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Powering the ISO124 with UCC2808

Jamaal_Charles
Genius 3245 points
Other Parts Discussed in Thread: ISO124, UCC2808-2, SN6501

Hello,

I need to design a supply with 2kV of isolation with a 12V or 24V input and a +/-15V output to power the ISO124.  I found this reference design, which uses the UCC2808-2.  In the PMP2534 reference design, pg 3, the FB and CS pin of UCC2808-2 were tied to GND, but I'm not clear on how it regulates the output voltage at +/-12Vdc?  Another issue with this reference design for me, is that the transformer only provides 500V isolation.  I'm having a really hard time finding a transformer with 2kV.  Any thoughts here?

If I want to get +/-15V, what components should I modify?  My guess is either RC to change the oscillator frequency or the transformer to change its windings.

In UCC2808 datasheet, pg 7, the application circuit shows that the FB pin provides output voltage feedback loop in terms of voltage regulation.  Is that correct?

If there is a different part I should use, I'm open to suggestions. 

Thanks!

  • 0
    Prodigy 5 points

    You are correct, it does NOT regulate it.
    Given a fixed input voltage, and fixed output voltage, an open loop PUSH PULL will provide the VOUT needed based off the TURNS RATIO of the transformer.
    The UCC3808 is running at max duty, 50% per channel, because FB is tied to ground.
    The duty cycle and transformer turns ratio directly determine the output voltage.

    CS tied to ground just does not implement any primary current limit protection.

    Regulation will depend on VIN tolerance and LOAD variations reflected in the winding losses of the transformer and PCB traces.
    Usually the load is fairly constant with the analog chain that follows it.

    So Vout in this case would be proportional to the range of VIN.

  • 0 in reply to Ed Walker
    Prodigy 5 points

    We have an SN6501 that is design just for this small isolated output needed for the ISO devices.
    It also explains the topology in the data sheet.

    http://www.ti.com/product/sn6501