There are two types of power supplies: linear power supplies and switching power supplies. Early devices used linear regulated power supplies, but with the development of power electronics technology, miniaturization, thinning, and lightweighting of power supplies have become the trend. Therefore, linear power supplies have gradually been replaced by switching power supplies in most applications. However, they have not been completely replaced; linear power supplies still have their place.
Let's take an example. Suppose we need a 5V/20W DC power supply, which can be achieved using both linear power supplies and switching power supplies, as follows.
Linear power supply scheme: First, the voltage is stepped down by the power frequency transformer (220V: 9V), then rectified by the full bridge to about 12V DC. After the 12V is filtered, it is output as 5V by the linear voltage regulator circuit, and then filtered again to provide power to the load.
Switching power supply solution: The input 220V is rectified into about 310V DC by EMI full-bridge rectifier. The power chip controls the switching transistor through PWM technology to cut the DC into an AC square wave. The energy is then transferred to the secondary side through a high-frequency transformer and filtered to output 5V DC.
A direct comparison of the physical prototypes made using the two methods: the left side shows a linear power supply circuit, and the right side shows a switching power supply circuit.
Based on the above comparison, the characteristics of linear power supplies and switching power supplies can be summarized as follows:
① Because the regulating transistor in the voltage regulator circuit is in the linear region, the voltage difference across the circuit is large, resulting in low conversion efficiency of only 35% to 60%. In contrast, the regulating transistor in the switching power supply is in the switching state, resulting in high conversion efficiency, generally reaching 80% to 90%.
② Due to the presence of the power frequency transformer and the significant heat generated by the regulating transistor requiring a heatsink, linear power supplies are large and bulky. In contrast, switching power supplies use high-frequency transformers, and since the switching transistors do not generate significant heat, they are smaller and more flexible.
③ Switching power supplies, due to their high operating frequency (20kHz～200kHz), have poor power supply noise and ripple, greater electromagnetic interference, and lower regulation compared to linear power supplies. Therefore, in some applications, such as high-quality audio equipment, linear power supplies still have their place.
④ Linear power supplies can only step down voltage, while switching power supplies can both step down and step up voltage.
