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UCC28704: Flyback Controller Zenner Diode requirement for "UCC28704DBVR-1" part,

Sakthivel B
Prodigy 60 points
Part Number: UCC28704

Tool/software:

Hi there,

we are designing a new product which requires a 230VAC to 5V DC with 2.5A of current rating, and we found that the flback controller part "UCC28704DBVR-1" is best suit for us,  

In that we have few doubts please answer the same.

1, We found there is a difference in EVM baord ("UCC28704EVM-724 10-W USB
Adapter") schematics and a TI work bench schematics, the Zenner Diode DZ in Work bench is not available in EVM baord. pelase confirm which schematics should we follow

      

2, The Maximum recommended voltage applied across the VDD = 38V as per the datasheet. But in both the schematics the VDD Pin is directly exposed to the 200V DC, Could you please confirm that do we need any other additional measures to overcome this issue ?

3, How to finalyse the trickle resistor value in the SLU603B Calculation sheet only the user input colum is there but there is no actual calculated value is available.

Thanks in advance for your response,

Sakthivel

  • 0
    TI__Guru* 82686 points

    Hello,

     

    Your inquiry will be reviewed in the order has been received.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 82686 points

    Hello,

    Please see my comments below.

    1, We found there is a difference in EVM baord ("UCC28704EVM-724 10-W USB
    Adapter") schematics and a TI work bench schematics, the Zenner Diode DZ in Work bench is not available in EVM baord. pelase confirm which schematics should we follow

    • Sometimes for safety engineers will add the Zener diodes to the VDD pin to protect the device from high voltage caused by such things as lightning strikes.
    • The EVM was not designed with this Zener because in this application it was not going to be exposed to lighting strikes.

          

    2, The Maximum recommended voltage applied across the VDD = 38V as per the datasheet. But in both the schematics the VDD Pin is directly exposed to the 200V DC, Could you please confirm that do we need any other additional measures to overcome this issue ?

    • In a normal application the VDD pin will trickle charge up through the trickle charge resistor until VDD reaches 21 V (VVDD(on). Once the device is active the current through the VDD pin will be 2.3 mA (Irun).  The current through RT that charged the CDD capacitor will be much lower than that and the VDD capacitor will not longer charge.  It will actually start discharging the CDD capacitor until the aux winding can supply to VDD.  So in all practical applications the  VDD voltage will not really see more than 21 V.

    3, How to finalyse the trickle resistor value in the SLU603B Calculation sheet only the user input colum is there but there is no actual calculated value is available.

    • In off line applications most, customer choose to use the 15.33M ohm for the trickle charge resistor (RT). However, if you wanted to dig into this deeper you could size it based on Irun minimum current.  RT > or = Vinmin/(1.65mA*10).  Please not that you may want to increase the resistance value to reduce power dissipation.

    Regards,