Boost Converter
What is it?
A boost converter is a DC-DC converter designed to increase the output voltage to a fixed value using a switching device, often a MOSFET. This crucial electronic component efficiently steps up voltage levels in various applications. While the output voltage is typically fixed, it can be adjusted with specific modifications and control techniques.
How does it work ?
The basic principle of a boost converter’s operation can be understood through two key stages:
- Switch-on period : During this stage, the input voltage (Vin) is applied across the inductor (L), causing the current through the inductor to increase linearly. This process stores energy in the inductor. Meanwhile, the diode (D) is reverse-biased, which prevents current from flowing to the load.
- Switch-off period : When switch S1 opens, the inductor current must continue to flow, causing the diode (D) to become forward-biased. The inductor then releases its stored energy to the load (R) and the output capacitor (C). During this period, the voltage across the inductor (VL) is equal to the difference between the output voltage (Vout) and the input voltage (Vin).
The output voltage of a boost converter can be controlled by adjusting the duty cycle of the switching waveform, enabling a higher output voltage than the input voltage. In the following sections, we will delve deeper into the circuit topology, key components, conduction modes, design considerations, efficiency, and applications of boost converters. This comprehensive exploration will provide a thorough understanding of their operation and uses in power electronics systems.
How to build it ?
In this boost converter topology, magnetic energy stored in the inductor transfers energy from a lower voltage DC source to a higher load voltage. When switch S is turned on, the inductor connects to the DC power supply E. The diode D is reverse-biased by the voltage at the load, which is supplied energy from the capacitor. With voltage applied, the current through the inductor increases linearly from its minimum to maximum value. A MOSFET will be utilized as the switch in this configuration.
This converter is a popular and widely used DC-DC converter topology designed to step up the input voltage to a higher output voltage. The basic circuit topology of a boost converter consists of the following key components:
Inductor (L): The inductor stores and releases energy throughout the switching cycles, playing a crucial role in energy storage during conversion and controlling current flow.
Switch (S): The switch, typically a MOSFET or IGBT, alternates between connecting and disconnecting the inductor to the load, managing the energy transfer between the input and output.
Diode (D): During the switch-off period, the diode permits current to flow only toward the output, preventing it from returning to the input side.
Output Capacitor (C): The output capacitor stabilizes the voltage across the load and smooths the output voltage, ensuring a consistent output voltage and filtering out high-frequency voltage ripples.
Load Resistor (R): The load resistor represents the electrical load powered by the boost converter, which must be designed to provide the necessary voltage and current to this load.
The basic operation of a boost converter can be summarized as follows: During the switch-on period, the inductor stores energy from the input voltage source while the diode prevents current flow to the load. When the switch is turned off, the inductor releases its stored energy through the diode, providing the load with a higher voltage than the input voltage. The output capacitor smooths the output voltage, delivering a stable supply to the load.
What modes it has?
In the analysis and design of boost converters, it is crucial to consider the two primary conduction modes: Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). These modes differ in the behavior of inductor current, which impacts the converter’s performance, efficiency, and design criteria.
Continuous Conduction Mode (CCM): In CCM, the inductor current remains positive and never reaches zero throughout the switching cycle. During the switch-on phase, the inductor current increases, storing energy in its magnetic field. When the switch is off, the inductor current decreases as it transfers stored energy to the output. The current waveform in CCM has a triangular shape, and the load current equals the average value of the inductor current.
CCM operation produces lower peak currents and output voltage ripple, enhancing efficiency and reducing stress on the converter’s components. However, maintaining continuous current flow requires larger inductor values, which can increase the converter’s size and cost.
Discontinuous Conduction Mode (DCM): In DCM, the inductor current drops to zero for a portion of the switching cycle, indicating that energy transfer to the output is complete before the next switch-on period begins. This creates a zero-current interval between the switch-off and the subsequent switch-on periods, resulting in a trapezoidal inductor current waveform.
DCM operation allows for smaller inductor values, leading to more compact and cost-effective converter designs. However, it results in larger peak currents, increased output voltage ripple, and lower efficiency compared to CCM. DCM is more commonly used in applications with a wide range of load conditions or light loads.
Components sizing
To accurately compute component sizing in a boost converter, it’s essential to set specific parameters, such as the inductor current ripple [math]\Delta I_{out}[/math] and voltage rippel [math]\Delta V_{out}[/math].
In out case: [math]\Delta I_{out}=20.51 mA[/math] and [math]\Delta V_{out}=0.89 mV[/math] with a switching frequency [math]F_s=50kHz[/math], an input Voltage of [math] 3V [/math] and output votage of [math]12V[/math].
Note that a voltage regulator, such as the LM7812, is added to eliminate any voltage ripple from the final output voltage.
So the formulas are :
- [math]D=\frac{T_{on}}{T_{on}+T_{off}}=\frac{V_{out}-V_{in}}{V_{out}}=\frac{12-3}{12}=\frac{9}{12}=75%[/math]. We chose to use 80% efficiency instead of 75%. This 5% difference accounts for the capacitive effect of the diode, which induces a delay.
- [math]C_{out}=T(1-D) \frac{\Delta I_{out}}{\Delta V_{out}}=92µF[/math]
- [math]L=\frac{V_{in} D}{\Delta I_{out} F_s }=2.34 mH[/math]
Conclusion
Boost converters are crucial in power electronics, efficiently stepping up input voltage to a higher output voltage. Their versatility in managing various load conditions and maintaining stable output through proper design and component selection makes them indispensable in many applications. Understanding the key components and operating principles allows designers to optimize boost converter performance, enhancing efficiency and reliability.
