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Jan 15 2018

Analysis of Switching Power Supply Principle

Warm hints: The word in this article is about 4800 and reading time is about 28 minutes.


Currently there mainly includes two types of power supply: linear power (linear) and switching power (switching). This paper is mainly about analysis of switching power supply principle,such as linear power,switching power supply; Transformer and PWM control circuit; Transient filter circuit analysis and etc.

Article coreSwitching Power Supply PrincipleTypeLinear; Switching
English nameSwitching Power Supply 





I.Linear power

VI.Secondary side

II.Switching power supply

6.1 sencondary side(1)

III.Active PFC circuit

6.2 Secondary side (2)

IV.Light tube

VII.Graphic switching power supply

V.Transformer and PWM control circuit

VIII.Transient filter circuit analysis


I.Linear power 

Currently there mainly includes two types of power supply: linear power (linear) and switching power (switching). The principle of linear power supply is to convert 127 V or 220 V mains into low voltage through a transformer, say 12V, and the converted low voltage remains AC. Then rectify through a series of diodes and Converting the low voltage AC to pulsating voltage (with 3 in Figures 1 and 2). The next step is to filter the pulsating voltage, complete with capacitors, and then convert the filtered low voltage AC to DC And 2 of the 4). At this time the resulting low-voltage DC is still not pure enough, there will be some fluctuations (such voltage fluctuations is what we often say that the ripple), so you also need to stabilize the diode or voltage rectifier circuit to correct . Finally, we can get pure low-voltage DC output (with 5 in Figures 1 and 2)

The standard linear power supply design

Figure 1: The standard linear power supply design

The standard linear power supply design

Figure 2: Waveform of linear power supply

Although linear power supplies are well suited for powering low-power devices such as cordless phones, game consoles such as PlayStation / Wii / Xbox, etc. Linear power supplies can be power hungry for high-power devices.

For a linear power supply, its internal capacitance and the size of the transformer are inversely proportional to the AC mains frequency: that is, the lower the input mains frequency, the larger the linear power supply will be needed for capacitors and transformers, and vice versa. Due to the current AC power frequency of 60 Hz (in some countries, 50 Hz), which is a relatively low frequency, the transformers and capacitors tend to be relatively tall. In addition, AC mains surge, the greater the linear power transformer head.

We can see that it would be a crazy move to build a linear power supply for the personal PC segment because of its size and weight. So that personal PC users are not suitable for linear power supply.

II.Switching power supply

Switching power supply can be a good solution to this problem through high-frequency switching mode. For high frequency switching power supplies, the AC input voltage can be boosted before entering the transformer (typically 50-60 KHz before boosting). As the input power increases, the head of components such as transformers and capacitors need not be as big as linear power supplies. This high frequency switching power supply is exactly what our personal PC and equipment like VCRs require. It should be noted that what we often refer to as a ’switching power supply‘ is actually an abbreviation for ‘high-frequency switching power supply’ and has nothing to do with the power supply itself turned off and on.

In fact, the end user's PC's power supply is a more optimized solution: The closed loop system (closed loop system) - responsible for controlling the switch circuit to obtain the feedback signal from the power output, and then more PC power consumption To increase or decrease the frequency of the voltage within a certain period in order to be able to adapt to the power transformer (this method is called PWM, Pulse Width Modulation). Therefore, the switching power supply can be adjusted according to the power consumption of the connected power devices, so as to allow the transformers and other components to take less energy and reduce the heat generation.

The other hand, linear power supply, its design philosophy is above the power, even if the load circuit does not require a lot of current. The consequence of this is that all the components work at full capacity, even when not necessary, resulting in much higher heat.

Voltage doubler and primary side rectifier circuit

As already mentioned above, the switching power supply mainly includes active PFC power supply and passive PFC power supply without PFC circuit, but equipped with a voltage doubler (voltage doubler). The voltage doubler uses two huge electrolytic capacitors, that is to say, if you see two large capacitors inside the power supply, it can basically determine that this is the power doubler. As we have already mentioned, the voltage doubler is only suitable for 127V voltage area.

Two huge electrolytic capacitor composed of voltage doublerTwo huge electrolytic capacitor composed of voltage doubler

Rectifier bridge can be seen on the side of the voltage doubler. Rectifier bridge can be composed of four diodes, it can be a single component, as shown below. High-end power rectifier bridge are generally placed in a special heat sink.

Rectifier bridge

There is usually a NTC thermistor on the primary side - a resistor that changes the resistance depending on the temperature. NTC thermistors are short for Negative Temperature Coefficient. Its role is mainly used to re-match the power supply when the temperature is low or high, and the ceramic disc capacitance is more similar.

