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UCC28950: PSFB converter design using UCC28950 + UCC27714 + UCC27524

Umit Anasiz
Prodigy 61 points
Part Number: UCC28950
Other Parts Discussed in Thread: UC3875

Tool/software:

Hi Team,
Good day!

For one of our customer we are designing power converter module using PSFB topology. 

Control IC we are considering as of now is UCC28950. The block diagram is as follows.

When we look at the design with the power stage designer tool, we see the Lsh inductor as below. Our question here is, does the Lsh inductor in this design have to be physically present? If an additional inductor is used, a standard type transformer will be sufficient.  

Is the transformer design made LLC type and is the Lsh inductor used inside the transformer?

I want to clarify the type of transformer that should be used in this topology. Do I have to use a special transformer like in the LLC structure in this topology? Or can we use a standard transformer?

Thank you for your help!

Best Regards,

Ümit

  • 0
    TI__Mastermind 32840 points

    Hello Ümit,

    Thank you for your question concerning the use of the "shim" inductor Lsh in the PSFB topology.

    The Typical Application Schematic diagram shown as Figure 54 does not specifically show a resonant inductor in series with the transformer, however such an inductance is definitely necessary in order to achieve zero-voltage switching.  Figure 54 infers that this inductance resides entirely within the transformer structure in the form of primary leakage inductance.   

    Since the value of Lsh must be precisely controlled and its value depends on accurate knowledge of the switched-node capacitances, it is difficult to design the transformer.
    In most cases, it is better to use a standard transformer with minimized leakage inductance and use a physical external inductor for the Lsh function.  This allows adjustment of Lsh value with actual capacitances during prototype debug, evaluation and optimization. 

    This App Note (involving an older PSFB controller UC3875) discusses the shim inductor ( as part of the total resonant inductance Lr) starting on page 3-4. 
    https://www.ti.com/lit/pdf/slup101   It is an old App-Note, but the ZVS concepts (referred to as ZVT at the time) are still valid for the UCC28950. 

    You are not required to use a special transformer (with all Lsh internally as controlled leakage) but you can if you wish.  Bu tusually it is not worth the trouble to keep Llk controlled, and an external Lsh makes design much easier.  Even LLC transformers are usually used with external resonant inductors to tune the resonance.

    Regards,
    Ulrich