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Self-Oscillating Converters

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

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Self-Oscillating Converters

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  1. Self-Oscillating Converters By: Andrew Gonzales EE136

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

  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

  4. Single transformer two transistor converter

  5. Single Transformer Converter

  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

  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

  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

  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.

  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.

  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

  12. Transformer Design (Step 5)Core gap (cont.) • From the graph we can determine the core gap at AL = 59 nH

  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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