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May 26 2018

Switch-Mode Power Supply Fundamentals (1)

Warm hints: The word in this article is about 3000 words and reading time is about 12 minutes.


Switch-mode power supply (SMPS) is a kind of power supply which uses modern power electronic technology to control the time ratio of switch on and off and to maintain a stable output voltage. The switching power supply is generally composed of pulse width modulation (PWM), control IC and MOSFET. With the development and innovation of power electronics technology, switch-mode power supply technology is also constantly innovating. At present, switch-mode power supply is widely used in almost all electronic devices because of its small size, light weight and high efficiency. It is an indispensable power supply mode for the rapid development of electronic information industry.

The switch-mode power supply fundamentals tutorials consist of five chapters: the type of topology, the relationship between efficiency with input & output and duty cycle, the definitions of synchronization and non-synchronization, characteristics of isolation and non-isolation, pulse width modulation and frequency conversion and other various control modes. It explains the basic concept in simple language and tends to establish a platform to exchange ideas and info for power designers.

Article CoreSMPS fundamentalsPurposeMaintain a stable output voltage
Switch-mode power supplyCategoryElectronic power supply
FunctionConverting & supplying powerFeatureEfficiency higher, smaller, lighter


CatalogsChapter 1 Switch-mode power supply fundamentals1.1.4 Volume comparison: linear power supplies vs. switching power supplies1.3.2 There are two main energy storage devices in the electronics industry1.8 Buck converter topology1.13 Buck-boost converter topology
1.1 Type of switch-mode Power supply1.1.5 Summary: linear voltage regulator vs. switching power supply1.4 Example: Simplified buck switching power supply1.9 Boost converter (boost switching regulator)1.14 Controller and regulator
1.1.1 The so-called linear regulators, also known as LDO, generally have two characteristics:1.2 What is a switching regulator?1.5 Type of switching Power supply (non-isolated)1.10 Boost converter (current and voltage waveforms)1.15 Summary of switching regulator
1.1.2  Switching regulator1.3 PWM (Pulse Width modulation)1.6 Working Model of Buck Converter (Buck Switching Regulator)1.11 Boost converter topology and circuit examples
1.1.3  Charge pumps1.3.1 A simple way to periodically change the turn-on and turn-off times of a switch1.7 Buck Converter Basics (Current and Voltage Waveforms)1.12 Buck-boost converters (current and voltage waveforms)


With the rapid development of power electronics technology, the relationship between power electronic equipment and people's work and life has been more and more close, and electronic equipment can not be separated from reliable power supply. In the 1980s, the computer power supply has fully realized switch-mode power supply. In the 1990s, switch-mode power supplies have been widely used in various fields of electronics and electrical equipment such as program-controlled switchboard, communication equipment, electronic test & control equipment and etc, which also promotes the rapid development of switch-mode power supply technology.

Compared with linear power supply, the cost of both them increase with the increasing of output power, but the growth rate is different between them. The cost of linear power supply is higher than that of switch-mode power supply at a certain output power point. With the development and innovation of power electronics technology and switch-mode power supply technology, this cost reversal point is increasingly moving to the low output power end, which provides a wide range of development space for switching power supply.


Chapter 1 Switch-mode power supply fundamentals

1.1 Type of switch-mode Power supply

1.1.1 The so-called linear regulators, also known as LDO, generally have two characteristics:

  • The transmission element works in the linear region, and it has no switch transition.

  • Limited to step-down conversion, booster applications are seldom to see.

1.1.2  Switching regulator

  • Transmission switches (field effect transistor) are fully turn on or off during each cycle;

  • It includes at least one energy storage component, such as an inductor or capacitor;

  • A variety of topologies (Buck, Boost, Buck/Boost etc.)

1.1.3  Charge pumps

  • Generally for low current applications

  • Transmission switches (for example FET and Triode), some of them are fully on, and some of them work in the linear region;

  • Only capacitors are used in the process of conversion or storage of energy, we can see that in some voltage doubler circuits.

Q: why do you use a switching regulator in some cases? Why not use LDO and charge pumps?

Well, we all known that energy will not just disappear, and the lost energy will eventually be transmitted in the form of heat, so that engineers will have a great challenge in their design, which means if losses will be eventually transferred by form of heat, which is also inevitable, then we shall need a larger heat sink in the circuit, also resulting in a larger size of the power supply and a low efficiency of the whole machine. But if we use a switch-mode power supply, it can not only improve the efficiency, but also reduce the design difficulty of thermal management. 

Now i would like to take an example to show the comparison of efficiency and size between linear power supply and switch-mode power supply:





From the point of view of their efficiency, a 12V input 3.3V / 2A output power supply, if realized by a linear voltage regulator, has only 28 percent output efficiency, but if it is done by switch-mode power supply, the output efficiency can reach more than 90 percent. Here, the advantages of using switch-mode power supply are obvious. Therefore, the efficiency of linear power supply is very low in the case of high-input low-output voltage. It is only suitable for situations where input-output voltage difference is relatively low. The losses of the linear regulator is 17.4 W, in contrast, the losses of switching regulator is only 0.73 W, both of which surely shall be eventually transmitted in a form of heat.

