power supply (cap input)

[jarthel]

some circuit uses a very small value cap as cap input. But when simulating this on PSUD2, it seems the voltage on the 1st cap is very unstable.

Is there a reason for using such a small value?

I was looking at diyparadiso and one of their designs uses .1uF.

Thank you for the help

[gingertube]

Jarthel,
The size of the input cap on a CLC filter is very important. With low values like 0.1uF the supply will act very much like an LC supply but with that 0.1uF at least providing a bit of high frequency (solid state diode switch noise for example) filtering.

In my latest amp (6V6 Ultralinear drawing about 170mA for 2 channels) I used 5V4 recifier, off a 370-0-370 secondary, then a small cap, a 5H 200mA choke to 100uF/450V electrolytic and from there a separate feed via 10 Ohms to 60uF polypropylene for each channel.

I found very small values gave a high output voltage (up around 360V) as I increased the cap the voltage dropped and provided a minimum (around 310V) with a value at around 1 to 1.5uF and then started to increase again with the voltage back at near the 360V mark with a 10uF cap. I used 2uF as the final value to give me 315 to 320V.

PSUDII modelled that behaviour very well.

I would hazard a guess that this is because with the smaller cap the rectifier (switcher), cap and inductor acts like a switching boost supply. With a larger cap the switching function does not occur or ooccurs to a lesser degree as the capacitors strored charge is then sufficient to maintain a current flow into the inductor when the rectifier switches off.

Does that make any sense?

Cheers,
Ian

Cheers,
Ian

[kopite]

hi Ian, I was just wondering if the increased voltage was due to the 1st small cap being a PP type with a very low esr? Using a larger electro with an increased esr gave me a lower voltage when simulating. Thanks,
Phil.