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UCC28740EVM-525: EVM Board

Vishwanath Singh
Intellectual 370 points
Part Number: UCC28740EVM-525
Other Parts Discussed in Thread: UCC28740

How to set Under Voltage limit.

As per datasheet given equation and Transformer Ratio-

 

UCC28740EVM Board BOM R2-105K and R3-27.4K

I am calculator this equation I am not found above resistance value.

Please suggest me What is  take value Npa,Nas and Vov 

  • 0
    TI__Mastermind 18805 points

    Hello Vishwananth,

    I will look into this and get back to you soon. 

    Thanks,

    Alexis

  • 0
    TI__Mastermind 18805 points

    Hello Vishwananth,

    Thank you for your patience. I'm unsure what value you used for Vov as that is decided by the user, therefore, you can have different resistances calculated from that. If you would like to solve for the EVM's Vov, I recommend using the resistor values already given with those turn ratios to calculate the Vov they used for the EVM.  

    Best Regards,

    Alexis

  • 0 in reply to Alexis_H
    Intellectual 370 points

    Hi Alexis

    1. What did you set Vov for EVM Board UCC28740EVM-525

    2. As per my assume I can set Vov any value. Example 5.5V or 6V or 7.5V 

    3. I can use EVM Board  50VDC Input with 5V/1.8 A what is value of R2,R3,C3 and C4. If other component value is change Please suggest me.

  • 0 in reply to Vishwanath Singh
    TI__Mastermind 18805 points

    Hello Vishwanath,

    I will look into this and get back to you soon. 

    Best Regards,

    Alexis

  • 0 in reply to Alexis_H
    Intellectual 370 points

    Hi Alexis

    Keep response as soon as possible

  • 0 in reply to Vishwanath Singh
    TI__Mastermind 18805 points

    Hello Vishwanath,

    Based on your questions, I think the UCC28740 Design Calculator can assist you in calculating what you need. (UCC28740-Q1 data sheet, product information and support | TI.com)   

    Best Regards,

    Alexis

  • 0 in reply to Alexis_H
    Intellectual 370 points

    Hello Alexis

    You didn't understand my question.

    1. What is set Vov on your EVM Board Design. Please tell me value.

    2.As per my assume I can set Vov any value. Example 5.5V or 6V or 7.5V then I will set R2,R3 as per design calculation(as above equation)

    3. No Change any Component, I have set EVM Board.I have found Efficiency 70% approx.But In datasheet EVM Board is given efficiency 81%.

  • 0 in reply to Vishwanath Singh
    TI__Mastermind 18805 points

    Hello Vishwanath,

    1. You can calculate for the Vov using the Resistor Divider equations as per attached in the datasheet with the Transformer Ratio given.

    Vov = [(Rs1/Rs2)*(Vovp) + Vovp + Nas * Vf ]/Nas
    Rs1 = 105k
    Rs2 = 27.4k
    Vovp = 4.6 (as shown in the datasheet)
    Vf = 0.4

    Nps = 13.66 (Turns Ratio for Primary to Secondary Winding)
    Npa = 3.9 (Turns Ratio for Primary to Auxiliary Winding)
    Nas = Nps/Npa (Turns Ratio for Auxiliary to Secondary Winding)

    Using those values above, you should get a value of ~6.7V for Vov.

    2. You can set Vov to those values and set R2,R3 as per the design calculation with the equation above.
    3. What are the test conditions when you are taking the efficiency/how are you taking the efficiency? The 81% efficiency given on the EVM User Guide is taken from when VIN is 115/230 VAC (the nominal value).

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Intellectual 370 points

    Hi

    I have done already RS1,RS2 and Vov calculation.

    I have test efficiency 230VAC 5V@1A Load and found efficiency 70%

    I have removed RT1 and again test 230VAC 5V@1A Load and found efficiency 77%.

    Please suggest me I will improve efficiency

  • +1 in reply to Vishwanath Singh
    TI__Genius 15616 points

    Hi Vishwanath,

    Doing some maths to estimate the losses at the RT1 resistor, I noticed that the efficiency drop should be very low:

    Assuming: efficiency 70% 230VAC 5V@1A load and RT1=10ohm (EVM BOM)

    IAC=Vout*Iout/(efficiency*VAC)=31mA, then Ploss=Iac^2*RT1=10mW approx., then 10mW/Pin*100=0.14%

    I would suggest to double check your efficiency calculations. According to the EVM the efficiency at 230VAC 5V@2.1A is 80%, so 77% is not very far from the target.

    Losses in a Flyback DCM converter is spread among switching losses, copper losses, leakage inductance losses, core losses, AC winding losses. UCC28740 has switching that reduces the copper losses at your MOSFET. Leakage inductance, core losses and AC winding losses depend on your transformer manufacturer. Switching losses depends on your switching frequency.

    80% efficiency is the target design goal int he EVM. Further increases could be achieved with a custom core and designed to operate at lower switching frequency overt the entire operating range.

    if you have any further questions, let me know replying to this thread.

  • 0 in reply to Manuel Alva Hernandez
    Intellectual 370 points

    Can I Design Low Voltage SMPS using Switching IC UCC28740D.

    If Yes Please suggest me.

    Input- 15VDC to 60VDC

    Output- 5V/1.5A

  • +1 in reply to Vishwanath Singh
    TI__Guru 50190 points

    Hi Vishwanath,

    The problem with using this controller for low VIN is the UVLO threshold:

    You need to have the means to get the controller started. Once running it can work down to 15V.

    A general-purpose PWM like the UCC28C55 could work for this range.