• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 290 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

UCC29910A: The problem of the voltage sampling circuit's voltage NPN transistor at the output end

Jon WANG
Intellectual 850 points

Part Number: UCC29910A

Tool/software:

Dears,

    My customer want to use UCC29910A for 500-900VDC input,320VDC/500W/1.6A Buck output.The reference circuit of UCC29910A 85V output evaluation board has an output terminal voltage sampling circuit, which uses NPN transistor. As the customer's output voltage is 320V, the customer wants to know what kind of NPN transistor specification needs to be changed for this sampling NPN transistor? If a high voltage NPN transistor is needed, it is difficult to find an NPN transistor of this specification. Is there any other voltage sampling circuit that can replace it?

  • 0
    TI__Mastermind 40580 points

    Hello Jon, 

    The level-shifting transistor inside the HS Bulk Sense block is a PNP device, not NPN. 
    Usually this controller is used in a "universal-line" AC input application, and PNP transistors with 400V+ ratings are relatively easy to find. 

    In your application, Vce must be rated for > 900V.  A P-channel MOSFET can also work, but the PNP is preferred because a) it is less expensive, and b) its Vbe drop is less of an error to voltage sensing than the Vgs drop of a MOSFET. 

    Although such a level-shifter requires a high-voltage semiconductor, the power dissipation is very small. Still, it may be difficult to find a physically small device due to the voltage rating.  There are isolating ICs that can be used to sample the output voltage and convey the signal across an isolating barrier to the low-voltage controller PGND reference, but these are much more expensive than a simple PNP.  

    I suggest to search through distributor inventory to find the lowest-cost part that meets the level-shift requirements. 

    Note: if a single device rated for >900V cannot be found, it is possible to cascode two lower-voltage parts in series to make up the total voltage requirement.

    Regards,
    Ulrich