UCC28881: Isolated flyback design - troubles with output power

Zdenek Jircík

Part Number: UCC28881

Tool/software:

Dear support

I'm testing my design of UCC28881 flyback voltage source. It works in case of lower input voltage cca up to 90VAC. Once it is barely reaching the requested output parameters (cca 24VDC / 0.15A), it is starting to limit switching somehow. Some protection is probably launching. There is in datashet I(limit)= 440mA. I expect that this value is RMS. I'm not reaching this limit so far. Could you please consider what is the issue?

Thank you

Zdenek

SCH+Transformer.zip

over 1 year ago

0 Harish Ramakrishnan5 over 1 year ago

TI__Mastermind 24375 points

Hi Zdenek

Thank you for reaching out.

The 440mA is peak current limit. I am thinking the primary indctance is on the lower side at 1.28mH. It looks like ton_limit is hit.

At 125 V (90Vac) min bulk voltage, Iin_avg = Pin/Vmin = 3.6/125 = 29 mA.

Iin_pk = (Iin_avg * 2) / Dmax = (29 mA * 2) / 0.485 = 120 mA - the UCC28881 Ipk w.c. min is 315 mA

Lpri = (Vmin * Dmax) / (Ipk * fmax) = (120 * 0.485) / (0.315 * 62 kHz) = 3 mH

This might be reason for this hitting the power limit.

Regards,

Harish

0 Zdenek Jircík over 1 year ago in reply to Harish Ramakrishnan5

Prodigy 20 points

Hello Harish

I will try to replace transformer with higher inductance in primary. My design is based on TI WEBENCH designer. It recomends primary inductance 0.88 mH.

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=C2A94BB7D0945C2C

I will give you a feedback asap. Thank you for your hint.

Zdenek

0 Harish Ramakrishnan5 over 1 year ago in reply to Zdenek Jircík

TI__Mastermind 24375 points

Hi Zdenek,

Sure, please let us know the feedback after replacing the transformer.

Regards,

Harish

Power management

Power management forum

UCC28881: Isolated flyback design - troubles with output power