LM25184: LM25184NGUR

Mubashir Alam

Part Number: LM25184

Hello TI Team,

We are implementing a DC-DC converter where our input voltage is 34 Volts and we are trying to get dual output of +22V and -6V. We are using the excel sheet given by the TI for LM25184.

Our transformer is a customer designed transformer with 1:5:1:0.278 +-3%. Currently the output voltage we are getting is +32V and -8.45V. It looks like there is a feedback problem, but I have checked with different resistances but still getting the same voltages. The design sheet is also attached here for the selected requirements. Can you please have a look and let me know what can be done to get the right voltages. Thanks

LM25184 DESIGN FILE.xlsx

3 months ago

0 Youhao Xi 3 months ago

TI__Guru 72795 points

Hi Mubashir,

Thank you for designing with the LM25184. I don't see obvious problem in your calculator. Did you apply any load to the output? There is a min load requirement which can be fulfilled by either the dummy load Rdmy in our calculator schematic and/or Vout Zener clamp.

If your circuit meets the min load requirement, and you still see over voltage, please share the following info:

Your own version of the schematic that you used to design your PCB. Please make sure all component values are accurately labeled. If some parts in the schematic are not used, please also indicate.
Your transformer datasheet.
Your load condition when observing the over voltage in the output.

Best Regards,

Youhao

0 Mubashir Alam 3 months ago

Prodigy 20 points

Hello Youhao,

Thanks for your fast reply. You are right, I didn't put the dummy load as I had another load but currently its not populated on the board. The reason for not putting dummy load was according to calculator, it was optional. But, I will put a dummy load.

However, does this means that even you are putting another IC as a load to LM25184, do you still need this dummy load for startup??

Also, please keep this thread open, when I will get the over voltage, I will provide you the information you requested. However, as there is snubber circuit with both outputs, I am not expecting an overshoot, what do you reckon?

Thanks for your help, mate.

+1 Youhao Xi 3 months ago in reply to Mubashir Alam

TI__Guru 72795 points

Hi Mubashir,

Sorry for the confusion. The dummy load is to keep the output voltage at the regulation level when your actual loads become very light or even no load. When you have another IC as the load, it should meet the min load requirement, or additional dummy load is still needed to prevent Vout from going higher. When you see 32V, it indicates that you did not install the Zener, either, right? You may save the dummy load resistor if you install the Zener. Of course you can use both of them.

The snubber circuit is to suppress the peak voltage ringing across the diode. If the snubber is selected properly, the ringing will be much damped.

Thanks,

Youhao

0 Mubashir Alam 3 months ago in reply to Youhao Xi

Prodigy 20 points

Thanks mate for your fast reply. I will let you know how it goes. Thanks

0 Youhao Xi 3 months ago in reply to Mubashir Alam

TI__Guru 72795 points

I am looking forward to your updates. Please post after you have results.

0 Mubashir Alam 3 months ago in reply to Youhao Xi

Prodigy 20 points

So, I got my desired voltages. I checked the ringing in output but there is no ringing found. So, there are three results attached. Image 0004 & 05 showing both +22V & -6 V while image 0006 showing the ripple in voltages.