Switching DC-DC Converter Design


Modern portable and battery-operated electronic devices such as laptops and smartphones use DC to DC converters to provide different levels of DC power to their several sub-circuits. These levels can be higher or lower than the supply voltage or battery voltage. Additionally, DC-DC converters also provide power to a battery whose voltage drops leading to space-saving by eliminating need of multiple batteries. Many DC-DC converters also regulate output voltage in some systems, regulate current through the LEDs in LED drivers, double or triple the output voltage, and maximize energy harvesting for photovoltaic systems and wind turbines.

DC-DC converters can be linear regulators (using linear conversion) or switching regulators (using switching techniques). Although linear regulators are inexpensive, simpler and do not generate switching noise, there are several downsides to using them.  They can only produce output regulated voltage lower than the input and are practical for low currents and power dissipations.

Designers use switching techniques instead of linear regulators in almost all portable and stationary applications. This is because switching circuits offer better efficiency, smaller components, and require less thermal management. Basic components of the switching circuit are a power switch, an inductor, and a diode to transfer energy from input to output, which can be rearranged to form a step-down (buck) converter, a step-up (boost) converter, or an inverter (flyback). Feedback and control circuitry is needed to regulate or maintain constant output. Most common method to regulate output voltage with respect to the input is pulse-width modulation (PWM). Earlier techniques used inefficient diodes with forward conduction losses and reverse recovery losses which dissipate heat. With wide availability of power semiconductor devices, MOSFETs have replaced diodes, leading to increased efficiencies well above 90%. This is because MOSFETs lower switching losses leading to reduced thermal management and heat sinks/fans, and require a less complicated drive circuit. This innovative method using MOSFETs is called synchronous rectification. There are, admittedly, some drawbacks of switching converters including complexity, electronic noise and cost, to some extent.

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More importantly, switching regulators can convert voltage to a higher or lower level than the input voltage according to their topology — Non-inverting regulators have output voltage with same polarity as the input. Step-up (Boost) topology provides output voltage higher than the input voltage. SEPIC provides output voltage lower or higher than the input. Step-down (Buck) topology provides output voltage lower than the input voltage. Inverting and non-inverting topologies can provide output voltage of opposite or same polarities as the input respectively. Inverting Buck-boost and True Buck-Boost topologies have the output voltage of opposite or same polarity as the input respectively, and can be lower or higher. Split-Pi (Boost-Buck) allows bidirectional voltage conversion with output of same polarity as the input and either lower or higher. This topology has applications in regenerative braking. In addition, a converter may be designed to operate in continuous current mode (where current never falls to zero) at high power, and in discontinuous current mode (where current may fall to zero) at low power.

Modern DC-to-DC converters are now available as integrated circuits (ICs) with minimum additional components. Advances in chip design have further cut down costs. Now complete hybrid circuit converter modules are available which are ready for use in an electronic assembly.

Mentioned below are reference designs of some DC-DC converters employing switching techniques. Take a look!

  • Tiny DC-DC Converter Design: This is a DC-DC converter reference design with 3.6V to 6V input and 3.3V output. The design is based on TPS62300, a high frequency synchronous step down converter optimized for small battery powered portable applications. Intended to minimize solution size, enhance battery life and maintain high efficiency, the system is capable of driving up to 500 mA (TPS62300) with high efficiency over 92%. More on this Reference Design
  • 12V output Switching DC/DC Converter: This reference design is an easy to design and cost effective solution with telecom input range 36V – 76V DC and output 12V with 200W power. Using advanced switching techniques such as Phase Shift Full Bridge (PSFB) topology lowers switching losses and enables efficiencies as high as 94%. This design is implemented using a single dsPIC33F “GS” digital-power DSCs from Microchip that provides the full digital control of the power conversion and system management functions. The reference design also supports the Full Bridge topology through minor hardware modifications. More on this Reference Design
  • Switching DC-DC Converter for Portable Applications: This reference design is a DC-DC voltage regulator designed for mobile products that require high light load efficiency with input from 8V-19V and 1.05V output. Featuring IR3473 voltage regulator in a very small 4×5 QFN Package, the design provides features such as programmable switching frequency, over-current protection, soft start, pre-Bias start up. More on this Reference Design
  • DC-DC Converter for Automotive LED Driver : Here is a reference design of a step-up/down, switch mode, DC-DC converter used for driving automotive LED applications. Based on MCP1630, a high-speed Pulse Width Modulator (PWM), the demo board provides a 350 mA (700 mA, with hardware modification) constant current source.  With a compact size and high output power, the design can operate in Buck (step-down) or Boost (step-up) mode. The board is able to sustain voltage stresses typically found in automotive products (about 42 V for 180 ms) and provides high efficiency over the entire operating range. More on this Reference Design
  • DC-DC Converter for Battery Charger: Here is a battery charger reference design using buck or a step-down DC-DC converter topology. Controlled by a Zilog’s Z8F042A MCU, the buck converter provides a regulated power source to charge battery in a number of modes such as constant current, constant voltage and constant current with a current limit. More on this Reference Design


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