Electronic – How safety capacitor replaces optocoupler


I see some phone chargers using safety capacitor (after some research I found its safety capacitor)But not optocoupler to isolate in and out . How safety capacitors replace the optocouplerenter image description here

Best Answer

It generally does not replace the function of an optocoupler, it's to control EMI by capacitively coupling the output ground with the input DC ground (after filter and rectifier etc.).

It should be a "Y" type safety capacitor. Usually it's around 1nF. For example, this one (from a Power Integrations datasheet):

enter image description here

The optocoupler function (when present) is to isolate the output voltage error (as a current) and send it to the controller which runs from the mains. If there is no optoisolator it may be using a transformer winding as feedback, which can work, but it's not as accurate.