1. Rectification: Incoming AC mains (e.g., 110V/220V) is first converted to DC.
2. Switching: A power transistor switches this DC on and off at high frequencies (typically tens to hundreds of kHz).
3. Transformation/Filtering: The chopped voltage passes through a small transformer or inductor to change voltage levels, followed by filters (capacitors/inductors) to smooth the DC output.
4. Feedback Control: A control circuit monitors the output voltage and adjusts the switching speed to maintain a stable output regardless of input changes

Core Working Principle

1. Input Rectification & Filtering: Incoming AC power is converted into high-voltage DC using a bridge rectifier and smoothed by capacitors.
2. High-Frequency Switching: A transistor (the "chopper") switches this DC on and off at very high frequencies (typically 20 kHz to 500 kHz).
3. Voltage Transformation: The resulting high-frequency pulse train is passed through a small transformer or inductor to step the voltage up or down.
4. Output Rectification & Filtering: The output of the transformer is rectified back to DC and smoothed to provide a stable, clean voltage for the load.
5. Feedback Control Loop: A control circuit monitors the output voltage and adjusts the Duty Cycle (the ratio of on-time to off-time) via Pulse Width Modulation (PWM) to maintain a constant output despite changes in input voltage or load.

Key Advantages

• High Efficiency: Typically 80% to 95% efficiency, reducing energy waste and heat generation.
• Compact & Lightweight: High-frequency operation allows for smaller transformers and components.
• Versatility: Can step-up (boost), step-down (buck), or invert voltage

Converter topology basics

Figure 1: A forward-mode or buck step-down switching converter

There are several different converter topologies in use, some of them more popular than others. Each has a slightly different configuration of magnetic components, such as inductors and transformers, and capacitors. Text-book topologies include the buck converter and the boost converter, and they are convenient examples to illustrate the basic principles of a switched-mode converter circuit.

A buck converter, also termed forward-mode, is used to step down the input voltage.

Figure 1 (right) shows a simplified circuit noting the use of an inductor (L) and capacitor (C) on the output circuit. The semiconductor switch (TR1) represents the fast switching action of a MOSFET driving into saturation or entirely off.

When TR1 is conducting, the diode (D) is reverse biased, and current flows to the load. This charges up the capacitor C, through the inductor (L) which opposes the flow, creating a magnetic field. When TR1 stops conduction, the magnetic field in L collapses, the diode (D) becomes forward biased forcing the current through the load, and during the same period the capacitor (C) also discharges its held charge to the load. The combination of the inductor and capacitor values create an LC filter that serves to smooth out any ripple created by the switching actions.

The boost converter (see Figure 2,  below) is another type of popular topology, this time suiting increasing or 'boosting' the input voltage to create a higher output voltage.

Unlike the buck converter, where the switching transistor is in series with the input voltage, in the boost circuit it’s in parallel with the input and connected to the input via an inductor. The capacitor continues to be across the load, serving to hold-up the output voltage while the transistor is conducting. The inductor’s collapsing field flows to the output while the transistor is off.

Figure 2: A simplified boost converter circuit

A combination of the buck and boost topologies is the buck/boost converter that’s capable of stepping up or down the input voltage.

Note that, from an aspect of safety, none of the above topologies uses a transformer - described as a non-isolated converter - to isolate the input voltage from the output. They also share a common ground connection.

There are several topologies in use for AC-DC power supplies that provide isolation and are highly energy efficient in operation, the most popular being the flyback and quasi-resonant methods.

The architecture of a typical switched-mode power supply

Figure 3: The functional blocks of an example AC-DC switched-mode power supply (SMPS)

Common Applications

• Consumer Electronics: Laptops, smartphone chargers, televisions.
• Computing: PC power supplies (PSU).
• Industrial: Motor drives, LED displays, automation equipment (PLC systems).

SMPS vs. Linear Power Supplies

While SMPS technology can introduce higher electrical noise (ripple) and requires more complex design, its superior efficiency and power density make it the dominant choice for modern electronics.