UCC28740: Capacitor between grounds (capacitive isolation)

Hello Catalin,

Thank you for your interest in the UCC28740 controller.

The capacitor C9 is often referred to as a "Y-Cap" and its purpose is to help shunt common-mode switching noise current from the output side of the transformer back to the input side. This noise current is generated when the high dv/dt edges of the primary switched-node voltage (on the MOSFET drain) couple across the primary-to secondary capacitances of the transformer windings.

By having a local bypass shunt capacitor between the primary and secondary grounds within the power supply, it avoids having the noise current travel down the output GND paths back to the primary through the building supply GND. That would be a tremendous conducted EMI problem. The Y-Cap shunts the path internally and considerably reduces (although does not eliminate) the conducted common-mode noise.

It's value is generally chosen empirically during prototype evaluation to select the lowest value necessary to meet the required EMI standard. Usually a place-holder part of arbitrary value (such as 1000pF or 2200pF or based on previous experience) is designed in to reserve the space and connection during pcb layout. Then the final value (along with other input line-filter component values) is determined during EMI evaluation testing.

This particular Y-cap value is rarely higher than 2200pF because it also allows a small amount of line-frequency current (50 or 60Hz) to flow from the line to the output, which a user may be able to touch. Safety standards usually limit this current to 5mA or less (depending on application and circumstances) and so the capacitor impedance at 120Hz (rectified line) must be high enough to limit this leakage current. Ordinary capacitors are not allowed to be used here. Special safety-rated Y-caps are designed for this and for line filtering purposes.

Regards,
Ulrich