UCC256302: UCC256302: abnormal output when no load

balasubramani r

Expert 1170 points

Part Number: UCC256302
Other Parts Discussed in Thread: TL431

Hi All,

I am debugging a LLC circuit with UCC256302.

when no load, the output is keep on oscillating from 17- 24VDC

could you advise some tips to me? what reason does cause the problems? thanks a lot.

BR,

Bala

over 6 years ago

0 Mike O' over 6 years ago

TI__Guru* 85536 points

Hello Bala,

Oscillation could be due loop stability and opto coupler TL431 feedback circuit is not setup correctly.

The following application brief gives some design guide lines to help make your design more stable.

You can find it at this web address

There is also and excel software design tool that you can use to check your design. You can find it at this web address.

There is also an evaluation module that you can order and evaluate to help with your design. It can be found at the following web address.

Regards,

0 balasubramani r over 6 years ago in reply to Mike O'

Expert 1170 points

Hi Mike,

This is happening only at no load condition. If i connect minimum load then this oscillation not at all happening. Please advise.

BR,

Bala.

0 Mike O' over 6 years ago in reply to balasubramani r

TI__Guru* 85536 points

Hello Bala,

Ben will be looking into this further for you to help resolve your issue.

Regards,

0 Benjamin Lough over 6 years ago in reply to Mike O'

TI__Mastermind 27085 points

Hi Bala,

At no load is the LLC in burst mode? Do you see VCC drop to 10.5V? What may be happening is that the controller is not switching often enough to keep VCC above the restart threshold. There are a couple ways to address this.

1. More capacitance on VCC

2. Reduce the burst threshold slightly by either adjusting the LL/SS resistor divider or adjusting the VCR capacitance

Best Regards,

Ben Lough

0 balasubramani r over 6 years ago in reply to Benjamin Lough

Expert 1170 points

Hi Mike,

Currently we disabled this burst mode settings in our design. Only soft start cap alone is there (0.22uF || 487K) in the LL/SS pin. And you are meaning increase VCC cap,but we have 1000uF already in place . Should i increase more ?

Please advise.

0 Benjamin Lough over 6 years ago in reply to balasubramani r

TI__Mastermind 27085 points

Hi Bala,

Could you measure the VCC voltage and LO at no load? Is VCC dropping to 10.5V? This would help confirm the dropout is due to infrequent switching. I would suggest installing a 470kΩ resistor from LL/SS pin to ground. This is to make sure the burst mode setting is correctly minimized.Another option we could look at is reducing the capacitance between VCR and ground. This will also help reduce the effective burst threshold.

Best Regards,

Ben Lough

0 balasubramani r over 6 years ago in reply to Benjamin Lough

Expert 1170 points

Hi Ben,

Let me try this changes and post if required more support.

BR,

Bala

0 Benjamin Lough over 6 years ago in reply to balasubramani r

TI__Mastermind 27085 points

Thanks Bala,

Let me know how it looks and we'll go from there.

Best Regards,

Ben Lough

0 Benjamin Lough over 6 years ago in reply to Benjamin Lough

TI__Mastermind 27085 points

Hi Bala,

Since this has moved to email, I will close this thread. Let's continue over email.

Best Regards,

Ben Lough

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UCC256302: UCC256302: abnormal output when no load