What Is a Switching Power Supply?

AC/DC power supplies convert high-voltage AC power from the mains into low-voltage DC power that is required by most electronics and light fixtures. There are two main types of power supplies that vary based on how they convert this power: linear and switching power supplies. Linear power supplies have been widely used in the past and are known for their simplistic design. However, switching power supplies, also referred to as switched-mode power supplies or switching-mode power supplies, have become increasingly popular due to their efficiency and compact size. Understanding the difference between the two will help show the innovation behind the switching power supply’s design.

The first step in understanding switching power supplies is to understand what a linear power supply is. These power supplies have been in use longer than their switched-mode counterparts and are commonly used in low-power applications. A linear power supply first steps down the high AC supply voltage to a lower AC voltage through a transformer¹. Following this, a rectifier² converts the AC voltage into a DC voltage, and an output filtering circuit³ smooths the waveform into a stabilized voltage. However, since the input voltage often fluctuates, the linear power supply modifies resistance using a linear regulator⁴ or transistor to maintain a constant output voltage. Any excess energy is dissipated as heat, making these systems inefficient. Wasted energy and a bulky transformer are the two main pain points for linear power supplies.

A switched-mode power supply (SMPS), on the other hand, efficiently converts electrical power using a different approach. First, the high-voltage AC power is rectified and filtered. To stabilize this input power, a high-frequency switch⁵ or chopper rapidly switches the input voltage on and off to convert it into a high-frequency pulse wave. The duty cycle, or the proportion of On-time to total time, is adjusted based on fluctuation in input voltage and results in a stable, high-frequency pseudo AC pulse wave. This is fed to a transformer to step down the voltage, which is then rectified and filtered again to produce a stable DC output. There are various types of switching regulators, with pulse width modulation (PWM) being the most common.

By adjusting the pulse width based on input voltage fluctuations, a switch mode power supply is able to eliminate wasted energy dissipated as heat, making them up to 70-98% efficient. Compared to the average 30-50% efficiency of linear power supplies, this is the main reason why many prefer using SMPS. Additionally, because the current passes through a high-frequency switch, the required transformer and capacitors⁶ can be much smaller to result in an overall more compact power supply design. However, the fast switching may generate noise in some power supplies, which is one of the drawbacks of using these devices. 

Switched-mode power supplies can step the input voltage up or down to accommodate large changes in the load and line power. However, linear power supply transformers can only step the voltage down, resulting in a fixed capacity.

Switching power supplies are used in a wide range of applications due to their high efficiency, compact size, and ability to handle a wide range of input voltages and frequencies. They are most commonly found in desktop power adaptors, phone power adaptors, computer SMPS units, and other electronic devices. In lighting applications, switching power supplies are frequently used with LED strip lights, panel lights, and sign lights to safely convert line voltage down to low-voltage DC power. Switching power supplies are also integral to certain industrial and medical devices where reliable power sources are critical.

1. Transformer: A transformer converts AC voltage up or down using primary and secondary coils. When the transformer receives a high-frequency pulse wave from the primary coil, this generates a magnetic field which induces a voltage in the secondary coil. Depending on the turn ratio of the coils, the transformer can either step up or step down the voltage.
2. Rectifier: A rectifier converts alternating current (AC) to direct current (DC) using semiconductor diodes. A full bridge rectifier utilizes four diodes (denoted by a triangle and line) to rectify both sides of the waveform. Power supplies with a full bridge have a higher average output voltage. A half-bridge rectifier only uses one diode to rectify one half-cycle of the waveform.
3. Filtering Circuit: In the filtering/smoothing circuit, a capacitor charges and discharges based on the waveform to reduce the ripple and produce a smooth DC output voltage. 
4. Linear regulator: A linear regulator maintains a constant output voltage despite variations in the input voltage or load conditions by adjusting the resistance in the circuit to dissipate excess voltage as heat.
5. Switching regulator: Also known as a high-frequency switch or chopper, a switching regulator rapidly turns the switch on and off to create a high-frequency pulse wave. The changes in pulse determine the average output voltage.
6. Capacitor: A capacitor stores energy from a rectified AC waveform. When the waveform reaches its peak, the capacitor charges until it reaches maximum capacity. As the waveform declines, the capacitor discharges to bridge the gap between each peak of the waveform. The result is a smooth DC voltage.