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UCC28C42-Q1: Webench simulation of UCC28C42-Q1 using TL431 instead of LMV431

Part Number: UCC28C42-Q1
Other Parts Discussed in Thread: LMV431, TL431, UCC28C42

Hi Team,

We would like to ask your help regarding the customer's inquiry below.

When I design the UCC28C42QDRQ1 with Webench the adjustable precision shunt regulator LMV431 is used. What do I need to change on the design when I use TL431 instead? I know the Vo is different. But how does this effect the design of the schematic?

Thank you for your support!

Regards,

Danilo

  • Hi Danilo,

    If I look at the Webench defaults at the bottom of the UCC28C42 product folder, I see this:

    When I run the default (report attached) I get this voltage regulator:

    The TL431 is a 2.5 V regulator and is suitable for a 12 V output. If you are seeing LMV431 populate the design, you most likely reduced the output voltage to something much less than 12 V? LMV431 is a 1.25 V regulator so if you reduced the Webench VOUT to something like 3.3 V, you would get LMV431. Webench is a conservative design tool that recommends components with sufficient margin to assure success. In your case, Webench decided your regulated output is too close to TL431 (2.5V). This decision comes down to the TL431 not being able to sink enough current through the opto to maintain sufficient regulation.

    If you decide to modify the design and choose a TL431, you are moving from a 1.25V to 2.5V shunt regulator and will need to change the VOUT feedback divider for sure, and perhaps the opto CTR resistors, and compensation and assure yourself you can achieve regulation with 2.5V as opposed to 1.25V.

    Have you downloaded and tried TI Power Stage Designer? This tool runs local on your PC and has a nice compensation section called "Loop Calculator" built in. Run it in parallel with Webench and see if this helps?

    Good luck with your design and thanks for connecting through E2E.

    Regards,

    Steve M

    5305.WBDesign23.pdf

  • Hi Steve,

    Thank you very much for your explanation. We have received further inquiry from our customer. According to the customer,

    I have another question about the AC to DC supply. there are values for the parts of the supply. Do you have a document on how to design the supply for the UCC28C42QDRQ1 Webench design? I'm talking about the EMI filter. thanks for the help!

    Regards,

    Danilo

  • Danilo,

    How to design the Webench design? The schematic, BOM and performance metrics are given, the customer needs to design the PCB, build it and test it. How to design the EMI filter for AC input depends upon what specification the customer is trying to test against. Radiated emissions, conducted emissions, CISPR32, CISPR25 (automotive) are a few examples. Webench gies component placeholders for the EMI filter but not values. Webench suggests the use of CM or DM filters but the design of these filters is iterative and non-trivial. Knowing the converter power level, AC input voltage, input RMS current and fundamental frequency is a good starting point for selecting a CM choke, input DM inductor and line caps. If the design allows the use of Y-caps across the isolation barrier, this can be a huge help for diverting CM noise. If the design if housed in a metal chassis, the use of X-caps between line/neutral/chassis can also help. Here are some materials you may find helpful:

    Bing Lu_Introduction to EMI in power supply designs.pdf

    slup400.pdf

    Steve

  • Hi Steve,

    Thank you very much for your explanation and for sharing the EMI presentation materials.

    Regards,

    Danilo