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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

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self oscillating converters

Self-Oscillating Converters

By:

Andrew Gonzales

EE136

introduction
INTRODUCTION
  • General Operating Principle
  • How the circuits work
  • Transformer Design for Converter
general operating principle
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
transformer design step 1 core size
Transformer Design (Step 1)Core Size
  • No fundamental equation linking transformer size to power rating.
  • Use nomograms provided by manufacturers to pick core size
transformer design step 2 primary turns
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
transformer design step 3 feedback and secondary turns
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

transformer design step 4 primary current
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.

transformer design step 5 core gap
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.

transformer design step 5 core gap cont
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

transformer design step 5 core gap cont1
Transformer Design (Step 5)Core gap (cont.)
  • From the graph we can determine the core gap at AL = 59 nH
conclusion
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