Harmonics in line currents introduce several problems, such as voltage-distortion, noise, and reduction in power quality. PFC circuits include a closed-loop feedback control to ensure unity PF and reduced THD by employing multi-loops for voltage and current control. The voltage loop regulates the output voltage, whereas the current loop forces the waveform of the input current to follow the sinusoidal waveform of the source voltage. Hence, providing a high PF and a low THD. Proportional-Integral-Derivative (PID) controllers are commonly used in industrial applications for closed-loop operation. PID controllers consist of low frequency (gain), an integrator, and a differentiator. Employing hysteresis current control (HCC) shows a better reduction of third, fifth, and seventh harmonics, which results in a much lower THD, especially in high power applications. HCC is one way of pulse-width-modulating a signal based on a reference signal and a given allowable margin value, whenever the actual value reaches the upper or lower margins, the controller gives an output 1 or 0, respectively. The block diagram of the closed-loop controller is below.

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