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UCC2897A: Active Clamp Forward (ACF) Transformer design

Tracy Song
Intellectual 350 points
Part Number: UCC2897A

I have two questions for TI：

1. Conventional positive excitation does not need to consider the inductance of the transformer, but active clamp forward (ACF) should calculate the inductance of the transformer, so as to obtain the appropriate excitation current to realize ZVS.Am I getting it right?


2. If my statement is correct, how to calculate the primary inductance of ACF and whether it is necessary to open the air gap?

3.Will the implementation of ZVS affect the efficiency of ACF when the efficiency is more than 90%?

  • 0
    TI__Mastermind 34465 points

    Hello 

    Yes you are correct. Magnetising inductance value is an important parametric value in the design of an active clamp converter.
    If you take the time to review 
    http://www.ti.com/lit/an/slua535/slua535.pdf
    this is explained in very great detail.

    Regards

    John

  • 0 in reply to John Griffin1
    Intellectual 350 points

    Hi,John
    The slua535.pdf does not describe the calculation method for the primary inductance of the transformer, but is given directly.I want to know if there is any formula to calculate the inductance?

  • 0 in reply to Tracy Song
    TI__Mastermind 34465 points

    Hi Tracy,
    A feature of the active clamp forward converters is that they utilize the magnetizing energy for soft switching of the power switch.
    While transformers for conventional forward converters do not have any airgap, transformers for active clamp circuits are usually gapped in order to reduce the magnetizing inductance. 

    If the transformer is not gapped, there is no impact on steady state operation of the active reset circuit, but the soft transition is difficult to achieve and the transformer is more exposed to the possibility of saturation under transient conditions.
    The peak magnetizing current of the transformer increases as a result of gapping.
    This magnetizing current adds to the reflected load current flowing through the main switch. It also increases circulating energy in the clamp circuit during the main switch off time. The amount of magetizing current is usually set experimentally as a fraction of the reflected load current.
    Some designs will use a rule of thumb as setting the magnetizing current as one half of the peak reflected load current.

    Setting a lower magnetizing current will result in a faster discharge of the resonant capacitor 
    The magnetizing inductance value should be chosen so as to enable soft turns off of the resonant FET.
    You could choose this to occur at max load.

    So this is what sets the magnetizing inductance value for your transformer.

    Regards

    John