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UCD3138: UCD3138

robin gangopadhya18
Mastermind 6296 points
Part Number: UCD3138

UCD3138 has an example implementation PFC

Curious question: if the line current is forced to be in phase with the line voltage, then the "converter" is presenting a " positive resistive " load to the ac system "on the average"..

Yet, the converter is a " constant power load (CPL)- which is characterized by a "negative" input impedance.

How do we reconcile these two?

Is it possible to instrument  for measuring the input impedance of  such a converter by code in the UCD? The display is of course the computer is it connected to   via the PMBus.

Then why not the loop response as well by code? Because all sensors are there in the circuit.

thnx

robin

  • 0
    TI__Expert 7260 points
    It is possible to measure the input impedance through UCD. You will need to measure Vin and Iin through ADC, calculate RMS value, and then calculate the input impedance assuming the power factor is 1. However this impedance does not include EMI filter.

    It is also possible to measure loop response through UCD, but that will take lots of CPU MIPS and may affect PFC performance.

    "Line current is forced to be in phase with the line voltage" means a "pure resistive" load, not a inductive or capacitive load, and it works like a "negative resistive" load.

    Regards,
    Bosheng
  • 0 in reply to Bosheng Sun
    Mastermind 6296 points
    Thnx Bosheng for the prompt reply.
    I find the Paradox of CPT vs "emulated resistance " in PFC is resolved by very clever reasoning. In fact, it is a reminder that "negative" is only a dynamic differential situation. AC source still drives power into an "emulated positive " impedance. It cannot be driving real power into a net "- resistance " & still be " in phase". One can still apply Middlebrook's theory that diff ac drive |Yof|>> |Yic| diff input of CPL|. So that Yic+Yic>0
    f for filter, c for converter for stability.