- Buck Converter Definition: A buck converter is a type of DC-DC converter that steps down a higher input voltage to a lower output voltage.
- Circuit Components: The main components include a switch (usually a MOSFET or IGBT), a diode, and an LC filter, each vital for the converter’s functionality.
- Pulse Width Modulation (PWM): PWM controls the timing of the switch in a buck converter, crucial for regulating output voltage and minimizing ripples.
- Modes of Operation: In Mode I, the switch is on and the diode is off, vice versa for Mode II, each mode essential for continuous current flow through the load.
- Steady State Analysis: Analysis of the buck converter in steady state demonstrates that the inductor current remains constant overall, ensuring a stable output.
DC-DC converters, often called Choppers, include the Buck converter, which steps down a higher input DC voltage to a lower specified output voltage. In continuous conduction the ideal relation is Vo = D Vin, where D is the switch duty cycle.
A typical Buck converter is shown below.
The input vvoltage source connects to a controllable solid-state device like a Power MOSFET or IGBT, which acts as a switch. Unlike a Thyristor, which requires extra commutation to turn off, a MOSFET or IGBT turns off when its gate drive falls below the threshold. Zero drain or collector voltage is not the turn-off command.
Alongside the main switch, a diode serves as the freewheel path. Both are linked to an LC filter sized to limit ripples in current and voltage. The worked circuit below uses a purely resistive load.
The input voltage is taken as constant. With a large L and C in continuous conduction, load voltage and load current stay nearly constant over a switching period, so the load can be treated as a current sink for first-order analysis.
Pulse Width Modulation (PWM) can hold a fixed switching frequency and vary on-time, or it can vary frequency. Fixed-frequency PWM is the usual choice for this converter, because a wide frequency range makes the LC filter harder to size.
Time based Modulation is mostly used for DC-DC converters. The switching frequency remains constant in this type of PWM modulation.
The Buck converter has two modes of operation. The first mode is when the switch is on and conducting.
Mode I : Switch is ON, Diode is OFF
The voltage across the capacitance in steady state is equal to the output voltage.
Let us say the switch is on for a time TON and is off for a time TOFF. We define the time period, T, as 
and the switching frequency,
Let us now define another term, the duty cycle, 
Let us now analyse the Buck converter in steady state operation for this mode using KVL.
Since the switch is closed for a time TON = DT we can say that Δt = DT.
While performing the analysis of the Buck converter, we have to keep in mind that
- The inductor current is continuous, made possible by selecting an appropriate value of L.
- The inductor current in steady state rises from a value with a positive slope to a maximum value during the ON state and then drops back down to the initial value with a negative slope. Therefore the net change of the inductor current over any one complete cycle is zero.
Mode II: Switch is OFF, Diode is ON
Here, the energy stored in the inductor is released to the load resistance and the output capacitor, which keeps the flow of current through the load. For analysis we keep the original conventions to analyse the circuit using KVL.
Let us now analyse the Buck converter in steady state operation for Mode II using KVL.
Since the switch is open for a time 
we can say that Δt = (1- D)T.
The net change of the inductor current over any one complete cycle is zero.
A circuit of a Buck converter and its waveforms is shown below.
The inductance, L, is 20mH and the C is 100µF, and the resistive load is 5Ω. The switching frequency is 1 kHz. The input voltage is 100V DC, and the duty cycle is 0.5.
The voltage waveforms are as shown above and the current waveforms are as shown in the figure below.
