Switching transformers generally refer to transformers used in switch-mode power supplies, operating in pulse mode with frequencies ranging from a few tens of Hertz to several hundred kilohertz. The core material used is typically ferrite. These transformers often operate in pulse mode at frequencies ranging from tens of Hertz to several kilohertz.

In terms of classification, switching transformers can be categorized based on the input voltage as single-ended or double-ended, and based on the output voltage as forward-excited or flyback. Single-ended and double-ended switching power supplies, or forward and flyback switching power supplies, all use switching transformers, each with different operating principles.

When the input voltage of the switching transformer is a DC pulse voltage, it is called a unipolar pulse input. A switching power supply operating with this unipolar pulse input is referred to as a unipolar excitation transformer switching power supply. When the input voltage of the switching transformer is an alternating pulse voltage, it is termed a bipolar pulse input. A switching power supply operating with this bipolar pulse input is known as a bipolar excitation transformer switching power supply. When the primary coil of the transformer is excited by a DC pulse voltage, the secondary coil outputs power.

This type of switching power supply is called a forward-excited transformer switching power supply. When the primary coil of the transformer is just excited by a DC pulse voltage, and the secondary coil does not provide power output to the load, only doing so after the excitation voltage on the primary coil of the transformer is turned off, this type of transformer-based switching power supply is called a flyback switching power supply.

Assuming the cross-sectional area of the switching transformer core is "s," when a rectangular pulse voltage with an amplitude "U" and a width "τ" is applied to the primary coil of the switching transformer, an excitation current will flow through the primary coil. Simultaneously, a magnetic field is generated in the core of the switching transformer, causing the transformer's core to be magnetized. Under the action of a magnetic field intensity "H," a magnetic flux density "B" is produced in the core, commonly referred to as magnetic flux. "B" represents the magnetic induction intensity B or magnetic flux φ influenced by the magnetic field intensity.

The excitation current refers to the current that magnetizes and demagnetizes the transformer core.

Operating Principle of the Switching Transformer

After 20V AC is rectified and filtered, it becomes 310V DC. This DC voltage is applied to the primary of the switching transformer and, in turn, to the collector of the switching transistor. Simultaneously, a certain bias voltage is applied to the base of the switching transistor, causing it to conduct. When the switching transistor conducts at the moment of power-on, a current is generated in the primary of the switching transformer connected to the collector of the switching transistor. This current creates a magnetic field in the core of the switching transformer, and this magnetic field induces a voltage in the feedback coil. This voltage is applied to the base of the switching transistor, reinforcing its conduction, thereby increasing the magnetic field and feedback voltage.

The switch continues to conduct until it saturates. After saturation, the current flowing through the collector of the switching transistor is large, but the amplitude of the current variation decreases until it no longer changes. The primary current of the transformer remains constant, the induced magnetic field disappears, and the feedback voltage vanishes.

As the feedback voltage disappears from the base of the switch, it quickly transitions from deep saturation to conduction state, and the collector current rapidly decreases. The decreased current generates a magnetic field in the core opposite to the aforementioned magnetic field. This opposite magnetic field induces a feedback voltage in the feedback coil opposite to the aforementioned magnetic field. This voltage is applied to the base of the switch, accelerating the transition of the switch from conduction to cutoff. After the switch is turned off, the current disappears, and the feedback voltage disappears. With the action of DC bias, the switch is turned off. This process repeats continuously, illustrating the working principle of the switching power supply.