III.Active PFC circuit

There is no doubt that this circuit can only be seen in the power supply with active PFC circuit. Figure 16 depicts a typical PFC circuit:

Active PFC circuit diagram

Active PFC circuit usually uses two power MOSFET lighting. These tubes are usually placed on the side of the heat sink. For ease of understanding, we used the letters to mark each MOSFET turn-on: S for Source, D for Drain and G for Gate.

The PFC diode is a power diode and is usually packaged in a power package similar to a power transistor. Both are long and resembled, and are also mounted on the primary heatsink, though the PFC diode has only two pins.

The inductance in the PFC circuit is the largest inductance in the power supply. the primary side filter capacitor is the largest electrolytic capacitor on the primary side of the active PFC supply. The resistor in Figure 16 is an NTC thermistor that changes resistance at a more temperature-dependent change, and acts like a second EMI NTC thermistor.

Active PFC control circuit is usually based on an IC integrated circuit, and sometimes this integrated circuit will also be responsible for controlling the PWM circuit (used to control the open tube closed). This type of integrated circuit is commonly referred to as ‘PFC / PWM combo’.

As usual, look at some examples. In Figure 17, we see the components better after removing the heatsink on the primary side. The left side is the EMI circuit of the transient filter circuit, which has already been described in detail above. On the left side, all are the components of the active PFC circuit. Since we have removed the heat sink, the PFC transistor and the PFC diode have not been seen on the picture. Also note that there is an X capacitor (brown element on the bottom of the rectifier bridge heatsink) between the rectifier bridge and the active PFC circuit. Often, olive-shaped thermistors, which resemble ceramic disk capacitors, have a rubber-covered wrap.

Active PFC components

Figure 18 shows the components on the primary heatsink. This power supply is equipped with two MOSFET power MOSFET and active PFC circuit power diode:

Open the light pipe, power diode

IV.Light tube

Switching power supply switching inverter level can have a variety of modes, we summarize a few situations:


the number of open tubes

Diode number

Capacitance quantity

Transformer pins

Single-transistor forward






Two-transistor forward






Half bridge






Full bridge












Of course, we are only analyzing how many components are needed in a given model. In fact, there are many constraints that engineers face when considering which model to adopt.

Two of the most popular modes at the moment are two-transistor forward and push-pull designs, both of which use two splitters. These are placed in the side of the heat sink on the open tube we have been introduced in the previous page, not to go into details here.

The following is the design of these five modes:

Single-transistor forward configurationTwo-transistor forward configuration

Half bridge configurationFull bridge configuration

Push-pull configuration


V.Transformer and PWM control circuit

Earlier we have already mentioned that a PC power supply would normally be equipped with three transformers: the largest one is the main transformer marked in Figure 3, 4 and 19-23, the primary side of which is connected to the switch and the second The secondary side is connected to the rectifier circuit and the filter circuit to supply the low-voltage DC output of the power supply (+ 12V, + 5V, + 3.3V, -12V, -5V).

The smallest transformer load + 5VSB output, usually also become a standby transformer, stands ready at any time, because this part of the output is always on, even if the PC is turned off.

The third transformer room isolator, the PWM control circuit and the open tube connected. Not all power supplies are equipped with this transformer, as some power supplies tend to have optocoupler integrated circuits with the same functionality.


This power supply uses an optocoupler integrated circuit, not a transformer

The PWM control circuit is based on an integrated circuit. Under normal circumstances, not equipped with active PFC power supply will use TL494 integrated circuit (Figure 26 uses a compatible DBL494 integrated chip). In power supplies with active PFC circuits, a chip that replaces the PWM and PFC control circuitry is sometimes used. The CM6800 chip is a good example of how well it integrates all the features of a PWM chip and PFC control circuit.

PWM control circuit

VI.Secondary side

6.1 sencondary side(1)

The last to introduce is the secondary side. On the secondary side, the output of the main transformer will be rectified and filtered, then the voltage required by the PC will be output. Rectifiers of -5 V and -12 V are done with normal diodes because they do not require high power and high current. But +3.3 V, +5 V and +12 V and other positive pressure rectifier tasks need to be carried out by the high-power Schottky rectifier bridge. The Schottky has three pins, the shape and power of the diodes are similar, but they are integrated within the two high-power diodes. Secondary rectification work can be completed by the power supply circuit structure, generally there may be two rectifier circuit structure, as shown in Figure 27:

Rectifier mode

Mode A will be more used in low-end entry-level power supplies, which require three pins from the transformer. Mode B is more used in high-end power supply, this model generally only need to equip two transformers, but the ferrite inductor must be big enough, so the high cost of this model, which is why the low-end power supply does not use this mode The main reason.

In addition, for the high-end power supply, in order to improve the maximum current output capability, these power supplies tend to use two diodes in series to the rectifier circuit maximum current output to double.

Both the high-side and low-side power supplies are equipped with full rectifier and filter circuitry for both +12 V and +5 V outputs, so at least two sets of rectifier circuits, shown in Figure 27, are required for all supplies.

