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Control Theory of Buck Converters

Last modified by Microchip on 2025/01/09 09:50

Overview: Step-down DC/DC (Buck) Converter

Buck Converter

Basic components: Input voltage (V_in), Output voltage (V_out), Inductor (L), Capacitor (C), MOSFET (Switch), and Diode.

ON-Time Operation

MOSFET State: When MOSFET is turned ON, it acts as a short circuit allowing energy to be stored in the inductor.
Current Path: Current flows from Vin through the inductor (Lout) to the load (Vout).

On time

Current Through Inductor

Current Rise: Current through the inductor increases linearly during ON time.
Equation: ΔI_L = (Vin - Vout) * (Ton / L)

On Time Operation

OFF-Time Operation

MOSFET State: When MOSFET turns OFF, it acts as an open circuit.
Current Path: Current flows through the freewheeling diode (LFET) to maintain continuity.
Equation: VL = -Vout (since Vin no longer influences the inductor).
The inductor voltage (VL) becomes negative due to the output voltage across the inductor.
Current Fall: Current through the inductor decreases linearly during OFF time.
Equation: ΔI_L = -(V_out * Toff / L)

MOSFET Switching Action

Switching Control: Controlled by PWM (Pulse Width Modulation).
OFF: MOSFET is turned OFF for a period (Toff), allowing the inductor to release stored energy.
The simplified circuit below shows MOSFET OFF state with current path through the diode.

Buck Operation

Control Theory in Buck Converters

Feedback Loop: Importance of feedback for maintaining stable output voltage.
Duty Cycle (D): Relationship between ON time (Ton) and switching period (T) for voltage regulation.

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