UCC28070

!U@W

Other Parts Discussed in Thread: UCC28070, UC3854

Hello,

I am designing a 3.5 kW / 380V PFC boost converter, using ucc28070. My AC line voltage range is 175V to 253V. For this specification, the max duty cycle is 0.34. When i calculate the R_DMX resistor for max duty cycle limit, I get negative value for the resistance(-42515 ohm).

How to design for this specification?

over 8 years ago

0 !U@W over 8 years ago

Prodigy 120 points

For current sensing, instead of CT's, can I use current sense resistors in the Emitter of IGBT.

0 Colin Gillmor over 8 years ago in reply to !U@W

TI__Mastermind 25570 points

Hello

The short answer of course is 'yes' - the controller doesn't care whether the CS signal comes from a current sensing resistor or a current transformer.

You will have to look at the dissipation in the current sensing resistors and also you will have to provide a good ground connection from the controller ground to the ground end of the resistors.

If you have a schematic and some specifications about the power level you are working at I can give some guidance on whether a resistor or transformer is a better choice.

Regards

Colin

0 !U@W over 8 years ago in reply to Colin Gillmor

Prodigy 120 points

Hello Colin,
In the application design note of UCC28070, 'UCC28070 300-W Interleaved PFC Pre-Regulator Design
Review' , the PFC inductor is designed at peak of low line voltage.
It is 140uH.Maximum inductance is mentioned as 350uH. How did they achieved it?

0 Colin Gillmor over 8 years ago in reply to !U@W

TI__Mastermind 25570 points

Hello
It is often useful to use a 'swinging choke' for the PFC inductor. This means that the inductance is a function of the current in the inductor - so for example at low line (90Vac for example) where the inductor current is greatest the inductance drops to 140uH. At high input voltages (230Vac for example) the current is significantly less and the inductance increases. A quick internet search on the term 'swinging choke' will show you what I mean.
Regards
Colin

0 !U@W over 8 years ago in reply to Colin Gillmor

Prodigy 120 points

Colin,

Can I test interleaved APFC circuit using UCC28070 on DC? Input is 320Vdc.
Also what happens if I connect SS pin to GND?

0 John Griffin1 over 8 years ago in reply to !U@W

TI__Mastermind 34465 points

You cannot run the UCC28070 from a DC supply.
The issue is that the quantized multiplier requires the VINAC input to fall below 0.7V for at least 50uS.
If this does not happen then the controller will not accurately measure the input voltage.
In answer to your second question, the controller will switch off if you connect the SS pin to GND.

Regards,
John Griffin

0 !U@W over 8 years ago in reply to John Griffin1

Prodigy 120 points

Thank you John.

0 !U@W over 8 years ago in reply to !U@W

Prodigy 120 points

Does UCC28070 require some voltage to be sensed on Current sense pins to start?

0 !U@W over 8 years ago in reply to !U@W

Prodigy 120 points

What happens if the output of the APFC is open circuit? Does the UCC 28070 start? Does it need some voltage to be sensed on Current sense pins for the starting few cycles?

0 John Griffin1 over 8 years ago in reply to !U@W

TI__Mastermind 34465 points

No
You may start up into an open circuit

0 !U@W over 8 years ago in reply to John Griffin1

Prodigy 120 points

I am using Shunt resistors in my design, instead of CTs. Should I disable the Current synthesis of UCC28070?

0 !U@W over 8 years ago in reply to !U@W

Prodigy 120 points

The Vref voltage has dropped to 2V from 6 V. Is the IC damaged while damaged? Need to change the IC and see?

0 John Griffin1 over 8 years ago in reply to !U@W

TI__Mastermind 34465 points

It sounds like the ic is damaged. I think you should change the ic

0 !U@W over 7 years ago in reply to Colin Gillmor

Prodigy 120 points

I am designing for 3.5 kw output power.DC voltage is 380Vdc. I had designed a circuit with current sense shunt resistors. Because of cost limitation, I am using shunt resistors. For this should I disable the current synthesis feature in the controller IC?
Give your inputs.

Awaiting reply

0 !U@W over 7 years ago in reply to !U@W

Prodigy 120 points

Can any one help me on this. Should the current synthesis of UCC28070 be disabled, if using current shunt resistors instead of CTS.?

0 John Griffin1 over 7 years ago in reply to !U@W

TI__Mastermind 34465 points

Can you attach a sketch of your application ?
The UCC28070 has been designed to utilize the current sensed when the MOSFET is turned on.
If your application is sensing this current only then you will still require the use of the current synthesizer.
If you are somehow sensing MOSFET current and diode current then you will not need the synthesizer.

Regards,
John

0 !U@W over 7 years ago in reply to John Griffin1

Prodigy 120 points

Hello,

The formula given in app note for calculating Rdmx gives negative value for duty cycle below 0.5. What is the correct formula for calculating the Rdmx for duty cycle of 0.34? Should that be Rmdx= Rrt(1-2Dmax)?
please clarify.

Awaiting reply

0 Israel Divan over 7 years ago in reply to Colin Gillmor

Prodigy 165 points

Mr Colin, hi. A swinging choke is a requirement for this project with UCC28070 or a simple toroidal choke with Sendust core 140uH will work well?

0 Peter Repka over 7 years ago in reply to John Griffin1

Prodigy 210 points

I was wondering if you could pull down the VinAC signal for 50uS at rep rate 100-120Hz to solve the problem
to trick the chip into thinking it has a modified sine wave as the input. would this work? I have an application
to use AC or DC input. The old guys here suggest that the UC3854 should be able to do this. I do not understand
why this chip needs this no signal time unless the multiplier is not actually an analog circuit inside

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