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UCC28600: Selection of OVP resistor

siddartha koganti34
Prodigy 70 points
Part Number: UCC28600

Hi all,

I am designing a AC/DC converter for 85 to 265VAC range for 3.3V output and 40W power. I have a doubt in selecting the OVP resistor as per the equation given in the design consideration. Vout(shutdown) in the equation of ROVP2 is given as allowed output over voltage level. Why is output overvoltage level on the secondary side considered in the equation when the winding ratio of primary and primary bias windings are taken into consideration. Or is it the primary bias winding maximum voltage which is to be considered for the calculation of this particular resistor. Please advise on this.

  • 0
    TI__Mastermind 40580 points
    Hello Siddartha,

    Thank you for your interest in the UCC28600. You have uncovered a mistake in Equation 4 of the rev-K version of this datasheet (DS).
    The term "Np" in the denominator of Equation 4 should be "Ns". This correction would then reflect the secondary output voltage to the bias winding by the proper ratio to be detected at the OVP input.

    A similar equation in the Design Procedure section (equation 22, sub-section 8.2.2.2) is already correct in this DS. Please use this one for your design calculations. I also checked the UCC28600 Design Calculator Tool (SLVC104J) and it is correct.

    Equation 4 had been correct in previous versions of the DS, but it appears to have been corrupted during an attempt to generalize the equations from specific turns numbers to turns-ratios. I believe this is because the specific turns number of a transformer's windings may not be known, but the turns-ratios are fundamental design parameters and should always be known or derived.
    Evidently, equation 22 was converted from turns to turns-ratio correctly, but equation 4 was not.

    I will submit this error to the appropriate group to be corrected as soon as possible in the next version of this DS. Thank you for catching it.

    Regards,
    Ulrich