AVR-Chat

Hello guys, I hope to get inputs. i am doing a a power supply card. 
input is 80V, output is variable from 0-76V (up to 500mA). i want my 
AVR to set the ouput voltage. i came accross high voltage variable 
regulators such as LM5008(national) and TL783 (TI). But outputs are 
controlled by variable resistors. Still thinking how to adjust it 
using MCU. Also im thinking of using a FET. Then using avr, adc, and 
opamp to set bias voltage on the gate terminal. But again problem 
arise when for example the output is set to 10V, means 70V drop across 
the FET. This will generate much heat. Anyone?