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UCC25630-1EVM-291: changed IC to UCC256304 work with UCC28056EVM-296

Part Number: UCC25630-1EVM-291
Other Parts Discussed in Thread: UCC256304, UCC28056EVM-296, UCC28056, PMP20612, LM8364

Dear all,

My customer is using UCC256301EVM-291. IC is changed to UCC256304.

And he connected to UCC28056EVM-296.

Two problems happened when AC input voltage slowly down.

1. The AC voltage when the output stops depends on the load value.

Output stops at AC78V (load 10A), AC43V (load 5A), AC23V(load 1A)

2. Output Hunching at 10A load. Waveform No.4

Not Hunching at 1A load (Waveform No.3) and 5A load (Waveform No.5)

The waveform is attached powerpoint 7 pages.

I need the reason why? and countermeasure with no parts addition.

Because the customer selected these ICs  at the point of view less parts count.

He need quick answer.UCC25630-1EVM-291wUCC28056EVM-296.pptx

  • Hi Doi-san,

    Thanks for your interest in UCC25630-1EVM. UCC28056 does not have a brownout feature. When you ramp down the input voltage, it will continue to try to deliver power to its output. The PFC will eventually hit cycle by cycle current limiting and the PFC output voltage will begin to droop as the line voltage is further decreased. When the PFC output voltage droops, this also means in the input voltage to the LLC also droops. The LLC will try to decrease its switching frequency to increase its gain to maintain regulation. If the PFC output voltage continues to drop, the LLC can enter OCP protection. This is likely the fault condition shown on slide 6 of your powerpoint. 

    When you changed the UCC25630-1EVM to install UCC256304, did you adjust the BLK divider ratio? You can adjust the BLK divider ratio so that once the PFC output voltage begins to droop, the LLC will shut off and you will avoid the droop in the LLC output voltage or the OCP faults.

    Best Regards,

    Ben Lough

  • Ben-san,

    UCC28056 have Brown-in AC85V fix  but do not have Brown-out.  Is this right?

    If Customer want  "Like Brown-out" at AC70V,  not depending on secondary load.

    Additional opAmp cct., such as "UCC28180- LMxxxx  NDA cct"  is required?

    Your recommended " PFC output drop -> sense Vbulkstop --> LLC Stop" can avoid hunching but the brown-out point is depends on load. It's not acceptable.

    Do you have any idea?

  • Hi Doi-san,

    Yes, UCC28056 has brown-in but does not have brown-out. An external circuit would be required to implement a brownout function. An example can be found in PMP20612 reference design using You can use this circuit to pull down the COMP pin of UCC28056 to disable the controller when brownout is detected.

    Best Regards,
    Ben Lough
  • Ben-san,

    I said by 1st sled "Because the customer selected these ICs at the point of view less parts count."
    UCC28056 has brown-in, so only needs brown-out.
    Do you know OPA's cost? I'ts cost is over half of UCC28056!
    Can you make Brown-out cct, do not use OPA , use a Mosfet.
  • Hi Doi-san,

    Here is a mosfet based brownout circuit.

    Best Regards,

    Ben Lough

  • Ben-san,
    Thank you but too much parts.
    This IC's merit is small space.
    Sxmsxng must need brownout, you know.
    I can not believe your IC specification plan
  • Ben-san,

    UCC28056 COMP pin is Input and Output terminal.

    If mosfet is on, COMP pin is connected to GND through Schottky Diode.  Is it ok for IC internal output cct?

    If ok, what is the recommended Schottky  type no.  and recommended Vf at it's current.

  • Hi Doi-san,

    The circuit needs to be able to pull the voltage of the COMP pin below 500mV. The diode/FET needs to be able to sink ~120uA out of the COMP pin. The diode will need to have Vf < 500mV and current rating >120uA. A number of small signal diodes should be able to fulfill this criteria. DB2L33500L1 is one example.

    Best Regards,
    Ben Lough