PIC12F675 Zeta Buck Boost Converter
Category: Switch Mode Power Supplies (SMPS)
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is simulated in Proteus 7.10. While code is written in MpLab X IDE using XC8 compiler. The microcontroller used in this project is PIC12F675. A zeta buck boost (step up and step down) converter is made by generating PWM through microcontroller. The working of this project is given below. 1. To understand the working of this circuit, first see previous circuit "PWM Generator With Duty Cycle Control". 2. In this circuit, a non-zeta buck boost converter called zeta converter is made using PIC12F675. 3. The output voltage Vo formula for zeta converter is Vo = D*Vin/(1-D) where D is duty cycle (0.1 to 0.9). 4. To derive the mosfet Q1 (IRF4905 pmos), a npn transistor Q2 (2N3904) is used, because Q1 should operate in saturation or cutt-off region, and mosfet needs 12V to go in saturation. 5. The input voltage of zeta buck boost converter is 12V, and output voltage can be lower or greater than 12V depending on duty cycle. 6. The resistor R1 is the load resistor, switching circuit should be operate with some load, otherwise the control is difficult. 7. The output voltage can be controlled by changing the duty cycle or mosfet Q1 through potentiometer RV1. 8. When the duty cycle of Q1 is less than 50%, the output voltage will be less than input voltage, that is circuit is in buck mode(use above equation for duty cycle and output voltage relation). 9. When the duty cycle of Q1 is greater than 50%, the output voltage will be greater than input voltage, that is circuit is in boost mode(use above equation for duty cycle and output voltage relation). 10. The frequency can also be changed by changing RV2. 11. The value of inductor is selected according to switching frequency, and it is inversely proportional. 12. The switching frequency selected for animation of this circuit is very low (almost 250Hz), in practical increase the switching frequency to at least 25Khz to reduce the size of inductor and cacpacitors. 13. The capacitors C1, C2, C3 and C5 are filter capacitors. 14. The LED D2 is used as indication of output voltages. 15. When the mosfet Q1 is ON, it connects the inductor L1 to +12Vdc, and inductor is charged and stores electrical energy in the form of current (i.e (1/2)*L*I^2), the capacitor C4 and inductor L2 also charges and the inductor L2 provides output current Io. 16. When the mosfet Q1 is OFF, both inductors releases its energy, L1 uses C4 and completes it path through diode D1, and inductor energy is added to the output voltage (as inductor tries to maintain its current direction). The inductor L2 again provides output current Io. 17. The current direction can be seen using arrows and colors. 18. The diode D1 is used to make the close loop for inductors to release its energy (when the mosfet is OFF) to output voltage. Application: 1. This circuit can be used to understand the concept of an zeta buck boost converter. 2. This circuit can be used as variable step-down and step up or zeta buck boost converter for DC input. 3. This circuit can be used as power LED driver through solar input to keep output voltage constant for solar input voltage variation. Download these files to perform more experiments. Note: Use FDAC app for output filter inductor and capacitor calculation.