Hooking up two voltage regulator in parallel.

Victor,

There are applications where two regulators can be used to load share, (www.ti.com/.../slva250.pdf) but this assumes you are using the same regulator. You cant put these two in parallel as they have different polarities so they can't share an output.
Can you describe the application more in depth? We may have another solution.
What is your input voltage?

Best,
Michael

Hi Michael:

What I am trying to do is from a +/- supply +44.5vdc unregulated (no load). Using a two positive variable voltage regulator get 40vdc @3 amps. 

Secondly getting -44.5vdc unregulated (no load). Using a two negative variable voltage regulator get -40vdc@ 3 amps.

I will look at the link.

Thanks,

Victor Morita

Victor,

That is what I thought you wanted to do, but just wanted to verify.
An additional concern I have about using these devices at such a high current is thermal performance. You will need to make sure to have them appropriately heatsinked.
I apologize for the delay in response.

Best,
Michael

Victor,

You could also consider hooking up two linear regulators in parallel through the technique specified in this TI Design: 

~ Aaron

Now I am in testing phase. I have tried the top hat heat sink, from avid  but the device goes into thermal overload in normal current draw. How big of a heat sink do I need?

Victor,

This is a very high power loss design so the heatsink will need to be very large to be able to handle 4.5V * 3A = 13.5W.
You should take a look at this app note on calculating thermal impedance and device temperature:
www.ti.com/.../slva462.pdf

Also, this is a very inefficient design. Might I suggest that you take a look at the following switching converter design in WEBENCH (please copy and paste the link into your browser):
webench.ti.com/.../webench5.cgi

Best,
Michael

Hi MIchael:

I have done alot of research.  How about this approach.

+ unregulated ---------------lm317hv------------------(+40volt@1.5amps)

                                                      ------------------------Gnd

- unregulated ----------------lm317hv -----------------Gnd

                                                     -------------------------(-40volt@1.5 amps).

The picture is a positive voltage regulator used as a negative regulator

Victor,

I don't understand the LM317 used in a negative input supply application. How exactly do you have the pins connected? This is not a recommended application for the lm317.

it is recommended to use a negative voltage regulator.

Best,
Michael

Hi Michael:

This approach is two separate + power supplies with transformer isolation (floating not tied to chassis gnd). One power supply is the + 40volt source and the second is the -40volt source. The second power supply"s plus is tied to the negative of the first. Thereby creating a new power ground and a + and minus supply.  I have taken two lab supplies and hooked up the unit in series with no chassis to minus tie in(floating). Hooked up the two power supplies in series to generate a plus and minus supply.  What do you think.