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UCC28742: UC28742 Start-Up

Farzaneh Faridi
Intellectual 555 points
Part Number: UCC28742

Dear members,

We used UCC28742 in our design and we faced to a question. We used TLP181 and LM4041 in the loop regulator. I will insert some photos  to make the problem more understandable. 

The probes are placed like in the following picture:

     · YELLOW --> FB pin      · GREEN --> Output Voltage      · PURPLE --> Reference input

As you can see, the yellow probe shows that during almost 3ms at the start-up of the chip PS the FB seems to be grounded internally by the UCC28742. is it right? this behavior makes the output voltage goes up of 5V and it could be dangerous for the loads.

I appreciate it if any one can help me understanding why it's happening. 

  • 0
    TI__Mastermind 26770 points

    Hello Farzaneh, 

    Thank you for your interest in the UCC28742 flyback controller.

    There is no function within the UCC28742 which grounds the FB input before start-up.  The yellow waveform is low because the opto-coupler is not being driven and R68 pulls FB low.  I believe that the yellow waveform stays low for ~3ms because that is the time it takes for the LM4041 shunt regulator to come into proper operation. (I assume that the AS431A regulator in the schematic is replaced by the LM4041.)  

    The purple waveform is the reference pin of the shunt regulator and should be 1.224V in LM4041, but at start-up this voltage goes up to 2.3V before the regulator starts to pull the reference back into specification.  Please investigate the biasing of the LM4041 to improve its start-up operation. 

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 555 points

    Dear Ulrich, 

    I changed my shunt regulator to "AZ431LANTR". As you can see in the following photo, this regulator starts regulating the voltage without delay. But still we can observe that FB pin stays 0 (grounded) for 3ms. 

    is there any other reason for that?

      · YELLOW --> FB pin      · GREEN --> Output Voltage      · PURPLE --> Reference input

    Regards

    Farzaneh

  • 0 in reply to Farzaneh Faridi
    TI__Mastermind 26770 points

    Hello Farzaneh, 

    To my knowledge, there is no active pull-down on FB internal to the controller.  I will check further to verify this, but it may take a day or two for me to get a reply from our design team. 

    Meanwhile, I think FB is low for 3ms because the opto-coupler is not driving any current until Vout gets very close to 5V.  I suggest to verify this by probing the cathode voltage on the AZ431 (pin 2) to see if it stays high (tracks Vout rising) until 5V, then suddenly drops low after 3ms.  
    While pin 2 is high, no current is driven through the opto-coupler and FB stays low.  When pin 2 drops low, then significant opto current is driven and the FB voltage jumps as shown.

    That's what I think is happening.  I hope you can check this. 

    Regards,
    Ulrich  

  • 0 in reply to Ulrich Goerke
    Intellectual 555 points

    Hello Ulrich,

    I have checked the cathode pin of the optocoupler (purple probe). your notes helped me to reach to the point. I've changed R71 to 261ohm.  As you can see in the picture below the behaviour of the FB pin (green probe) has been changed.

     

    It makes sense. My question is how can I manage the R to have the most adequate function of the chip?

    Regards

    Farzaneh

  • +1 in reply to Farzaneh Faridi
    TI__Mastermind 26770 points

    Hello Farzaneh, 

    I am a little confused.  The purple trace appears to be the REF pin of the shunt regulator (sitting at ~1.24V).  It looks like the yellow trace may be the regulator's cathode voltage.  And I think the blue trace is Vout.  Please correct me if I assumed wrong.  I suggest to keep the voltage node colors consistent from one scope capture to another, and to set Vout, cathode voltage, and reference voltage to 1V/div with all 0V levels set on the same horizontal line.  This will allow easier scaling between traces and reveal more detail.

    In any case, 261 ohms seems not in the typical value range for this resistor's function.  It may be that R71 is also very low.  R71 is sized to provide the minimum current for the shunt regulator when the opto-coupler is cut off (at maximum output power).  The AZ431 only requires a maximum of 80uA to function.  The opto-coupler is generally cut off when the diode Vf < 0.5V, so 0.5V/681R = 734uA minimum.  I suggest to drop this current to about 100uA, so 0.5V/100uA = 5kR, so R70 can be 5.1kR, for example. 

    With R70 = 5.1K, then R71 can be raised to a higher value and still provide sufficient diode current for the CTR to the output collector current.  The FB current of the UCC28742 ranges from 0uA at full load to ~26uA at no-load. At that current, Vfb = ~1V, so R68 current = 1V/4kR = 250uA.  Total emitter current = 26+250=276uA. Despite low CTR at uA levels, it doesn't take much diode current to regulate at no load.    

    I see that the TLP181 is an obsolete coupler.  TLP183 is a possible replacement, or TLP383 which has higher CTR at low diode currents (or other devices, of course).  From the datasheet, TLP181 has a CTR range of 40% to 100% at 276uA, then the LED current must be at least 690uA through the LED.  At that current, Vf is ~0.95V, so R70 current would be 186uA. Total R71 current would be 690+186 = 876uA.  If the lowest shunt voltage = ~1.3Vka, then 5Vout - 1.2Vf - 1.3Vka = 2.5V across R71, so R71 maximum = 2.5V/880uA = 2.84kR.  The assumptions made in this example calculation need to be verified before using these values, but this is part of the process. 

    Regards,

    Ulrich

  • +1 in reply to Ulrich Goerke
    Intellectual 555 points

    Dear Ulrich,

    Thanks for your fantastic notes which helped me to get the best result of the desin function ever. I changed the values as follow:

    R71=750R (as it was previously in the design)

    R70=5k11

    and you can see the result in the photo below,

    · YELLOW --> FB pin      · GREEN --> Opto-coupler-cathode      · PURPLE --> Reference input    · Blue --> Output Voltage

    Regards

    Farzaneh