Quote Originally Posted by G H Z View Post
So when we do voltmods we are changing the way the controller signals the switching of the FETS? Which makes a FET heat up more, faster or slower switching? What makes higher/lower voltage - faster or slower switching? What do the SMD's (the one's we often mod) typically do in the circut?

Excellent post Johnny
Thanks G H Z :cheers:

To answer questions:

When you vmod for more voltage or control over the voltage you are simply changing the feedback voltage. This is a very simple circuit, just two resistors in series between the actual output voltage and ground. The values of the two resistors defines the setpoint voltage that is measured between the resistors (see the site below)

http://www.electronics2000.co.uk/calc/calcdiv.htm

The vmod basically soldering in parallel with the lower resistor, which means that the lower that variable resistor goes the lower the voltage at V2 (for the image in the link) is. Now if the perceived voltage is lower then the control circuit will attempt to increase the output voltage to compensate, for example:

we set 1.5volts in the bios -> controller set 1.5volts and feedback loop shows i.5volts

we add a vmod so that the feedback loop now sees 1.35volts (10% reduction) so 1.5v in BIOS controller attempts to set 1.5volts but feed back says its 1.35 volts so controller ups the output to 1.65 volts so that the feedback loop now shows 1.5volts, as is the requirement in the BIOS.

Now that is various simplifications in there, things like OVP and feedback stability have been ignored but I hope you get a general feel for whats going on

Technically, in regards to your comments on switching its to do with how long the FETs are either on or off for a given time, called the duty cycle. Ideally you want to be switching as fast as you can to limit ripple somewhat ( although the act of switching will inherently create spikes in the output that will be dealt with according to how good your smoothing circuit is tuned) but the bigger the voltage the longer the switches will have to be "ON" for a given cycle.

While this will have something to do with the increase in heat I'd personally say its because the cpu or whatever you're supplying is drawing more current because you're running it faster.