a Constant Power consumption design

[Shahriar]

Hi

I have a SMPS that its output varies from 4V up to 32V. As you know at the output of most SMPS there must be a Load to Dissipate some power. For my SMPS, it should be at least 2-3W Because this SMPS is to be used as a Laboratory Supply, I have to put a load at the output. I can not use a single resistor.
i.e.

Vi = 4V  & R=8ohm    ---> P=V^2 / R = 2W

But if  Vi = 32V then P= V^2 / R =  128W 

So I have to use a constant power consumption circuit. I designed one but it dissipate 8W at Vi=18V and dissipate 2-3W at Vi =32V and dissipate 3W when Vi=4V.
I have not build it yet. These writing are from my calculates.
This Non-Linearity is because  P is proportional to Vi^2.

Does anybody have a good Idea?
The schematic is attached
My numbers are not very Sharp

ThanX
Shahriar

ConstantPower.pdf

[audioguru]

Hi Shahriar,
I don't like to see your zener diode starved of current when the supply voltage is only 4V, and cooking when the supply voltage is 32V, so I tried it with an LM317 regulator adjusted to 1.25V. I used a resistor on its output to hold its output voltage down. Then I added a common emitter transistor to reduce the load voltage when the supply voltage increased.
I didn't have the correct resistor ratios so it didn't work.

I'm sorry I don't have enough time now to sketch it nor re-calculate it but I hope you know what I mean. Try it, I think it will give a constant power load. ;D

[ante]

A MOSFET glued to an NTC (and o few other parts) will draw a constant wattage if the room temperature is reasonably constant. 8)