Analysis of Single-Ended Forward Excitation Transformer[Copy link]
In a switching power supply, the excitation circuit is very important, especially when MOSFET is used as the main switch tube. The excitation circuit sometimes uses a single-ended forward circuit to excite the main switch tube, as shown in Figure 1. The circuit uses an excitation transformer to achieve voltage pulse conversion and isolation. The design of the excitation transformer is similar to that of a general pulse transformer. However, due to the high frequency of the switching power supply, the pulse interval time is very short, and the duty cycle usually reaches 50%. In addition, the transformer magnetic state must be reset before the next pulse arrives. In order to achieve magnetic reset within the pulse interval time, the pure magnetizing inductance of the transformer cannot be large, and the feedback winding NP2 and the clamping diode D1 are used to effectively limit the kickback amplitude and protect the field effect tube V1 from being broken down by the kickback voltage. Only when V1 is not damaged can the magnetizing inductance and R1 value be changed to achieve the magnetic reset of the transformer.
2 Analysis of Recoil Characteristics and Magnetic Reset Recoil characteristics and magnetic reset are transient processes of releasing magnetic field energy stored in the transformer core and electric field energy stored in equivalent capacitors. In the circuit of Figure 1, at the moment when the pulse starts to drop, the pulse source is disconnected, and the internal resistance of the pulse source and the load resistance both change. The equivalent circuit can be used to analyze the recoil and magnetic reset characteristics.
At the moment when the pulse starts to drop, the pulse source and the load are disconnected successively. At this time, the energy stored in the excitation inductance and the capacitor begins to discharge. The capacitor C discharges through R1′L△. When the electric energy stored in the capacitor is completely discharged, the voltage pulse drops to zero. The time required for the discharge is the so-called trailing edge time. Due to the existence of inductance in the circuit, the current does not stop, and the capacitor is reversely charged to form the so-called kickback voltage; circuit analysis shows that the characteristics of this transient process are completely dependent on the parameters of the parallel circuit, and generally there will be three states: underdamped oscillation, critically damped oscillation and overdamped oscillation.
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