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Voltage loop can be evaluated and re-tuned in a similar way. Figure 13-3 shows voltage loop PI control coefficients and corresponding bode plots.
The voltage loop PI control is implemented with software and has the below form,
There are two sets of the PI coefficients for voltage loop control. In normal operation, the control is with Linear Coefficients. In transient when the PFC output bulk voltage exceeds the defined Error Threshold, for example, 16.0 V, as shown, the PI control coefficients are changed to Non-Linear Coefficients to achieve better transient response and to eliminate the output large deviation faster. The output error threshold is usually within 5% of the output set point, or within 20 V on 390-VDC output.