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Design of a PS for a train display supply, 5V, 80W with the following voltage range input: nominal inpu110V DC +-30% = 77-143VDC

Posted by: Peter Michel on Wed, 05/13/2020

I have two questions regarding the design:

1. Is the out put current limitation a result of the PI Expert SW or is the chip itself limiting the current? If yes, what are possibilities of finding a work around to increase the output current to 16A?

2. How sure is it that the converter really works at full load down to a input voltage of +77V DC?

Thank you in advance and Best Regards

Peter Michel

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Gesendet: Dienstag, 12. Mai 2020 12:17
Betreff: AC-DC Converters :: new comment for your post.

Hi Peter Michel,

You have received a comment on: "Design of a PS for a train display supply, 5V, 80W with the following voltage range input: nominal inpu130V DC +-30% = 77-143VDC"

Design of a PS for a train display supply, 5V, 80W Dear Peter.

Thank you for reaching out to us.
Your desing input and out spec are rather challenging given the relavely low input voltage and rather high output current required. This combination may lead the power supply to operate in deep CCM (Continues current mode
operation) and in case if the Device Current limit is not enough, KP may drop below 0.1, hence the error message you are experiencing.
Just as a clarification, PI Expert only allows you to specify a maximum output current of 15A for LY6 product family.

I noticed in your report that you are using LT6079 which is not the hihgest current limit device.
Try swithing to LTY6070 with Increased current limit. if you are still getting the error message, increase your VOR to around 120V. you should then be able to input 15A output current. Once you have entered the new output current, double chck your device size as in some instances PI Expert reselects the device based on the total output power.
At this point, now that you were able to enter the high current output spec, I would strongly racomend to optimize your desing by means of the inbuild optimization.
You can optimize your desing by clicking either on the *target icon* (also called Optimize button) or on the icon which resembles a *magic wand* (also called Auto design button). The difference between the two buttons is that the first one optimizes your desing while retaining the frozen parameters and components, the latter discards the user selections and optimizes the desing with no restrictions.

if you click on Auto desing PI Expert will be reselcting the output capacitors and device size. In order to avoid that you could follow these steps.
1) if you have frozen either FS or VOR go to Key Parameters functional dialog, on the left side of the screen, and unfreeze them. Most likely the desing will worries though.
2) Now Click on the Optimize button (the target icon). PI Expert will optimize your desing keeping the selected Device. LTY6070 and the selected output cap Configuration

I have created a simial desing to your spec for you to have a look. here is the share link.
LT6070_5V_15A [1]

Hope this helps.
Best regards
PI-Star Lord

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