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Question about Current-sense Transformer reset design of UCC28070

Randy Chen
Intellectual 2295 points
Other Parts Discussed in Thread: UCC28070

As shown on the Figure 25. Possible Reset Networks in the UCC28070 datasheet, any detail calculations/formulas for these 3 types of Reset network?

What are the pros and cons for these 3 types of Reset network?

Any preference of reset type in the application?

Thanks and best regards,

Randy

  • 0
    TI__Mastermind 34465 points

    Randy,

    The current transformer can be reset with either of the three methods shown in Figure 25

    • The most popular is the method shown in the middle  section (diode with  resistor) and this method is described in SLUA479B.

    It requires that VuS(on)max = VuS(off) max . Since Dmax is set and the max sensed voltage is known then the max reset voltage can be calculated.

    (and also kept to a safe voltage level). DDesign details are available at

    • The capacitive method of reset ( with CRST and RRST) takes advantage of a resonant type of reset of the current transformer.

    This method avoids the hard switching of the previous method with its associated EMI signature.
    For most applications CRST is only a few pF and RRST is about 10 times the value of the current sense resistor. The exact values depend on the CT parasitics.

    • The final method of CT reset is achieved with a zener clamp on the reverse sensed voltage
      This method is self explanatory with the reverse voltage being clamped by the zener.  This method is not used a lot.
      It adds a higher component cost and the power dissipated in the zener is very dependent on the leakage inductance of the CT.

    Hopes this helps.

    John Griffin