LM7800 - Sharing the Load

[olsenn]

Hi Guys,

I need to be able to regulate a source down from 18-20 volts to 15 volts, at 2 amps. I have several LM7815CV (15V) regulators; however, the datasheet says they can only source up to 1.2A. I have tried placing two of these in parallel, but one of the IC's does more of the work than the other. Just for fun, I have also tried placing three LM7805 (5V) regulators in series, but still the last one gets too hot for comfort.

Is there a way that I can evenly share the power dissipation among 2+ regulators? I only have small heat syncs available, so ideally I would like to split it over 3 or 4 of these to keep the heat density down. I'd appreciate any ideas.

[c4757p]

For that much current and voltage drop, you really ought to investigate using something like a MC34063 switching regulator. By itself it handles 1.5A peak, and the datasheet shows you exactly what to do to boost that with an external transistor. You don't have to do a bit of design work yourself, just follow the datasheet.

[olsenn]

I'd really rather not go that route; I figure I didn't stock up on transistors, diodes, regulators... for nothing; there has to be some way of rigging up something that will dissipate 10 watts with what I have.

I do appreciate the suggestion though; I will entertain any ideas!

[c4757p]

LM317's datasheet (well, one of them, at least) has an example circuit to boost the capacity with an external PNP power transistor. Should work the same for 7812.

[mariush]

See the following datasheet, it shows lots of schematics including a couple or more for high current.

http://www.st.com/web/en/resource/technical/document/datasheet/CD00000444.pdf

I can't find the datasheet right now, but I remember reading somewhere that it's possible to just use about 20cm of cable as a sort of balancing resistor between them, but don't take what I say as true, it may have been in regard to a switching regulator.

[BravoV]

You need "identical" ballast resistors at each regulator output to balance the loading at those regulators.

Usually low resistance resistors to reduce power lost, or use identical pcb traces as the low resistors if you're going to make/etch your own pcb.

[c4757p]

Usually low resistance resistors to reduce power lost, or use identical pcb traces as the low resistors if you're going to make/etch your own pcb.

I'd use some pretty long traces for that - coiled, maybe. You've got to overcome manufacturing differences from regulator to regulator.

Probably better to just use one of the circuits the datasheet gives to ballast it with an external transistor.

[olsenn]

Thanks guys for the advice. Using a modified circuit design in the (my) LM317 datasheet, I can now use a cheap TO-92 LM7815 to regulate the voltage, and an NPN transitor of my choice to dissipate the power. This is helpful since I DO have power transistors that handle the current/power I need, but I am still at a loss for heat (no pun intended). Is there a way then to split the power evenly between two BJT's?

[darko31]

Why not just strap on a big heatsink to one of the regulators, it will have to dissipate the same amount of heat whether it be one regulator or ten of them. Might as well use just one.

A note from the datasheet

The LM78XX series is available in an aluminum TO-3 package which will allow over 1.0A load current if adequate heat
sinking is provided.

I'm sure if you have TO-220 version of 78XX, that they can also dissipate 10W with proper heatsink.

[alm]

You can put BJTs in parallel. Make sure to put a small value resistor (typically in the order of 0.1 ohm) in series with the emitter to ensure even load sharing.

Note that using an external pass transistor with an LM78xx defeats the over temperature and over current protection. There are circuits (I think one is in the National datasheet) that still allow over current protection to work.