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UCC28711: Support Request – Issues with UCC28711DR Flyback Design, CS/VS pin

Vikas Rawani
Prodigy 10 points
Part Number: UCC28711

Tool/software:

Hello TI Team,

I am facing multiple issues with my UCC28711DR-based flyback design(50W) and would appreciate your guidance.

Schematic:
Schematic

Layout:

Background

Design Specs:  Input is 40V-800V DC, 50W Power.

  1. Initially, I tested the circuit using a controlled DC source at the DC bus.

    • The IC started switching at around 55 VDC (possibly due to peaks).

    • However, it kept restarting repeatedly, so I increased the DC bus to 100 VDC.

    • Waveforms for this condition are attached (Fig1, Fig2, Fig3).

  2. My auxiliary and secondary windings are designed for the same output voltage (15 V).



Fig1

Fig2

Fig3

Problems Observed

Problem 1:

  • The output is regulating at 23 VDC instead of 15 VDC, even though the number of turns for both the auxiliary and secondary windings (Naux and Nsec_15V) are identical.

Problem 2:

  • The VS pin seems very sensitive.

  • When I touch the divider node (between RS1 and RS2) — not directly the pin — the output voltage sometimes increases, and then the IC turns off and restarts.

  • Even when not touching, bringing a metal tweezer close to this point causes disturbances.

  • I am unsure whether this sensitivity is expected behavior of the VS pin, or if it is due to my PCB layout.

Problem 3:

  • After modifying the CS pin divider, the IC kept restarting, so I checked the CS waveform (attached).

  • To rule out the DC source, I switched to an AC variac connected to a diode bridge:

    1. Applied single-phase AC at 60 VAC (DC bus ~80 V).

    2. The IC continued to restart.

    3. Changed the VS divider (RS1 = 51 kΩ, RS2 = 22 kΩ) to increase current at the VS pin.

  • Divider values were calculated as per the datasheet table.



Fig4

Fig5

Fig6


Table calculation was followed as per pdf: /cfs-file/__key/communityserver-discussions-components-files/196/slua783.pdf

VAUX_RMS_measured 12.8 V
RS2 22 k
RS1 51 k
DIVIDER RATIO_calculated 0.301369863  
VS PIN_calculated 3.857534247 V
VDC_OUT_measured 20.89 V
VAUX_PEAK_measured 30 V
VS PIN PEAK_calculated 9.04109589 V
VS line-sense run current (IVSL_RUN_NEEDED) 225 uA
VS line-sense stop current  IVSL(stop) 80 uA
Absolute Maximum Peak current, VS (I_VS) -1200 uA
IVSL_RUN_CAL 250.9803922 uA
IVSL_RUN_CAL(from peak) 588.2352941 uA

Maximum switching frequency  fSW(max) 100 kHz
desired time for tON fault is 1 / FSW(max) 10 us
LPRI_designed 70 uH
IPRI_measured 0.1 A
VBLK_measured 40 V
for the first cycle, tON_calculated = (LPRI × IPRI ) / VBLK 0.07 us
Minimum CS threshold voltage  VCST(min) 195 mV
Maximum CS threshold voltage  VCST(max) 780 mV
RCS 235 mohm
Min peak primary current IPP(min)_calculated 0.829787234 A
Max peak primary current IPP(max)_calculated 3.319148936 A

Current Status

  • Unfortunately, one IC was damaged while I was measuring voltage at the VS pin.

  • Although I did not short it, the multimeter probe introduced noise which caused the IC to fail.

  • So far, I have lost five ICs this way. I am waiting for replacement ICs to continue testing.

Request

Please advise on:

  1. Why the output voltage is regulating much higher (23 V) than the expected 15 V.

  2. Why the VS pin is so sensitive to nearby objects/noise, and whether this is normal or a layout-related issue.

  3. How to stabilize operation and avoid continuous restarts.

Any recommendations to help me operate the UCC28711DR in a normal and stable manner would be greatly appreciated.

Thank you.

Best regards,
VIKAS



  • 0
    TI__Guru* 85826 points

    Hello,

     

    Please see my comments below.

    1. Why the output voltage is regulating much higher (23 V) than the expected 15 V.
      • More than likely your VS resistor divider is not setup correctly.
      • The following link will bring you to an excel design tool for this product that you can use to setup this resistor divider correctly. https://www.ti.com/tool/download/SLUC590
      • Once you make this adjustment to the VS resistor divider should be able to get the design to regulate correctly.
      • Please note that over filtering the VS pin can also cause regulation issues. It is recommended that use less than 3.3 pF of capacitance on this pin if it requires filtering.

     

    1. Why the VS pin is so sensitive to nearby objects/noise, and whether this is normal or a layout-related issue.
      • The VS pin is used to regulate the output voltage during the transformer demagnetizing time (tDmag). It is also used to check the input voltage when the primary FET is on.  So, if there is noise on this pin it will react to it.
      • During tDmag I a see there is a lot of ringing after the leakage spike. This can lead to issue with accidently triggering OVP.
    2. How to stabilize operation and avoid continuous restarts.
      • If your design it restarting it is most likely due to an OVP, OCP or input UVLO fault. The most common issues for this is noise.  The following link will bring you to an application note that discusses how primary fault sensing that the UCC28711 uses.  The issues with it and how to resolve them with filtering, snubbing and clamping.  https://www.ti.com/lit/pdf/sluaac5
      • The high side resistor in your VS divider to the resistor sets the input voltage where the design turns on. It sounds like that is not setup correctly.  The excel tool mentioned earlier can be used to select the resistor to turn on at the proper input voltage.

     

    Regards,