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UCC28730: Equation (14) for two output system

Part Number: UCC28730

Hi team,

Customer is considering to use UCC28730 to two output system.

In two output system, equation (14) should be modified to summing up two output powers, as follows.

Lp=2×{(Vocv1+Vf1+Vocbc1) ×Iocc1+( Vocv2+Vf2+Vocbc2) ×Iocc2} / (Ipp(max)**2×fmax×ηxfmr)

Is it correct and all?

  • Hello Yoshwa-san,

    Yes, you are correct, but not complete. For a two-output system, equation (14) should be modified as you indicate to sum up the total output power. In general, this should be expanded for an N-output system, where the sum of all of the output’s powers is used to calculate Lp.

    That is not the full story, however.
    In a single output system, Iocc is defined as the output current level where constant current regulation begins. This output current is reflected as a peak limit on the primary side, and controlled by the UCC28730 by equation (13). Rcs of equation (13), in turn, was calculated by equation (12) which assumes a single output current Iocc.

    In a multi-output system, it is incorrect to expect fixed Iocc1, Iocc2, … Ioccn values for each output, because constant current regulation depends on regulating the peak primary current, not any individual output current. To design this system, the power levels of each output must be reflected to a single output (usually to the one with the maximum power) to obtain an equivalent maximum rated output current (Iout). This equivalent Iout is then increased by some percentage (usually +5~10%) to determine the equivalent constant current limit Iocc_eqv which determines the peak primary current Ipp based on equation (6). Equations (7) and (8) also use this equivalent Iocc_eqv value for standby power estimate and bulk capacitor calculation in equation (9). In general for an N-output system, the total equivalent Iocc_eqv value should be substituted into each equation that uses Iocc (or Pout or Pin) under the assumption of a single output calculation.
    (In similar fashion, equations (20) through (25) dealing with Cout and Cvdd should be modified to account for multiple outputs. An equivalent Cout_eqv can be calculated, then split up proportionately to each output according to their relative rated power levels and scaled using the turns-ratios. )

    If all outputs are fully loaded to their rated levels and overloads are applied to each output in the same proportion as the rated loading, then the Iocc level for each output will be the same percentage over the rated current for that output as the equivalent Iocc_eqv for the total system.

    If all outputs are fully loaded to their rated levels and an overload is applied to only one output “X”, then the constant current point, Ioccx, of output “X” will be a greater percentage than that of Iocc_eqv.
    The total overload allowed by the Ipp limit on the primary side will all be delivered to the one output with the overload applied.

    If all outputs are not loaded to their rated levels and an overload is applied to only one output “X”, then the constant current point, Ioccx, of output “X” will be a much much greater percentage than that of Iocc_eqv. Again, the total overload allowed by the Ipp limit on the primary side will all be delivered to the one output with the overload applied. Light loads on the other outputs means more current can be delivered to the overloaded one.

    I hope this clarifies the situation, which applies not just to the UCC28730, but to all DCM flyback systems using a single primary-side controller.

    Regards,
    Ulrich