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Bridge Converters and Faraday Screens

Bridge Converters and Faraday Screens. By Paul Wilson. Outline. Bridge Converters Introduction Transformer Design example Problems with Bridge Converter Design Possible Solutions Schematic Simplorer Simulation. Outline. Faraday Screens Difficulties in switchmode power supply design

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Bridge Converters and Faraday Screens

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  1. Bridge Converters and Faraday Screens By Paul Wilson

  2. Outline • Bridge Converters • Introduction • Transformer Design example • Problems with Bridge Converter Design • Possible Solutions • Schematic • Simplorer Simulation

  3. Outline • Faraday Screens • Difficulties in switchmode power supply design • Applications to switches • Differences between Faraday Screens and Safety Screens

  4. Advantages Single primary winding Works in well defined conditions Flyback energy recovery without energy recovery winding Disadvantages Cost More parts Bridge Converters

  5. Design Example • Straightforward design • Minimize magnetization current • Specifications

  6. Step 1 • Select Core Size Initial efficiency of 75% therefore transmitted power will be 500/0.75 = 667 W

  7. Step 2 . Usually it is understood that minimum loss (maximum efficiency) will be found near the point where the copper and core losses are equal, this is shown in figure 5 at 70 mT where the core loss is 44%. 80 mT is normally assumed as the optimum choice. • Select optimum flux density

  8. Step 2 cont. A 100 mT is recommended by the manufacturer is not far from optimum and this higher value will be used to reduce the number of turns

  9. Step 3 • Calculate Primary Voltage Vcc Vcc = 90 x 1.3 x 1.9 = 222 VDC

  10. Step 4 • Maximum on period

  11. Step 5 • Calculate Primary turns

  12. Step 6 • Calculate secondary turns

  13. Problems • Staircase Saturation • Transient Saturation Effects Solutions • Adding fast acting current limits to power transistors • Controlling slew rate of control amplifier • Forced Flux Density Balancing

  14. Circuit Schematic

  15. Simplorer Simulation

  16. Waveforms

  17. Faraday Screens • Difficulties in switchmode power supply design • Reducing RFI and EMI noise • Components that are thermally linked to chassis • Applied to high-frequency and high-voltage

  18. Applications to Switching Devices • Components that are thermally linked to chassis Principles of Faraday Screens • Returns current to “star” points

  19. TO3 Transistor Example

  20. Faraday Screens Vs Safety Screens • Location • Rating • Structure Faraday Screens Safety Screens • Return to input or output circuits • Made of light weight copper • Small Current • Return to chassis • Made of more durable material • Rated to 3 times supply fuse rating

  21. Faraday Screen Considerations • Only use when necessary • Application results in lower power efficiency

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