1. Maximum Duty Cycle Dmax of MOSFET Switch Operation:
This formula helps determine the maximum duty cycle that a MOSFET can handle without causing saturation or instability in the circuit. Here, Vor represents the reflected voltage from the secondary side to the primary side. When the input is AC 220V, this value is typically around 135V. VminDC refers to the minimum DC voltage after rectification, and VDS is the voltage across the drain and source when the MOSFET is turned on, usually taken as 10V for safety margins.
2. Peak Primary Winding Current IPK:
The peak current in the primary winding of the transformer is calculated using this equation. η stands for the efficiency of the transformer, while Po is the output power in watts. This parameter is crucial for selecting the right MOSFET and ensuring the transformer doesn't overheat during operation.
3. Primary Inductance LP of the Transformer:
This formula calculates the primary inductance of the transformer, which plays a key role in determining the energy storage capability and switching performance. Ts is the switching period, and LP is measured in henrys. A properly sized inductance ensures smooth operation and reduces losses.
4. Air Gap lg of the Transformer:
The air gap in the magnetic core affects the inductance and magnetic saturation. It's calculated based on the effective cross-sectional area (Ae), the change in magnetic flux density (ΔB), and the primary inductance (LP) and peak current (IPK). Proper air gap design is essential for maintaining stable operation and reducing core losses.
5. Transformer Core Selection:
Flyback transformers are typically small, so ferrite cores are commonly used due to their high permeability and low losses at high frequencies. The power capacity of the core (AP) is calculated using the window area (AQ), effective core area (Ae), output power (Po), switching frequency (fs), and other parameters. A slightly larger core is often chosen to ensure sufficient margin and better utilization of the core window.
6. Number of Turns on the Primary Side NP:
This formula determines the number of turns on the primary side of the transformer. ΔB is the change in magnetic induction, Ae is the effective core area, and Ts is the switching period. Accurate turn calculation ensures proper voltage transformation and efficient energy transfer.
7. Number of Turns on the Secondary Side NS:
The secondary turns are calculated based on the desired output voltage and the forward voltage drop (VD) of the secondary diode. This ensures the correct voltage regulation and minimizes losses in the rectifier stage.
8. Selection of Power Switching Transistor:
The minimum voltage stress (UDS) on the switch is critical for choosing an appropriate MOSFET. The breakdown voltage should be slightly higher than the calculated value to account for voltage spikes and ensure reliable operation under varying load conditions.
9. Copper Loss PCU:
Copper loss is calculated based on the resistance of the windings and the current flowing through them. For the primary winding, the peak current (IPK) is used, while for the secondary, the output current (Io) is considered. Minimizing copper loss improves overall efficiency.
10. Core Loss:
Core loss depends on factors like operating frequency, magnetic induction, and core material. For bipolar switching, the core loss is calculated using the specific loss per kilogram (Pb) and the core mass (Gc). In unipolar switching, the core operates on half the hysteresis loop, so the loss is roughly halved. Total loss is the sum of copper and core losses, which directly impacts the efficiency and thermal performance of the transformer.
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