SLUAAF9 September   2021 UCC28782

 

  1.   Trademarks
  2. 1 Initial Board Visual Inspection and Start-up Check
  3. 2 Typical System Operating Waveforms
    1. 2.1 SBP2 Mode
    2. 2.2 SBP1 Mode
    3. 2.3 LPM mode
    4. 2.4 LPM to ABM Mode Transition
    5. 2.5 ABM Mode
    6. 2.6 ABM to AAM Mode Transition
    7. 2.7 AAM Mode
  4. 3Typical System Protection Waveforms
    1. 3.1 Over-Power Protection (OPP)
    2. 3.2 Output Overvoltage Protection (OVP)
    3. 3.3 Output Short-Circuit Protection (SCP)
  5. 4Common Issues and Solutions
    1. 4.1 VDD Boost Converter Survival Mode
      1. 4.1.1 Survival Mode Due To Boost Inductor DCR Too High.
      2. 4.1.2 System Stays In Survival Mode When the Output Voltage is Low Such as 3.3V/5V/9V
    2. 4.2 10% Load Efficiency Might not Meet spec. in USB-PD Application, Especially at 5 V/9 V Output Condition
    3. 4.3 Transient
      1. 4.3.1 BIN/BSW Pin Damage During LPM to ABM Mode Transition
      2. 4.3.2 SR MOSFET VDS Overstress at Survival Mode
      3. 4.3.3 SR MOSFET Vds Voltage Overstress Due to PWMH Partial Turn On At Load Transition
  6. 5References

3.3 Output Short-Circuit Protection (SCP)

For SCP, VDD voltage needs to drop to VDD(OFF), the auxiliary winding voltage can reflects output voltage. If output been shorted, the AUX winding voltage during PWMH on time is also been clamped to a low voltage. Then UCC28782 internal current threshold (VCST) can reflect the VS pin voltage at Vvs (sm2). This can be reflected by capturing transformer primary winding current which equal to VVS(SM2) divide current sense resistor Rcs. CH4 in Figure 3-6 shows VS pin is at VVS(SM2) stage.

GUID-20210804-CA0I-7QMV-P6LL-P4MGFM3HBBPC-low.png Figure 3-6 SCP Delay to Action
GUID-20210804-CA0I-KXR4-XVNM-TBXJBJRXQHRX-low.png Figure 3-7 SCP Action

CH3: Low side FET Vds, CH4: Primary current, CH2: VDD

Additional protections details and corresponding solutions can be found in Debugging UCC28780 ACF Converter Start-up Issues. application note.