Electronic – Low Profile Power Dissipation


I have a project where my power supplies are giving me a bit of a headache. I have three voltage regulators that I am trying to run from a single bench power supply. There are two 3.3V regulators, and one 12V regulator. The current draw for the 12V regulator is up to 500 mA, and each 3.3V regulator peaks at 150 mA.

When I hook up about 14V to the inputs of these regulators, the 3.3V regulators start to cook. Doing the math, that's about 1.7W of power dissipation for each of the 3.3V regulators. They're in a TO-220 package pointing straight up, and they got too hot to touch. I was able to throw a random heatsink I found on the tab and cool things down, but the top of the heatsink was about 2.5 inches high – very flimsy and cumbersome.

I really don't want to sacrifice the single power supply input, and I also really don't want to go to an on-board switching supply for either the 12V or 3.3V supplies.
What are some passive package+? solutions for dissipating up to 2.0W of heat that are physically secure and low profile (shorter than 1 inch)?

My reasons for not wanting a switching supply are:

  1. I don't want to deal with switching noise in the analog parts of my design
  2. I'm less comfortable with designing a switching regulator than using a linear regulator
  3. The PCB that I'm working on is big and expensive. I don't have the time/budget to "try out" a regulator. A linear regulator with enhanced heat dissipation seems a lot safer way to avoid making coasters than a switch mode regulator.

Best Answer

If you're stuck using linear regulators, you can put big power resistors in series with the inputs of the regulators to drop the voltage and share some of the heat dissipation. You're dissipating the same amount of power either way, but it might make it easier to fit on your board or whatever.

If you're using an LM7833, for instance, and supplying 150 mA, the datasheet says the dropout is 2 V, so the input voltage has to always be above 5.3 V. From a 14 V supply at 150 mA, this is a 2 W, 56 Ω resistor. The resistor just needs air circulation around it, not a heatsink, and then your regulator only needs a 500 mW heatsink instead. The highest power dissipated in the regulator will be at the current when resistor and regulator are both dissipating the same power, which in this case is about 95 mA.