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UCC28810 switching frequancy

Muhammed Reda
Prodigy 50 points
Other Parts Discussed in Thread: UCC28810, UCC28811

Hi all ,

I am using UCC28810 as my buck flyback controller and i found an application note PMP2698 which is a 50 watt LED Driver , I made a PCB for it and it is work fine but there is a noisy sound from the flyback transformer (which i made it  look like G084231LF which it is mentioned in the appliaction note)  so i think problem that the switching frequency of my transformer is not correct , So i am asking what is  operating switching of the UCC28810 and how i can change it  ?

Thanks in advance  

  • 0
    TI__Mastermind 28655 points
    The switching frequency is dependent on the transformer and operating conditions(inductance, turn ratio, input and output voltage, load). The test report for PMP2698 has a drain waveform running about 30 KHz. Increasing the input voltage and/or decreasing the load will increase the switching frequency. The switching frequency is also not constant. It will probably be lowest when the input voltage (portion of the rectified AC) is low. The current will also be low so there will be less energy to emit noise.

    Decreasing the inductance will also increase the switching frequency. Hand built transformers can also emit more noise because the core may be able to move.

    If you measure the drain waveform with an oscilloscope you can see the switching frequency over the entire line cycle to see if it is in the audible range.
  • 0 in reply to Irwin Nederbragt
    Prodigy 50 points
    Dear Irwin Nederbragt ,

    Thanks in your quick reply .

    When i saw the drain waveform the frequency is changed between 5 kHZ and 100 kHZ so my question if i worked in the same operating conditions for PMP2698 the drain waveform frequency must be fixed at 30 khz or it will work in range ?

    Thanks in advance .
  • 0 in reply to Muhammed Reda
    TI__Mastermind 28655 points

    The drain waveform is not fixed at 30 KHz.  This is a critical conduction mode flyback, this means the switching frequency will change over the line cycle.  It would be good to know if the switching frequency in the audible range is near the zero crossing of the AC.  There is little energy being transferred and should not cause too much audible noise if near the zero crossing.  If the switching frequency drops into the audible frequency range near the peak of the AC waveform there is a lot of energy being transferred and the flux level in the transformer is the highest.  Also, the transformer can emit audible noise even if the switching frequency is above the audible range.  It also has a lot to do with how the transformer was made.  A hand made transformer with a taped core set would behave much different than a production varnished transformer.

  • 0 in reply to Irwin Nederbragt
    Prodigy 50 points
    Dear Irwin Nederbragt ,

    Thanks in your quick reply .

    Ok i got the point and i will change the transformer but i have another questions , when i open UCC28810 datasheet there is an application circuit which they used a circuit called Leading Edge Blanking Circuit on the TZE Pin (page 18 of UCC28810 datasheet ) and i didn't put it in my design ,my question this circuit can be a solution for this audible noise ?

    Another problem that the circuit take about 3 seconds to startup when i switch on the power i didn't know why this delay ? there is any suggested startup cct. ?

    Thanks in advanced .
  • 0 in reply to Muhammed Reda
    Prodigy 50 points
    For the delay problem when i change the Vcc cap (which is C6 in the PMP2698) form 100uf to 4.7 uf the delay is decreased to 0.5 second , does this change has a bad effect on the IC ?
    i found a high voltage startup circuit on UCC28810 datasheet to make but it very large

    For the audible noise problem: when i change the voltage divider on Vins pin to make the driver work on universal range and i mad the input voltage about 110v there is no audible noise as i think the switching frequency increased and the minimum frequency is higher than the audible frequency range but at 230 v there is audible noise so it also from transformer ?? as i must be sure before order a new one as it take time .

    Sorry for disturbance .
    Thanks in advance .
  • 0 in reply to Muhammed Reda
    TI__Mastermind 28655 points
    You will be able to tell watching the drain waveform or the TZE waveform during start-up. If you read about the circuit on page 18 you will know what to look for. This is more for a turn-on runaway issue. This should not be an issue during normal operation.
  • 0 in reply to Muhammed Reda
    TI__Mastermind 28655 points

    Changing to 4.7 uF should not be an issue as long as VCC does not drop below UVLO.  The larger capacitors generally go with triac dimming because the converter is off for many mS and the capacitor has to supply VCC during that time.  Reducing the resistor values feeding the capacitor will also speed up turn-on but will increase losses, especially at high input voltage.  For quick turn-on an added start-up MOSFET/transistor circuit can be added but then I would switch to the UCC28811 with the lower turn-on threshold.

    The switching frequency should be lower at 110 VAC input.  It may be useful to really look at the drain waveform when it is audible to make sure it is switching correct.  This is transition mode so the current should ramp up, then MOSFET turn-off, then energy in the transformer dumped to the secondary side.  As soon as the energy is gone the voltage on the windings should drop and trigger TZE.  There should be very little time when there is no current in the transformer winding (zero volts across the windings).

  • 0 in reply to Irwin Nederbragt
    Prodigy 50 points
    Dear Irwin Nederbragt

    1-For the delay problem it is solved by reducing the resistor values feeding the capacitor on the vcc as it will charged fast in this case.
    2-For the audible noise problem when i change the transformer with the same specs but Impregnate it in varnish it is steel the transformer emit noise .
    I think if i decrease the primary inductance from 470uH(like in the application note PMP2689) to 240uH with the same turns ratio it will increase the switching frequency and make it fare a way from the audible noise range , does this changes will be a solution for the noise problem and it will not change any thing of the circuit operation ?

    Thanks for your great efforts with me.
  • 0 in reply to Muhammed Reda
    TI__Mastermind 28655 points

    Hello,

    Cutting the inductance in half will increase the switching frequency.  The other good thing is the only change to the transformer design should be the core gap (for testing anyway) getting larger.  May want to optimize the transformer design when it works as you like.

    The circuit operation will change.  There will be more switching loss due to more switching transitions.  The peak currents may go up slightly due to fixed delays not dropping though the switching frequency is increasing.  It should all, hopefully, be minor.