ZHCSE71I september   2015  – august 2023 SN6505A , SN6505B

PRODUCTION DATA  

  1.   1
  2. 特性
  3. 应用
  4. 说明
  5. Revision History
  6. Pin Configuration and Functions
  7. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Typical Characteristics, SN6505A
    8. 6.8 Typical Characteristics, SN6505B
  8. Parameter Measurement Information
  9. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Push-Pull Converter
      2. 8.3.2 Core Magnetization
    4. 8.4 Device Functional Modes
      1. 8.4.1 Start-Up Mode
        1. 8.4.1.1 Soft-Start
      2. 8.4.2 Operating Mode
      3. 8.4.3 Shutdown-Mode
      4. 8.4.4 Spread Spectrum Clocking
      5. 8.4.5 External Clock Mode
  10. 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. 9.2.2.1 Drive Capability
        2. 9.2.2.2 LDO Selection
        3. 9.2.2.3 Diode Selection
        4. 9.2.2.4 Capacitor Selection
        5. 9.2.2.5 Transformer Selection
          1. 9.2.2.5.1 V-t Product Calculation
          2. 9.2.2.5.2 Turns Ratio Estimate
          3. 9.2.2.5.3 Recommended Transformers
      3. 9.2.3 Application Curves
      4. 9.2.4 System Examples
        1. 9.2.4.1 Higher Output Voltage Designs
        2. 9.2.4.2 Application Circuits
  11. 10Power Supply Recommendations
  12. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  13. 12Device and Documentation Support
    1. 12.1 Device Support
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Related Links
    4. 12.4 Receiving Notification of Documentation Updates
    5. 12.5 Community Resources
    6. 12.6 Trademarks
  14. 13Mechanical, Packaging, and Orderable Information

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Push-Pull Converter

Push-pull converters require transformers with center-taps to transfer power from the primary to the secondary (see Figure 8-1).

GUID-40D1B18F-485A-421A-94A4-40D290C2BD69-low.gifFigure 8-1 Switching Cycles of a Push-Pull Converter

When Q1 conducts, VIN drives a current through the lower half of the primary to ground, thus creating a negative voltage potential at the lower primary end with regards to the VIN potential at the center-tap.

At the same time the voltage across the upper half of the primary is such that the upper primary end is positive with regards to the center-tap in order to maintain the previously established current flow through Q2, which now has turned high-impedance. The two voltage sources, each of which equaling VIN, appear in series and cause a voltage potential at the open end of the primary of 2×VIN with regards to ground.

Per dot convention the same voltage polarities that occur at the primary also occur at the secondary. The positive potential of the upper secondary end therefore forward biases diode CR1. The secondary current starting from the upper secondary end flows through CR1, charges capacitor C, and returns through the load impedance RL back to the center-tap.

When Q2 conducts, Q1 goes high-impedance and the voltage polarities at the primary and secondary reverse. Now the lower end of the primary presents the open end with a 2×VIN potential against ground. In this case CR2 is forward biased while CR1 is reverse biased and current flows from the lower secondary end through CR2, charging the capacitor and returning through the load to the center-tap.