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UCC3895: Output voltage to high at light loads at minimum level shift

Frank Wachsmann
Prodigy 30 points

Part Number: UCC3895
Other Parts Discussed in Thread: UCC2895, LM5046, UCC28950

Dear colleagues,

we use the UCC3895 (UCC2895) for a high voltage source with an input voltage of 360V DC.

The output transformer has a high winding ratio (from 360V to 10kV), so with the lowest level shift from the controller,

the output voltage at light loads behind the full bridge rectifier is always to high (600V DC).

Is there a possibility to reduce the minimum level shift or skipping pulses like burst mode?

Maybe I have to switch to an other controller to get the output regulated.

Thanks for your help.

Frank

  • 0
    TI__Mastermind 25570 points

    Hello Frank

    This is my understanding of your system. You are not able to trim the output voltage of the system below 600V at light loads. I assume that you can do this at heavier loads and that the maximum output voltage is 10kV.

    The UC3895 includes a 'No Load Comparator' which compares the EAOUT signal to 0.5V. If EAOUT drops below 500mV then switching stops, and remains stopped until EAOUT rises above 600mV. This mechanism should allow control all the way down to zero volts. I would check the EAOUT signal to see if it is behaving 'properly'. My first thought is that there is enough electrical noise in the system while it is switching to prevent this mechanism from operating.

    TI does have other PSFB controllers - including the UCC28950 and LM5046 - and these may or may not be more suitable. The UCC28950 in particular has an in-built burst mode which is triggered when the duty cycle drops below a user settable minimum (TMIN). It was intended for secondary side control but has been successfully used as a primary side controller by many users.


    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 30 points

    Hello Collin,

    thanks for the fast answer. This sounds plausible.

    We will check the signals around the EAOUT pin to get the no load comparator working properly.

    I will inform you about the results.

    Kind regards,

    Frank

  • 0 in reply to Frank Wachsmann
    TI__Mastermind 25570 points
    Hello Frank

    You are welcome. Unfortunately Plausible and True aren't quit the same ! so please let me know if you need any further help on this.

    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Prodigy 30 points
    Hello Collin,

    we have found a solution for our problem with the high minimum output voltage (600V) at the beginning of the level shift.

    As we implemented the regulation of the UCC3895 controller with an FPGA, the setpoint for the EAP input comes with an PWM through an R-C lowpass filter. The time constant is too low to regulate the output proper down to 50 volts for example.

    We use the soft start pin SS to trim the minimum output voltage. We put a transistor (collector) to the SS pin and drive this transistor with a pwm of 5kHz. So we have a easy tool to set the min. output of our HV transformer down to several volts :-)

    Thank you for assistance.
  • 0 in reply to Frank Wachsmann
    TI__Mastermind 25570 points

    Hello Frank

    That's excellent news. I'd advise you to check that the system remains stable when under the control of the SS pin, because this is an unconventional (not necessarily wrong just unconventional), use of the SS pin.

    Regards

    Colin