UCC256403: Could an unloaded -12V output of LLC Half-Bridge stay within regulation?

Austin Allen

Part Number: UCC256403
Other Parts Discussed in Thread: PMP40379, PFCLLCSREVM034

Hello all,

A customer of mine has a question regarding the regulation capabilities of the UCC256403 under certain circumstances.

Above is a capture of the LLC Half Bridge schematic. The feedback loop is set to monitor the +12V.

The question is, if the +12V output is loaded to 10A, and the -12V is unloaded, can -12V output stay regulated within +/-3%? And vice versa, if -12V is the one loaded and +12V is unloaded, can the +12V output stay regulated?

Thank you for your help!

Best,

Austin Allen

over 4 years ago

0 Benjamin Lough over 4 years ago

TI__Mastermind 27095 points

Hi Austin,

Does the negative rail also need to support up to 10A load?

Could you also share what is the application? Something we have done before for audio loads that are 180° out of phase is reference the feedback circuitry to the negative rail and connected the top of the FB resistor divider to the positive rail. PMP40379 is one example of this.

Best Regards,

Ben Lough

0 Van Do1 over 4 years ago

Prodigy 50 points

hi Austin and Ben

this is Van Do. i have the main LLC control like TI model in simlis, i added secondary winding to get -12VDC out, i wonder how good the regulation of the -12V load from 0 to 3A, while the controller +12VDC OUT is able to load up to 10A? please advise. i am think adding another winding for -5V/8A too

thank

Van

0 Benjamin Lough over 4 years ago in reply to Van Do1

TI__Mastermind 27095 points

Hi Van Do,

My initial estimate would be +/-10% for the unregulated -12V output across load depending on how tight the secondary windings are coupled. Please note this estimate is not unique to UCC25640x, you will see similar regulation performance for any LLC here.

Do the output rails need to be isolated from each other? If +/-3% is absolute must, it may worthwhile to do a flyback or inverting buck-boost using the +12V rail as the input to achieve the negative outputs.

Best Regards,

Ben Lough

0 Van Do1 over 4 years ago in reply to Benjamin Lough

Prodigy 50 points

Hi Ben

I am looking at the appnote and EM for ucc256403, I don't see the start-up waveform with transformer input current. can you share that waveform or you might have it somewhere I have not get to it yet?

Thanks

Van

0 Benjamin Lough over 4 years ago in reply to Van Do1

TI__Mastermind 27095 points

Hi Van Do,

It looks like I don't have a waveform for resonant current during startup handy. Please give me a couple days to capture the waveform and share with you.

Could you also confirm which EVM you want this waveform?

https://www.ti.com/lit/ug/sluubx3b/sluubx3b.pdf

https://www.ti.com/lit/ug/sluuc46/sluuc46.pdf?ts=1611097178290

Best Regards,

Ben Lough

0 Van Do1 over 4 years ago in reply to Benjamin Lough

Prodigy 50 points

Ben

please capture the start up waveform for transformer current from the PFCLLCSREVM034, thank

Van

0 Don Dapkus over 4 years ago in reply to Van Do1

TI__Guru**** 189401 points

Thanks, Van. Give Ben a couple days to collect this waveform for you.

0 Van Do1 over 4 years ago in reply to Don Dapkus

Prodigy 50 points

hi Ben

do you have any idea why my LLC ucc256403 stays in the start up mode ( more like hick up), vout never come up, SS was trying to ramp up to certain level then drop down?

Condition: VIn 250V or higher, 100mA load

Lr=82uHm LM=400um, Cr=15nF both to VIn and Pgnd

VCR capacitor top = 68pF, and lower cap 2200pf or 9400pf

Isns, and SS pin config, same as your TI eval board

please let me know

Van Do

0 Benjamin Lough over 4 years ago in reply to Van Do1

TI__Mastermind 27095 points

Hi Van Do,

Is the converter attempting to restart every 1s? This would indicate this is likely either OCP or OVP tripping and causing a fault. How much output capacitance do you have on your board? This may give a different startup profile for the resonant current compared to the EVM board.

I would suggest to first determine if this is OCP or OVP. You can try depopulating the top BW resistor and see if the converter is able to start up. If yes, then the issue is likely more margin is needed for the BW divider setting. If the converter continues to struggle to start up then this would indicate that the converter is likely tripping OCP. To address this, we can adjust the VCR cap or adjust LL/SS to have a smaller Vssinit or adjust ISNS.

Best Regards,

Ben Lough