how smps starts when power on signal comes from ATX

[makarand1907]

can any1 explain me how smps starts after active low signal on power_on pin of atx? wat actualy happens inside smps???
intel motherboard guide states that if we press the poweron switch for less than 5 seconds,it turns on.if we press for more than 5 seconds,it turns off.how this works?

 

[betwixt]

On a mother board, there are several power inputs. One of them is always on so the SMPS has at least one output which is present whenever the AC power is applied. Usually this is a very low power 3.3V or 5V output and on the mother board it helps to keep the real-time clock running and also powers a circuit to monitor the front panel switch. When the switch is pressed it produces a signal sent back to the SMPS to tell it to switch on it's main outputs.

Really, the front panel switch should be called 'standby' rather than power as it doesn't control ALL the PSU outputs. Note that the power control is entirely hardware controlled, with no main supplies it is not possible for software to run and monitor the switch. You may also notice that the switch still works on dead mother boards for the same reason.

Brian.