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# UCC28070: About abnormal behavior by AC line voltage drops

Part Number: UCC28070

Hi Team,

Our customer is evaluating their board.

If heavy output current occurs, FET is stopped.

Therefore, they investigate this cause.

[Investigation content]

If heavy output current occurs, AC input voltage drops by Resistance component of AC line on their board.

And, when VINAC reached around 1.0V(Level1 Threshold) by AC input voltage drops, FET is stopped (they guess that UCC28070s Gate driver is stopped).

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We have three questions.

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[Q1]

If AC input voltage extremely(or unintentionally) descends, does UCC28070 operate abnormally?

(For example, “Gate driver is stopped”, “VCCs UVLO works” etc.)

[Q2]

If VINAC pin voltage extremely(or unintentionally) descends, does UCC28070 operate abnormally (Gate driver is stopped etc.)?

I understand that A change of QVFF levels does not have the problem.

However, could you please let us know if you have other concern?

[Q3]

If AC input voltage extremely(or unintentionally) descends, does UCC28070 affect VCC pin?

(For example, “Current flows from AC line(or other) into VCC pin” etc.)

We evaluated UCC28070EVM.

Then, I let input AC voltage line descend intentionally by addition of the unnecessary resistance.

As a result, DC power supply is stopped.

We guess that “Current flows from AC line(or other) into VCC pin”.

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Regards,

Hide

• Hello Hide,

VINAC is one input to the multiplier block of the UCC28070 and you can see from the block schematic of the data sheet that

the output current of the multiplier is given by this equation:

When VINAC drops below 0.7V the gate drive turns off.

This is normal behaviour for the multiplier and it occurs for every cycle of the VINAC.

For good power factor the gate drive should be off at this time.

Regards

John

• Hi John-san,

I can find “Zerocrossings are defined as VVINAC falling below 0.7 V for at least 50 μs, typically.” in the datasheet.

So, if VVINAC falling below 0.7 V for at least 50 μs, typically, the gate drive turns off.

Is my understanding correct?

Regards,

Hide

• Hello Hide-san,

Yes. That is correct.

Regards

John

• Hi John-san,

Thank you for the information and kind support.

Regards,

Hide