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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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

Single transformer two transistor converter

Single Transformer Converter

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

Advantages

Low cost, simplicity, and small size

Disadvantages

Frequency instability due to changes in the magnetic properties of the core, load or applied voltage