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UCC2891 - VDD pin is not charging upto 13V

Kurian Polachan
Prodigy 60 points
Other Parts Discussed in Thread: UCC2891

Hi,

I am trying to test UCC2891 PWM output before using the chip to build the dc-dc converter. In my bare bones circuit i have placed the following components Rdel, Ron, Roff, Rin1, Rin2. All other pins are made floating.

I also connected a 10uF capacitor at the VDD pin. When i turn ON the supply 24V @Vin pin the voltage at VDD pin goes upto 5V and stays at this point.

As per the datasheet the VDD pin voltage needs to go upto 13.5V for the device to turn ON.

Can somebody figure out the reason why this is the case ?

Also, the device consumes excessive current if i try to connect VDD pin directly to the supply of 24V.

Thanks,

Kurian Polachan

  • 0
    TI__Expert 5900 points

    Hi Polachan,

    From the spec,

    "

    During initial power up the JFET is on and charges the CBIAS and CHF capacitors connected to the VDD pin (pin
    14). The VDD pin is monitored by the controller’s undervoltage lockout circuit to ensure proper biasing before
    the operation is enabled. When the VDD voltage reaches approximately 12.7 V (UVLO turn-on threshold) the
    UVLO circuit enables the rest of the controller .At that time, the JFET is turned off and 5 V appears on the VREF

    terminal (pin 4).

    "

    That means you need an auxiliary bias for VDD in real converter design. 

    Have you see the transient waveforms of VDD using oscilloscope ?

    Another suggestion is you should at least add a capacitor to VREF. I do not know whether you have added it because i can not see your schematic.

    Regards

  • 0 in reply to Kening Gao
    Prodigy 60 points

    Attaching the schematic,

    Configuration, Vin = 24V, FB = 3V (i have connected FB to LINEUV pin for testing purpose)

    Issues Observed,

    1. Chip gets heated up, when VDD pin is connected to external supply of 24V or even 13V

    2. Even after following step 1, Vref is found to be at 0V, 'Out' voltage is at 0V and Aux voltage is at VDD Level

    3. I am not finding any switching activity at Out / Aux pins

    It will be great if you could figure out the problem in the schematic.

  • 0 in reply to Kurian Polachan
    TI__Expert 5900 points

    Can you post the scopes of VDD and VREF.

    Another thing is I do not know what this barely board can help.

    My suggestion is to design a real board. This chip has been widely used in real converters.

    Regards

  • 0 in reply to Kening Gao
    Prodigy 60 points

    Hello Kening,

    VDD is at 13V, since 13V supply is directly connected to this pin. Vref is still at 0V, No switching activity at OUT or AUX pin.

    Also, my intention is to check/study UCC2891 behavior before proceeding with incorporating this PWM controller in the real converter.This will help me to quickly isolate any problems in the later stages of the controller development.

    My Questions,

    1. Is the schematic electrically & functionally correct, ie, can i connect VDD directly to a 13V supply

    2. Why doesn't i see any switching activity on OUT or AUX pin

  • 0 in reply to Kurian Polachan
    Prodigy 60 points

    Kening,

    Identified the problem, circuit is correct. Chip i was using some how got damaged, i am getting proper output waveforms after replacing the chip.


    Thanks for the help.