## 2 SMPS Overview  
The SMPS converts AC to DC power by using switching devices, inductors, and capacitors. They deliver stable power to many industrial, commercial, and domestic electrical systems proficiently, as they contain rectifier circuits, consequently drawing non-sinusoidal current from the grid. As a result, they have low power factor and power quality [18]. SMPS are common in computers and battery chargers of different home appliances. A typical computer SMPS operates at 220V and draws input current with a total current harmonic distortion (THD\({}_{i}\)) of 80%, and the power factor around 0.6 [19]. The block diagram of an SMPS is shown in Figure 1. The electromagnetic interference (EMI) filter reduces the high-frequency noise, and the inrush current limiter protects the circuit from the initial current surge due to capacitor charging. Most high power SMPS also contains a power factor correction circuits (PFC) to improve voltage regulations. The rectifier converts the input AC power to DC using switching devices like a diode, Insulated gate bipolar transistor IGBT, or MOSFET. The DC to DC converter is used to convert the input DC from one voltage level to another. The latest development in power electronics has enabled us to produce more efficient DC power supplies with improved power factor and stable output [20]. Modern design possesses low conduction losses and synthesizes near sinusoidal current waveform leading to fewer problems related to power quality. The SMPS are classified based on the converters, power factor correction circuits, and type of switching control strategy [21]. The converter topology classification includes buck, boost, buck-boost, and multilevel converters [3]. The control strategies include pulse width modulation (PWM), proportional integral derivative (PID), sliding mode control (SMC), adaptive control, and neural network controllers. Many other control strategies are also employed and discussed in the literature [22]. The single-stage power supplies are widespread as they implement power conversion using a single switching circuit and simple control [23]. However, in computers and other  
Figure 1: Block diagram of a typical switch-mode power supply. advanced applications, the single-stage power supplies are inadequate because of the stress across the switch and poor voltage regulations [22]. The SMPS in computers provides multiple DC outputs, and PFC circuits are used in the first stage for improving power factor and harmonics in the input current [24; 25]. The impact of computer power supplies and other electronic equipment on the network has been discussed in [26]. Similar studies have also estimated the impact of computer power supplies and other new harmonic sources on the grid [27; 28]. Most computer power supplies incorporate passive PFC circuits due to strict emission standards [29]. Researchers have proposed several designs with improved PFC for SMPS used in computer applications [30; 31]  
SMPS can be classified based on the type of filter circuits. Mostly low power SMPS (less than 75W) does not contain any PFC circuits. The high power SMPS may contain active or passive PFC. The devices with passive PFC contain a large inductor to smooth the variations in the current. On the other hand, devices with active PFC encompass an additional DC-DC converter. The SMPS with active PFC generates fewer harmonics than SMPS with passive PFC. We have selected six single-phase SMPS from different manufacturers in the range of 30 W to 120 W. Table 1 shows the detailed specification of all six SMPS. The power supplies are categorized into two types based on the shape of the input current waveforms. Figure 2 shows the current waveform of both types of SMPS. Type 1 power supplies include passive filtering circuits to improve the power factor. The current waveform drawn by type 2 power supplies shows that they contain typical rectifier circuits without filters.