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UCC25600: UCC256404 TL431 Value Calculator

Part Number: UCC25600
Other Parts Discussed in Thread: TL431, UCC256404, UCC28056, PMP22087

This might be a useful addition you could add to your Excel Spreadsheet UCC25640x Design Calculator and others.

I want to use the standard TL431 type feedback control to a UCC256404 design. I would assume that most designers would do the same.

In my case, I want values for a 5.0V output, rather than the 12V output values shown.

From the circuit values shown in the application notes, I can't determine what Opto current you are targeting.

What might it be?

While in other designs, a 1k resistor is placed across the OPTO LED, the UCC256404 application schematic shows 3k. No doubt there is a reason for this.

Furthermore, to provide appropriate RC values for compensation would ensure designers are implementing the UCC25640x optimally.

To have an additional Excel page with a TL431 value calculator might be very useful.

The TI Excel spreadsheets I am using (UCC28056 and UCC256404) are excellent and I will trust what it suggests. To extend this to the TL431 would complete the package.



  • Hi Peter,

    Thank you for the feedback. Yes, we are looking to add compensation guidelines to the calculator in future revisions. We did not target a specific opto-coupler current but wanted to maximize the control loop bandwidth while still having sufficient phase margin. The total fast lane current is split between the opto-coupler and the resistor in parallel to the opto. The smaller the resistance, the more current is diverted to the resistor instead of the opto. Adjusting the resistance shifts the gain curve up and down which is useful for optimizing the loop response. With the UCC25640x LLC controller family, the feedback current source inside the device is quite small, only ~85uA not including the clamping current. This means that the opto-coupler will typically have fairly low current during operation (depends on the CTR but generally speaking it will be fairly low current). The next revision of the EVM will use type III to increase the bandwidth and phase margin a little more. Below is some test data from the UCC25640EVM using type III.

    Bode Plot Data for Peter

    Best Regards,

    Ben Lough

  • Hi Ben

    Sorry for the late response. I got caught up in other tasks and want to give you the feedback below.

    I'll look forward to the TL431 calculator. Actually I was hoping for values for a 5-volt output. More specifically, the values across the opto-coupler. That's why I wanted to know the currents you were using in the feedback network.

    I've been working on bringing my combined UCC28056(PFC)  UCC256404 (QR Controller) alive.

    Here are some comments on the excellent spreadsheets.

    UCC28056x Spreadsheet

    Cell 38 needs more vertical spacing.



    UCC25640x Spreadsheet

    Appreciate that most of us are not SMPS specialist. We are desiring that the SMPS can be built from spreadsheet information and work optimally. The less time required to be poking around high-voltage electronics, the more comfortable I feel.

    These spreadsheets are also excellent marketing for these parts. The less guesswork needed because of your tools, the more attractive your parts are for a product.


    Line 50 Select Ln and Qe.

    I’m having a lot of trouble understanding this section.


    Cell 56 – Selected Primary Inductance Ratio Ln(selected). (Ratio between Lm (larger) and Lr (smaller)?

    It isn’t obvious what the ratio Ln is for (it’s not an integer) and why you should or should not just select a mid-range value of n=4.5?



    The MG(peak)VS Qe plot on the right.

    It isn’t indicated, where the “sweet spot” we should be targeting. Is it on the curve and not on the linear section (vertical or horizontal?). This plot need a shaded area to suggest the optimal selection point.

    Why are we being given so much flexibility in this area?

    I feel that with “a lot of thought”, you’d automate this entire section. There is too much risk of customers not getting it ideal.


    LLC Gain Curve – This needs an example waveform that we should be targeting. I can adjust the two values and get all sort of results that might meet the criteria. Yet they might be poor choices.

    How many divisions above Mg(max) should our blue waveform go?

    Maybe you can adjust the parameters in my attached file to show me the preferred waveform I should be getting? Better still put an image on E2E so everyone can see.

    I don't know whether you can extract my Excel spreadsheet below.

    UCC25640x Design Calculator PRB.xlsx

    My SMPS has multiple outputs (40V and 5V//5V) and so I am more interested in the OUTPUT POWER parameter rather than a specific VOUT parameter. Obviously VOUT is used for the feedback network. Is VOUT being specifically used for any of the LLC calculations?

    Are there are waveforms that should observe under certain conditions to ensure the converter is working optimally?



  • Hi Peter,

    Apologies for the delayed response and thank you for the feedback regarding the calculator tools.

    Ln refers to the ratio between the magnetizing inductnace Lm and the resonant inductance Lr. Larger Ln shortens the required operating frequency range but has more circulating current and will result in higher conduction losses on the primary. Qe refers to the quality factor of the resonant tank. Smaller Qe gives higher peak gain but also requires higher switching frequency variation to achieve a given gain range. The optimization of the LLC power stage is often an iterative process and is normally driven by application level requirements such as size, weight, thermal budget, etc. A common practice is to start with Ln=5 and Qe=0.5 and iterate as needed from there.

    Regarding the gain curve, the curve must become greater than the maximum required gain at some point along the curve before the capacitive region where the gain curve begins to decrease as switching frequency decreases. An ideal gain curve will be able to pass through both Mg(max) and Mg(min) within the desired frequency range with some margin above Mg(max).

    I would suggest decreasing Lm slightly to 500uH. This will give a bit more margin on Mg(max)

    Note that the operating rage should be chosen based on the application needs. Some designs favor operating above resonance due to the smaller circulating current on the primary but the rectifiers would not have zero current switching. Operating below the resonant frequency will have the benefit of ZCS transitions on the secondary but the circulating current on the primary is higher. This is a tradeoff.

    Vout would come into play for calculating Re. The rail with the highest output power is used for the power stage calculation in this case.

    What I would suggest is checking the switch node waveform and making sure the LLC is achieving full ZVS switching on the primary under all operating conditions. I also would suggest taking a look at the test report for PMP22087. It will give you an idea of what the waveforms should look like:

    Best Regards,

    Ben Lough