Negative Voltage on LMZ14202H

Take a look at this app. note

http://www.ti.com/lit/an/snva425/snva425.pdf

I did read that app note already, but it's dealing with postive to negative power supplies. I am in need of a negative to negative supply which is why I can't use that app note.

I am sorry, my mistake.

If the transformer provides two isolated windings, without a common connection, you could stack two power supplies to create your solution (I guess you know this already). If the input voltage has a common point, I don't know an answer to your question.

Here's how the system is designed, maybe it'll help....There are 2x 120VAC to 26VAC transformers, each full wave rectified to 26VDC. All the orginal manufacturer did was take the second transformer and roll the leads to create the negative power supply. The DC commons on both power supplies are tied to each other so I have 3 wires, +26VDC, -26VDC and Common. Because of the way the entire system is designed I cannot put any additional load on the positive side, the -20VDC has to come from the -26VDC supply. While a simple LM117 linear regulator would work I am also dealing with a thermal requirement hence why I want to go with a switcher instead.

I will disclosed the fact that this entire system was designed in the 1960s so it was designed much differently than modern industrial systems.

If you have two different transformers then you are in good shape.

1) Do NOT connect together the output of each transformer

2) Make two separate, identic power supplies. Let's call them VA and VB

3) Connect the output of VA- to VB+

4) Your finall output will be:

  +20V = VA+

   GND = Common of VA- and VB+

  -20V = VB-

Do not connect GND to the transformers secondary side in any way.

Hello there,

I am from the applications team that developed the LMZ family of modules. Let me first reply to the initial question in the thread.

If the target system has a common ground on the +/- 26V source then the negative input / negative output supply would be called a "negative buck."  Typically you use a boost topology switching IC as the basis for this stage. You also have to apply a current mirror on the feedback path to the boost converter since the internal error amp and reference of the boost converter are tied to -VIN (26V).  I'm not aware of any module solutons that can accomplish a negative buck. If you would like a schematic of a negative buck using a discrete IC such as LM5000 with a discrete inductor let us know by reply and we will draw one up and post.

On the other hand, if the the +/- 26V source has isolated source grounds then there are several ways you can configure conventional bucks and join the regulated outputs to give you +/-20V output. (As described in the other posts on this tread.)

Alan Martin  TI- SVA

I

Alan,

I would greatly appericate if you could post a schematic for the LM5000. I believe there may be a common ground I cannot see / access due to the pure size of this system.

Thanks,

Doug

Hello Doug,

This was a fun exercise for a Friday afternoon.

Greatly appericated Alan. What would you recommend for a bucking regulator on the positive side? I was thinking LM5088 but that's just because that's what WEBENCH recommended.

Hello!

   following this thread I've posted, I find that Douglas' need is similar to mine. In the thread I've post my need is a little bit more explained but what I need to do is a SMPS. I have a rectifier with +25 V, Common and -25 V outputs. I need to "boost" those voltages: from 25 V to 50 V and from -25 V to -50 V. The first one is an easy one, for the other I did not find a solution yet. I need an IC that uses the negative rail with an output more negative than that rail.

   The normal boost (from 25 V to 50 V) i'm thinking of doing it with the LM3478. Now, the other one, from -25 V to -50 V, is it possible to do it with the LM3478 or the LM3477?

Regards!