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UCC256304: Bias winding and auxiliary bias

Part Number: UCC256304
Other Parts Discussed in Thread: UCC25640EVM-020, PMP22087

I'm designing some LLC resonant power supply with PFC and I decide to use controllers from TI.

Out voltage will be 48V and power around 800 - 900 W.

I took a look at datasheet of UCC25630x and UCC25640x families and they  look like perfect controllers.

I also took a look at the suggested project on TI projects for LLC converters.

Maybe I'm wrong, but I noticed that most of those project never use the bias winding of the transformer to provide the power supply to the LLC controller and PFC controller, while the transformer has in most case the bias winding used only as BW feedback only.

I wonder if there is any reason to prefer auxiliary external supply for the controllers and what are the possible issues that I may experience if using the bias winding of the transformer to power up the controllers IC?


  • Paolo

    First I'd like to make sure that it's clear on the differences between UCC25630x and UCC25640x.  UCC25640x is a generational, pin-to-pin compatible improvement over UCC25630x.  Details of improvements are in the following app note

    Some variants of UCC25630x/40x are designed to use an auxiliary bias power supply, while others have integrated high voltage startup and self bias thru an auxiliary winding.  Table 5 (page 3) on the UCC25640x datasheet has details. 

    Designs like the EVM UCC25640EVM-020 and PMP22087 show UCC25640x configured in a way where they are powered by bias winding on the transformer.  The advantage of such a configuration is that it eliminates the requirement for a bias supply on the input to provide power to the IC.  UCC25630x/40x take care of the main design challenge of startup, since they both have integrated HV FET that charges up VDD to enable UCC25630x/40x to start operating long enough for the bias winding to build up voltage and start proving power to the IC.  Details on this are in section 8.4.1 (page 33) of the datasheet.  When you start your design we recommend that you use our excel design calculator and follow layout recommendations in section 11 (page 65-67) to insure your design works well. 

    Some designs may prefer to use an external bias power supply for various system level reasons.  For example they could require such a converter for a separate reason, and it is found that simplifying the transformer construction to eliminate the aux winding and associated bias power circuitry is preferred.

    Best Regards,


  • Eric,

    thank you so much for your answer.


    Best regards