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UCC28063A: Fooling the UCC28063A to believe that Vin is lower than the actual value to limit fmax to a reasonable value

Stephen Oberholzer
Intellectual 895 points
Part Number: UCC28063A

Good morning,

I am designing a 400W PFC with the UCC28063A.

My Vin range will be 184V RMS to 265V RMS.

To keep the inductor values small, I would like to use values of 114uH which will give a peak current of 7.58A at  Vin = 184V and fmin = 100kHz.

My problem is that for a fmin = 100kHz, RT = 22.8k and fmax will then be 2.9MHz which is way too high.

To circumvent this problem, I would like to increase the value of RT to 121k which will limit fmax to 550kHz.

According to the RT equation in the datasheet (equation 48), which relates switching frequency to input voltage, fmin will then be 18.8kHz at Vin = 184V which is also unacceptable because it is in the audible frequency range.

What can I do to make the UCC28063A believe that Vin,min = 85V (although it will be in reality 184V) so that the switching frequency will be 39kHz at a theoretical Vin of 85V (for RT = 121k) and so that the switching frequency at 184V input will be much higher, preferably at 100kHz? My problem is that equation 48 gives a switching frequency of 18.8kHz at Vin = 184V with RT = 121k. Should the frequency not increase with higher Vin? Why does it decrease? What will the exact frequency be at 184V input?

  • 0
    TI__Mastermind 35530 points
    Hello Stephan,

    I've notified the appropriate team of your questions and they should be responding soon.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    TI__Mastermind 34465 points

    Hello Stephan,

    Unfortunately the value of inductance you choose is just too small for your application.
    Equation 48 of the data sheet shows that the minimum switching frequency is 236kHz with an inductance of 114uH.
    This value is set by two of your design requirements:

    1. The max duty cycle value (which in turn is set by the DC output voltage and the peak input at low line
    2. The input power and the value of the inductance you chose.

    This is the nature of a transition mode pfc controller.

    The solution is to use a larger value of inductance.

    Regards,

    John

  • 0
    TI__Genius 9790 points

    Hi, Stephan

    UCC28063A is 2-phase interleaved transition mode PFC controller. The PFC MOSFET ON time is fixed at certain condition. But the switch frequency is decreased vs input voltage (Vin) increase, because the PFC MOSFET OFF time is increasing while input voltage (Vin) increasing, it is transition PFC theory.

    Back to your request, you cannot fool UCC28063A to believe the Vin=85V (although it will be 184V), because UCC28063A doesn't need Vin signal to control. The minimum switching frequency is only related to PFC inductance and Vin and VOUT of PFC. If you want to increase mimimum switching frequncy:

    1. increase PFC inductance

    2. increase Output voltage of PFC Vout

    Regards,

    Yunsheng 

  • 0 in reply to John Griffin1
    Intellectual 895 points

    Hello John,

    Thank you for your prompt reply and explanation.

    I fully understand what you say.

    Please consider the following: In an attempt to lower the inductance, I do my design for the full input range between 85V and 265V AC (although I will only be working between 184V and 265V AC)

    I choose fmin = 45kHz, Pout = 400W, efficiency = 0.92 and Vout = 390V.

    Equation 19 in the datasheet then gives L as: L = 254uH.

    According to equation 48 in the datasheet, the frequency at Vin = 184V will then be: f = 102kHz at full load.

    I then use equation 49 in the datasheet to calculate RT.

    Using Vinmin = 85V and fmin = 45kHz, RT works out to be: Rt = 105k

    fmax will then be: 633kHz according to equation 50.

    Am I correct in my approach?

    Please let me have your urgent reply.

    Thank you,

    Stephan.

  • 0 in reply to Yunsheng Qu
    Intellectual 895 points
    Hi Yunsheng,
    Thank you for replying so soon to me.
    I do not quite understand that you say fs (switching frequency) decreases with increasing Vin.
    In the datasheet of the UCC28063A fs is chosen as a minimum value of 45kHz at Vin = 85V (Table 2 in the datasheet)
    The actual value works out to be slightly lower as 39.2kHz in equation 48 of the datasheet.
    As I understand it, this is the minimum switching frequency.
    The purpose of choosing fmin is to also prevent the converter from switching in the audible range.
    If fs then lowers with increasing Vin, the converter can enter the audible range at high Vin.
    Where am I wrong in my reasoning?
    Please also see the reply I sent to Mr Griffin.
    Does the on-time not also adapt in conjuction with the off-time?
    Waiting to hear from you.
    Regards,
    Stephan
  • 0 in reply to Stephen Oberholzer
    TI__Genius 9790 points

    Hi, Stephan

    At a certain condition (like 85Vac input and full load output), output of voltage loop Vcomp could be constant since the voltage loop of PFC is pretty low. Because turn on time of MOSFET TON=K*(Vcomp-125mV), Ton is constant at such condition.

    Because input voltage is sinusoidal, even input RMS voltage is 85V. here suppose Vout=390V.

    when input voltage is 85V*sin(30)=42.5V, Ipk =42.5V*Ton/L=(390-42.5)V*Toff_1/L

    when input voltage is 85V*sin(90)=120.19V,  Ipk =120.19V*Ton/L=(390-120.19)V*Toff_2/L.

    Combine above too condition, (Ton+Toff_1)<(Ton+Toff_2). So switching frequency is decreased with increased Vin.

    And the Minimum frequency is designed at peak voltage of minimum input and full load.

    regards,

    yunsheng 

  • 0 in reply to Yunsheng Qu
    TI__Mastermind 34465 points

    Hello Stephen,


    You are correct in your understanding of these equations.

    If you calculate an L value for an 85V to 264V application and use a range of 184V to 264V then the minimum switching frequency increases.

    It looks like you were just using the wrong value of RT in your initial question and that gave you such a high switching frequency

    Sorry I missed that !

    Regards,

    John