• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 62 subscribers
  • Views 195 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

UCC28711: UCC28711 Circuit design calculations

UCC28711: RCD+Z Clamp Design for TIDA-00173

IgorR
Prodigy 85 points
Part Number: UCC28711
Other Parts Discussed in Thread: PMP22288

I am using TIDA-00173 reference design guide and I do not see any mention of how the RCD+Z clamp was selected for the design.

I would like to know more details about component selection for the clamp.

Thanks

  • 0
    TI__Mastermind 23165 points

    Hi IgorR,

    Thank you for the query on UCC28711.

    The maximum input voltage is 1200. Considering the turns ratio is around 12, the 24V reflected voltage is around 288V. The voltage across the primary is 1200+288+leakage spike. D20 and D21 have clamping voltage of around 171V depending on the peak current into the clamp. 

    The bottom MOSFET has a voltage rating of 900V and it is essentially clamped to around 700V through the D16 and D15 in the schematic (which can be adjusted if more margin is needed for the top MOSFET). So the two TVS diodes essentially clamp to 1200V+171*2 = 1540V (leaving some margin for leakage spike).

    D19 and D21 are ultra fast diodes (two of them in series to handle the voltage rating).

    R4 and C33 needs is tuned based on the leakage spike ringing which is observed. Generally we start with resistor between 100-500k and capacitance from 470pf to 5nf and tune the system.

    Please refer the following document for detailed design of snubber circuit.

    https://www.omicron-lab.com/fileadmin/assets/Training_and_Events/Power_Analysis_and_Design-Symposium/2022/Presentation_BirichaDigital.pdf

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 85 points

    Thanks Harish.

    Can you also explain the purpose of the R24 C37 and D18 components in the VFB signal path shown below.

  • 0 in reply to IgorR
    TI__Mastermind 23165 points

    Hi IgorR,

    Thank you again for the query.

    Ideally there only needs to be resistive divider from the aux winding to Vs pin. If your application has high voltage input, it is necessary to protect Vs pin and to ensure that this pin is protected against maximum current of 1.2mA.

    So generally we recommend the following circuit for this purpose. You can refer PMP22288 for this.

    Coming back to your question on the requirement of above components, from my limited experience on testing this board, I had some issues with the Vs pin without the capacitor and putting this 1000pf stabilized the waveform on Vs and it was able regulate better. So probably it had some layout issues, in terms on noise into this pin. So with a proper layout, these components should not be needed. You can only use the schematic snippet I have attached above.

    Regards,

    Harish