I am designing a buck converter with PWM circuitry for a school project to drive a MOSFET switch to keep my output voltage constant. The specs are:
- input voltage of 20v DC +/- 4
- output voltage of 5v DC
- Maximum output current of 1A
- No more than 10% change in voltage over entire range of input voltage and load
- Min. 80% efficiency at full load
- No micro-controllers can be used
I am having trouble keeping the voltage constant with the PWM circuitry. When simulating the circuit, the voltage goes to around 5.7 to 6v. Can you please help me correct the PWM circuitry and/or the inductor and capacitor values? They may not be correct.
EDIT: The lm555 Timer with the BC557BG transistor creates a ramp voltage by repeatedly charging and discharging capacitor C6. The 741 Op-Amp is set up as a differential amplifier with a gain of 1, which will subtract a sample voltage from the output with a reference voltage of 5v. The output of the 741 Op-Amp goes into an input of the lm311n voltage comparator, and the ramp voltage into the other input, the output should be a square wave with varying duty cycle to drive the MOSFET. Click for full-size. EDIT:
The main Buck converter
The calculations I used to select inductor and capacitor values.
I solved these values by setting LIR to 0.2 and dV to 75mV.
Summing amp and voltage comparator
The output voltage does not stay around 5v, it slowly increases to 10v