This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28711: Current drain from AUX winding

Part Number: UCC28711
Other Parts Discussed in Thread: TIDA-00173

Hello,

In our application, we also need to draw current from the auxiliary winding (approx. 50mA / 15V). But if we load the AUX output, the voltage drops to 10V (with 50mA) immediately. We use a similar connection as in TIDA-00173 (VAUX).

Some Informations:

Input voltage 380 - 420V
Output 15 V / 0.5A
Auxiliary Output 15 V / 0.05A
Primary Inductance 2.37 mH
Switching frequency 90 kHz

Do you have any tips how to fix this?

Thank you

  • Hi,

    You need to design your transformer with very good coupling between the AUX and the OUTPUT. The drop is most likely due to the bad coupling since the AUX is cross regulated so if the coupling not good then the drop is high. Also, did you try to add load on your output so see if the AUX voltage regulation gets better. It is another way to improve the AUX voltage regulation to reduce the leakage effect from a load on the output.

  • Hi,

    This is what we know for such similar issues. 

  • Hello,

    I rewound the transformer and add 4.7k ohm resistor to output, but problem is even worse.

    When I load the aux output with 5mA, the output drops to about 11V. When I load the output with 10mA, the output starts to oscillate between 11V and 0V, see the waveform from the oscilloscope.

    This waveform is on the capacitor C27:

    This waveform is on the capacitor C29:

    Thank you

  • Hi,

    What is your +15V output at these conditions?

  • Hi,

    I mean what are +15VOUT and +15V_PFC when you loaded 5mA to which and where did you put 4.7k resistor?

  • Hi,

    both waveforms are from +15V_PFC. 4.7k resistor is on +15VOUT and 5mA load is on +15V_PFC.
    If i load 10mA od +15V_PFC, on +15VOUT is 18.2V.

  • Hi,

    It looks this confirms the issue due to the transformer two windings not coupled tight enough. Basically the +15VOUT is with a voltage higher than the intended regulation point 15V and this voltage is sensed by the controller so to decide to make narrower or no pulses, it means the controller does not know the drop of +15V_PFC. 

    A load of 4.7k on +15VOUT is too small to have proper cross regulation. You may try to increase the +15VOUT load to > 50% of its rated load, then loading +15V_PFC with 5mA to see if the situation gets better.

    Also, you can try to short R32 to see any improvement.

  • Hi,

    It looks this confirms the issue due to the transformer two windings not coupled tight enough. Basically the +15VOUT is with a voltage higher than the intended regulation point 15V and this voltage is sensed by the controller so to decide to make narrower or no pulses, it means the controller does not know the drop of +15V_PFC. 

    A load of 4.7k on +15VOUT is too small to have proper cross regulation. You may try to increase the +15VOUT load to > 50% of its rated load, then loading +15V_PFC with 5mA to see if the situation gets better.

    Also, you can try to short R32 to see any improvement.

  • Hi,

    As no communication for a long time, I will close this thread. If needed, you can re-open it.