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UC2854B: Understanding Current Limiting in UC2854

Sundar Doraiswamy
Prodigy 30 points
Part Number: UC2854B
Other Parts Discussed in Thread: UC2854, UC2854A

Tool/software:

We would like to understand the current limiting mechanism of the UC2854B PFC controller.

In our design:

  • The multiplier block receives inputs from:
    • The DC output voltage through a resistor divider connected to the VSENSE pin.
    • The DC input voltage through a resistor divider connected to the IAC and VRMS pins.
  • Current sensing is implemented using a 2 mΩ shunt resistor (Rsh) placed in series with the boost inductor. The voltage across this resistor is connected to the ISENSE pin to measure the instantaneous inductor current.
  • This signal is processed through the current error amplifier, whose output is compared against the PWM ramp to control the duty cycle.

We are specifically interested in understanding:

  1. The function of the ISENSE pin in relation to the current control loop.
  2. The role of the PKLMT (Peak Limit) function in the UC2854.
  3. How this peak current limiting is implemented and how it behaves during normal operation and overcurrent conditions.
  4. How will the multiplier work.

Our application requires boosting the input voltage from a minimum of 50V up to 410V, and we would like to ensure that the current limiting mechanism is properly configured to protect the circuit and operate efficiently under varying input conditions.

Kindly explain how the ISENSE and PKLMT functionalities work in detail, specifically in the context of our setup.

  • 0
    TI__Mastermind 40580 points

    Hello Sundar, 

    Although the UC2854B is very similar to the UC2854, there are several improvements in the UC2854B that make it different enough from the UC2854 that the "B" suffix is important to apply to the part number when referring to the controller of interest.  Casual mention of "UC2854" in your second question may be confusing in some other context. 


    Let me address your questions about PKLMT first.
    The function of PKLMT is to shut off gate-drive to the boost MOSFET if the peak inductor current sensed exceeds a level determined by the designer.  
    It works with a negative current-sense polarity and PKLMT is triggered when the voltage at PKLMT is pulled from a positive offset down to 0V. 
    During normal operation, PKLMT does not operate.  
    If the peak current exceeds the programmed limit, PKLMT will immediately (after propagation delay) shut off the gate drive. 

    Since it is a peak detection without slope compensation, duty cycles over 50% may result in sub-harmonic oscillation with possible audible noise.  However, it is a protection feature so audible noise should be of lower concern than the operation of PKLMT in the first place.  In normal operation, inductor current peaks are always lower than the PKLMT threshold and the controller works in average-current mode control, so slope compensation is not necessary and there should be no audible noise.|

    The ISENSE input detects the inductor current as a voltage and the difference of this current-sense voltage from the multiplier reference voltage generates an averaged error signal which is used to generate the PWM pulses. 

    This app-note explains in detail how both of these functions work: https://www.ti.com/lit/an/slua144/slua144.pdf?ts=1748884332996 
    Although it was written for the UC2854, it is also applicable to the UC2854A and UC2854B devices.  However, in the example design procedure the specific parameters for the UCC2854B should be used in place of those of the UC2854, where they differ in value. 

    2mR is a very small value for a current shunt resistor, which implies very high current. This app-note describes alternate methods of current sensing: 
    https://www.ti.com/lit/an/slua172/slua172.pdf?ts=1748884242779
    This may help to achieve a higher signal to noise ratio for current-sensing without excessive power loss. 

    Regards,
    Ulrich