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1. Self-Oscillating Converters By:
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.
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 Explain How this circuit operatesExplain How this circuit operates
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 (? period t = 16.5 ?s)
Core area Ae 20.1 mm?2
Supply Voltage Vcc 100 V
Flux density swing DB 250 mT
Np = = 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.
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
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 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.
10. Transformer Design (Step 5)Core Gap 2 ways to calculate core gap
By Calculation and Published data
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:
Lp = = 6.4mH
We can then use this value to calculate the AL factor (nH/turn2)
AL = = 59 nH/turn
12. Transformer Design (Step 5)Core gap (cont.) From the graph we can determine the core gap at AL = 59 nH
13. Conclusion Applications
Auxiliary power for larger power converters
Stand-by power source in off line power supplies
Low cost, simplicity, and small size
Frequency instability due to changes in the magnetic properties of the core, load or applied voltage