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UC3842 Aux power topology

Brian Wang0928
Genius 12790 points
Other Parts Discussed in Thread: UC3842

In the UC3842 datasheet, there are some questions on an application circuit.

What's the purpose of the D3 and C4? Why there two diode rectifier in the circuit?

What's the purpose of the R9?

Regards,

Brian

  • 0
    TI__Mastermind 31580 points
    Hi Brian,

    I have asked one of the engineers to respond to your post.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Expert 4015 points

    Hi Brian, this circuit is not in our datasheet. It does appear in some of our competitors datasheets for their equivalent part. Considering that many of the part values don't have units it's hard to understand some parts of the circuit. In any case I don't understand the reasoning behind the part of the circuit you have highlighted being put in.

    I recommend you ask the owners of the datasheet, or perhaps more useful would be to refer to the TI datasheet which has a fully explained application and implementation section, section 9, with an example schematic.

    Regards,

    Billy

  • 0 in reply to Billy Long
    TI__Mastermind 34465 points

    This circuit uses the bias winding to regulate the output voltage.

    The 68 Ohm load on the bias ensures that the effects of leakage inductance of the winding are eliminated.

    Since the power supply will not start up with a 68 Ohm load directly connected to the 56k bias resistor, the startup is isolated from the bias Vcc with the second diode.

    You can also see that Vcc is divided down and applied to EA- input.

    This is a low cost method of primary side regulation.