Howto repair switch mode power supplies #2: Stand-by circuit and its functionality

In the second video in the series of repairing switch mode power supplies (SMPSs) we look at the importance and the functionality of the so called Stand-by circuit. Namely, the low power Stand-by circuit is responsible for generating the Stand-by voltage, which should be always generated by the power s
upply unit (PSU) as soon as your device is plugged into the power grid. This is because the stand-by
voltage is feeding those circuits (most of the time microcontrollers) which are checking the state of
your device and they are waiting for a turn-ON signal. After a microcontroller receives the turn-ON
signal, again the stand-by power is used to send the wake-up signal to the high power portion of the
Switch Mode Power Supply.
Therefore, whenever you try to repair a switch mode PSU, firs you have to make sure, that the Stand-by power section of the PSU is working properly, and it is providing the right voltages to the stand-by circuit. This is because if there is no stand-by power, the device will never able to turn on.
The Stand-by circuit it mostly formed by a small transformer, a medium-power FET transistor and an in
tegrated circuit, which is controlling the gate electrode of the FET.
Thus, if you are able to spot a small transformer on the printed circuit board of your switch mode PSU you are trying to repair, you can be almost sure that it is the stand-by transformer.

In this video I introduce the notion of pulse width modulation (PWM), but this is the most important
topic for switch mode PSUs, so I will cover it in the follow-up video.