Switching-Mode Power Supply Design Tutorial

A tutorial on switching-mode power supply design by Jerrold Foutz 

What is a switching-mode power supply?

A power supply is a buffer circuit that provides power with the characteristics required by the load from a primary power source with characteristics incompatible with the load. It makes the load compatible with its power source.

Example: A power source might be the 60 Hz, single phase, 120 V ac power found in a home in the United States or the 50 Hz, single phase, 220 V ac found in the United Kingdom. The load might be a logic circuit in a personal computer that requires regulated 5 V dc power. The power supply is the circuit that makes the 120 V ac or 220 V ac power source and 5 V dc load compatible.

A power supply is sometimes called a power converter and the process is called power conversion. It is also sometimes called a power conditioner and the process is called power conditioning. The Power Sources Manufacturers Association's (PSMA) Handbook of Standardized Terminology for the Power Sources Industry gives this definition of a power supply.

Power Supply -- A device for the conversion of available power of one set of characteristics to another set of characteristics to meet specified requirements. Typical application of power supplies include to convert raw input power to a controlled or stabilized voltage and/or current for the operation of electronic equipment.

Power supplies belong to the field of power electronics, the use of electronics for the control and conversion of electrical power. The IEEE Power Electronics Society provides a more formal definition of power electronics in their constitution.

Power Electronics -- This technology encompasses the effective use of electronic components, the application of circuit theory and design techniques, and the development of analytical tools toward efficient electronic conversion, control, and conditioning of electric power.

A switching-mode power supply is a power supply that provides the power supply function through low loss components such as capacitors, inductors, and transformers -- and the use of switches that are in one of two states, on or off. The advantage is that the switch dissipates very little power in either of these two states and power conversion can be accomplished with minimal power loss, which equates to high efficiency. The term switchmode was widely used for this type of power supply until Motorola, Inc., who used the trademark SWITCHMODE TM for products aimed at the switching-mode power supply market, started to enforce their trademark. Then more generic terms had to be found. I started using the term switching-mode power supply to avoid infringing on the trademark. Others used the term switching power supply, which seems to be the more popular term. PSMA does not define either switching-mode power supply or switching power supply, but does define switching regulator.

Switching Regulator -- A switching circuit that operates in a closed loop system to regulate the power supply output.

The design problem

Because of its emphasis on efficiency, switching-mode power supply design minimizes the use of lossy components such as resistors and uses components that are ideally lossless such as switches, capacitors, inductors, and transformers. The primary design problem is how to interconnect these components and control the switches so the desired results are obtained. The secondary design problem is to select, design, or overcome the performance characteristics of less than ideal components. Protection techniques and parts derating are used to circumvent the fact that real parts tend to fail when overstressed.

The design process is successful when a proper topology and control has been chosen that exceed the performance requirements and when protection techniques, parts selection, and derating have been used that exceed the required reliability. Both normal and abnormal operating environments the circuit will encounter over its useful life must be considered throughout the design process.

Note that the design goal is to exceed, not to just meet, the performance and reliability requirements. This comes from the philosophy that given fixed resources the engineer's task is to get the most from these resources. For example, if analysis shows that it will take 25 parts worth $50 and one month development time to just meet requirements, the goal then shifts from just meeting requirements to getting the absolute best performance out of these parts in the allotted time -- exceeding and not just meeting requirements if this is possible. This is not "gold plating" but just good engineering. Because of the critical nature of power supplies in all equipment, this approach improves both the performance and reliability of the total system at no additional cost. It often keeps the power supply off the critical-path schedule when increasing requirements might force a redesign resulting in a schedule slip.


Power conversion circuits are often classified in four categories.

ac-ac converters (example: frequency changers, cycloconverters)
ac-dc converters (example: rectifiers, off-line converters)
dc-ac converters (also called inverters)
dc-dc converters (also called converters)

The term converter or power converter is used for all these categories or for dc-dc converters only, the meaning is usually clear from the context.

All of these converters may be open-loop circuits or use feedback to provide regulation.

The scope of this tutorial is dc-dc converters and a special type of ac-dc converter called an off-line converter or off-line power supply. In off-line converters the ac voltage is rectified to dc directly off the ac power line and filtered with no isolation transformer and then processed with a dc-dc converter that provides isolation at the switching frequency. Since the switching frequency is much higher than the line frequency the isolation transformer and output filter are greatly reduced in size and weight. The switching frequency is usually 20 kHz or higher to place any audio noise from the switching beyond the range of human hearing. Regulation of output voltage, current, or power is assumed, because that's where the fun begins. Also the rectification process may or may not include power factor correction or harmonic current suppression techniques.

The PSMA Handbook defines off line power supply, power factor, and power factor correction as:

Off Line Power Supply -- 1) A power supply in which the ac line voltage is rectified and filtered without using a line frequency isolation transformer. 2) A power supply switched into service upon line loss to provide power to the load without significant interruption. Also called Off Line Switchers (OLS).

Power Factor -- The ratio of total active power to total apparent power in volt-amperes in an ac circuit, where voltage and current are rms values and include the effects of harmonics as well as the effects of phase displacement. If both voltage and current are sinusoidal, power factor is the cosine of the angle between them.

Power Factor Correction -- 1) Technique of increasing the power factor so that the phase angle between the voltage and current approaches zero in an ac circuit. 2) Addition of capacitors to an inductive circuit to offset reactance.

For the purposes of this tutorial, power factor is real power (average power) divided by apparent power (rms voltage times rms current) and power factor correction is making this ratio approach one.

Tutorial goals

The intended audience for this tutorial include power supply circuit designers, those learning the profession, and those that use power supplies. The treatment is mostly descriptive, using mathematics only as required and graphics only if they are needed to understand the concept being described.

This tutorial has always been planned as a method of tying together the collection of problems discussed in the SMPS Technology Knowledge Base. This was going to occur after about 100 problems were entered into the Knowledge Base. However, I have gotten so much email from graduate students and new engineers around the world with basic questions on switching-mode power supplies, I decided to start the tutorial and develop the tutorial and problems concurrently.

Upon completion of this tutorial it is hoped that the reader will have an appreciation of the design and application problems associated with switching-mode power supplies and the modeling, analysis, and testing techniques used to resolve them. An important part of the tutorial is identifying the printed technical literature and web resources that can be consulted for a more in-depth study of the techniques introduced. Techniques introduced will include circuit-averaging, state-space-averaging, state-plane analysis, impedance graph analysis, optimization, the derivation and classification of various topologies, SPICE simulation, other computer programs, testing, and the relationship of power supply design to system design - the tradeoffs involved. Layout, EMI (Electro-Magnetic Interference), and thermal design -- all skills required by a well-rounded power supply designer -- will be covered also. Links to the SMPS Technology Knowledge Base of problems and solutions are an important part of this introductory tutorial.

Webmaster and editor: Jerrold Foutz
Original: December 9, 1998, revised October 7, 2007