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Y1 Capacitor in SMPS

Posted by: RVA15 on
Dear Sir/ Maddam, What is the purpose of the Y1 capacitor that connects the ground from the primary and secondary side of the transformer in SMPS? This capacitor can be seen in figure 11 of the document lnk500.pdf. Is this capacitor optional? Regards, Ricardo

Comments

Submitted by PI_Crusher on 10/15/2008

Hello Ricardo,

One general reason to use this capacitor is to minimize the amount of noise transferred between secondary and primary side. For this case, the use of this Y1 capacitor makes sense for AC powered devices, or for special applications where the HV DC source must be protected against noise.

The second benefit is for so called ESD/Surge protection (sparks and/or voltage pulses). Without this capacitor, the whole high speed and high voltage stress is applied directly to the transformer isolation. The transformer must be designed very carefully with triple insulated wire and very big spaces between exposed conductive points. The Y capacitor connected between secondary and primary GND provide a lower impedance path to short-circuit voltage pulses directly to GND.

For low power PI products, you will see many times the buzz-words “noise cancellation techniques”. Our power products combined with special transformer construction are able to comply with EMI regulatory requests without the use of this expensive Y capacitor. What you need to do is to use heavy insolated wire for the primary side, and triple isolated wire for the secondary side (ESD/Surge protection). Only for extreme cases with higher power levels you must use that capacitor for EMI compliance.

Cheers,
PI_Crusher

Submitted by FD_cklee on 06/29/2009

Hi PI_Crusher,

I saw the post about the benefit of using Y1 capacitor. It is indeed a very useful information.

But I'm still not sure if the Y1 is really necessary when it comes with safety issue.

I'm using a SMPS for a medical device which needs to pass IEC60601-1. Indeed, the isolation issue lies on the use of Y capacitor on the SMPS. The IEC60601-1 explicitly states that the use of Y capacitor only provide ONE MOPP (means of patient protection) and actually, TWO MOPP is required. In short, using Y capacitor, you never archieve the isolation requirement.

I was told to remove the Y cap and say the whole issue can be solved.

My question is whether this Y cap is needed as per the IEC60950-1 or IEC60601-1. Or it will affect the EMC testing?

ck