TNY256 - what is that "Y1 Safety" capacitor for ?

[nowhere]

Hello,

I stumbled upon TNY256 chip: http://www.powerint.com/sites/defaul...ocs/tny256.pdf .

The chip is apparently used in many of those small switching power supplies plugged directly into electrical outlet.

On page 7 of the datasheet I see "Figure 12. 5.5 W AC Adapter with Universal Input (85-265 VAC)." - typical application circuit. And on page 8 "Figure 13. Recommended PC Layout for TinySwitch without Under-Voltage Lock Out Resistor.".

I don't understand why/what for "C5 2200 pF Y1 Safety" on page 7 is needed. I.e. I don't understand why capacitive connection between primary and secondary network is needed, especially taking into account that there is already parasitic connection through windings of T1 transformer.

Or that's the purpose - to protect from T1 parasitic capacitance of from breakdown, i.e. from possible electrical discharge going through insulation between primary and secondary winding of T1 ?

[wrenchone]

Its to reduce common mode conducted/radiated EMI.

[nowhere]

Originally Posted by wrenchone

Its to reduce common mode conducted/radiated EMI.

Your reply doesn't explain the word "safety". The word is typically used in conjunction with implied danger, and what is that implied danger is the thing I want to find out.

[DF96]

The text says it is part of an EMI filter. Not sure how this works, but as the cap connects the output to live mains it has to be a safety cap.

[nowhere]

Originally Posted by DF96

The text says it is part of an EMI filter. Not sure how this works, but as the cap connects the output to live mains it has to be a safety cap.

Yes and no. The "no" part is that EMI most of the times is mentioned in conjunction with intentionally introduced frequency jitter and just one time in conjunction with the capacitor in question.

[wrenchone]

The cap has to be rated for safety - an ordinary disc cap is not OK, even if it's rated for high voltage. In this case, a Y1 rating is needed, meaning that it is certified by the various safety agencies for to safely bridge the primary-secondary barrier and is 100% tested for a high withstand voltage. You can find out more about safety and EMI issues by checking out AN-15 at the PI web site. And yes, iit's an EMI filter cap and is instrumental in reducing common mode EMI. The transfomer parasitic capacitance from primary to secondary actually is the cause of a lot of the common mode EMI, the transformer primary being a prinncipal source of noise. The cap forms a short return path from secondary return to primary. Proper transformer construction is also instrumental in reducing common mode EMI.

[DF96]

The jitter spreads out the interference, thus getting it under the required level set by the relevant EMC standard. This seems to be the modern way of 'reducing' interference - just spread it out so more spectrum users get a bit less interference each. I think this could be regarded as an example of malicious compliance. Spark transmitters are banned, yet jittered SMPS have a similar effect and are accepted.

[wrenchone]

Jitter only gives you about a 5-6dB reduction in EMI at best. It can help to reduce the size of some EMI components (assuming layout and everything else is good, but it does not render an EMI filter unnecessary.

[Antoinel]

The engineer(s) who designed this circuit will be the best judges on the value of said capacitor. Is it for health safety [connected between RTN and power line] and/or EMI reduction?.

[wrenchone]

Excuse me, I am one of the engineers...The capacitor is in the circuit for the purpose of EMI reduction, and needs to be proply safety rated (Y1) because it bridges the primary-to-secondary barrier. It has no safety benefit. It one of the first things to add for common mode noise reduction.