Can the Y1 cap be replaced with a Y2 caps in refernce design EPR-85

Posted by: mixignal on 08/20/2014

Hi,

I am looking to design a low power AC/DC converter into a LCD application I am developing.

The reference design EPR-85, an engineering prototype report for a 2W charger using a Linkswitch-LP (LNK564P) seems a good fit for me.

I noticed in the ref. design the Y1 capacitor (C4 in Figure 3. Pg-7) is a HV surge capacitor with a failure rating of 8 kV (Y1). This capacitor is connected between the two isolated ground to keep the common-mode noise to the minimum which is a sensitive issue for the LCD application.

My question is: since most surge standards(eg. IEC 1000-4-5 ) for AC/DC mains application are only upto 2kV, what is the reason for choosing a 8kV surge capacitor ? Can I choose a Y2 cap which is compliant upto 5 kV ?

Thanks in advance for all your help.

-SR

Comments

Hi,

Yes, you could use Y2 capacitor if the requirement is less.

Please refer to the PI application note for the safety requirement:

http://www.powerint.com/sites/default/files/product-docs/an15.pdf

Thank you very much for the app note reference. That was immensely helpful.

Looks I can use the Y2 caps (two in series) at the expense of higher cost and higher count.

Hi,

Thanks again for your excellent app note reference AN15.

I had another question regarding the Y-caps.

On page-9 of the app note (AN15) it is mentioned that the maximum value of the Y-caps should be less than 2.8nF to limit the leakage current at 250VAC/60Hz to 250uA.

If I have two Y-caps is parallel so do they need to pass the leakage test in parallel or seprately? In other words, can I have two 2.8nF caps or 1.4nF each ?

Thanks in advance.

Hi,

If you have the leakage requirement, then the total Y capacitor value should be less than 2.8nF.

For example, if you put two 2.2nF capacitors in parallel, then the total value of these two capacitor is 4.4nF which is over 2.8nF. The it is not able to meet the leakage current requirement.