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UCC28056: Inductor for Boost operation.

Rene Schilling Johnson
Prodigy 70 points
Part Number: UCC28056

Dear TI Team,

Would you have a suggestion of how to design my inductor if I want to use UCC28056 as a DC/DC - converter (Boost only, without PFC) ?

My current problem is that I am not entirely sure how the CrM/DCM Control Principle of UCC28056 works. I need to find an ideal frequency, so that I can determine the correct boost inductor needed to boost my minimum Voltage (210 VDC) to my 390 VDC output. The problem that I have is that UCC28056 can change both the frequency and also the ON-Time to regulate my output to 200 W at 390 VDC. I believe that UCC28056 only uses frequency modulation for PFC purpose and  the ON-time will change to regulate output-power and voltage regulation purposes.

Could you tell me weather I am correct with my assumptions ? Also, would you have a suggestion for my switching frequency of UCC28056 so that I can design and determine the inductor needed for my application ?

Thank you very much.

Sincerely,

René

  • 0
    TI__Guru 74095 points
    One of our experts will reply to you.
  • 0 in reply to Hong Huang
    TI__Prodigy 750 points
    Hi,

    Thanks for your interest. The UCC28056 is designed to be a PFC. Hence for a sinusoidal input voltage, the PFC shapes the cycle average current to be sinusoidal. The UCC28056 can be configured as a DC input system as well. For a DC input, the line current is shaped to be DC.

    The TONMAX for the input is 12.9us. This is assumed to be the case for a 120V peak line voltage. For a VIN of 210V, UCC28056 will work with TONMAX4 ( as per page 7 of datasheet). Hence assuming that the system will be in CRM in full load, then for a 390V output, the switching period of operation can be calculated as:

    TPER = TONMAX4 * (VOUT)/(VOUT - VIN)

    TPER = ~10.4us

    AT full load, the GFFx level is calculated based on the (VIN/120)^2 and look for the closest GFF level in page 7.

    Hope this helps

    At a conceptual level. UCC28056 modulates TON to provide a constant resistance Re to the input. The frequency of operation is achieved based on the efficiency considerations by switching between CRM and DCM.

    VA
  • 0 in reply to Hong Huang
    Prodigy 70 points

    Hello Hong,

    Thank you for your precise and decisive answer. What you told me about the On-Time was exactly what I was hoping for.

    On another note, could you also tell me how exactly UCC28056 can tell, weather it senses the current or the voltage at the ZCD/CS Pin ? In the Datasheet you say that both can be done at the same pin because during the OFF-Time the current through the FET is close to zero, hency why you only meassure the voltage across the transistor. On the other hand, during the ON-Time the voltage across the FET is close to zero and the current can be meassured. Still, how does UCC28056 tell weather it meassures current or voltage ? Both signals are sensed with voltage threshhold levels at UCC28056, meaning that both the current AND voltage are beeing detected as voltage threshhold levels at the ZCD/CS - Pin of UCC28056. Is there a logic implemented that tells UCC28056 that during the ON-time it meassures the current and during OFF-time it meassures the voltage ?

    I look forward to your answer.

    Greetings,

    René

  • 0 in reply to Rene Schilling Johnson
    TI__Prodigy 750 points
    Rene,

    You are correct - The same information is sent to both the internal circuits which do the current sense and the ZCD detection. There is internal logic which knows whether you are in the TON state and when the inductor current is demagnetizing. Based on the state, we have the ability to reconstruct and blank the relevant signal and make decisions on current sense and ZCD.

    For example, the Current sense comparator logic is only active when the MOSFET is ON ( and once the blanking time is complete). The output of this comparator is blanked in blanked in every other sub-period of the switching cycle. Likewise, the ZCD logic is completely blanked until the MOSFET is turned OFF. This resolution is done internally in the waveform generator block show in page 13 of the datasheet.

    Hope this helps
    V.A.