We all know that: 

device operating temperature = device temperature rise + ambient temperature;

temperature rise = thermal resistance × losses.

And we suppose the thermal resistance of the device is θ=35℃/W, then the temperature rise of device is 35 ℃ × 17.4W=609 ℃, and the temperature rise of switching regulator is 35 ℃ × 0.73W=25.55 ℃.

It can be seen that switching regulators can work at 60~70℃ and there is no problem with them. In contrary to this, LDO has a very serious heating problem, which means its thermal resistance has to be reduced, but the thermal resistance depends on the heat dissipation area, that is, the larger the heat dissipation area, the smaller the thermal resistance, so the heat dissipation area of LDO has to be very large to reduce its thermal resistance and its temperature rise (see chart below).

1.1.4 Volume comparison: linear power supplies vs. switching power supplies



The red marked places show a 2.5W LDO and a 6W switching power supply, the power difference between the two being 2.4 times, but the area of the SMPS is less than a quarter of that of the LDO, which means the losses of the switching power supply has been greatly reduced and capable of withstanding higher thermal resistance, therefore reducing the area of heat dissipation.

Again, if the voltage difference between input and output is relatively low, LDO shall be used, but in the opposite case (the voltage difference between input and output is relatively high), switching power supply instead will be recommended. Of course, the switching power supply also has its disadvantages, its output will have noise, ring or jump while the LDO will not. In some cases where the load is very sensitive to the voltage of the power supply, an LDO can be added after the switching power supply. For example, we want to convert 5V to 1.2V, if we use an LDO directly, the efficiency may be only 20%, but if we first convert the 5V to 1.5V by using an switching power supply and then convert the 1.5V to 1.2V using an LDO, wouldn't it be more effective? That is the optimized design.

1.1.5 Summary: linear voltage regulator vs. switching power supply

(1) Switch-mode power supply

  1. it can increase the voltage (boost)

  2. and lower (buck) or even invert the voltage

  3. High efficiency and power density

(2)Linear voltage regulator

  1. it can only decrease the voltage (buck)

  2. The output voltage is relatively stable


1.2 What is a switching regulator?



Switching regulator, some may call it regulator, voltage stabilizer or voltage regulator. To achieve voltage stabilization, it is necessary to control the system (negative feedback). From the theory of automatic control, we know that when the voltage rises, it can be lowered by negative feedback, and when the voltage drops, it can be lifted up by the same way, thus forming a controlled loop. The block diagram in the figure demonstrates the use of PWM (Pulse Width modulation), and of course there are others such as: PFM (Pulse-Frequency Modulation), phase-shift modulation, etc.

1.3 PWM (Pulse Width modulation)

1.3.1 A simple way to periodically change the turn-on and turn-off times of a switch



Duty cycle, it is the ratio of ON time (Ton) to the time period T of the pulse waveform, that is 

ton ( turn-on time) + toff ( turn-off time ) = T (time period)

D (Duty cycle) =ton / T.

However, we cannot use a single pulse output, so we need a method of stabilizing and controlling the energy flow, to store energy during switch-on and to provide energy during switch-off with many pulses, switching at high frequencies and thus achieving a smooth voltage.

1.3.2 There are two main energy storage devices in the electronics industry



1.4 Example: Simplified buck switching power supply



The figure is a simplified step-down switching power supply, and in order to facilitate the analysis of the circuit, we will leave out the feedback control section first.

State 1: When S1 is closed, the input energy is supplied from capacitor C1 through S1 → inductor L1 → capacitor C2 → load RL. At this time, inductor L1 is also storing energy, and the voltage applied to L1 can be obtained as: Vin-Vo =L*di/dton.

State 2: When S2 is turned off, the energy is no longer obtained from the input, but through a freewheeling loop that energy stored in the inductor L1 → capacitor C2 → load RL → diode D1. At this time, it can be obtained: L*di/dtoff= Vo, then we finally get Vo/Vin=D, where Vo is always less than Vin because duty cycle D ≤ 1.

The role of each device:

  1. the input capacitor (C1) is used to stabilize the input voltage;

  2. the output capacitor (C2) is used to stabilize the output voltage;

  3. The clamp diode (D1) provides a current path for the inductor when the switch is open;

  4. The inductor (L1) is used to store the energy that will be delivered to the load.

1.5 Type of switching Power supply (non-isolated)



1.6 Working Model of Buck Converter (Buck Switching Regulator)



The switching power supply is a closed-loop control system. We can compare the current of the switching power supply to the flow of water, the input capacitor is a high and large reservoir, and the output capacitor is a low and small reservoir. A small cup of water is transferred from the large pool to the small one, and the fixed amount of water in the small pool will be achieved by controlling the interval between the transfer and the amount of water in the cup. When the level of water output is low, the amount of water in the cup will be increased, and when the level of water output is high, the amount of water in the cup will be decreased.

