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UCC28056: Choices for bias supply (Vcc) for PFC

PAUL MESSICK
Prodigy 90 points
Part Number: UCC28056
Other Parts Discussed in Thread: LM5021, TIDA-010047, UCC28780

I'm designing a relatively low power SMPS that needs PFC to keep line THD low, but has a high output voltage requirement (250VDC). My initial block diagram is a UCC28056 followed by a LM5021 as a flyback converter. The UCC28056 looks easy to use, but it requires an 11V to 34V supply, independent from the rest of the power supply. None of the examples I find are clear on how best to do this. The UCC28056 eval board shows a 12V input from the outside world, and some other examples get this bias supply from other switchers in the system.

I'd like to have a simple design here. Since the supply current is < 2ma, is there any problem just using a simple zener to supply this from the DC side of the rectifier? I'm not trying to grind every mW out of the design--this is pro-audio gear that has a power switch, so it isn't connected all the time.

  • 0
    TI__Genius 9140 points

    Hi Paul,

    just use simple zener from DC side also works for VCC in UCC28056, or you can also use RC start up circuit comcon used in Flyback.

    Please take  the standby power into consideration.

  • 0 in reply to David Huang33
    Prodigy 90 points

    Thanks, David. That's exactly what I thought, but want to avoid any gotcha's.

  • 0 in reply to PAUL MESSICK
    Prodigy 90 points

    So I'm still a little confused. My application is a universal supply, so using a zener turns out to be a non-starter. Looking through various EVM schematics that use the LM5021 I see various ways to power the chip. In many of them, there is only a cap to ground on the Vcc pin, with no other connections. In at least one, there is a connection for an external bench supply. In many of them, there is a resistor connecting between the CS pin and Vcc.

    How exactly do I supply Vcc to this part in a real standalone, shippable power supply?

  • 0 in reply to PAUL MESSICK
    Prodigy 90 points

    Sorry, I mean to be referring to the UCC28056 PFC controller, not the LM5021...

  • 0 in reply to PAUL MESSICK
    TI__Genius 9140 points

    Hi Paul,

    can you confirm there is a reference design connect VCC to CS through a resistor?

    my understanding is CS (pin 2)is for ocp and ZCD detecting, it should not be connected to VCC.

  • 0 in reply to David Huang33
    Prodigy 90 points

    This is what happens when I try and do two designs at once: one with an LM5021 and one with UCC28056. I've mixed up the two here. You are correct: CS does not connect to Vcc in any way on UCC28056, but it does on the LM5021 reference designs.

    So, back to the UCC28056. The examples I find either expect Vcc to be supplied from an external bench supply, or derive it from LLC controllers or other downstream sources. For example, this circuit from the TIDA-010047 reference design:

    This is hard to parse: AC_IN doesn't appear to be a signal anywhere in the schematic, and the anode of D19 is tied to VDD3 which appears to come from the downstream UCC28780 flyback controller. This circuit looks promising, but it's not easy to see how to use it in the general case. I've also seen an EVM on the TI site (which I can't seem to find now) that uses a small buck switcher to generate the 12V. That also seems quite complicated when the task is to supply 2ma for Vcc.

    So I'm still looking for a best-practice Vcc supply for my particular design.

    Paul

  • 0 in reply to PAUL MESSICK
    TI__Genius 9140 points

    Hi Paul,

    yes, any circuit has its benefit and drawback, and it is hard to say whether it is good or it is bad.

    For UCC28056, most the the case uses downstream to supply it.

    Please allow me to close the post. If you have further questions, please create a new post, thank.