SLUAAJ7 June   2022 UCC256402 , UCC256403 , UCC256404

 

  1.   Abstract
  2.   Trademarks
  3. 1UCC25640x Selection Guide
  4. 2UCC25640x Features Brief Overview
    1. 2.1 High Voltage(HV) Startup
      1. 2.1.1 HV Startup Procedure
      2. 2.1.2 HV Startup with External Bias
      3. 2.1.3 HV Start-up, VCC, X-cap Discharge Internal Block Diagram
      4. 2.1.4 HV Startup External Resistor
    2. 2.2 XCAP Discharge
      1. 2.2.1 IEC Standards
      2. 2.2.2 Detecting AC presence
      3. 2.2.3 Test Current Injection for Zero Crossing Detection
      4. 2.2.4 Typical Waveforms of HV Startup and XCAP Discharge
    3. 2.3 Feedback Chain
      1. 2.3.1 FBreplica Generation
      2. 2.3.2 Vcomp Signal and Threshold Voltages
      3. 2.3.3 FB Pin Voltage Typical Waveform at no Load
    4. 2.4 Hybrid Hysteretic Control and VCR Pin Voltage and Gate Pulse Generation
      1. 2.4.1 Hybrid Hysteretic Control
      2. 2.4.2 VCR Pin Voltage
      3. 2.4.3 VCR Typical Waveform
    5. 2.5 Soft Start
      1. 2.5.1 Soft Start Timing
      2. 2.5.2 Soft Start Initial Voltage Programming
    6. 2.6 Burst Mode
      1. 2.6.1 Burst Patterns
      2. 2.6.2 BMTL/BMTH Ratio Programming
      3. 2.6.3 BMTH Generation
      4. 2.6.4 Interpreting BMTL and BMTH
      5. 2.6.5 Soft On or Off
      6. 2.6.6 Operation when Burst Mode Disabled
      7. 2.6.7 Typical Waveforms
    7. 2.7 Adaptive Dead Time Control
    8. 2.8 Fault Management
      1. 2.8.1 OCP Protection
      2. 2.8.2 OCP Fault Typical Waveforms
      3. 2.8.3 Over Voltage Protection using Bias Winding (BW OVP)
      4. 2.8.4 Restart or Latch
    9. 2.9 ZCS Region Prevention Scheme
      1. 2.9.1 ZCS Effects
      2. 2.9.2 ZCS Detection and Prevention and Disabling
  5. 3UCC25640x Power Up Guidelines and Debugging Notes
    1. 3.1  Power Up Procedure
    2. 3.2  HV Pin
    3. 3.3  VCC Pin
    4. 3.4  BLK Pin
    5. 3.5  FB Pin
    6. 3.6  ISNS Pin
    7. 3.7  VCR Pin
    8. 3.8  BW Pin
    9. 3.9  LL/SS Pin
    10. 3.10 LO Pin
    11. 3.11 RVCC Pin
    12. 3.12 HS, HO, HB Pins
  6. 4References

2.4.2 VCR Pin Voltage

The VCR pin voltage is one of the important signals for this LLC controller as this signal determines turn on instants of the both low side and high side switches.

  • The resonant capacitor voltage is sensed using lossless capacitor divider (Cup and Cdw) as shown in Figure 2-16.
  • The VCR waveform is composed of two components: the sampled resonant capacitor voltage and the voltage due to internal frequency compensation current. This frequency compensation is added using two well-balanced current sources (Iramp) as shown in Figure 2-16.
  • The added compensation will make the VCR signal in the form of a triangle voltage superimposed on the sinusoidal resonant capacitor voltage (Vr). Equation 5 gives the relation between change in VCR pin voltage and the change in resonant capacitor voltage and the frequency compensation current.
  • Equation 7 gives the relation between change in VCR pin voltage and the Vcomp signal and average input current and slope compensation current. When the output power is high, converter operates similar to charge control as the input average current component given in Equation 7 dominates the added frequency component. The slope compensation has a small impact on transfer function. The overall transfer function will be approximately first order plant transfer function similar to charge control. When output power is low, converter operates in direct frequency control as the input average current will be very small. These operations will be true assuming we set the proper mix between the sampled voltage and the compensation current in the VCR pin signal. This needs proper selection of Cup and Cdw capacitors.
  • Figure 2-17 shows that when VCR > VTH, high side gate signal (HO) turns off and when VCR < VTL, high side gate signal (LO) turns off. These thresholds are related to Vcomp signal as given in Equation 2 and Equation 3. The turn on instants of the HO and LO signals depends on the adaptive dead time control given in the Section 2.7.
  • VCR pin also has a common mode voltage (VCM) of 3 V and the peak to peak voltage should be less than 6vV at the minimum operating input voltage with maximum load.
  • The VCR voltage affects the initial switching frequency profile during startup so it is important to make sure there is sufficient margin to avoid tripping OCP when starting into maximum load. The initial switching frequency also impacts the startup timing. Section 2 of the UCC25630x Practical Design Guidelines gives more detail regarding the impact of VCR voltage on startup behavior.