LM5185-Q1: Minimal power output on output of transformer, positive ringing choked

Hi SK,

Thank you for designing with the LM5185.  We developed a design calculator available at https://www.ti.com/tool/download/LM5185-DESIGN-CALC. Sorry I am not sure if your excel spreadsheet is correct or not.  Please use our design calculator to check you design. 

I did a quick run of the design calculator and it seems you have wrong selections for the current sense resistor, which should be ~25mOhm, but you choose 50mOhm which causes your circuit to trigger the current limit.  In addition, the transformer inductance seems to be lower than recommended by the calculator, although it can work. 

Hope this clarifies.

Regards,

Youhao

Hi Youhao,

I changed out the current sense resistor for a 20mOhm one and swapped the compensation components for the ones on the dev kit. This yielded this result:

The blue being the input and the yellow the output. Thoughts?

I'll try some other things in the meantime.

Thanks,

S

Hi SK,

What do you mean by "The bule being the input..."?    Was it the MOSFET drain voltage? 

It seems you have our EVM kit, right?  Have you ever run the EVM as was before you making any changes?  Were your scope picture obtained from the modified EVM, or from your own circuit board?

Regards,

Youhao

Hi Youhao,

I wrote that entirely incorrectly, sorry. The blue trace is the positive output of the secondary side of the transformer, whereas the yellow trace is where we expect 15V (after the snubber, at TP1 on our schematic).

The EVM has not been modified (aside from removing a capacitor) and still works normally.

The scope pictures are all from our own circuit board.

Regards,

S

That's okay.  I also have typos in my msg  :-) 

What are these three components on your board now?  Did you update the values per our design calculator suggestions?

Thanks,

Youhao

We have the following components (red Xs are removed):

The calculator suggests R18 = 2.6 kOhm, C18 = 416 pF, C19 = 62 nF (but we don't have this, so I have tried 47 nF and 100 nF, neither of which worked, but a high pitched ringing could be heard with either).

Hi SK,

Could you check if your C18 and C19 swapped onboard?  Your schematic marking looks correct but your above description shows C18=416pF which should be on C19.

Once you confirmed C18=100nF, and C19=416pF, please try to raise R18 to  24.9k and see if the operation is stable with Vout regulated at 15V.

Thanks,

Youhao

Hi Youhao,

I seem to be making all sorts of typos! I meant to type C19 = 430pF and C18 = 100nF. The components on the board do match the schematic. I will try changing R18 tomorrow and update you.

Thanks,

S

Please let me know tomorrow.  Note that I will be out of office throughout next week.  Hope to resolve the issue in this week.

If you still have problem, please capture VIN, Vds of the FET, Vcs, and Vcomp on the same scope picture. Please use a probe of short ground lead for Vcs signal.  

Thanks,

Youhao

Hi Youhao,

We are still seeing issues. We do not have enough probes on our scope to measure all of those, so I have measured everything with VIN (a.k.a. FLY_IN+). We have VIN & Vcs, VIN & Vcomp, VIN and Vd (the drain voltage with respect to the ground at TP5), and separately we have Vds.

Hi SK,

Sorry to ask this:  could you double check your R12, and it is 154k but not 15.4k?  If R12 is correct, then I would like to ask you to ship your board to me so I can debug in our lab.  Is this possible?   If yes, please ship out ASAP, and I can work on it after the Thanksgiving.

Thanks,

Youhao

Hi Youhao,

R12 is in fact 154k. I removed it from one board and measured it. To what address should I ship the board?

Thanks,

S

I will email you...