A Buck converter steps down a DC voltage from the input to the output. The circuit operation depends on the conduction state of the MOSFET:
- On-state: The current through the inductor increases and the diode blocks.
- Off-state: Since the current through the inductor can not abruptly change the diode must carry the current so it commutates and begins conducting. Energy is transferred from the inductor to the capacitor resulting in a decreasing inductor current. During steady state the circuit is said to operate:
- in discontinuous conduction mode if the inductor current reaches zero and
- in continuous conduction mode if the inductor current never reaches zero.
The circuit has two limits of operation. For a PWM duty cycle D ➝ 0 the output voltage equals zero, and for D ➝ 1 the output voltage equals Vin. In between those limits the output voltage in continuous conduction mode is given by: Vout = D · Vin.
Note that the parallel combination of inductor and capacitor as shown above acts as a second order low pass filter reducing the voltage ripple at the output.
- Change the PWM duty cycle of the converter from 0.3 to 0.5 and observe how the average output voltage changes from 7.2 V to 12 V.
- Change the resistance value from 5 Ω to 50 Ω and observe how the converter goes into discontinuous conduction mode.