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Self-Oscillating Converters

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Self-Oscillating Converters

By:

Andrew Gonzales

EE136

- General Operating Principle
- How the circuits work
- Transformer Design for Converter

- 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

- No fundamental equation linking transformer size to power rating.
- Use nomograms provided by manufacturers to pick core size

- 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

- 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

- 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.

- 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.

- 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

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

- 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