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UCC24612: Conditions for turning on

Hirotsugu Kobayashi
Mastermind 8915 points
Part Number: UCC24612
Other Parts Discussed in Thread: UCC28782,

Tool/software:

Hi team,

I am using UCC28782 and UCC24612.

When the AC input is low, it operates stably, but when the AC input becomes high, pulses are missing or false on occurs.

① At this time, ID is not flowing, so I understand that VTHGON cannot be detected.
② Even though there is a sufficient ID flowing, it does not turn on. Why is that?
③ What are the conditions for turning on at this timing?

  • 0
    TI__Guru 73375 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 73375 points

    Hello,

    There is a minimum off time that is calculated based on the previous cycle.   Your drain current looks quite unstable and the conduction time seems to change every switching cycle.  This might be what is causing the gate driver to misbehave.

    Is your output voltage stable during this?

    I think if you remove the instability in rectifier current the gate driver will work properly.

    Regards, 

  • 0 in reply to Mike O'
    Mastermind 8915 points

    Hi Mike,

    I understand that this is due to instability.
    Could you please explain at what point Toff blank is set and causes this behavior?

  • 0 in reply to Hirotsugu Kobayashi
    TI__Guru 73375 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 73375 points

    Hello,

    The Toff blank time is set every previous cycle based on conduction and  off time.

    Regards,

  • 0 in reply to Mike O'
    Mastermind 8915 points

    Hi Mike,

    The previous Toff is about 5us, so Toff blanking time is 3.5us,

    However, even though current is flowing, Toff is extended by 12us.

    What is the factor that blocks ON?

  • 0 in reply to Hirotsugu Kobayashi
    TI__Guru 73375 points

    Hello,

    If Toff time is 3.5 us the only thing that would prevent it from turning on is not seeing a Vds greater than 240 mV at the VD and VS pins of the UCC24612. 

    Regards,

  • 0 in reply to Hirotsugu Kobayashi
    TI__Mastermind 31130 points

    Hello Kobayashi-san, 

    Similar to my reply in your other E2E question (ref:  https://e2e.ti.com/support/site-support-group/site-support/f/site-support-forum/1368998/ucc24612-pulse-skip ), I think the complex timing conditions for on and off blanking result in this behavior.  

    Basically, I think the UCC24612 is getting confused by the different timing rules it must operate by due to the changing conditions that it detects at the VD and VS pins.  

    In this case, I think there are 3 different conditions that result in the 3 different on/off states:

    Point 1:  Off time is longer than minimum on-time so next on-time is skipped (same as in the other E2E references above).  
    Point 2:  This small current peak would have turned on the SR FET, but was skipped by Point 1.  However, I think it triggered another minimum off-time interval.
    Point 3:  This is the off-time blanking set up by Point 2.  After the blanking expires, the SR finally does turn on.  It is abruptly shut off by next primary on-time.
    Point 4:  Because of body-diode vs. minimum off time condition, next SR on pulse is skipped again. 

    After that, the timings become "normal" again and many switching cycles behave as expected.  Sooner or later this abnormal set of conditions may occur again.  I think if the resonant current can become more regular (instead of varying cycle-by-cycle), the SR controller can operate consistently. 
    Again, this would become an ACF design issue to minimize variations in the currents.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Mastermind 8915 points

    Hi Ulrich-san,

    Thanks for the explanation.
    What is the off-time blanking in point 3 based on?
    Is it because Tring was set with the skipped Ton?
    Or is it because Toff_max was set?

  • 0 in reply to Hirotsugu Kobayashi
    TI__Mastermind 31130 points

    Hello Kobayashi-san, 

    I'm sorry, I am not exactly sure of the answer. 
    I think the off-time blanking in point 3 is based on Toff_max being set.  

    The datasheet specifies that Toff_max = 3.68us, and I believe that that includes the total 2.2*Tring time allowable, if Tring was set.

    But on the 10us/div time scale, 3.68us is less than the off time that can be seen between the skipped turn-on point 2 and the actual turn-on point 3.   
    So it looks like Toff_max alone does not account for all of the time between points 2 and 3.  

    So, I think there is some operating mode in the SR controller that I do not understand, and it is not described in the datasheet.
    Or, it may be possible that the 2.2*Tring timer is independent of the Toff_max time limit. 

    I'm afraid that I don't have a better answer than that. 

    Regard,
    Ulrich