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UCC2889: question

Benjamin Fons
Prodigy 30 points
Part Number: UCC2889

Have several questions on the UCC2889.
Using the UCC2889 as the controller in an offline bias supply in dual flyback configuration, similar to app note u-149. In the application, line voltage range is 600 - 1500V. Setting Ron so that minimum current into Ton is greater than 220 uA, but UCC2889 still goes into line voltage fault mode with .5msec off time pulses.
1st question, what is the minimum current into Ton pin for IC to operate and not be in line voltage fault? Should current be sum of startup current and the minimum i (Ton) current for line fault?
2nd question, if the VCC pin is connected to an external voltage, why does the Ton period change compared to when the VCC pin is only connected to a bypass cap?
3rd question, when the controller is running, why would the measured voltage across the output cap in 1st flyback stage be twice the calculated voltage based on the vout = vin*d/(1-d)? The calculated voltage is 85V and the measured voltage is 130V. Let me know what info on the setup is needed to address these questions.

  • 0
    TI__Expert 6080 points

    Hello Benjamin,

    I will have to get back to you on this after consulting it with the design team within the next week.

    Thanks,

    Sonal

  • 0 in reply to Sonal Singh
    TI__Expert 6080 points

    Hello Benjamin,

    Please find my comments below:

    1. Ton is basically a kind of line voltage control: it has these major functions; Ct discharge (Ton current is routed to Vcc), Ct charge (current through Ton is used to charge Ct and little current is used to sense the line voltage(220uA) is used for that. When the Drive is high the current is consumed from Ct timing cap. Please refer to the ICHG Vs Line voltage graph in the datasheet to check this value. 
    2. Normally, Vcc is not directly connected to an external source. If it is the case the only precaution from my understanding is to limit the Vcc current (<5mA) to avoid overstressing the part.
    3. The application note suggests that the output of the first flyback topology will be Vin*(d/1-d)  as you suggested. However, the output of a cascaded stage would have a higher output voltage because the duty cycle will be doubled. I am not quite sure about your question here, but can you share the schematic and your design parameters for a better understanding of your issue.

    Regards,

    Sonal