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 A e 20.1 mm­ 2  Supply Voltage V cc 100 V  Flux density swing DB 250 mT  N p = = 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.  N fb = = 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.  N s = = 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 V cc = 100 V to be I m = = 43 mA  The peak current can be calculated as  I peak = 4 x I mean = 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. I p = 1.5 x I peak = 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: L p = = 6.4mH We can then use this value to calculate the A L factor (nH/turn 2 ) A L = = 59 nH/turn

12. Transformer Design (Step 5) Core gap (cont.)  From the graph we can determine the core gap at A L = 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