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UCC28060 Stability Analysis

vidan jovanovic
Prodigy 40 points
Other Parts Discussed in Thread: UCC28060, UCC28063, UCC38050

I want to optimize closed loop performance for pfc UCC28060 with regard to phase margin and cross over (0dB) frequency.

Design goal is to reach more than 50 deg of phase margin with cross over at about 10 hz.

output capacity at 660uF, vout at 400 volts and 800 watt load.

Does this IC have feedforward function and what is the internal gain with respect to voltage output beyond the internal transconductance amplifier?

Information is not clearly represented as in UC38050 data and application notes where each block is identified.

Thank you for help,

Vidan Jovanovic  

  • 0
    TI__Expert 3200 points
    Vidan,

    I will request for an Application engineer to respond to your questions.
  • 0 in reply to Brian Johnson96
    TI__Mastermind 34465 points

    Vidan,

    The information on loop compensation of the UCC28060 is detailed in page 29 of the data sheet.

    The design is optimized to provide a 45 degree phase margin at 1/5 the line frequency.

    You can use Equation 43 to 47 to calculate loop compensation components or use the calculator tool for this part:

    Most users of this device will use this tool to calculate the error amp components (Rz, Cz and Cp) and optimize the design by
    either step load response or by extracting the Bode plot with a network analyzer.

    Also you should note that the gate drive on time varies with the error amp output voltage by a factor called the KT.
    There are two values of KT depending on whether the input voltage is in a low line or high line range. (detailed in page 17 of data sheet)
    This may result in a non-linearity in the case of a large signal input line transient.
    You may wish to consider using the UCC28063 ( in which case KT is fixed) if your design has a wide ranging input voltage.
    The UCC28063 has a PSpice model that you may find useful for evaluation.

    Regards
    John Griffin

  • 0 in reply to John Griffin1
    Prodigy 40 points
    John,
    Thank you for your fast response.
    Yes, I did see those equations. However those equations do not cover the variations if the output capacitor is increased or if the output voltage is changed.(it is only for particular design) In addition I want to increase phase margin, I do not want to work with 45 deg.
    I need the gain equation for Vout/Voerror , final output and error amplifier output relationship. I am sure your design engineer can provide this information. In addition, is there any feedforward functionality for this part?
    If not, system has to work with gain variation of 3 to 1 as line voltage changes.
    Thank you for your support,
    Vidan Jovanovic
  • 0 in reply to vidan jovanovic
    TI__Mastermind 34465 points
    Vidan,
    The input voltage VINAC is only used for brownout detection and recovery. It is not part of the feedback loop.

    The gain Vout/Vcomp = (400/6)*Gm*Rp ,where Gm is the error amp trans conductance and Rp is the resistive load on VCOMP.
    Equation 3 in the data sheet,
    Ton = KT*(VCOMP - 0.125)
    sets the relationship between gain and MOSFET on time.
    Hope this helps.
    Regards
    John
  • 0 in reply to John Griffin1
    Prodigy 40 points
    John,
    You are doing very good so far.
    My request is for something that is not on the data sheets or application notes for the device. I am asking for Transfer Function relating the final output voltage and its capacitance(typically around 400 volts) to transconductance amplifier output. Since there is no feedforward this function will depend on ac input voltage as well. It will depend on kT resistor. It is basically a Boost Transfer function.
    Please, ask your design engineers to provide this information. (UCC38050 is an example where TI provides transfer functions)
    Thank you for your support,
    Vidan Jovanovic
  • 0 in reply to vidan jovanovic
    Prodigy 40 points

    JOHN,

    TI document SLUU138A provides voltage loop  equations 21 through 24.(This is for UCC38050)

    Can we obtain similar equations for UCC28060?

    Thank You,

    Vidan Jovanovic