SLUSD37E October   2017  – November 2019 UCC28056


  1. Features
  2. Applications
  3. Description
    1.     No Load Power
      1.      Device Images
        1.       Simplified Application
  4. Revision History
  5. Device Comparison Tables
  6. Pin Configuration and Functions
    1.     Pin Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 CrM/DCM Control Principle
      2. 8.3.2 Line Voltage Feed-Forward
        1. Peak Line Voltage Detection
      3. 8.3.3 Valley Switching and CrM/DCM Hysteresis
        1. Valley Delay Adjustment
      4. 8.3.4 Transconductance Amplifier with Transient Speed-up Function
      5. 8.3.5 Faults and Protections
        1. Supply Undervoltage Lockout
        2. Two Level Over-Current Protection
          1. Cycle-by-Cycle Current Limit Ocp1
          2. Ocp2 Gross Over-Current or CCM Protection
        3. Output Over-Voltage Protection
          1. First Level Output Over-Voltage Protection (Ovp1)
          2. Second Level Over-Voltage Protection (Ovp2)
        4. Thermal Shutdown Protection
        5. Line Under-Voltage or Brown-In
      6. 8.3.6 High-Current Driver
    4. 8.4 Controller Functional Modes
      1. 8.4.1 Burst Mode Operation
      2. 8.4.2 Soft Start
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. Custom Design With WEBENCH® Tools
        2. Power Stage Design
          1. Boost Inductor Design
          2. Boost Switch Selection
          3. Boost Diode Selection
          4. Output Capacitor Selection
        3. ZCD/CS Pin
          1. Voltage Spikes on the ZCD/CS pin Waveform
        4. VOSNS Pin
        5. Voltage Loop Compensation
          1. Plant Model
          2. Compensator Design
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 VOSNS Pin
      2. 11.1.2 ZCD/CS Pin
      3. 11.1.3 VCC Pin
      4. 11.1.4 GND Pin
      5. 11.1.5 DRV Pin
      6. 11.1.6 COMP Pin
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Custom Design With WEBENCH® Tools
    2. 12.2 Receiving Notification of Documentation Updates
    3. 12.3 Community Resources
    4. 12.4 Trademarks
    5. 12.5 Electrostatic Discharge Caution
    6. 12.6 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

Boost Diode Selection

The Boost diode carries the Boost inductor current while the switch is OFF (TDCH), and carries zero current while the switch is ON (TON). Equation 34 calculates The RMS diode current over a single switching cycle, at angle θ in the Line half-cycle .

Equation 34. UCC28056 eq-34.gif

Equation 35 describes the duty cycle of Boost diode conduction for ideal transition mode operation .

Equation 35. UCC28056 eq-35.gif

Equation 36 describes the RMS Boost diode current across a complete Line half-cycle .

Equation 36. UCC28056 eq-36.gif

The maximum RMS current in the Boost diode occurs at maximum load and minimum Line.

Equation 37. UCC28056 eq-37.gif

Conduction power loss in the Boost diode is primarily a function of the average output current.

Equation 38. UCC28056 eq-38.gif

Use the previous calculations and these guidelines to select the Boost diode:

  • Ensure that the Boost diode voltage rating exceeds the maximum output voltage. Under transient or Line surge testing the output voltage may rise far above its normal regulation level.
  • The Boost diode must have average and RMS current ratings that are higher than the numbers calculated by Equation 37 and Equation 38.
  • Diodes are available with a range of different speed/recovery charge. Fast diodes, with low reverse recovery charge, typically have higher forward voltage drop. Fast diodes have higher conduction loss but lower switching loss. Slow diodes, with high reverse recovery charge, typically have lower forward voltage drop. Slow diodes have lower conduction loss but higher switching loss. Ensure maximum efficiency by matching the diode speed rating to the application.
  • When Line voltage is first applied, to the Boost converter input, an uncontrolled current flows through the Boost diode while the output capacitor charges to the Line voltage peak level. The charging current is limited only by the impedance of the Line and EMI filter stage, and may reach a very high magnitude during the output capacitor charging period. Any diode carrying this current must be rated to carry this non-repetative surge current. It is normal practice to add a bypass diode to divert most of this charging current away from the Boost diode. The bypass diode can be a slow type with lower forward voltage drop. It is therefore cheaper and more robust than the faster Boost diode.
  • For this example design the STTH5L06 diode from STMicroelectronics® was selected. This diode has a voltage rating of 600 V and an average current rating of 5 A. It has a forward voltage drop of approximately 0.85 V giving a conduction loss in the Boost diode, of less than 0.5 W.