AVR-Chat

On Nov 18, 2008, at 8:01 PM, magzky02 wrote:

>
> 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?
>
>
> 
That means LOTs  of heat! Suppose the input is 80V, output 50V, load  
current 500ma. Voltage difference is 30V. Power dissipation in the  
linear pass device is 30V*0.5A=15W. That, friend, is a LOT of power  
and a LOT of heat.  Suppose output is 20V and current is 500ma.  
Voltage drop is 60V. Dissipated power in a linear pass device is 30W.  
That is even more than a LOT of heat.

Either you will need a VERY big heat sink or you will need a high  
efficiency switch-mode regulator.

Jim



[Non-text portions of this message have been removed]