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.
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