Maxx Coral

4/3/2015

Switch Mode Power Supplies

Abstract

This application note explains the basic theory on the principles of how switch mode power
supplies operate. This article is for the engineer who has minimal experience with switch mode
power supplies. A brief discussion is given to the advantages and disadvantages of a linear
regulator versus a switch mode power supply. The principles of a switch mode power supply
will be learned and can then be applied to choosing and design a supply. Example circuit
schematics along with figures and simulation data will enhance an engineer's knowledge of the
operation of switch mode supplies. The strengths and weaknesses of different switch mode
power supply solutions will be shown.
Introduction

Most electronic circuit designs that are created need a voltage rail to be increased, decreased
or regulated for the operation of the integrated circuit (IC) to perform in a correct and reliable
fashion. With the endless different solutions that are currently present, it can be difficult to
know what the best option for your current design application. Every solution will have
different efficiencies, transient responses, output regulation, deliverable power and cost and it
is the engineer who in the end needs to make a finally decision based on the many different
variables.

Linear Regulator

The first question you need to ask yourself is which power converter is right for your design?
Linear regulators can simply a power converter design. Some linear regulators are as simple as a
three pin IC, with an input and output ripple capacitor as seen in Figure 1.

Figure 1: Linear Regulator Schematic

The simplicity of a linear regulator does not come without a trade-off. Relative to a switch
mode power supply, a linear regulator is less efficient. The regulators usually operates around
25% efficient when your output voltage is about half your input voltage. The other 75% of the
power is dissipated in thermal energy. This dissipation of thermal energy leads to larger IC
package size and required heat sinks to keep the IC operating in a proper temperature range.
When space on a printed circuit board (PCB) is critical to a design, a linear regulator as a power
convertor is not the best choice. When your output voltage is near the input voltage, the
efficiency does increase. Another drawback of a linear regulator is it only works when a larger
DC voltage needs to be stepped-down to a lesser DC voltage. Thus any application requiring a
step-up in voltage cannot use such a regulator.

There are applications where linear regulators provide greater performance for a power
converter than their switching counterparts. For low power designs that are not space limited,
and not thermally constrained, a linear regulator can be a simple low cost solution. The cost of
components and design time are greatly reduced. With linear regulators no external inductor is
needed which leads to less electromagnetic interference (EMI) issues for other nearby circuit
elements. EMI is less likely to be a problem. Since the linear regulators do not have a switching
frequency there is no switching related electromagnetic radiation being generated by the IC.
Finally, linear regulators have faster transient responses due to their internal feed-back loop.

Switch Mode Power Supply

What if linear regulators will not work in your design? Then consider using a switch mode
power supply. A switch mode power supply (SMPS) are generally chosen by designer for their
increased efficiency. SMPS efficiency can be greater than 90%, which means smaller package
optimizing the space on a PCB. With only 10% of the power lost to thermally energy, heat sinks
can be eliminated from the design again saving space and cost. There are three main topologies
for SMPS and they are a buck converter, boost converter, and buck/boost converter which is
sometimes called inverting regulator.

Theory of Operation

A SMPS has the ability to convert a DC input voltage to different DC output voltage values,
depending on how the four main discrete elements are arranged. The main elements in a SMPS
are an inductor, a capacitor, a diode, and a transistor. As mentioned previously, these four
elements can up the three main topologies used in industry and they are a buck converter,
boost converter, and buck/boost converter. Simplified examples of these topologies can be
seen in Figure 2.

Figure 2: Switch Mode Power Supply Topologies

The SMPS operates by controlling the path of current that charges the inductor, differently than
the path of current that is discharged from the inductor. The transistor being used as a switch
being able to change its impedance to high and low states and the diode works well together to
achieve the unique current flow of each topology. A microcontroller of some sort would be
connected to the transistor to control the switching of its state. The microcontroller would
pulse width modulate the base of a BJT or the gate of a MOSFET to create a square wave. This
square wave in turns controls the duty cycle and switching frequency of the SMPS. The
switching controls the ramping of current into and out of the inductor, which in turns is related
directly to the output voltage. Current that is stored in the inductor during the charging cycle, is
pumped into the load capacitor and resistor when the transistor's state changes causing the
inductor's discharging cycle. When the inductor is charging and not delivering power to the load
resistance, the output capacitor holds the output voltage near the desired voltage. Since the
capacitor cannot hold the DC voltage output completely constant, a ripple voltage appears
across the load. This ripple voltage is directly dependent on switching frequency, duty cycle,
peak inductor current and the capacitor's capacitance value. When a minimal ripple voltage is
required a low pass filter can filter out the high frequency ripple voltage that is riding on the DC
voltage, leaving a near perfect DC output voltage. An interesting phenomenon occurs in a
SMPS, since the inductor is changing between absorbing power and delivering power. Since the
current cannot change direction instantaneously in an inductor, one will see the voltage across
its terminals will change polarity instantaneously, as seen in Figure 3.

Figure 3: Inductor Voltage and Current Graphs

Buck Converter

When a design specification requires that the output voltage be less than the input voltage, an
engineer would want to choose a buck converter topology, as seen in Figure 2.

Boost Converter

When a design specification requires that the output voltage be greater than the input voltage,
an engineer would want to choose a boost converter topology, as seen in Figure 2.
Buck-Boost Convert or Inverting Regulator

When a design specification requires that the output voltage be greater in magnitude than the
input voltage yet an inverted sign, an engineer would want to choose a buck-boost converter
topology, as seen in Figure 2.

Summary

In summary linear regulators and switch mode power supplies both have their own place in
many different electronic applications. Linear regulators are simple to use and easy to quickly
test with, but have a draw back when it comes to efficiency and package space. Switch mode
power supplies offer a versatile variety of different output voltages, but are more costly to
design and buy. SMPS also will radiate more electromagnetic noise which can be problem for
nearby circuitry. Engineers and designers need to look at the given design specifications and
make an informed decision on which power converter is right for their design.
 References

1. Corporation, Linear Technology. AN140 - Basic Concepts of Linear Regulator and Switching Mode Power Supplies (n.d.): n. pag. Basic Concepts of
Linear Regulator and Switching Mode Power Supplies. Oct. 2013. Web.
2. "An Introduction to Switch-Mode Power Supplies." - Application Note. N.p., n.d. Web. 02 Apr. 2015.
3. Dr. Gregory M. Wierzba "ECE-402 Application of Analog Integrated Circuits" - Spring 2014