UCC28056: how UCC28056 decide constant on time and duty cycle at start up

Hello,

Our office is closed for a US holiday today. We will get back to you on your question shortly.

Best regards,

Don

Hi Fred,

I'm not sure I understand your first question, but from what I can tell, section 8.3.2.1 in the datasheet answers that: "Prior to the start of switching operation, at power - up or after a Burst - OFF period, the ZCD/CS pin voltage is sampled and used to select the appropriate starting GFF level". TONMAX0-7 will then be chosen based on that GFF level

To your second question, can you please clarify your question.

Thank you,

Ray

Hi Ray,

1.  I assume the feed-forward voltage is being compared at this peak detector, 

before any switching, since the Vin is exactly the ZCD voltage. does that mean ZCD has to sample through out whole 60Hz cycle( or maybe half) to know the peak input voltage(ex.230V) and decide the correspond GFF?

2. as for second question, it said before it decide the correspond  GFF, it will apply VFF to COMP voltage(Vco),

but I don't really get what it means in the datasheet, would you elaborate why does it apply VFF to Vco?

Hi Ray,

thanks a lot, I know why PFC needs to integrate the square of Vin to cancel the line voltage influence.

want to finalize this with last question.

so you mean the Vco acutally represent the square of Vin part,

I saw there're PDem and Gff before the the Ton/TDCM, does that mean PDem is the square Vin part you mentioned that from Vco?

got it , thanks Ray

Hi Ray.

sorry for updating this forum again, recently customer bump into new issue, and it's the gating issue to our design-in so need to consult with you.

1. I know GFF will be fixed at very beginning when you can directly sense Vac by ZCD/CS pin,

but I wonder if PFC operate normally for a while, then the Vac increase from 110V to 264V,

will the Ton/Gff change accordingly or it will be still and fixed?

2. and since we don't have Vac pin like UCC28064A, is there a tradeoff or side effect especially when high input voltage like 264V?

(duty cycle will be very low)

Hi Fred,

1. GFF/TON will update when the input voltage changes per the VFFxRise and VFFxFall thresholds.

2. Both PFC controllers were designed to work over the Universal voltage range of 90-264Vac. The UCC28056 uses discrete levels for determining Ton (starts with Max TON for lowest VIN. As VIN increases, TON decreases to maintain same power envelope). TON for the UCC28064A is analog (does not use discrete levels) and can be shifted though selection of RTSET. However, this wouldn't result in a limitation at high Vin.

Please describe the customer issue so I can help. You may want to consider opening an E2E on the TI INTERNAL forum so that info can be shared confidentially.

Regards,

Ray

Hi Ray,

I'll create another internal forum later just for customer issue.

sorry I thought feed forward voltage means the Vin has to be sense directly at the very beginning 

(both side of inductor is Vinac)

since once PFC starts to operate the VDS voltage will be close to Vout instead of Vinac,

so does it means the "Vin" triggering  VFFxRise  doesn't have to be sensed directly (the real Vin),

instead it can also accept synthesized Vin so that even PFC is already operating it can still change Ton/GFF, is that right?

regards,

Fred

Hi Ray,

1.so I would take it as yes that GFF will change during the operation according to the Vrec if we increase the Vac rms value, right?

2. but the difference is during startup stage, the Vinac is sense directly through inductor instead of rectifying and synthesized,

since there's no current flow through inductor at that time, right?