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Original question:

UC3875: inconsistant duty cycle value in UC3876 pin C and D

UC3875: inconsistent duty cycle value in UC3875 pin C and D

Akihisa Tamazaki
Genius 11365 points
Part Number: UC3875
Other Parts Discussed in Thread: UCC28951, UCC28950, UCC28250, UC2825

Hello team,

My customer has a related question with the following E2E. 

https://e2e.ti.com/support/power-management/f/196/t/598673

The schematics of the customer is as almost same as the above E2E.  The answer this thread is that "the error amplifier is confirgured as a voltage follower so the EAOUT is following the NI (EA+). So if you the NI is not consistent, the duty cycle will be changed accordingly because the duty cycle is controlled by the EAOUT. You should keep the EAOUT consistent if you want the duty cycle consistent."

However it is hard for the customer to keep EA+ consistent because there is noise that is difficult to get rid of in EA+.
In this case,  in realizing feedback control with UC3875, is it possible to make the circuit when using an error amplifier other than the voltage follower?
Would you please advise?

Best Regards,
Akihisa Tamazaki

  • 0
    TI__Guru 72195 points

    Hello,

    I reviewed that post and it seems that they were evaluating duty cycle control with an open loop test circuit.  There should be no problems setting this amplifier up with traditional type 2 compensation.

    If this is a new design for a phase shifted full bridge design, I suggest that you use the UCC28950 or UCC28951.  These are new devices and we have many design tools that can help you with your design.  The following link will bring you to the available design tools for TI's latest PSFB controllers.

    Regards,

  • 0 in reply to Mike O'
    TI__Genius 11365 points

    Hello Mike-san,

    Thank you for your reply.
    The customer cannot use UCC28950 / UCC28951 because the customer selected that the switching frequency is 25kHz.

    The customer is evaluating the following block in pptx file. 
    /cfs-file/__key/communityserver-discussions-components-files/196/IC.pptx

    The customer connects  EAOUT with EA-, and EA+ is connected to the feedback from the output of this system. The customer thinks that it is hard for the customer to keep EA+ consistent because there is noise that is difficult to get rid of in EA+.

    To avoid inconsistent duty cycle value in UC3875 pin C and D, do you have any idea? Your advise would be so appreciated.

    Best Regards,
    Akihisa Tamazaki


  • 0 in reply to Akihisa Tamazaki
    TI__Guru 72195 points

    Hello,

    From the schematic it looks like the customer is running the design open loop.

    The amplifier output is a low impendence and should not be noise sensitive.  The input is not low impedance and could be noise senseative.

    They could add low pass RC filter to the input of the voltage follower.  1k ohm and 330 pF should do it.

    25 kHz is a low switching frequency you might consider a regular full bridge using the UC2825 or the UCC28250 that does not have ZVS.

    This controller may be easier to use at these low frequencies. 

    Regards,

  • 0 in reply to Mike O'
    TI__Genius 11365 points

    Hello Mike-san,

    Thank you for your support.

    The customer will try to add RC filter to the input of the voltage follower.

    Best Regards,
    Akihisa Tamazaki