I need to get 5V 3A from a 6-12V input without a heat sink or fan.

I was doing some final checks on a circuit I've been working on the last few months and I realized I made a major boo-boo with my power calculations.

I was planning on using this regulator:
http://www.micrel.com/_PDF/mic29150.pdf

And when I did the power calculations I used the junction to case thermal resistance. But I now know that only tells me how fast the regulator can move heat into a perfect heat sink, and my 4" sq PCB is not a perfect heat sink, by a long shot even though I have a large copper plane on the bottom and thermal vias. (I am using the surface mount version of the regulator.)

In my hours of research into this, I came across this page with someone who had a similar issue with a similarly sized board:

Long story short, they determined they could dissipate around 2W with a board that size.

But with 12V in 5V 3A out, I'm looking at having to dissipate 21W. (12-5 = 7V * 3A = 21W)

Even at 7.4V in 5V 2A out, I'd still be looking at 4.8W.

I'd have to go with the bare minimum of 6V in, 5V 2A out to get the power dissipated down to 2W. And though that might just work for most of my needs, I'd still end up with a fairly warm board.

The reason for such a wide voltage range is I designed this board to be used in large and small devices and with a variety of batteries. For example one might want to place the board in a small handheld device with only enough room for 4 AAA's. Or one might want to place the board in a large prop with an amplifier that needs a 12V input and they would prefer not to have to use two sets batteries to power both the amp and the board.

So, I need to know what my options are. Aside from the requirement that I can't have a large heat sink or fan (a very small heat sink might be okay but they seem expensive and I'm not sure how much of a different one would make in an enclosed space with not much air flow), I also need the design to be fairly simple and low cost.

The obvious solution which came to mind was to use a buck regulator. The problem is, they tend to be complex, expensive, and I can't find one that can output 5V with a 6V input.

For example, the simplest one I've found is the Simple Switcher:

Sticking one of these on my board would end up costing me $1000 more than the regulator I have on there now. And it indicated the minimum input voltage is 7.5V for a 5V output. I could swallow the $1000 cost, but the 7.5V input is a real sticking point.

The reason 7.5V is a sticking point is I wanted to put these in some handheld devices powered by alkaline batteries but there's only enough room for 4 AAA cells and those will drop to 4.8V after 2 minutes when drawing just 1A. And a 9v fares even worse. Another device I want to power would require around 500mA since it would not be playing audio constantly, and I could fit 6 AA's in that, but those would drop below 7.5V after only 18 minutes.

I'd consider the use of LiPos, but I can't find any discharge curves showing what voltage they start at and when they'd actually drop below 7.5V for reasonable discharge rates. The only graphs I've found are ridiculous stress tests in what I believe is the 50A-100A range. (They use C to define discharge rate, and as far as I know the amps would be the maH of the battery * C, which gives some ridiculous amperage numbers on these tests the RC guys are doing.) Plus a 7.5V LiPo isn't gonna fit in the smaller of the two devices anyway. I'd need to use three of these battery packs on Sparkfun and wire them in series:

Two probably wouldn't cut it at just 7.4v.

Anyway back on the subject of regulators, I also found this one:

But that one still requires 7V and has a large inductor that would take up space on my board I really don't have. I realize I may have to give in on the space issue, but the 7V thing is still a problem.

That brings me to this:
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=4319

I doubt that this UBEC (universal batteyr elimination circuit) is a buck-boost regulator since it needs at least 5.5V, but it lists a 93% efficiency, so it can't be a linear regulator either. And if it were a boost regulator I'd expect it to require a higher input voltage like all the rest I've found. Is this some kind of hybrid linear boost regulator? I have yet to find any chip with which I could make something similar, but I guess I'm gonna go back on Digikey shortly and have another look around.

Another thing that strikes me about this is how cheap it is. It's less than the price of the simple switcher! $1.50 less. And that's at Qty 1. I'm comparing to the cost of 250 Simple Switchers! What's up with that?

Simply purchasing these for use with my boards instead of rolling my own power supply would seem to be a decent solution to my problem. But they are a bit large and I would think I could get them much smaller and cheaper by rolling my own and using ceramic caps. As is it would make things a bit cramped wit the batteries also being stuffed inside my smallest device.

Anyway, if you have any suggestions, please let me know. I wanted to have this board ready to go out to the PCB house by Monday and this has thrown a real monkey wrench into those plans.

I doubt that this UBEC (universal batteyr elimination circuit) is a buck-boost regulator since it needs at least 5.5V, but it lists a 93% efficiency, so it can't be a linear regulator either. And if it were a boost regulator I'd expect it to require a higher input voltage like all the rest I've found. Is this some kind of hybrid linear boost regulator? I have yet to find any chip with which I could make something similar, but I guess I'm gonna go back on Digikey shortly and have another look around.

It is a switching regulator, the link states that it uses a 300khz switching rate. Other features given are:

Features.
Over current and over heat protection.
Switching frequency 300khz.
Chip Efficiency is 92%. This UBEC utilises switching technology to gain a much higher efficiency than liniar BEC. Which means less heat and longer battery life.
The UBEC is equiped with a noise reduction alloy housing to reduce noise even further.
Incorrect polarity safety feature ensures the UBEC will not be destroyed should the battery be connected incorrectly.

All in all if that BEC works as rated you would have a hard time duplicating it at near that cost in any DIY project.

Lefty

But how could a switching regulator output 5v with a 5.5v input? Or even a 6v input? The lowest I've been able to find is 7v.

But how could a switching regulator output 5v with a 5.5v input?

That is how they work, they charge up an inductor and then discharge it. When the input and output are close then there stability issues but you can do it.

The lowest I've been able to find is 7v.

The higher the voltage difference the easier it is to design.

What would happen with a regulator like that Simple Switcher when you drop below 7.5v? Does it just stop working?

I'm going through around 30 different parts from TI's site looking at them and I came across this one just now:

Unfortunately they don't provide a convenient schematic with a 5V output and listing the input range, nor do they seem to have any tables showing what output voltage you can get with a particular input, but strangely they do have a graph on page 10, figure 10, which appears to show a 5V output with a 4.5V input.

Unfortunately they don't provide a convenient schematic with a 5V output and listing the input range,

That it is because it is up to the skill of the designer in laying out the circuit, choosing the components and so on. It is not a function of the chip itself.

I've got a lead on something...

I was looking at thus UBEC:

And one of the people that bought it had one of the leads come off and took it apart, and someone else recognized it as a KIS3R33S module. I'm not sure what that is yet, but apparently they're all over ebay.

And this is the chip used in them:

Two people tested those BECs, and got the following results:

5.5V at 0.5A
5.45V at 1A
5.35V at 2A
5.15V at 4A
5.02V at 5A

Amp Volt
0 5.5
1 5.4
2 5.28
3 5.17
4 5.08
5 4.96
6 4.79
7 4.6

Is that normal for a buck regulator? I'm not sure if my circuit can take 5.4V, but I don't want to have to use one of these set to 6V and feed that into a linear regulator. Then I'd be limited to 2A.

I know the atmega has a max rating of 6V. I guess I'll have to check the dac and amp. A 5.5V supply would probably cause some clipping in my amp though since it expects 5V. Then again maybe because Vin on the amp would be higher as well it would just result in a higher output voltage to the speaker. My SD card should be fine since I have a 3V regulator for that...

Well, it looks like everything in my circuit should be okay running of 5.5V, and since that will only be the case when not much current is being drawn, additional power dissipation associated with it won't be an issue either.