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UCC28730-Q1: DC-DC PSU design

Dhaval Ravat
Prodigy 20 points
Part Number: UCC28730-Q1
Other Parts Discussed in Thread: UCC28781-Q1, UCC28C50-Q1, UCC28782, UCC28781, UCC28730

Hey TI Team,
I hope you are doing well. I needed your recommendation on using UCC28730-Q1 for DC-DC applications such that it meets the below-given specifications. Efficiency as well as quiescent current are the key parameters in my design. The quiescent current I'm aiming for is around 500uA at no load condition, which from the datasheet, I think is possible as the standby power of the device is around 5mW (Correct me if I'm wrong). I'm evaluating 2 TI ICs for the DC-DC converter: one is the UCC28730-Q1 and another is the UCC28781-Q1. I want to use this thread for my understanding of using UCC 28730-Q1 or UCC28781-Q1 to meet the below-mentioned specs. 

Flyback Converter Specifications

Sr. No.

 Parameter Min Typ Max Units
1 Input voltage 60 100 150 V
2 Output voltage 12.5 13 13.5 V
3 Power 30 W
4 Current 2.2 2.3 2.4 A
5 Efficiency > 85 %
6 Isolation 3 kV

Note: I was previously using UCC28C50-Q1 (PSR feedback) to meet the above specs, but the design with UCC28C50-Q1 had very poor output regulation at no load, and the quiescent current was also around ~6mA.

What parameter should I change in the calculator of UCC28730 and UCC28782 (in place of UCC28781) to use it as a DC-DC controller. Currently, when I set the frequency to 0 (for UCC28730), a few cells of the recommended value show errors. If you have a DC-DC calculator for the same, can you please share it with me?

Regards, 

  • 0
    TI__Mastermind 22915 points

    Hi Dhaval,

    Thank you for the query on UCC28730.

    The UCC28730 can help to achieve low standby power it because it is able to switch at extremely low frequency (32hz) to minimize bias power.  However, many other system level consideration factor into the total stand-by loss, including leakage currents of the semiconductors and bulk capacitors.  

    You cannot put frequency 0Hz as this is not recommended by the tool. Please download a copy of the calculator from the website, it should be having values which are used in EVM board.


    For a 30W design, <5mW may not be achievable, but <15 - 20mW might be achievable with diligent design work.    

    Similar analysis from UCC28781 EVM shows less than 25mW should be possible (with Xcap disabled).

    Please through the EVMs for choice of components selection.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 20 points

    Hello Harish,

    Thanks for the response. Yes, I did the calculation for Pstby for UCC28730 and it was around 14mW. For component selection for UCC28730, can you share a calculator that can calculate for the DC-DC converter? As the datasheet has a lot of parameters that are associated with AC-DC so does the calculator for UCC28730 which is available online. As for the EVMs, I'm referring to PMP22288 as this is the closed DC-DC design which I can find with this particular IC. 

  • 0 in reply to Dhaval Ravat
    TI__Mastermind 22915 points

    Hi Dhaval,

    The datasheet calculator does not provide an accurate representation of standby loss calculation. For th DC-DC case just multiply the AC voltage term by sqrt(2) and I have attached an unlocked claculator for your reference.

    Thank you

    Regards,

    HarishSLUC579_UCC28730 Design Calculator.xlsx

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 20 points
    Hey Harish
    PFA below: Calculations done on the unlocked calculator
    A few doubts that I had with the calculator are: 
    1. For Rs1, Rs2 and Rlc values, it takes reference from the 'LOOKUP TABLES AND DROPDOWN LIST'. If I calculated the Rs1, Rs2 and Rlc values from the datasheet, I would get a different value compared to what the calculator recommended.
    2. What exactly is "Minimum Allowable Output Voltage Due to Load Step" Is this value the delta output drop (when Iocc is reached) or the output voltage (when Iocc is reached) ? 
    3. The value of the input capacitor recommended by the WEBench for the same specs is very different from what the calculator suggested.
    4. Since I'm not planning to use any wake up IC for my design, will the size of the output capacitor recommended by the calculator be enough for the design? I use the simpler formula COUT > Iocc/(Vcq*fmax) for my calculations, and hence I'm using a 650uF 50V capacitor over there.
    Regards,
    Dhaval Ravat
  • 0 in reply to Dhaval Ravat
    TI__Mastermind 22915 points

    Hi Dhaval,

    Thank you for reaching out.

    1. There is only a slight difference. For example Rvs1 is 58k as per the formula in datasheet. Whereas the one in excel sheet optimizes based on available resistor values. It is there in a sheet titled "LOOKUP TABLES" which I have attached. So as long as your able to find resistors close to datasheet it should be fine. Even the calculator should not provide a lot of difference to performance

    2. That depends on your application and allowable dip at the time of transient. It is going to affect the cap value chosen.

    3. Please follow the datasheet or calculator

    4. Without a wakeup IC you might need a big cap at the output. Generally this is around 1.5mF-2mF max.

    3022.UCC28730_13V_30W(Calculations).xlsx

    Regards,

    Harish