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UCC28711: Intermittent operation of Flyback Converter

Part Number: UCC28711

Hello,

I designed a flyback converter using the UCC28711. The DC input voltage ranges from 150V-1000V. And the output voltage consists of 2 outputs of 17V each. Unfortunately, the circuit does not respond as expected. In particular, the constant voltage regulation does not stop on the desired voltage. Even though, the calculations of Rs1 & Rs2 are correct (the voltage on Vs is 4.05 when the output voltage is 17V), the IC keeps charging  the output until 22V, then it stops and when the output discharges to 10V in starts charging again until it reaches 23V. At this point the overvoltage protection is activated and the IC swithces off, until Vdd reaches the Vdd turn-off threshold (8.1V). Then, the process starts again with the aforementioned pattern.

Is it possible that the problems occur because the NTC pin is left floating?

Can the ripple on the Vauxiliary create confusion to the Vs pin during the sampling time?

The following pdf files include the waveforms of the converter:

Blue=Vdd pin-Channel A

Red=DRV pin-Channel B

Green=Vauxiliary-Channel C

Yellow=Output Voltage-Channel D

The file ti1 presents the waveforms of the above measured signals. At 1.712ms is the point where the IC detects the overvoltage, even though it reaches the  desired regulated voltage at 0.462ms

The file ti2 presents the same waveforms but zoomed in to the moment where the output voltage reaches 17V. I would liek to hear your opinion regarding the shape of Vauxiliary (Green) and how it can affect the sampling of the Vs pin. 

ti1.pdfti2.pdf

Thanks in advance.

  • Gerasimos

    If you are not using NTC pin function we recommend to leave it floating as you have already done.

    If you have not already, we recommend you use the excel design calculator http://www.ti.com/lit/zip/sluc590 to verify that all the external components in your design are correct.

    Please note that for VS pin measurement we don't recommend probing the pin directly.  We recommend that you measure the aux winding voltage.  The parasitic capacitance on a probe can interfere with operation if directly connected to VS pin.

    From your waveforms it looks like the waveform that the VS pin is measuring looks to be incorrect.  The waveform should look like figure 14 (page 13) of the UCC28711 datasheet.  The turn on point is not always on the valley as expected, and instead varies from cycle to cycle.  Below are a few recommendations to improve VS waveform among others.

    • It is recommended to have <4pF of capacitance on VS pin.  Remove any external capacitor if there is one.
    • Verify that the transformer design follows best recommended practice for multiple output, coupling thru  - see details in this video https://training.ti.com/multiple-output-flybacks-how-improve-cross-regulation?context=1134585-1139235-1134623
    • Verify that layout follows recommendation in section 11.2 (page 25) of the UCC28711 datasheet.  If it does not noise can be injected into the key nodes which will interfere with operation.
    • Increase output capacitance - there may be insufficient output capacitance which is causing the output voltage to overshoot

    Best Regards,

    Eric

  • Dear Eric,

    Thank you for your quick reply. I do not measure the Vs pin. I measure the Anode of the Auxiliary winding diode (Vauxiliary-Green). In the file ti2.pdf that I sent you can see that there is something weird with the Vauxiliary (Green). In particular, various oscillations take place after the switch-off of the mosfet. Have you ever seen something similar? 

    Best Regards.

  • Gerasimos

    Thank you for clarifying how you measured VS waveform.

    Vauxiliary waveform does not look correct.  It should look like Figure 14 (page 13) of the UCC28711 datasheet.  The turn on point does not track the valley.  Typically distortion such as this is caused by excessive leakage inductance in the transformer or layout error that injects excessive noise.  If you continue to have issues with your design, we recommend that you use an RCD snubber instead of a Zener clamp (this app note has details on how to design the clamp http://www.ti.com/lit/an/snva744/snva744.pdf).  

    I recommend you verify your design using the steps I highlighted earlier to identify what is causing this signal to be distorted.  If you cannot resolve the issue, please share your schematic and PCB layout.

    Best Regards,

    Eric

  • Dear Eric,

    Thank you for your usefull advices so far. I changed the snubber circuit to RC snubber (previously it was Diode-Zener), and there was some progress. Specifically, when I changed the ultrafast diode of the snubber to a standard, the chip stopped guiding the converter to output overvoltage which led to switch-off until the undervoltage lockout of 8V in Vdd and then switching on again through the HV pin. Nevertheless, now I face different problems. In particular, as you can see in the files that I have attached, when the output reaches the desirable limit (18V) the IC stops regulating for a long period. In the meantime the output discharges a lot. Some times it discharges to 16V, others to 13V and some others even lower to 7.68V, without any particular pattern. Have you ever faced a similar problem?

    The following files include the waveforms :

    Blue: Vdd 

    Red: IC driver pin (DRV)

    Green: Voltage on the auxilliary winding, on the anode of the auxiliary winding diode

    Yellow: Output voltage

    As you can observe in the first file, the IC sends pulses until the voltage reaches the desirable value of 18V. Then it stops for along period, usually until the voltage goes to 8V. It is clear that the there is no OverVoltage Protection, since the Vdd is not let to discharge until the Undervoltage lockout of 8V, restarting the HV circuit. 

    4331.ti1.pdf

    The secomd file presents the same waveforms as the first file, but in more detail, zooming in. 0777.ti2.pdf

    Thanks in advance for your time and your useful advices.

  • Gerasimos

    In your zoomed in waveform from the file "0777.ti2.pdf" it is not zoomed in enough to verify that the aux winding signal is now correct.  In the earlier waveform you shared the turn on point is not always on the valley as expected, and instead varies from cycle to cycle.  This is incorrect, it should look like figure 14 (page 13) of the UCC28711 datasheet.  If the waveform is incorrect, UCC28711 may think the output voltage is at a different point than it is actually at, which may cause the improper behavior that you are seeing.

    Another recommendation is to verify that you have sufficient output capacitance.  Generally we recommend 100uF/W of capacitance to insure stability

    Best Regards,

    Eric