what determines power rating?

[bruceyang]

I am trying to down tune the output voltage of a 90w 4.5amp 20v lenovo power supply.

If I down adjust it to 12v, what would be the power rating? I am hoping it stays at 90w and the output current can reach 6amp.

[dmill89]

Re: what determines power rating?

I am trying to down tune the output voltage of a 90w 4.5amp 20v lenovo power supply.

If I down adjust it to 12v, what would be the power rating? I am hoping it stays at 90w and the output current can reach 6amp.

The switching transistors and output rectifiers are rated in amps so the maximum amperage is can output is not going to increase with the same components in place (it is possible that the components are over-spec'ed and it can do more than 4.5A but what ever the true max amperage is will stay the same regardless of output voltage). 4.5A at 12V is approximately 55w.

[budm]

Re: what determines power rating?

If you try to draw 6A through the output rectifier diodes, since the Vf of the diode is basically constant, lets say 0.9V (you need to look at the spec) so at 4.5A it will dissipate about 4W, but if you draw 6A, then the power dissipation will be 5.4W, so the heatsink, the diode , the transformer winding, the capacitance of the output filter caps must be design to handle more current through the device. The primary will not be affected because it is designed to be able to supply 90W at the output side.