Y capacitor connection in TOPswitch?
Hi, Page 21 of the TOP264 datasheet says
QUOTE>>>The Y capacitor should be connected close to the secondary output return pin(s) and the positive primary DC input pin of the transformer. If the Y capacitor is returned to the negative end of the input bulk capacitor (rather than the positive end) a dedicated trace must be used to make this connection. This is to “steer” leakage currents away from the SOURCE pins in case of a common-mode surge event.<<<UNQUOTE
….this surely doesn’t add up?.....
The Y capacitor should be connected to the quiet node of the primary, and the quiet node of the secondary, and the reason is so that the switching currents which went through the transformer pri_to_sec capacitance can return to primary side.
A common mode surge is between line and earth…..this would not have any effect on the isolation gap y capacitor, because the isolated secondary may well not be connected to earth. Surely you agree?
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thanks...ayk, you cannot (should not) connect y cap to the bulk pri cap negative, because this does not connect directly to the transformer winding.
Hi,
As you have mentioned, Y-cap should be connected to quiet nodes of primary and secondary. Because of the high capacitance of bulk capacitors, it can easily absorb noise. So you can consider both Bulk+ and Bulk- as quiet nodes. Having said that you can try both and select which one gives better results in EMI and Surge.
In some cases, yes I agree. But for some applications where the output return is connected to the PSU chassis, then on a common mode surge the isolation y-cap capacitor will see the line-to-earth surge.