy) I used a PI filter.
However after the PI filter , I need an electrolytic capacitor (330uf) to help with some transients. The problem is that the output of PI filter has a capacitor that its in direct correlation with the L…so by adding this electro capacitor I will change that correlation…so any pointers on what to do ?
Define your maximum ripple voltage: ΔV (also called dV, V ripple). Look at image below and I guess you should figure it out.
Use this formula to determine minimum capacitor capacity.
\$C = \frac{I}{Vpp * 2f}\$
where: C is capacitor capacity in Farads (1F = 1000000uF), I - current, Vpp (or delta V) - voltage change on capacitor when it is being discharged, f - frequency before bridge
This method can be found in wikipedia article about ripple, just in diffrent form (formula on wikipedia calculates ripple at given capacitor).
Image source: wikipedia article mentioned above + me
Use capacitor with voltage rating at least a bit higher than your transformer amplitude, not RMS voltage. Amplitude for sinuoid after full bridge is:
\$Vpp = Vrms * \sqrt{2}\$
In your case it will be about 17,8V, so you cant use 16V capacitor.
Also notice, that transformer open circuit (no load) voltage may be much higher than nominal.
A 39000uF cap and a transformer winding is way too much for a filter.
I suspect your power supply is either defective, poorly-designed, or being operated way out of spec. The spikes are happening at a rate of ~85-90 kHz, which could be the switching frequency. The higher-frequency ringing afterward is clearly due to the spikes. If you can zoom in on the spikes with your scope, it might tell you (and us) more. A link to the Amazon page or a datasheet would also be helpful.
Regardless, your options are:
The Photon's suggestion of trying a larger load is also a good one.
Best Answer
The extra capacitance will obviously change the characteristics (i.e., frequency response) of your Pi filter, but the key is, it won't make it worse for your application in any way. Go ahead and hook it up.