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LM5023: NOT GET OUTPUT PWM PULSE IN PIn 7 LM5023 IC

SATHISHKUMAR vishwanatha
Intellectual 495 points
Part Number: LM5023

Hi,

Find the attached schematic.

Red mentioned area component we are soldered,

and given external +12V dc supply in D3 anode pin.

Output PIn 7 is 0V, Not getting PWM pulses.

Kindly suggest what will do next.

Regards,

Sathishkumar.

  • 0
    TI__Mastermind 40530 points

    Hello Satishkumar,

    Thank you for your interest in the LM5023 QR-flyback controller. 

    This controller requires 12.8V (typical) at the VCC pin to start up, but max spec can be as high as 13.5V.  To guarantee this value after the D3 diode, you would need to apply almost15V to the anode of D3.  (I assume that the transformer aux winding is not connected.)  But the QR input has an OVP threshold at 3V and ~15V at D3 may exceed the OVP at QR input.

    Instead, I recommend to apply 14V to the cathode of D3, not anode.  

    In addition, I recommend to install R13 and R14.  Without the current sense resistors, CS is essentially an open circuit. The CS input has an internal pull-up offset added based on QR current. Although it should be zero, there may be some leakage which could raise the internal voltage above the 0.5V turn-off threshold.  To avoid this, please provide a DC path for CS leakage current to GND through R13 (and R14). 

    Hopefully that should allow your open-loop controller to start switching.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 495 points

    Hi Ulrich,

    We soldered R13 and R14, We have apply +15V to D3 anode but we not seen any changes.

    But when we apply voltage D3 cathode +14VDC we get this following results,

    Input Voltage +14V D3 cathode

    Pin Number

    Pin name

    Voltage

    1

    QR

    0V

    2

    VSD

    650mV

    3

    SS

    5.5V

    4

    COMP

    5.85V

    5

    CS

    0V

    6

    GND

    GND

    7

    OUT

    13.7V

    8

    VCC

    13.7V

    In the output pin we get only 13.7V only,

    We are not getting the PWM pulses,

    Kindly reply,

    Regards,

    Sathishkumar.

  • 0 in reply to Ulrich Goerke
    Intellectual 495 points

    Hi Ulrich,

    We soldered R13 and R14, We have apply +15V to D3 anode but we not seen any changes.

    But when we apply voltage D3 cathode +14VDC we get this following results,

    Input Voltage +14V D3 cathode

    Pin Number

    Pin name

    Voltage

    1

    QR

    0V

    2

    VSD

    650mV

    3

    SS

    5.5V

    4

    COMP

    5.85V

    5

    CS

    0V

    6

    GND

    GND

    7

    OUT

    13.7V

    8

    VCC

    13.7V

    In the output pin we get only 13.7V only,

    We are not getting the PWM pulses,

    Kindly reply,

    Regards,

    Sathishkumar.

  • 0 in reply to SATHISHKUMAR vishwanatha
    TI__Mastermind 40530 points

    Hello Satishkumar,

     

    I'm glad that you got a high output voltage condition on the OUT pin.  I thought the LM5023 might oscillate at the 130kHz clamp frequency, but further inspection of the block diagram shows that it is not internally forced to do that.  The LM5023 operates in quasi-resonant transition mode, which means that it will begin a new PWM cycle after detecting (at the QR input) the down-ward resonant ring after demagnetization of the previous switching cycle.   

     

    The device does not oscillate by itself from an internal clock.  It oscillates based on external conditions, dependent on the transformer inductance, input voltage and feedback COMP level.  (It does have a frequency detector which prevents operation above 130kHz.)

     

    To get the controller to oscillate and produce PWM pulses in an open-loop condition, you must provide inputs to the CS and QR pins that emulate a closed-loop in-circuit situation.  A rising voltage ramp at CS can be generated using a resistor from OUT to CS, since CS has a capacitor on it.  Remove R12 (499ohm) to prevent loading of the ramp.

     

    I suggest to apply 1.7V to the COMP input. This will result in 0.33V at the PWM comparator negative input.  The CS voltage can ramp up to 0.33V at the PWM positive input which will reset the OUT flip-flop and terminate the OUT voltage.  To get oscillation, QR has to go positive above 0.35V and then back to zero again. When QR falls below 0.35V, it triggers the next OUT pulse.  QR must not go above 3V or OVP will latch off the chip Enable and you’ll have to cycle VCC to clear it.

     

    Connecting 4.7Meg from OUT to CS will ramp Vcs above the 0.33V PWM threshold developed by the 1.7V at COMP input. With 100pF at C16, the ramp time should be about 11us.

     

    Connecting 39k from OUT to QR will pull it up to about 2V. Add a 220pF cap from QR to GND to extend the QR voltage fall time for a few micro seconds to achieve and off time. 

     

    These changes to the open-loop circuit should result in oscillation at approximately 60-80kHz, I think.
    Please give that a try, but remember to restore the circuit to the design schematic condition before operating in closed-loop.

     

    Regards,
     Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 495 points

    Hi Ulrich,

    It is working at 70Khz,

    can please share your email id, so i can contact throw the email,

    i need suggestion regarding testing and transformer design,

    Regards,

    Sathishkumar.

  • 0 in reply to SATHISHKUMAR vishwanatha
    TI__Mastermind 40530 points

    Hello Sathishkumar,

    I'm glad that you are getting the PWM now.

    Please contact me by ulrich_goerke@ti.com for your other questions.  I will close this thread.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 495 points

    Hi,

    We implemented as per your suggestion ( refer attached is the referenced document which you have advised us), but still it is going into hick up mode.

    Requesting to help us to over come this issue.

    We are under tremendous to solve this issue, hence requesting to look into and suggest us to move further.

    Thanks

    Sathish

    to ti 03-07-2019 (modified).docx

  • 0 in reply to Ulrich Goerke
    Intellectual 495 points

    Hi,

    We implemented as per your suggestion ( refer attached is the referenced document which you have advised us), but still it is going into hick up mode.

    Requesting to help us to over come this issue.

    We are under tremendous to solve this issue, hence requesting to look into and suggest us to move further.

    Thanks

    Sathish

    4186.to ti 03-07-2019 (modified).docx

  • 0 in reply to Ulrich Goerke
    Intellectual 495 points

    Hi,

    We implemented as per your suggestion ( refer attached is the referenced document which you have advised us), but still it is going into hick up mode.

    Requesting to help us to over come this issue.

    We are under tremendous to solve this issue, hence requesting to look into and suggest us to move further.

    Thanks

    Sathish

    5875.to ti 03-07-2019 (modified).docx

  • 0 in reply to SATHISHKUMAR vishwanatha
    TI__Mastermind 40530 points

    Hello Sathishkumar,

    Since we are dealing with this issue via email, I cannot leave this thread open on this forum.  I will close it.
    If you have other issues not related to this one, please start another thread with a new post.

    Thank you,

    Ulrich