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UCC3895: Varying phase shift and duty ratio

Roy Bell13
Prodigy 20 points
Part Number: UCC3895
Other Parts Discussed in Thread: UCC28950, UCC2895

I work in Kilby Labs in Santa Clara.

I would like to use this component as it looks like it has the capability to output two pwm wavforms--each with its own complimentary waveform (a total of 4 signals).

From my understanding, you should be able to vary:

1. the duty ratio of OUTA/B and OUTC/D

2. the phase shift between OUTA and OUTC

The datasheet does not show how I may do this. Any help would be greatly appreciated at this point.

-Roy

  • 0
    TI__Mastermind 25570 points
    Hi Roy

    1. The duty cycle ratio for the OUTA/OUTB pair and for the OUTC/OUTD pair is fixed at 50% - you won't be able to vary it. This is the nature of the PSFB - the duty cycle imposed on the transformer is set by the phase shift between the two pairs (OUTA/OUTB and OUTC/OUTD)

    2. The phase shift between the two pairs is set by the feedback system - and you will get the same phase between OUTA/OUTD and OUTB/OUTC - these are not independently variable.

    How to do this? - configure the device for voltage mode control - in this mode the duty cycle is set by the voltage at the error amplifier output - so configure the error amplifier as a voltage follower - (Connect EAN to EAOUT) and control the duty cycle with the voltage at EAP.

    Hope this helps - depends on what you are doing but a simpler and more flexible solution may be to use a microcontroller or even some discrete logic - or two PWM controllers.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    TI__Prodigy 20 points

    Hi Collin,

    The line in the datasheet that reads "0% to 100% Duty-Cycle Control" lead me to believe that this chip had the capability that I wanted, but your explanation clears things up.

    Do you know of a TI chip that can output 2 PWM signals and their complimentary signals (a total of 4 signals), where you can adjust frequency, duty ratio, phase shift, and dead time?

    If not, is there a chip that outputs a pwm signal and that can be synchronized to an external clock? This way, I can use two of them and phase shift them externally.

    -Roy

  • 0 in reply to Roy Bell13
    TI__Mastermind 25570 points
    Hi Roy

    thinking abut it some more - you could try this -
    Use two UCC28950 devices
    Set them up in voltage mode - use the same voltage applied to each devices EA input to control the duty cycle.

    Generate a SYNC signal - with variable width - any decent signal generator could do this. The frequency of the SYNC signal controls the frequency of your system and the duty cycle will adjust the phase shift between the two UCC28950 devices.

    Synchronise one of the UCC2895 to the falling edge of the SYNC signal - the UCC28950 synchronises on the falling edge - and use this to SYNC one of the two devices
    Synchronise the other UCC28950 to the falling edge of an inverted version of the SYNC signal


    AND the OUTA/OUTD pair of each of the UCC28950 devices - the AND function will give you a PWM signal
    Control the phase between the ANDed signals of the two UCC28950 devices by adjusting the width of the SYNC signal

    Should be easy ! - this is all hardware based - you may find it easier to program a microcontroller to do it for you but ...

    Regards
    Colin