LM5169FQEVM - Strange behavior of secondary isolated output

Thomas Kofoed-Hansen

Other Parts Discussed in Thread: LM5169

I have tried testing the LM5169 using the above mentioned EVM board.

What I see is that the primary output is stable even with no load but the secondary side is very dependent on the load. The board is equipped with an 1k secondary load which gives a nice and stable 10V as expected.

But changing this load in either direction makes the voltage rise and drop significantly. Lower load makes output voltage rises up to twice the set voltage and higher load makes the voltage drop more or less linearly.

What is the reason for this?

over 1 year ago

0 Marshall_Beck over 1 year ago

TI__Mastermind 28940 points

Hello Thomas,

The flybuck has a minimum load requirement, so, if the load is decreased, the output flying is speaking to that.

In regards to the output dropping with increased load, can you reference fig7-2 of attached doc and if the performance differs from the typical curve, can you quantize the difference?

Below

https://www.ti.com/lit/ug/snvu757/snvu757.pdf?ts=1651766026266&ref_url=https%253A%252F%252Fwww.google.com%252F

0 Thomas Kofoed-Hansen over 1 year ago in reply to Marshall_Beck

Prodigy 180 points

That is not at all what we see. When I say that the output drops or rises then I mean several volts, so that when we get near max load there is almost no voltage left, and when the load decreases we can reach almost twice the set output voltage.

I will try to make some screen shots and maybe draw some curves as those in the User Guide. What we read and see in the user guide is also what we want and expect but NOT what we see.

0 Andrew Kutzler over 1 year ago in reply to Thomas Kofoed-Hansen

TI__Genius 9980 points

Hi Thomas,

Yes screenshots would be very much appreciated.

Thanks,
Andrew

0 Andrew Kutzler over 1 year ago in reply to Andrew Kutzler

TI__Genius 9980 points

Hi Thomas,

I haven't heard from you in a while.

I'm going to assume this issue is resolved.

Thanks,
Andrew

Power management

Power management forum

LM5169FQEVM - Strange behavior of secondary isolated output