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UCC28C40: Problems with start up - insufficient VDD and possible output disable.

Part Number: UCC28C40
Other Parts Discussed in Thread: TPS5405

Hello,

I'm having some trouble getting the UCC28C40 running from high voltage DC in a flyback converter very similar to the one on page 24 of the datasheet. When I connect a DC voltage of 120V to the input capacitor, the resulting V_DD is only 2.84V, whereas I am looking to supply the chip with 12V. This is achieved using a startup resistor R_START = 360kOhm. I have measured the output on pin 6 to be a constant, but noisy 76mV (during writing it dropped from 100mV).

My hypothesis is that the output is being forced low which results in the auxiliary winding of the flyback transformer not receiving any voltage to power the controller. This is further supported by observations that lowering R_START will allow the controller to draw enough current to raise VDD above the UVLO ON threshold of 7V.

One of the output disable conditions is being met - The voltage on pin 1 (COMP) is 0 ("Enable and Disable" on page 19 of the datasheet). I have checked the connections on my PCB with the schematic and there are no incorrect connections. Furthermore, COMP is not being accidentally shorted to ground. 

What could be forcing COMP low? Any suggestions for further troubleshooting?

Regards,

Nikola A. Žikić

  • Nikola,
    Thanks for your interest in TI here. This is a very popular controller for a broad range of applications. I'll have an applications engineer assist you with this debug.

    To better assist you quickly could you provide a schematic of your design and transformer details along with any waveforms of the issue you are seeing?

    Regards
    John
  • Hi John,

    Certainly, here is the schematic with the controller IC on the left, gate driver in the middle, and optoisolated compensator on the right. 

    The transformer used is the Coilcraft GA3366-BL.

    Here are the scope traces for each pin when 120V is applied to the power supply input:

    COMP:

    FB:

    CS:

    RT/CT:

    OUT:

    VDD:

    VREF:

    MOSFET DRAIN:

    I have tried connecting the desired VDD of 12V directly to the VDD pin using a DC bench top power supply.

    These are the waveforms I am observing:

    COMP:

    FB:

    CS:

    RT/CT:

    OUT:

    VDD:

    VREF:

    MOSFET DRAIN:

    COMP and CS do not fulfil the disable conditions, and yet there is still no output when 12V is directly applied to the chip.

    Any assistance you can provide would be greatly appreciated.

    Regards,

    Nikola

  • Hello Nikola,
    Could you send a more complete schematic with transformer connections?

    I am curious at what your input and output power requirements are?

    In the mean time you could try powering the IC with an external bias supply just to see if you can get the converter switching. You should be able to do this open loop without even having an input voltage. I would do this as sanity check just to make sure there is not something pulling down on the COMP pin.

    If you do have switching then you can apply a small input voltage say 30V and check the polarities of your transformer connections; as well as, you could check your turns ratio to make sure the aux wining can apply enough voltage to keep the IC enabled.

    Regards,

    Mike
  • Hello Mike,

    Here is the complete schematic as both an EAGLE CAD file and image. /cfs-file/__key/communityserver-discussions-components-files/196/6428.Board2_5F00_updated.sch

    Input and output specs are:

    Input - 85-265Vrms from mains at 50-60 Hz

    Output - 19.5VDC, 4.5A maximum

         5V DC, 1A maximum from TPS5405 buck converter on secondary.

    I have already attempted to supply the IC with an external bias voltage with the results shown in my previous post. COMP was high, CS was low, and the output was low too.

    An attempt was made to power the circuit with an input of 120VDC, and with no gate driver. The VDD waveform is shown below. VDD oscillates above and below the UVLO ON threshold of 7V.

  • Hello Gentlemen,

    Thank you very much for your time, patience, and willingness to help.

    Upon comparison with a friend's schematic using the same chip, I removed the ramp capacitor from the slope compensation circuit and bridged the pads with solder. For good measure I replaced the controller IC with a new specimen. It works now.

    Once again, thank you very much and kind regards.

    Nikola