LM431: Higher ripple on AC/DC power supply output?

Hello Ant,

Were all the devices purchased from TI or its authorized distributers? Thanks.

Hi Masoud,

My understanding is that the lot is coming from an official distributor, I will let customer comment directly about this.
A picture of a T&R sticker is provided so I guess it should be possible to check.

The PowerDesignService team is advising to do the below:
The output has a ripple at ~ 2.7 kHz, which means it looks the converter is unstable.

Probably with the first lot of LM431, the converter does not oscillate because its phase and gain margin is on the edge.
On the new lot of LM431 there is may be a slight change in the gain, and with higher gain, that would trigger the oscillation.

The following stability test would need to be run to understand the system margin better:
Get a bode plot measurement with the working LM431,
If the customer doesn’t have any network analyzer, they can perform a load transient on the output, and check Vout variation versus load step.
From the working system you should be able to understand how much margin you have and if some more tuning is needed to prevent the system to oscillate.

Also the below presentation from the "TI power design seminar" could give some tips about "stability issue and feedback using opto and TL431":
   video + PDF:  https://training.ti.com/common-mistakes-flyback-power-supplies-and-how-fix-them 
   PDF:              "Common mistakes in flyback power supplies and how to fix them - SLUP398" (2020) 
                          available from https://www.ti.com/design-resources/seminars/power-supply-design-seminar-psds/psds-resources.html? 

A.

Hello Ant,

Could you please share the customer's schematic for the team to look at the LM431 implementation?

Thanks,

Walter

Hello Walter,
As mentioned the team is asking first to make the stability analysis on the working board.
This is to understand if there are enough design margins at all in the original design.

Have such an analysis be done already when the HW got qualified during R&D?

Thanks,

Ant.