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UCC28070 Vout dropping at lower Vac range

Evan Shultz
Intellectual 285 points
Other Parts Discussed in Thread: UCC28070

We have a 6kW UCCC28070-based PFC supply. Vout is 403Vdc, and it should operate throughout the full universal input range.

At 160Vac and above, it will deliver 6kW stably. At 120Vac and below, power drops off because Vout sags. I can only deliver ~5kW @ 120Vac. After a burst of @ 5.5kW for a couple seconds, Vout falls to ~380Vdc and the power drops.

If I trigger my scope on Vout falling to 380V, I see that the CSA and CSB pins are completely below 3V. The duty cycle is ~50%. So it doesn't appear to me the UCC28070 is pushing the supply as hard as it can to maintain a regulated Vout. The traces are as follows: Yel - PFC Vout, Cyan - CSB, Mag - Vac, Grn - Iac.

The max duty cycle clamp is set to 95%. I adjusted the voltage going to CSx and PKLMT to no avail.

What could prevent the controller from cranking up the duty cycle and/or allowing higher FET current once Vout has fallen out of regulation? Because this is line voltage dependent, it seems like there's a power limit somewhere in the system.

  • 0
    Intellectual 285 points

    We started probing around on various IC pins that may tell us interesting info, starting with VSENSE and then moving to VAO, IMO (if we can measure it), and CAO.

    At the peak of the AC cycle, where IAC is at maximum, VAO was pegged at 5V. VENSE had fallen to 2.8V. So the voltage loop was trying as hard as it could.

    Pages 24-25 of the datasheet indicate that a higher RIMO should allow greater input (and thus output) power. This also makes sense when considering the multiplier output is a current going onto RIMO. A bigger RIMO means a higher voltage, and a higher trip/reference point on the CAs.

    Increasing RIMO increased the output power as expected, and resulted in reaching the current limit (which resulted in a frightening hash/buzz noise). So apparently the problem is that RIMO was too small. Next step, revisiting calculations to find out where we went wrong, and investigating raising the current limit.

    While it seems that we're on the right path, additional assistance and/or tips would be very welcome.