UCC28070A: 【Repost】Could you tell me about UCC28070 ?

Hi Masumi-san,

I cannot find that document number, what is the title of the document?

Thanks

Peter

Thank you for contacting me.

"JAJA216" was a Japanese version of the document.
The English version corresponds to the document of "SLUA479B".

How is progress progressing to this response?

About this matter, we will meet with customer at 14:00～ tomorrow(11/14) in Japan time, so please let me know just the progress.

Hello Pokkun,

Sorry it has taken so long to answer this question.

The value of Crr is usually selected by test.

The volt-sec product during the forward conduction time of the current transformer must equal the volt-sec product during the reverse bias time of the diode DRA, DRB.
Otherwise T1,T2 will saturate.
So you can select the value of Rr as perhaps 10 to 20 times the value of the burden resistor RSA,RSB
If you make Rr too high the reset voltage is excessive and will cause the diodes DRA,DRB to breakdown. You make it high enough so the power dissipated is not excessive. and the reverse voltage is acceptable. It is possible to short out Crr but this also cause un-needed power dissipation.

So Crr is selected so that there is just enough volt-sec balance.
For most applications 4.7pF is sufficient and of course you verify correct operation of the waveforms with an oscilloscope

Regards

John

Thanks for the reply, John-san.
I replied that Crr capacity value is sufficient 4.7pF to customer.
The customer received this answer and asked additional questions on the following two points.
Please tell me.

① How can you calculate the potential difference across the ends of Crr?
※I saw the EVM but it seemed that it exceeded the withstand pressure,
　so I would like to calculate a problemless value from the viewpoint of selecting parts.

② Does not the Crr capacitance value (4.7 pF) need to be changed depending on the PFC frequency?
※ The design is proceeding with the PFC frequency at 40 kHz.

Best regards,
Pokkun

Hello Pokun,
(1) The max voltage across Crr (Voff) is calculated by this equation:
Vout*Ton(max) = Voff*Toff(min)
Vout is the sensed voltage that you design for CSA/CSB
Ton(max) is calculated from the switching frequency and Dmax (max duty cycle set)
Toff(min) is calculated from Ton(max) and the switching period.
This gives you the voltage rating for Crr and the reverse voltage rating of the rectifier diode

(2) yes the value of Crr will change with frequency.
You need to make sure that the time constant set by Crr*R is equal to or greater than Toff.
This ensure that the transformer is reset and does not saturate.


Regards
John

Thank you for the answer, John-san.

>(2) yes the value of Crr will change with frequency.
>You need to make sure that the time constant set by Crr*R is equal to or greater >than Toff.
>This ensure that the transformer is reset and does not saturate.

⇒Is it no problem with a fixed value of Crr=4.7pF ?

Best regards,

Pokkun

Hello Pokkun-san,

I recommend that you do your initial tests with 4.7pF.

You should check the scope waveforms and make sure everything is acceptable.

You can adjust the value upwards if needed.

Regards

John