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 SizeInitial 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

E1

TransprimaryTransecondary1

TransSecondary2

S1

S2

S3

S4

D1

D2

C1 R1L1

PWM

pwm1

PWM

pwm2

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