What is a Switching Power Supply

With the advancement and innovation of power electronics technology, switching power supply technology continues to evolve. Currently, switching power supplies are widely used in nearly all electronic devices due to their compact size, light weight, and high efficiency. They are an indispensable power supply method for the rapid development of the electronic information industry.

A switching power supply is a type of power supply that utilizes modern power electronics technology to control the switching time ratio of switching transistors while maintaining a stable output voltage. Switching power supplies are typically composed of pulse width modulation (PWM) control integrated circuits and field-effect transistors.

Compared to linear power supplies, switching power supplies directly rectify AC input into DC. Under the action of a high-frequency oscillation circuit, the switching transistor controls current flow to generate high-frequency pulsed current. Utilizing inductors (high-frequency transformers), they deliver stable low-voltage DC output.

Since the size of the transformer core is inversely proportional to the square of the switching power supply's operating frequency, higher frequencies allow for smaller cores. This significantly reduces the size of the transformer, thereby decreasing the power supply's weight and volume. Because it directly controls DC, this type of power supply is much more efficient than linear power supplies. This energy savings makes it highly popular. However, it also has drawbacks: complex circuitry, difficult maintenance, and significant circuit noise. The high noise level makes it unsuitable for certain low-noise circuits.

Characteristics of Switching Power Supplies

Switching power supplies typically consist of pulse width modulation (PWM) control integrated circuits and field-effect transistors (FETs). With advancements in power electronics technology, switching power supplies have gained widespread adoption in nearly all electronic devices due to their compact size, light weight, and high efficiency, underscoring their importance.

Classification of Switching Power Supplies

Based on the connection method of switching devices in the circuit, switching power supplies are generally categorized into series switching power supplies, parallel switching power supplies, and transformer-based switching power supplies.

Among these, transformer-based switching power supplies can be further divided into push-pull, half-bridge, and full-bridge types. Depending on the phase relationship between the transformer's excitation and output voltage, they can be classified as forward excitation, flyback, single excitation, and dual excitation types.

Differences Between Switching Power Supplies and Conventional Power Supplies

Conventional power supplies are typically linear power supplies. A linear power supply refers to a power supply where the regulating transistor operates in a linear state. However, this differs from switching power supplies. The switching transistor (in switching power supplies, we commonly refer to it as the regulating transistor or switching transistor) operates in two states: ON—with very low resistance, and OFF—with very high resistance.

Switching power supplies represent a relatively new type of power supply. They offer advantages such as high efficiency, lightweight design, the ability to step up or down voltage, and high output power. However, due to the circuit operating in a switching state, they generate relatively higher noise levels.

Example: Buck Switching Power Supply

Let's briefly discuss the working principle of a buck switching power supply: The circuit consists of a switch (typically a transistor or field-effect transistor in actual circuits), a freewheeling diode, a storage inductor, a filter capacitor, and other components.

When the switch closes, power flows through the switch and inductor to the load, while storing part of the energy in the inductor and capacitor. Due to the inductor's self-inductance, the current increases gradually after the switch closes—meaning the output voltage doesn't immediately reach the supply voltage.

After a period, the switch closes. Because of the inductor's self-inductance (the current in the inductor exhibits an inertial effect), the circuit current remains constant—flowing continuously from left to right. This current flows through the load, returns to ground, flows to the positive terminal of the freewheeling diode, passes through the diode, and returns to the left end of the inductor, thereby forming a loop.

The output voltage can be controlled by regulating the switch's on and off times (i.e., pulse width modulation). If the on/off times are controlled by monitoring the output voltage to maintain a constant output voltage, the purpose of voltage regulation is achieved.

Conventional power supplies and switching power supplies share the same voltage regulation principle.

The primary operating principle of a standard half-bridge switching power supply involves alternating conduction of the upper and lower bridge switches (typically VMOS at high frequencies). Initially, current flows through the upper bridge switch. By leveraging the inductive coil's energy storage capability, electrical energy accumulates within the coil. Subsequently, the upper bridge switch closes while the lower bridge switch opens, allowing the capacitor to discharge continuously. Subsequently, the lower bridge switch is closed while the upper bridge switch is opened to allow current flow. This cycle repeats continuously. The term “switching power supply” originates from the necessity for these two switches to alternately activate and deactivate.

Linear power supplies operate differently. Without switching mechanisms, the power supply circuit continuously discharges energy. Excessive discharge inevitably leads to leakage, explaining why many linear power supplies exhibit significant heat dissipation in their regulating circuits. Essentially, all electrical energy is consumed and entirely converted into thermal energy. From this perspective, linear power supplies exhibit low conversion efficiency. When heat generation is high, component lifespan inevitably decreases, impacting overall performance.

Key Difference: Operating Mode

The power regulation transistor in a linear power supply always operates in the saturation region, with continuous current flowing through it. Due to the power dissipation of the regulation transistor...