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RE: Power supply design for low power PC

It looks like Bubba2 takes 12V at 4A, that takes a different design, this on is only good up to about 2A. Conceptually its the same but has different part values. I'd have to run the numbers to come up with a good design for that.

There will always be heat dissipated in a linear regulator. The whole concept is that they are turning the excess voltage between the raw supply and the output into heat. How much heat depends on several factors: the current the load pulls, and the difference between the average voltage of the raw supply and the output voltage. That "average" in the above is critical.

What comes out of the raw supply is DC with some ripple. Each regulator design has what is called the drop out voltage, the raw supply must never go below the droput voltage. In the case of the LT1084, the drop out voltage is 1-1.5V depending on the current. So for a 12V reg the lowest point of the raw supply must be above 13.5V. So even if you had zero ripple coming out of the raw supply you would still have 1.5VX4A = 6watts to dissipate from the reg. Of course you always have ripple on the raw supply. So the raw supply has to be high enough up that the lowest point is high enough, but the power dissipation comes from the average voltage. So the lower the ripple the closer the average is to the lowest point. I've designed these supplies so the ripple is pretty low (.5V or so) so the average is only slightly greater than the low point. So with the raw supply as low as possible you will have about 8watts.

You also have to worry about main voltage and transformer specs etc. If a supply is designed for 115V and your wall voltage is 120 what comes out of the transformer will be higher, so more power will be dissipated by the regulator. Also the transformers don't come in infinately small steps in voltage rating so you have to choose one that is high enough, but it might wind up being a volt or two higher than necessary.

If its really important to get the voltage reg to have as low a power dissipation as possible you can run the transformer off a variac and look at the waveform going into the reg with a scope and adjust the variac until the lowest part of the waveform is just above the dropout voltage. But then if the mains voltage drops due to the neighbor's air conditioner trurning on you go out of regulation. Thats why I always build some margin into my designs and live with the heat.

John S.


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  • RE: Power supply design for low power PC - John Swenson 13:50:06 01/06/09 (0)

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