Self-Oscillating Converters

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Self-Oscillating Converters

• By
• Andrew Gonzales
• EE136

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INTRODUCTION

• General Operating Principle
• How the circuits work
• Transformer Design for Converter

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General Operating Principle

• Switching action
• Maintained by positive feedback from a winding on
  the main transformer.
• Frequency
• Controlled either by saturation of the main or
  subsidiary transformer
• Controlled by a drive clamping action

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Single transformer two transistor converter
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Single Transformer Converter
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Transformer Design (Step 1)Core Size

• No fundamental equation linking transformer size
  to power rating.
• Use nomograms provided by manufacturers to pick
  core size

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Transformer Design (Step 2)Primary Turns

• Assuming the following parameters
• Frequency 30 kHz (½ period t
  16.5 ?s)
• Core area Ae 20.1 mm2
• Supply Voltage Vcc 100 V
• Flux density swing DB 250 mT
• Np
  330 turns

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Transformer Design (Step 3)Feedback and
Secondary turns

• We want the feedback voltage to be at least 3 V
  to make sure we have an adequate feed back factor
  for the fast switching of Q1.
•  
• Nfb 9.9 turns  
• The secondary voltage should be 12.6 V because
  we want the output voltage to be 12 V and there
  is a 0.6 V diode loss.
•  
• Ns 42 turns

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Transformer Design (Step 4)Primary current

• Assuming 70 efficiency and output power of 3 W,
  our input power should be 4.3 W. Which gives the
  mean input current at Vcc 100 V to be
•  
• Im 43 mA
•  
• The peak current can be calculated as
•  
• Ipeak 4 x Imean 172 mA
•  
• The actual collector current must exceed this
  calculated mean current by at least 50 to make
  sure that the diode D2 remains in conduction
  during the complete flyback period.
• Ip 1.5 x Ipeak 258 mA.

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Transformer Design (Step 5)Core Gap

• 2 ways to calculate core gap
• Empirical method
• By Calculation and Published data

• Empirical method
• Use a temporary gap and and operate with a
  dummy load at the required power. Adjust the gap
  for the required period.

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Transformer Design (Step 5)Core Gap (cont.)

• By Calculation and Published Data
• We first calculate the required
  inductance of the transformer using the following
  formula
•  
• Lp 6.4mH
• We can then use this value to
  calculate the AL factor (nH/turn2)
•  
• AL
  59 nH/turn

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Transformer Design (Step 5)Core gap (cont.)

• From the graph we can determine the core gap at
  AL 59 nH

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Conclusion

• Applications
• Auxiliary power for larger power converters
• Stand-by power source in off line power supplies
• Advantages
• Low cost, simplicity, and small size
• Disadvantages
• Frequency instability due to changes in the
  magnetic properties of the core, load or applied
  voltage