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UCC256402: Minimum switching frequency setting for UCC25640x

yathavakannan viswa
Prodigy 70 points
Part Number: UCC256402
Other Parts Discussed in Thread: UCC25661, LMG2640, LMG2650

Tool/software:

Hello,


I'm working on a project with the following requirements:
· Input voltage (Vin): 270V - 330V
· Output voltage (Vout): 50V
· Power: 400W


                         Initially, I considered a forward converter, but due to power requirements, I opted for an LLC converter to minimize board dimensions. However, LLC converters can be challenging with wide input ranges.
To address this, I plan to use a fixed switching frequency in the LLC converter and add an external buck converter for regulation.

To further reduce board size, I'd like to set the switching frequency to around 800kHz.

                      The issue is that the controller doesn't provide a minimum switching frequency setting. I'm wondering if it's possible to set the minimum switching frequency to nearly 700kHz? Alternatively, do you have any suggestions or solutions that could meet my requirements?

  • 0
    TI__Guru 50040 points

    Hi Yathavakannan,

    Most LLC controllers are optimized to run around 100kHz. Our newest LLC the UCC25661 series is the only one that can come close as you can set the minimum resonant frequency to 700kHz. 

    I hope this answers your question. 

  • 0 in reply to JCHK
    Prodigy 70 points

    Thank you for your response.

    You mentioned that it can operate at a minimum resonant frequency of 700kHz, which aligns with my requirements.

    However, I need to operate the controller between 700kHz to 1MHz. The UCC25661's maximum switching frequency is 750kHz, which is lower than my desired upper limit. Therefore, I've been considering the UCC256402 as an alternative. Can you confirm if the UCC256402 can meet my requirements of operating between 700kHz to 1MHz? Or are there any other options or considerations I should be aware of?

  • 0 in reply to yathavakannan viswa
    TI__Guru 50040 points

    Hi Yathavahannan,

    The minimum resonant frequency of the UCC25661 is 700kHz. The maximum frequency is well over 1MHz. When designing an LLC you set the minimum frequency with the full load power. At lighter loads, the frequency will increase. 

    No the UCC256402 can not run this fast. The UCC256612 is your best option. 

    Your biggest issue is that while the higher switching frequency can help to reduce the volume of the magnetics, you still need to get the heat out of the transformer. 

    Does this answer your question?

  • 0 in reply to JCHK
    Prodigy 70 points

    Hi JCHK, 

           Thank you for your previous response. I have a few follow-up questions regarding the UCC256612 controller:

    1. The datasheet mentions a switching frequency range of 50kHz to 750kHz. However, I need to operate at 1MHz. Is it possible to operate the controller at 1MHz, or are there any limitations or considerations I should be aware of?

    2. The datasheet also mentions a minimum gate ON time of 250ns both the controller as you mentioned (UCC256402 & UCC256612). Could you please clarify what this value represents and how it affects the operation of the controller?

               Additionally, I plan to use the LMG2640 Half-bridge GaN FET. Is this part sufficient for my application, or would you recommend any other alternatives? Are there any specific considerations or factors I should take into account when selecting a GaN FET for my design?

  • 0 in reply to yathavakannan viswa
    TI__Guru 50040 points

    Hi Yathavakanna,

    The datasheet is mentioniong the full-load resonant frequency. As you reduce the load, the frequency goes higher. Here is an example from the excel calculator tool:

    Full Calculator is here:

    UCC25661x_Design_Calculator_Rev2.0 E2E 73025.xlsx

    For 2. The minimum on time is the shortest possible gate pulse width.

    As for GaN, There are several options to choose and balance cost between efficiency (i.e. Rdson). 

  • 0 in reply to JCHK
    Prodigy 70 points

    Hi JCHK, 

             What is the maximum switching frequency of this controller?

  • +1 in reply to yathavakannan viswa
    TI__Guru 50040 points

    Hi Yathavakannan,

    I have seen up to 1.6MHz at startup.

    Regards,

  • 0 in reply to yathavakannan viswa
    Prodigy 70 points

    Hi JCHK,

             Based on my understanding, the LMG2650 includes an additional high-side gate input pin (GDH) with reference to the switch-node (SW), which would allow to directly connect the HO pin of the UCC256612 to GDH pin of the GANFET.

           Alternatively, The LMG2640 has a level shifter input high gate pin with reference to the ground. In this case, can I connect the HS pin of the controller IC directly to the ground(Without bootstrap) instead of switch-node(SW). 

    Which FET would be better choice for the application of 450W.

  • 0 in reply to yathavakannan viswa
    TI__Guru 50040 points

    Hi Yathavakannan,

    Yes with the UCC256612 you would connect HO to the GDH pin. Here is how to make the connections for our half-bridge GaN:

    AGND of LMG2650 is connected with  GND of 25661,

    SW node of LMG2650 is connected with HS node of 25661,

    INL of LMG is connected with LO of 25661,

    INH of LMG is shorted with AGND of LMG,

    GDH of LMG is connected with HO of 25661

    And both LMG and UCC25661 should use bootstrap cap and diode.

    The LMG2650 with the lower RDSON might be your best bet

    450W might be the the edge of what you can support with the LMG2650. Another option could be using two of the LMG3411.  This would be the most efficient option.