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UCC28950: Current transformer reset

Catalin Muntean
Intellectual 280 points
Part Number: UCC28950

Hi everyone,

In the datasheet, it is recommended that the reset resistor of the current transformer to be 100*Rsense resistor.

Can anyone tell me how one arrives to such a conclusion ?

Why do not use a zener diode to reset ?

Here is my approach:

minimum off time>5*time constant (Lcurrent xformer/Rreset)

Thank you for your time !

  • 0
    TI__Mastermind 34465 points

    Hello Catalin,

    You are correct that a zener may be used to reset the current transformer an in fact many users do so.

    For proper reset of the magnetic core:

    Von * D(max) = Vreset * (1-Dmax)

    Vreset = Von *D(max)/(1-Dmax)

    So at max duty cycle you can get a very large voltage across the rectifier diode and the diode needs to be rated for this.

    The app note recommends a 100:1 ratio between the reset and the load resistors in order to limit the reverse voltage to 100 times  theforward bias voltage of the transformer. This should occur at duty cycle , D = 0.99

    Other users use a small RC filter to reset the transformer.

    It is always best to built, test and verify your results !

    Regards

    John

  • 0
    TI__Expert 3095 points
    Hi Catalin Muntean,
    I appreciate that there are many different ways to reset a current transformer.
    A resistor is likely to be simplest and cheapest. It also has the advantage that the reset voltage increases with the magnetizing current so there is in-built protection against saturation of the current transformer in abnormal conditions. In addition it will tend to damp out spikes and oscillations.
    I like your calculation method but it also has limitations. Strictly we only need to apply sufficient volt-second product to bring the magnetizing current back to its starting level, we do not need to take it all the way close to zero. If the off time is short and the magnetizing inductance is large it may not be practical to employ your calculation method.
    I hope that you find this answer of interest.
    Thanks
    Joe Leisten