# Self-Oscillating Converters - PowerPoint PPT Presentation

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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

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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

• Controlled either by saturation of the main or subsidiary transformer

• Controlled by a drive clamping action

### 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

• Assuming the following parameters:

• Frequency = 30 kHz (½ period t = 16.5 s)

• Core area Ae 20.1 mm­2

• Supply Voltage Vcc100 V

• Flux density swing DB 250 mT

• Np = = 330 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

• 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

• 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.)

• 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 (cont.)

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

### Conclusion

• Applications

• Auxiliary power for larger power converters

• Stand-by power source in off line power supplies

• Low cost, simplicity, and small size