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Colin Wonfor

Dear Colin, Solid organic polymer capacitors such as the Sanyo Oscon series have a very good reputation for digital decoupling and I'd like to use some when refurbishing an old CD player of mine as I've got plenty in my parts stash. Before I go fitting any I've got a couple of questions: 

1)   A typical situation might be decoupling the power supply pins of a digital IC. Let's say the original capacitors are a 22uF electrolytic and a 100nF ceramic bypass closest to the pin. If using an Oscon should the ceramic bypass be kept or removed?

Keep it in as at high frequency its impedance will be lower that the ESR of the Oscon.

I've read some people advising that the small ceramic cap should be removed as it would have a tendency to resonate with the Oscon. Obviously if the ceramic was removed the Oscon could be moved to the position previously occupied by the ceramic closest to the power pin.

Firstly I doubt that cap is resonating with the Oscon both being passive devices, secondly fit a X7R or a X5R type. Not COG or NPO. Now what does happen due to the ESR being very low and on some regulators the inrush current to the cap is to high cause internal current shut down only to restart and then again shut down, some designer get over this problem with a current limiting resistor but this defeat the object of having a low ESR Cap. I have tried choke but again small one tend to have high DC resistance, leaving the track long sometime works but again defeating the object. So we have to compromise and select a) The right PSU device and b) the right cap fun this often comes with long term experience.

2)   I've seen several examples where people using Oscons in place of a standard electrolytic significantly reduce the capacitance value. So for example the original electrolytic may be 100uF and the Oscon they select as a replacement is only 47uF or smaller. Why the reduction in capacitance? Are there any sensible rules of thumb for this? Thanks, Mike.

Mike at 50Hz / 100Hz energy must be stored so we need are large value to fill in the gaps in the ripple, but most cheap caps have very poor ESR at any current now if they are not used to store energy but to decouple noise etc. we need low ESR because the impedance at the frequency we are trying to depress the impedance of the cap will be lower than the ESR, but due to the ESR we can never achieve that impedance. No I love these caps big mean and good.

So as you can see a 20uF at 50V has a ESR less than 3mR. Nice

Thank you for the question.



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