1.7 Buck Converter Basics (Current and Voltage Waveforms)



When the switch is opened, the energy is transferred from the input to the output, and the current is inclined upward. It is like the water of the cup in the model sent to the small poll. When the water in the small pool is up enough, the switch will be shut off, the inductor, load and the diode form a natural freewheeling loop, and the current begins to decrease linearly; when the water in the small pool is down to a certain degree, the switch will be turned on again—with such a high frequency turn-on and turn-off, a stable output voltage is formed then.

1.8 Buck converter topology



The above is a circuit structure in which we can sample the output voltage through the two resistor voltage divider, and then a comparator comparing with the reference: turn on the MOS if the output is less than the reference and turn off the MOS if the output is larger than the reference.

The following is an example of a circuit using LM22670 chip, which is a typical asynchronous step-down converter because it uses a fast recovery diode or Schottky diode. But why? Well, the parasitic parameters and leakage inductance of the diode will cause a high-voltage oscillation when the MOS transistor is turned on. This oscillation will eventually lead to high-voltage damage and very large switching losses of the SW pin of the chip, resulting in very low efficiency, so it is generally to use fast recovery or Schottky diodes.

1.9 Boost converter (boost switching regulator)



The boost converter can also be compared with the model of the water flow. The only difference with buck converter is that it sends the lower water flow to a high place, which i would like to use topology diagram and waveform diagram to analyze.

1.10 Boost converter (current and voltage waveforms)



The left figure shows the topology of the Boost converter. As we mentioned earlier, the inductor L is an energy storage device. When the switch is turned on, the input voltage charges the inductor and the formed loop is: Input Vi → Inductor L → switch Q; When the switch is turned off, the input energy and the inductor energy together to provide energy to the output, and the resulting loop is: input Vi → inductor L → diode D → capacitor C → load RL, the output voltage at this time is definitely higher than the input voltage, so as to achieve boost.

1.11 Boost converter topology and circuit examples



The control loop of the boost converter shown in the figure above is sampled through a voltage divider resistor, then compared with the reference source through an error comparator, and finally outputs PWM. It should be noted that when the chip does not work, the circuit has naturally formed a loop from the input → inductor → diode → capacitor → load, so if it is not in the synchronous boost topology, a switching circuit should be added to the part of input. Otherwise, the battery will run out for no use.

1.12 Buck-boost converters (current and voltage waveforms)



State 1: The switch is turned on, diode D is cut off in reverse and inductor is storing energy, so the current loop is : input Vin→switch tube Q →inductor L;

State 2: The switch is turned off and the diode D being using is diode freewheeling in forward, so the current loop is : inductor L→capacitor C→load RL→diode D.

About the output, when is boost and when is buck operation?

We can know from the formula Vo=Vin×D/(1-D) that when D=0.5, Vo=Vin; when D<0.5, Vo<Vin; and when D>0.5, Vo>Vin. Also we can see that this kind of topology can easily get a negative voltage, so we can use this method to achieve negative voltage when we don't want to use an isolation transformer.

1.13 Buck-boost converter topology



The figure above is a circuit of negative voltage output realized by using TPS5430DDA. The pins of LM22670 and TPS5430DDA

 are the same, so they are interchangeable.


1.14 Controller and regulator



The figure above is a distinction between a controller and a regulator. ICs with integrated switches are commonly referred to as regulators, ICs that require external switches are called controllers instead. The description in the diagram we can only use as a reference, many voltage regulators can now be larger than 3A, and many thermal resistors are as low as 10℃/W. However, many high-power switching power supplies still require controllers and external MOS transistors.

Comparison between controller and regulator:



1.15 Summary of switching regulator


Here is a video tutorial series on the design of power converter circuits.

Part 1 deals with AC to AC and AC to DC converters.

SMPS Tutorial (1): Introduction - Switched Mode Power Supplies and Power Conversion

Book Recommendation

Switch-Mode Power Supplies Spice Simulations and Practical Designs

It is a comprehensive resource on using SPICE as a power conversion design companion. This book uniquely bridges analysis and market reality to teach the development and marketing of state-of-the art switching converters. Invaluable to both the graduating student and the experienced design engineer, this guide explains how to derive founding equations of the most popular converters…design safe, reliable converters through numerous practical examples…and utilize SPICE simulations to virtually breadboard a converter on the PC before using the soldering iron.

by Christophe Basso

Power Converters with Digital Filter Feedback Control 1st Edition

This book presents a logical sequence that leads to the identification, extraction, formulation, conversion, and implementation for the control function needed in electrical power equipment systems. Also it builds a bridge for moving a power converter with conventional analog feedback to one with modern digital filter control and enlists the state space averaging technique to identify the core control function in analytical, close form in s-domain (Laplace). It is a useful reference for all professionals and electrical engineers engaged in electrical power equipment/systems design, integration, and management.

by Keng C. Wu

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