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Self-Oscillating Converters






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Self-Oscillating Converters. By: Andrew Gonzales EE136. INTRODUCTION. General Operating Principle How the circuits work Transformer Design for Converter. General Operating Principle. Switching action Maintained by positive feedback from a winding on the main transformer. Frequency
Self-Oscillating Converters

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Slide 1

Self-Oscillating Converters

By:

Andrew Gonzales

EE136

Slide 2

INTRODUCTION

  • General Operating Principle

  • How the circuits work

  • Transformer Design for Converter

Slide 3

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

Slide 4

Single transformer two transistor converter

Slide 5

Single Transformer Converter

Slide 6

Transformer Design (Step 1)Core Size

  • No fundamental equation linking transformer size to power rating.

  • Use nomograms provided by manufacturers to pick core size

Slide 7

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

Slide 8

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

Slide 9

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.

Slide 10

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.

Slide 11

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

Slide 12

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

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

Slide 13

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


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