Closed-Loop Control System for Buck Converter Figure 6. Output voltage (vout) (red) and reference voltage (vref) (green). DESIGN OF BUCK CONVERTER USING PROPORTIONAL AND INTEGRAL (PI) CONTROLLER A PI controller is a special case of a PID controller, but with no use of the derivative (D) part of the error. So control signal U for a PI controller is given below U = K p + K i Δ dt (5) where Kp, Ki is proportional, integral gain, Δ is the error or deviation of actual measured value (PV) from the SP Δ = SP − PV (6) The PI controller transfer function C(s) is given below C (s) = K p + Ki / S (7) As carried out with an integral controller, MATLAB is used to calculate Kp and Ki of the PI controller. General approach to PI tuning: 1. Firstly set Ki= zero. 2. Increase KP until desired response has been obtained 3. Add integral gain and modify Ki until the removed steady state error. To determine PI gain constants, the position of the root locus for the buck converter is modified online as shown in Figure 7(a) to get Kp&Ki equal to 4&415 at minimum settling time=0.00746 s and rise time=0.00425 s as shown in Figure 7(b). Then mikroC software is used for programming the microcontroller to control the MOSFET of the buck converter based on previous Kp&Ki values. And so the programmed microcontroller controls the constructed buck converter circuit in Proteus software as indicated in Figure 8. 34 Figure 7. (a) Root locus and open-loop bode plots of buck converter. (b) Step response of buck converter. IEEE A&E SYSTEMS MAGAZINE MARCH 2017