LM5121-Q1: Alternate wide and narrow duty cycles resulting in higher voltage ripple

selvam perumal

Intellectual 515 points

Part Number: LM5121-Q1
Other Parts Discussed in Thread: LM5121

Boost converter

Input : 5V

Output : 10V@ 6A

Switching frequency:450 Khz

Output ripple is observed around 400mV.After probing Switch node,Wide and narrow duty cycles observed alternately.

Changed the Rslope value as calculated from eqn 6 in datasheet Page:16

Still the issue is not solved.

Any other parameter to be re-looked for alternate wide and narrow duty cycles.Boost converter.pdf

over 2 years ago

0 Haroon Wardak over 2 years ago

TI__Genius 10375 points

Dear Selvam,

Thank you for using TI products.

Could you please share the image of the output ripple measured in an Oscilloscope?

Usually the probing method could be the source of those ripples, please check the below link：

https://e2e.ti.com/blogs_/b/powerhouse/posts/how-you-measure-your-ripple-can-make-you-or-break-you

BR,

Haroon

0 selvam perumal over 2 years ago in reply to Haroon Wardak

Intellectual 515 points

Hi Haroon,

It doesn't look like measurement issue.We are also measuring other lower voltages (5V) in the same board using ripple tip and measured around 50mV.

In 10V, we observed 400mV due to alternate wide and narrow duty cycles.

Believe,rectifying the alternate wide and narrow duty cycles will solve the problem.

Refer the attached Output ripple and Switch node waveform.

With regards,

Selvam.

0 Haroon Wardak over 2 years ago in reply to selvam perumal

TI__Genius 10375 points

Hi Selvam,

I see, May I know what type of output Capacitors you are using? output capacitance has a huge influence on the output voltage ripples, especially if it is electrolytic capacitors with higher ESR.

Also, may I know why are you opting for alternating duty cycles, is there a reason for that?

BR,

Haroon

0 selvam perumal over 2 years ago in reply to Haroon Wardak

Intellectual 515 points

Hi Haroon,

Tantalum capacitor(100uF,25V,ESR:50 mohm) and 4 nos of 22uF ceramic capacitor in parallel.

We are not opting for alternating duty cycles.

We would also like to avoid alternating duty cycles to reduce ripple.

Kindly suggest how to get rid of alternating duty cycles.

With regards,

Selvam.

0 Haroon Wardak over 2 years ago in reply to selvam perumal

TI__Genius 10375 points

Hi Selvam,

I think your output capacitor might be the cause of the problem, the electrolytic capacitors have higher ESR and that could increase the output ripples.

But regarding the alternating duty cycles, that is usually related to the sub-harmonic oscillation, so it is possible that you do not have enough slope compensation such that you can eliminate the subharmonic oscillation. From your schematics I see that your RSense is 0.035 ohm, which is too high and your R slope does not add up when you calculate using the R slope equation (equation 6 on the LM5121 datasheet), try switching your Rsense to 4 mohm or you can use our WEBENCH tool for design reference here:

https://webench.ti.com/power-designer/switching-regulator/customize/6?VinMin=5&VinMax=5&O1V=10&O1I=6&base_pn=LM5121-Q1&AppType=POWER&Flavor=None&op_TA=30&origin=pf_panel&lang_chosen=en-US&optfactor=3&Topology=Boost&flavor=None&VoltageOption=None

BR,

Haroon

0 Haroon Wardak over 2 years ago in reply to Haroon Wardak

TI__Genius 10375 points

Hi Selvam,

Should I go ahead and close this thread if your problem has been solved.

BR,

Haroon

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LM5121-Q1: Alternate wide and narrow duty cycles resulting in higher voltage ripple