A buck-boost converter is a type of DC-DC converter that can step down or step up an input voltage to provide a regulated output voltage. In a discontinuous conduction mode (DCM), the load current is low enough that the inductor current falls to zero during a portion of the switching cycle. This mode of operation is distinct from the continuous conduction mode (CCM), where the inductor current never reaches zero.
In DCM, the buck-boost converter controls its output voltage through the regulation of its duty cycle, which is the ratio of the time the switch is on (conducting) to the total switching period. The control loop of the converter adjusts this duty cycle to maintain the desired output voltage.
Here's a general overview of how a buck-boost converter operates and controls its output voltage in DCM:
Input Voltage and Inductor Charging: During the "on" time of the switching cycle, the switch (usually a transistor) is turned on, connecting the input voltage source to the inductor and load. Current starts flowing through the inductor, storing energy in its magnetic field. This energy is transferred to the output during the "off" time.
Inductor Discharging: When the switch is turned off, the inductor discharges energy into the load. The energy stored in the inductor's magnetic field helps maintain current flow through the load. As the load draws current, the inductor current decreases. In DCM, this current eventually reaches zero before the next switching cycle begins.
Voltage Feedback Control: The buck-boost converter's control circuit continuously monitors the output voltage. This is typically done through a voltage feedback loop that compares the actual output voltage to a reference voltage (setpoint). If the output voltage deviates from the desired value, the control circuit adjusts the duty cycle to correct the voltage.
Duty Cycle Adjustment: To regulate the output voltage, the control circuit adjusts the duty cycle. If the output voltage is too low, the duty cycle is increased. This allows the inductor to store more energy during each switching cycle and deliver a higher average current to the load. If the output voltage is too high, the duty cycle is decreased, reducing the energy delivered to the load.
Closed-Loop Control: The control loop continuously adjusts the duty cycle based on the voltage feedback. This closed-loop control mechanism ensures that the output voltage remains within a certain tolerance of the desired value, even with variations in load and input voltage.
In summary, in the discontinuous conduction mode of a buck-boost converter, the output voltage is controlled by adjusting the duty cycle of the switching operation. This adjustment is based on a feedback loop that compares the actual output voltage to a reference voltage and modifies the duty cycle to maintain the desired output voltage despite changes in load and input conditions.