UCC28950: Variable-frequency mode

HAITANG LIU

Part Number: UCC28950

The IC ucc28950 is desinged for fixed-frequency LLC phase-shift control, but I am wondering if we can make the IC work in variable-frequency mode by change the old connecting way? In fixed-frequency mode, the RT pin is connected to ground via a resistor, what if connecting to a DAC port through a resistor? Will the IC work in varieble-freqeucny mode?

Thank you for your reply.

over 3 years ago

0 Yunsheng Qu over 3 years ago

TI__Genius 9790 points

Hi, Haitang

As you know, UCC28950 is designed for fixed frequency operation, it can be configured as voltage mode and Peak current mode. RT pin current define switching frequency by pulling RT pin to 2.5V internal. It is not recommended to let UCC28950 in variable frequency mode.

Yunsheng

0 HAITANG LIU over 3 years ago in reply to Yunsheng Qu

Prodigy 30 points

Thank you very much. I got a new problem, Vref is 7.5 V, rather than 5V as the datasheet descript. Do you have any idea of that issue?

0 Yunsheng Qu over 3 years ago in reply to HAITANG LIU

TI__Genius 9790 points

Hi, Haitang

VREF is 7.5V is not right behavior, could you try a new UCC28950?

Yunsheng

0 HAITANG LIU over 3 years ago in reply to Yunsheng Qu

Prodigy 30 points

It has been solved by connecting a 1uF capacitor between VREF and GROUND. Thank you very much.

0 HAITANG LIU over 3 years ago in reply to Yunsheng Qu

Prodigy 30 points

By the way, the IC can really work in variable frequency mode by changing the RT voltage. But we have not gotten the proper way to utilizing this feature.

+1 Yunsheng Qu over 3 years ago in reply to HAITANG LIU

TI__Genius 9790 points

Hi, Haitang

good to know you have solved your concern, i am going to close this thread.

Yunsheng

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UCC28950: Variable-frequency mode