Analysis of the influence of voltage loop on PFC system performance

Design principle of voltage loop bandwidth

In order to analyze the voltage loop pair PFC The impact of performance , With no bridge based on average current control BoostPFC Take an example for analysis , Here's the picture 4-1 Shown . The voltage loop in the figure will sample the output bus voltage value β ⋅vo And voltage reference Vref Comparison , Through the voltage ring PI Regulator to keep the output voltage stable , At the same time, the capacitance and sampling resistance in the voltage loop feedback loop form a low-pass filter , Used to attenuate the harmonic component of output voltage .
When the set cut-off frequency of the low-pass filter is very low （ The band width of the corresponding voltage loop is very narrow ）, The output of the voltage loop is a direct flow （ Set to VEA）, It multiplies the sampled value of the input voltage and generates the reference value of the inductive current , In the ideal state of current loop （ Compared with voltage loop , The current loop bandwidth is usually set very wide , It has little effect on the dynamic response speed ）, Inductive current iL No distortion is also sinusoidal , Input current iin Equal to inductive current iL, namely ：



By formula (4-4) It can be seen that , Output current io(t) It's made up of two parts ： DC component and second harmonic component , The second harmonic component of the current flows through the output capacitor , When the grid frequency is 50Hz This current corresponds to 100Hz Output voltage ripple . If the output voltage ripple is not limited , It will affect the sinusoidal degree of the inductor current reference value through the voltage loop , Thus, the input current is distorted , Therefore, it is necessary to reduce the voltage loop bandwidth to suppress the second harmonic . In general PFC Voltage loop design in progress , The cut-off frequency of the low-pass filter is often set at 10～20Hz Between , Used to attenuate the second harmonic component contained in the output voltage sampling .