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We are using UCC256404 in one of the Project. IC functions as per expectation. One of the requirement of Power supply is to provide 22A at step loading. No Load (0A) to 22A sudden step change.
During no load, IC enters in burst mode and we observed, if step load is applied then sometimes o/p collapses completely and then recovers back immediately (within a sec) and sometimes it does not collapse and provides good transient response.
Further detail investigation showed that during no load burst mode operation, RVCC is pulled to low. RVCC is a square wave with 1sec Frequency. see below image for details.
We identified at 15mA o/p Load RVCC is continuous and hence when we connect 1k resistor across o/p the RVCC remains ON always and Converter gives excellent transient response for multiple step loads in shrt time which is way above the actual requirement. See below Waveform with 1k added across 18V o/p
Blue : op current, Yellow: RVCC, Purple :Vout
with Above Details there are few questions
1. When IC is operating in burst Mode, why RVCC is fluctuating. I went through datasheet again to see if i can find any reference, but i could not find anything related to RVCC pin behavior during burst mode operation. It is Normal ?
2. we have a NTC based over temperature detection circuit, which turns OFF LLC Converter by Pulling Down BLK pin of UCC256404. This circuit misbehaves when LLC Stops operating. as RVCC also varies and goes to Zero the hysteresis designed for the Over temperature ckt does not operate.
What needs to be done to make Sure RVCC will keep its regulation level, if UCC256404 is powered.
You can see in 1st image, during burst mode, O/P remains at 18V, and number of turns in transformer winding for VCC and o/p voltage are same, so i expect VCC also to remain at 18V, which is well above the 13V of RVCC.
Hi Abhishek,
The RVCC will be zero in case of a fault state and the controller will restart after 1sec.
So, you need to identify which fault being triggered.
Please see the section 3 of the app note to identify the fault in case of the burst mode: https://www.ti.com/lit/pdf/slua834
Please let me know once you figure out the fault.
Regards
Manikanta P
i checked at no load VCC is going down
it reaches to 9.9V which is enough to generate the Fault.
i also looked at burst operation of Converter and the burst packets are not symmetrical, resulting in uneven charging of cap on Aux winding.
i think it gets discharged to lower value and results in RVCC Down.
O/P has 4mF capacitor so, o/p variation is minimum but Aux winding has 330uF cap, and it is getting discharged enough to turn off RVCC.
even for 500mA load, it comes in regulation and RVCC remains stable at 13V.
i am thinking to update the threshold of burst mode, to see if this adds the symmetry in SW node waveform during no load.
Do you have any other suggestion ?
Hi Abhishek,
Regards
Manikanta P