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UCC28070: About VAO pin

Hide
Guru 21045 points
Part Number: UCC28070

UCC28070 - About VAO pin

 

Hi Team,

 

I have three questions for VAO pin.

 

---------

[Q1]

I understand that VAO pin is Voltage Amplifier Output.

And “VVAO represents the total output power of the PFC preregulator.”.

 

I understand that Voltage Amplifier don’t monitor the output current but monitor the output voltage.

However, behavior of VVAO is as follows.

-if output current increase, output voltage decrease and VVAO increase

-if output current decrease, output voltage increase and VVAO decrease

( VVAO : 1V at zero current and full output current is 4.2V.)

 

Therefore, VVAO can represent total output power.

Is my understanding correct?

---------

 

[Q2]

I understand that “the UCC28070 has been designed to improve its transient response by pulling up on the output of the voltage amplifier (VVAO) with

an additional 100 μA of current in the event the voltage on VSENSE drops below 93% of regulation (2.79 V).”

 

so, if VVAO is clamped at 5V and this status continues being maintained, output current is abnormal(over current).

Is my understanding correct?

 

---------

[Q3]

I understand that If VVAO is more than 4.2V (for example, VVAO is 4.5V), output current is abnormal(over current).

Is my understanding correct?

 

<datasheet page.27>

Although the maximum VVAO is clamped at 5 V, at full load VVAO may vary around an

approximate center point of 4.2 V to compensate for the effects of the quantized feedforward

voltage in the multiplier stage (see Linear Multiplier and Quantized Voltage Feed

Forward for details). Therefore, 4.2 V is the proper voltage to use to represent maximum

output power when performing voltage-loop gain calculations.

---------

 

Regards,

Hide

  • 0
    TI__Mastermind 34465 points

    Hello Hide,

    I will respond to your questions in the order you wrote :

    Q1 : Your understanding is wrong.
    VAO relates to the difference between the required output voltage and the actual output voltage
    VAO has nothing to do with the output current. It is the voltage error amplifier output and has nothing to do with current.
    The voltage on the multiplier output IMO sets the power output

    Q2 : Your understanding is wrong
    The 100uA pull up current only exists when VSENSE is below 2.79V.
    The closed loop nature of the pfc will adjust the pwm so that VSENSE increases to 3V steady state.

    Q3:  I think that Page 27 relates to the control loop equations and it is not quite right clearly worded.
    The IMO output is maximum at full power but the EAO is only maximum at full load and low line .

    Regards

    John

  • 0 in reply to John Griffin1
    Guru 21045 points

    Hi John-san,

     

    Thank you for your response.

    I have additional question.

     

    Currently, we are evaluating UCC28070 at the following conditions.

    Case1; Vin=85V, Vo=390V, Io=around 2.5A, f=62kHz

    Case2; Vin=200V, Vo=390V, Io=around 3.0A, f=62kHz

     

    If Case1, peak current limit works.

    If Case2, VAO is 5V clamp, so Power limit works.

     

    We would like to evade “Power limit works” at Case2.

    However, we can’t understand the reason why Power limit works on at Case2.

    Could you please let us know if you have any idea for this reason?

    (For example, when VAO is 5V, maximum PFC output pawer so over current limit condition etc. )

     

    And, we would like to reduce VAO voltage (form 5V to normal voltage).

    Could you please let us know if you have any recommended method?

     

    <Datasheet>

    The output of the voltage-error amplifier (VVAO) is clamped at 5 V, which represents the maximum PFC output

    power. This value is used to calculate the maximum reference current at the IMO pin, and sets a limit for the

    maximum input power allowed (and, as a consequence, limits maximum output power).

     

    Regards,

    Hide

  • 0 in reply to Hide
    TI__Mastermind 34465 points

    Hello Hide,

    It sounds like you are using the wrong components values for correct operation of the UCC28070.

    Please use the Excel calculator to helpy you with your design.

    It is located here:

    https://www.ti.com/product/UCC28070/toolssoftware

    I recommend that you pay attention to the selection of the IMO resistor value.

    Regards

    John

  • 0 in reply to John Griffin1
    Guru 21045 points

    Hi John-san,

    Thank you for always kind support.

    I greatly appreciate your cooperation.

    Regards,

    Hide

  • 0 in reply to Hide
    TI__Mastermind 34465 points

    Hi Hide,

    No problem. Let me know how you get on.

    Regards

    John