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UC2843A: Startup

toktam khani
Prodigy 90 points
Part Number: UC2843A

Hi,

i am using UC2843A for a low voltage flyback. (20V .. 60V input and 24V output with 2.7A and 200KHz). i choose all the parameters as in Webench design recomendation. 

Vref (5V) is constant and OK. RC pin is also OK. CS pin is also OK (i have slop compensation and i can see a fraction of my RC voltage in this pin).

Problem is that with activation of VREF pin, the comp pin is also activated and rises from 500mV to 800mV. it does not reach VOL of error amplifier and won't let IC to generate any PWM signal on the out pin. 

How can i correct this? i have a parallel 1nF and 10KOhm between comp and feedback with 5KOhm between feedback and secondary side opto. i also tried different resistor combination. this resistor should be able to sink the error amplifier current and there is a minimum value for it. 10KOhm should be OK. what is the startup process of Comp pin?

Thanks in advance

  • 0
    TI__Expert 7900 points

    Hello Toktam,

    Usually an issue at startup is because of the choice of values for the HV startup components delivering charge to the VDD pin/capacitor.  You should measure the VDD and COMP and OUT pins with a 'scope during startup.  I suspect you're getting a short burst of switching, but VDD voltage is dropping below the UV Stop threshold, which causes COMP to be pulled low.

    Eric

  • 0 in reply to Eric Reicher
    Prodigy 90 points

    Hi Eric,

    i already measure everything with Osci. at startup. the Vref is constant and does not goes down. if the Vref goes down then it has effect on my RC pin also. but everything is fine at CS, RC and Vref pin.

    Toktam

  • 0 in reply to toktam khani
    TI__Expert 7900 points

    Not the VREF (5V) pin.  The VDD pin is incoming voltage (from VIN) with Start/Stop thresholds.

    Eric

  • 0 in reply to Eric Reicher
    Prodigy 90 points

    I already had worked with this IC and this time is not working. For your information: at the startup, the consumption of the IC is around 1mA. passing UVLO the consumption rises to 14mA and if the startup circuit is not able to deliver the energy, voltage at Vcc drops to the UVLO-turn off. at this point the VREF turns also OFF --> if there is a problem at VCC voltage you can see it also in VREF pin in the way that  it will be pulse form. 

    Also i talked about VREF pin beacuse the positive pin of error amplifier comes from voltage divider of VREF pin and if VREF does not exist the error amplifier does not have a reference voltage of 2.5V.

    again: i measured simultaneously voltages at COPM, VCC, VREF and CS pin at the startup using the trigger function of my Osci.(abbreviation of Oscilloscope) for 1s/div to be sure that the voltage is constant and stays constant. 

    normally voltage at comp starts from for example 500mV and rises up quickly to LO level (which is maximum 1.1V) and IC starts to generate PWM. do or don't you know why my comp pin is not rising although the voltage is there? does it mean that there is another fault? i could not find any data in datasheet.

    Toktam

  • 0 in reply to toktam khani
    TI__Expert 7900 points

    COMP will not rise if there is a fault condition.  The only fault I know of is the VCC pin falling below the UV Stop threshold.  You need to examine COMP, VCC, VREF, and OUT with a 'scope at a small enough time base to see any switching.  1 s/div does not have enough resolution to see a burst of switching.

    Can you include your schematic and 'scope pictures at small time base to show gate pulses?  It's a huge stretch to expect anyone to troubleshoot your circuit with brief description.  Measuring DC voltages will not show an issue.

    When COMP is at 800mV what is the value at VCC?

    Did you make any changes to Webench recommended components?

    Regards,

    Eric

  • 0 in reply to Eric Reicher
    TI__Expert 7900 points

    I used Webench with your design targets: 20-60Vin, 24Vo, 2.7Aout, 200kHz.

    Notice Rstart1, Rstart2, and CVCC:  5.36k, 5.36k, and 220uF.

    The UVLO Start/Stop thresholds of the UC2843A are only 8.4V/7.6V, typical.  This is a window of only 0.8V.  So, the 220uF capacitor on the VCC pin is critical to maintain VCC and allow reliable startup.

    Regards,

    Eric