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UCC28634: What is the purpose of R_HV?

Piotr Wyderski
Prodigy 150 points
Part Number: UCC28634
Other Parts Discussed in Thread: UCC28630, UCC28710

I am designing a 1kV V_IN flyback based on UCC28634 and have a problem with the startup circuit. The part has a steerable 700V current source connected to the HV pin, which is 300V too low in my application. I presume that it internally is a depletion-mode MOSFET or a JFET. So I am planning to cascode it with an external depletion-mode MOSFET with the necessary ratings: gate to VDD, source to HV through a current-limiting resistor and drain to the input voltage. The resistor would be calculated to source 1mA or so, and the MOSFET would be steerable by the IC. But the datasheet demands a rather low tolerance resistor connected to HV (180..220k, table 7.3). What is the purpose of that part? If indeed inside the IC there is a DMOS/JFET current source, then the additional resistance in its drain connected to HV should have no observable purpose. Do I still need this resistor in my external transistor-based startup circuit?

  • 0
    TI__Guru* 85696 points

    Hello,

    That resistor is used to control the discharge time of the Xcap if the UCC28630/33 devices are used.  It is explained on page 16 of the data sheet.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 150 points

    Mike O' said:

    Hello,

    That resistor is used to control the discharge time of the Xcap if the UCC28630/33 devices are used.  It is explained on page 16 of the data sheet.

    Regards,

    Hi Mike,

    but this is the UCC38634 part and the datasheet makes no exception here: the resistor seems to be always required.

    Best regards, Piotr

  • 0 in reply to Piotr Wyderski
    TI__Guru* 85696 points

    Hello Piotr,

    The systems engineer that designed the device must have had a reason for doing adding the resistor.  I am not sure exactly what that was.

    We do have many other PSR flyback controllers with internal startup that don't require the resistor.  You might take a look a the UCC28710/1/2/3 devices that don't have this resistor requirement on the HV pin.

    Regards,