ENEL701 Assignment 2B Design of a Self-Oscillating Flyback Converter
Ratings: 18 V dc to 12 V dc (+/- 0.8 V) at 3.06 W
Due date: 4th June 2020
Design a self oscillating converter and simulate it in Microcap. The design must be based on the circuit topology given in the lecture notes, and as shown below. The reference material provided can be used as a guide. For the simulations assume the controlled switch can be implemented using a MJE240 BJT transistor, the inductance of winding W1 must be 500 ?H, W1 is fitted with 31 turns and the core of the flyback magnetic component is an ETD29 type. The number of turns on the W2 and Auxiliary windings must be determined. A report must be submitted answering the following:
1. State the value of all components used in the circuit and show the circuit diagram.
2. What determines the on-time of the converter?
3. What determines the off-time of the converter?
4. Estimate the core gap size that is required to ensure the inductance of W1 is 500 ?H with 31 turns fitted.
5. What turns ratios were used between the W1, W2 and Auxiliary windings, and state why these values were chosen.
6. What type of diode is connected in the output of the W2 winding, and what are its ratings? State why this particular diode was chosen.
7. If a short circuit is placed across the output of the converter, will the converter circuit be damaged? Justify your answer.
8. What is the peak flux density in the core of the flyback circuit magnetic component? Show how this flux density was determined.
9. State the efficiency of the converter, and show how the efficiency was determined.
Note:
1. It is recommended that the operating frequency of the converter be less than 30 kHz, to avoid excessive switching losses in the BJT.
2. A three winding transformer can be implemented in Microcap by component type XFMR2.
a. NOTE – make RATIO 1 a negative number and RATIO 2 a positive number to get winding directions/dot orientation correct (the dots in the circuit diagram won’t change, but the orientation will be correct).
3. Higher marks are given for higher efficiency levels.