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 Applications to switches Differences between Faraday Screens and Safety Screens

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

Design Example Straightforward design Minimize magnetization current Specifications

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

Step 2 Select optimum flux density. 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.

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

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

Step 4 Maximum on period

Step 5 Calculate Primary turns

Step 6 Calculate secondary turns

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

Circuit Schematic

Simplorer Simulation

Waveforms

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

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

TO3 Transistor Example

Faraday Screens Vs Safety Screens Location Rating Structure 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 Faraday Screens Safety Screens

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