UCC25600: Inrush current

user5339789

Part Number: UCC25600

Hi,

We are trying to use the UCC25600 to LLC power supply, now under debugging.

But when start up, device found inrush current as overcurrent, then goes shout-down rapidlly during 2-3ms.

We are looking also SS pin (as start-up enable), but seems to be kept low level (around 0.6 to 1.0V) even if we try to start.

Well one question is, this is right operation using to LLC ?

We guess resonant capacitor is one of factor to occur such as issues. However we will not change this capacitor7s value and types.

Therefore we try to control DC current level using RC network between OC pin and current sense circuit, we can avoid this issue.

So Do you think it is better resolution?

Best regards, Keiichi

over 5 years ago

0 Hong Huang over 5 years ago

TI__Guru 73475 points

Can you provide your mentioned circuit drawing so we can have a closer look to see if any possible concerns?

0 user5339789 over 5 years ago in reply to Hong Huang

Intellectual 590 points

Thank you or your quick reply.

Are you sure to find a file I attached? This is our engineer's schematics of your chip and current sense circuit.

0 Hong Huang over 5 years ago in reply to user5339789

TI__Guru 73475 points

Sorry, I cannot find your file. Can you re-submit it?

0 user5339789 over 5 years ago in reply to Hong Huang

Intellectual 590 points

Thanks, I will try again, LLC_PS_20-b_sch.pdf

0 Hong Huang over 5 years ago in reply to user5339789

TI__Guru 73475 points

If you arrange R35 and R36 to avoid sensing inrush current, that is ok. But that current sensing circuit will also not sense the same level of current if that level current in your design should trigger current protection. But if you are ok with that current, then it is ok.

It looks C11 is your resonant capacitor. As you only use one resonant capacitor, the inrush current is usually high. If possible you can add another 4.7uF and form a half-bridge which can reduce inrush current. If you do so, your current sensing can be designed with much smaller inrush current.

So there are trade-offs that require you to make decision.

0 user5339789 over 5 years ago in reply to Hong Huang

Intellectual 590 points

Dear Hong Huang,

In spite of sending resolved, ultimate customer asked again simply.

In fact LLC power supply with the UCC25600 generally, Inrush current will be occured basically, then designer has to consider RC network to avoid such as problem, is that correct?

0 Hong Huang over 5 years ago in reply to user5339789

TI__Guru 73475 points

The inrush current is mainly due to using single resonant capacitor. If using two resonant capacitors in half-bridge, the inrush current will be much smaller. This is a common behavior and not related to a device such as UCC25600.

0 user5339789 over 5 years ago in reply to Hong Huang

Intellectual 590 points

Thank you for your ccomment.

Could you please tell me reason why to be needed two capacitors for resonant capacitor?

We can not see any recommendations and notices of this method on the datasheet.

0 Hong Huang over 5 years ago in reply to user5339789

TI__Guru 73475 points

When using two capacitors in half-bridge, the voltage on each capacitor is established right away and to be half of input voltage. But with single capacitor, the half-input voltage on single capacitor needs sometime to build up, so there is a transient which causing high startup current.

Again, high rush current in LLC with single capacitor is common for this converter topology, which is not due to a particular LLC controller. Such knowledge can be found in various application notes and design guidelines. That is why such thing is not needed usually listed a datasheet for LLC controller.