Y-capacitor

Posted by: theswissguy on 02/18/2009

In the very well-written "AN-40", page 7, the following paragraph seems not so clear to us: "Designs using a Y-capacitor require the EMI filter impedance to be placed on the appropriate side of the input. Therefore when Y capacitor is returned to the DC rail, the fusible resistor(s)/Filterfuse should be placed in the opposite side of the input". Any interpretation of this paragraph would be highly appreciated.

Comments

Hello theswissguy,

Sorry for this paragraph. All you have to do is replace “impedance” with “component”. You will read “Designs using a Y-capacitor require the EMI filter component to be placed on the appropriate side of the input.” Check attached picture.

Cheers,
PI_Crusher

Dear PI_crusher, Thanks for the response. However, the small sketch that you included makes us a bit puzzled: in all application circuits that you have about PI-devices, the Y-cap is connected on the SAME side as the inductor, contrary to your statement. Please refer to the attached sketch. Your comments are highly appreciated.

Hello theswissguy,

I agree with you, some of our older applications are with the Y capacitor connected on the opposite side. All our designs are tested for EMI compliance and we confirmed some EMI improvement if this capacitor is connected to the AC line without filter components. Most of our low power applications are not using Y capacitors anyway, and the newer schematics are with the change we are discussing here.

Cheers,
PI_Crusher