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UCC28700: UCC28700 and Rlc and Rcs values

MIKHAIL SALOV
Expert 6950 points
Part Number: UCC28700
Other Parts Discussed in Thread: PMP10415, PMP

Good day Colleagues,


My customer develops a power supply up to 700V based on UCC28700. 

He confused with the Rlc and Rsc values. He calculated it with our Excel design table but he looked and PMP10415 design and the value of R18 is 511Ohm what is too low. Datasheet example shows it around 5kOhm.

Could you tell me, is it wrong or how to calculate these values correctly? Are there any trade offs between these values? And what customer has to have in mind for calculations?

Thank you, Mikhail

  • 0
    TI__Expert 4015 points
    I would follow the procedure in the datasheet to calculate the values of Rlc and Rcs.
    The purpose of the Rlc is to maintain constant current limit over line input voltage by injecting a current proportional to the line input voltage across Rlc during the ON time which essentially puts a pedestal on the CS pin waveform.
    There is not really a trade off to be made here, the Rcs resistor dictates the power flow and current limit point. The Rlc just tweaks the current limit point with line voltage.
  • 0 in reply to Billy Long
    TI__Expert 6950 points

    Thank you, Billy,


    Do you know why the PMP design has such a low resistor value? it's 511 Ohm versus 5kOhm (and even few kOhm in another designs) what the datasheet shows.

    It really confuses the customer.

    Thank you, Mikhail

  • 0 in reply to MIKHAIL SALOV
    TI__Mastermind 40580 points
    Hello Mikhail,

    Please allow me to venture a possible reason for the low Rlc value. The PMP10415 design appears to be intended to demonstrate an application for multi-output flyback using a SiC MOSFET at high input voltage. The focus is on the high voltage application, not on current limit accuracy. In fact, the UCC28700 family is optimized for line compensation over the so-called "universal line" range of 85Vac to 265Vac, whereas this PMP design is targeted to start at the high end of that range, on up to 800Vdc on the bulk rail. This level is way outside of the line compensation range of the IC, so the actual value of Rlc becomes irrelevant. The compensation current out of the CS pin will be stuck at its maximum level.

    The design calculator may give a 5K result, but the calculator was not designed to accommodate this application. That value will impress too much offset voltage to the current sense signal. I believe the PMP designers simply placed the 511R resistance there to provide for potential noise filtering with an RC on the CS pin (C not populated, unless necessary). As it is, the maximum line compensation current at CS cannot exceed 48uA, since it is limited by the absolute maximum current rating at the VS input of -1.2mA, with a ratio of 1/25 to the CS pin. 48uA x 511R = at most 24.5mV of offset to the CS voltage.
    I misspoke earlier about the Rlc value being irrelevant. If it were left at the calculated value of 5K, the offset would be about 0.25V which is 1/3 the range of the CS sense threshold. This would severely affect the Rcs, Lm, and other major system-level parameters of the design. In the usual universal line applications, a few-K ohm at Rlc makes sense, since the compensation current range is also significantly lower, and compensation offset is in the 10s of mV range.

    Why the designers might need ~24mV of offset at CS, I don't know. I think it is merely a placeholder and could as well have been zero ohms. In either case, it most certainly has no function of "traditional" line compensation over the 300Vdc to 800Vdc input range.

    Regards,
    Ulrich