When to and when not to use a Poly?

[Rangefinder]

The use of Poly is interesting. One Question bothers me though. When is the substitution of a Poly is acceptable and when it is not. That is in which circuits does it work and which does it not or is to be avoided. Second Q: what if one replaces a cap with exactly the same value in Poly?
No value adjustments.

[c_hegge]

Re: When to and when not to use a Poly?

Q1. Polys can almost always be used in CPU and GPU VRMs, ie. on graphics cards and for the motherboard caps near the CPU. For the other caps on a motherboard, it will depend on the board. Some motherboards will handle it better than others. NEVER poly-mod a power supply

Q2. If you are going to poly mod the other non-VRM caps on a motherboard, the capacitance should always be kept the same. The half-capacitance rule only applies to VRMs. IDK what would happen if you kept the capacitance the same there, but generally a poly of the same value as an electrolytic will be physically wider and may not fit and some common values for electrolytics used in VRMs (eg, 1500uF 16v, 3300uF 6.3v) aren't available in polys.

[PCBONEZ]

Re: When to and when not to use a Poly?

If you are wanting a simple answer that fits all situations there isn't one.

The topics have been covered here multiple times [use the search feature] but I understand why it's confusing.
I don't have time today for the whole spiel but here's the basics.

Caps do things with DC and AC simultaneously using the same [other] parts in the circuit.
The cap has to keep the AC circuit -and- the DC circuit 'happy' at the same time which means you have to look at what a prospective poly will do twice, once for the AC and once for the DC.

Caps acting as filters for ripple from switching are affected primarily by the cap's ESR and any inductors in the circuit.
Caps acting as 'smoothers' [compensating for DC fluctuations: aka 'transients', which are -way- lower frequency changes than switching ripple] are affected primarily by the uF. In this case uF is 'storage capacity'.
Caps acting as EMI filters [much higher frequency than switching ripple] are affected primarily by the uF and any inductors in the circuit.

You have to figure out which of those 3 jobs the cap is responsible for [and it may be all three although usually just two] and do the circuit analysis for each job to see if a poly will work.

There are situations where you should not:
- change uF
- change [even lower] ESR drastically
Thus you have to do the circuit analysis and electronics math for each cap to know if a poly is 'okay' there.

CPU VRMs are a well known mod, already been worked out many times, and the situation doesn't change much from board to board.
Keep total ESR the same or less than original.
[For skt 423, 478, 775, 771] Keep total uF in Vcore over 5000uF [for each CPU] and more is better.
Keep total uF on +12v side over 1320uF [4x330uF or 3x470uF poly are good there.]

What other caps do changes MUCH from one board to another and even from cap to cap on the same board so for those break out your scratch paper, calculator and pencils and start tracing and drawing out circuits to figure out what they do and hence what matters there.

You will need to know this eventually.
https://www.badcaps.net/forum/showpo...4&postcount=25
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