What is a Buck converter?
A buck converter is an energy-efficient DC-DC (direct current) converter used to step down the voltage from higher to lower levels. It’s based on the principle of pulse-width modulation (PWM). It uses lossless components like inductors, capacitors, and switches to achieve high efficiency. When contrasted with an LDO (Low Dropout Regulator), a buck converter demonstrates notably higher efficiency due to its theoretical absence of lossy components. In an LDO, efficiency is compromised due to voltage drop across a variable resistor used for regulating the dropout voltage.
Buck converter circuit diagram
A basic buck converter comprises essential components like a MOSFET switch, an inductor, a diode (or a second MOSFET), and a capacitor, as depicted in Figure 2. When augmented with a control circuit, this setup transforms into a buck regulator. The control circuit monitors the output voltage, compares it with a reference value, and autonomously adjusts the duty cycle to achieve the desired output voltage.
The fundamental configuration of a buck converter comprises the following elements:
- Inductor (L): The inductor stores and releases energy throughout switching cycles. Its primary function is to accumulate energy and provide current to the load when the switch is turned on. It is connected to the input source. When the switch is turned off, it still provides energy to the load while losing its own energy.
- Switch (Q1): The switch controls the current flow into the inductor. The time duration for which the switch is turned on decides how much of the inductor’s energy is built. If the switch is not used and the inductor is permanently connected to the input voltage, the output will keep rising without control.
- Diode (D): It acts as an automatic switch. It automatically turns off and isolates the input and output nodes when the inductor charges. It turns on automatically when the switch is turned off, and the inductor releases energy.
- Output capacitor (C): The output capacitor is used to smooth out the ripple at the load.
Operating principle of Buck converter
In this section, a Non-synchronous buck converter is discussed. It is the foundation of the buck converter topologies. The operation can be understood by analyzing the condition when the switch is ON and the switch is OFF separately.
Switch in ON
In a buck converter, when the switch (typically a MOSFET) is turned on, it allows current to flow from the input source (usually a higher voltage) through the inductor and the switch to the output load (typically a lower voltage) and output capacitor. Diode D1 is reverse-biased because the cathode voltage is higher than the anode voltage.
During this phase, current builds up in the inductor. The inductor resists the sudden change in current flow. The inductor stores energy in the form of a magnetic field. Energy from the input source is transferred to the inductor during this ON period. The voltage across the inductor is the difference between the input voltage and the output voltage, as shown in Fig 3, with the assumption that the voltage drop across the switch Q1 is negligible.
$$\Delta{}I_{L}=\cfrac{V_{in}-V_{out}}{L}T_{on}$$
The capacitor is used to smooth out the output waveform and reduce the ripple caused by the triangular nature of the inductor current.
The power dissipation through an ideal switch is zero. Real switches have some finite small, non-zero ON resistance, so power dissipation is I2R. Since there is no current flow through the diode, the power dissipation through the diode is zero. So, during the switch ON phase, there is no power dissipation in the converter (except the load).
Switch is OFF
When the switch in a buck converter turns off, the inductor keeps the current’s direction the same. To do that, it induces a negative voltage at the cathode of the diode, turning the diode ON and forcing the diode to conduct.
The voltage across the inductor is reversed. The inductor’s left terminal is set to 0V automatically (assuming the diode is ideal) while the right terminal is at the desired Vout. The inductor is bleeding energy in the form of current. The voltage across the inductor is constant, so the rate of drop in current is also constant, having a negative slope of Vout/L.
$$\Delta{}I_L=-\cfrac{V_{out}}{L}t_{off}$$
The capacitor also provides the energy to load and simultaneously get charged by the inductor’s current. If the inductor’s current drops to zero, it is called discontinuous conduction mode; otherwise, it is called continuous conduction mode.
Buck converter equation and duty cycle
The output voltage is fixed using an IC controller. If the load is fixed, the current is also fixed. To maintain the same average current to a fixed load, the change in current through the inductor during the turn-on period has to be the same as the change in the turn-off period. It means ΔIL(on)=ΔIL(off):
$$|\Delta{}I_L(on)|=|\Delta{}I_L(off)|$$
$$\because{} V_{out} < V_{in}$$
$$\cfrac{V_{in}-V_{out}}{L}T_{on}=-\left(\cfrac{-V_{out}}{L}T_{off}\right)$$
$$\cfrac{V_{out}}{V_{in}}=\cfrac{T_{on}}{T_{on}+T_{off}}$$
Let duty cycle is D, which is :
$$D=\cfrac{T_{on}}{T_{on}+T_{off}}$$
Therefore, the buck equation in terms of duty cycle:
$$\cfrac{V_{out}}{V_{in}}=D$$
Buck converter waveform
Input Voltage (Vin): The input voltage is the voltage supplied to the buck converter. It remains relatively constant during each switching cycle.
Output Voltage (Vout): The output voltage is the voltage provided to the load. It is usually filtered to reduce ripple using an output capacitor. In the Buck converter, it is lower than the input voltage, as shown in the above figure.
Switch Node Voltage (Vsw): This is the voltage at the switch (usually a transistor) when turned on and off. When the switch is on, the voltage at the switch node is close to the input voltage. When the switch is off, the voltage drops and depends on the inductor and capacitor values.
Inductor Current (IL): The inductor current ramps up when the switch is on and ramps down when the switch is off. The inductor current waveform is generally triangular in shape. It is continuous in a well-designed buck converter, which never drops to zero during a switching cycle.
Diode Current (ID): The diode conducts when the switch is off, allowing the inductor current to flow through it. The diode current waveform is typically sawtooth.
Switching Frequency (fsw): The frequency at which the switch turns on and off. It is typically in the tens of kilohertz to megahertz range.
Buck regulator integrated circuits
Types of Buck converter
Buck converters can be classified into two categories based on how the switching is implemented :
Non-synchronous buck converter
A non-synchronous buck converter has only one switch for energizing the inductor while it depends on a diode to automatically turn on and de-energize the inductor when the switch is turned off. It is simple in construction but the efficiency is lower than synchronous buck converter due to VDIL power dissipation. Fig 2 represents a non-synchronous buck converter.
Synchronous Buck converter
It typically consists of two switches, usually MOSFETs – a high-side (Q1) and a low-side (Q2) switch. These switches alternate between ON and OFF states to regulate the flow of current. Since both the switches are controlled in sync, this topology is called synchronous buck converter.
The main advantage of synchronous buck converter is the removal of diode drop when switch Q1 is off. Instead, Q2’s Ron carries the current and eliminates VDIL power dissipation.
There is a small catch to it. The control circuit becomes complex because Q1 and Q2 should not be ON at the same time. If not careful, during transition, this short can happen. If it happens, a large current can flow through Vin to ground through Q1 and Q2 leading the overheating and permanent damage. To prevent that, non-overlapping control signals are used to drive Q1 and Q2′ s gate.
Between the time when Q1 turns OFF and Q2 turns ON, the body diode of Q2 turns ON allowing the current to flow. For this small moment, diode dissipation is there.