For 3.3V output, there are three options to choose from:

  • +5 V output in the increase of a 3.3V voltage regulator, many low-end power supply are used in this design. Add a complete rectifier and filter as shown in Figure 27 for the 3.3 V output, but share a transformer with the 5 V rectifier. This is a more common high-end power supply design.

  • Adopt a complete independent 3.3V rectifier circuit and filter circuit. This program is very rare, only in a few top-level fever-class power may occur, such as An anti-US Galaxy 1000W.

  • 3.3V output is often limited by the 5V output because the 3.3V output is usually a fully public 5V rectifier circuit (common in low-side power supply) or partially shared (common in high-end power supply). This is why many power supplies are famous in the nameplate ‘3.3V and 5V combined output’.

Figure 28 below shows the secondary side of a low-end power supply. Here we can see the integrated circuit responsible for generating the PG signal. Under normal circumstances, low-end power supply will use the LM339 integrated circuit.

Secondary side

In addition, we can also see some electrolytic capacitors (the head of these capacitors is much smaller than the capacitor of the voltage doubler or active PFC circuit) and the inductor. These components are mainly responsible for the filtering function.

In order to more clearly observe the power, we remove the fly line and the filter coil on the power supply, as shown in Figure 29. Here we can see some small diodes, mainly for -12 V and -5 V rectification, through the current is very small (this power supply as long as 0.5A). Other voltage output current of at least 1A, which requires the power diode rectifier.

-12 V and -5V negative voltage rectifier diodes

6.2 Secondary side (2)

Figure 30 below shows the components on the secondary heatsink of the low-side power supply:

Secondary heat sink on the components

From left to right:

  • Regulator IC chip - although it has three pins and looks very similar to the transistor, but it is IC chip. This power supply uses a 7805 regulator (5V regulator), responsible for the regulator + 5VSB. As we have already mentioned before, the + 5VSB uses a separate output circuit because it still requires a +5 V output to + 5VSB even when the PC is powered down. This is why the + 5VSB output is also commonly referred to as ‘standby output'. The 7805 IC provides up to 1A of current output.

  •  power MOSFET transistor, mainly responsible for 3.3V output. The power MOSFET model is PHP45N03LT, allowing up to 45A of current through. As we have already mentioned, only the low-side power supply will use a 3.3V regulator that is shared with 5V.

  •  Power Schottky rectifier, made up of two diodes. The Schottky power supply for this model is the STPR1620CT, which allows up to 8A of current per diode (16A total). This power Schottky rectifier is usually used for 12V output.

  • another power Schottky rectifier. The power supply model is E83-004, the maximum allow 60A current through. This power rectifier is often used for +5 V and + 3.3 V output. Because the +5 V and + 3.3 V outputs use the same rectifier, their sum can not exceed the rectifier's current limit. This is what we often call the concept of federated output. In other words, the 3.3V output comes from the 5V output. And other output is different, the transformer does not have 3.3V output. This design is commonly used in low-end power supplies. High-end power supplies typically use separate +3.3 V and +5 V outputs.

Let's look at the secondary side of the high-end power supply main components:

Components on the secondary side of the high-end power supply

Components on the secondary side of the high-end power supply

We can see that

Two parallel 12V Schottky rectifier power output. Low-end power often has only one such rectifier. This design naturally allows the rectifier maximum current output to be doubled. This power supply uses two STPS6045CW Schottky rectifier, each can run up to 60A current.

  • A Schottky rectifier responsible for 5V output. This power supply uses the STPS60L30CW rectifier, the maximum allow 60A current through.

  • A Schottky rectifier responsible for 3.3V output, which is the main difference between high-side and low-side power supplies (low-side power often does not have a separate 3.3V output). This power supply uses the STPS30L30CT Schottky, the maximum allow 30A current through.

  • a power protection circuit regulator. This is also a symbol of high-end power supply.

  • The main point is that the above mentioned maximum current output is only relative to a single component. The maximum current output of a power supply actually depends on the quality of many of the components connected to it, such as the coil inductance, the thickness of the transformer, wires, PCB board width, and so on. We can get the maximum theoretical power of the power supply by multiplying the maximum current of the rectifier with the output voltage. For example, the maximum power output of the 12V output of the power supply in Figure 30 should be 16A * 12V = 192W.

VII.Graphic switching power supply

The following figure 3 and 4 describes the switching power supply PWM feedback mechanism. Figure 3 depicts a low-cost power supply without a PFC (Power Factor Correction) circuit. Figure 4 depicts the mid-to-high-end power supply using an active PFC design.

Power Supply Without PFC Circuit

Figure 3: Power Supply Without PFC Circuit

Power Supply with PFC Circuit

Figure 4: Power Supply with PFC Circuit

By comparing Figure 3 and Figure 4 we can see the difference between the two: one with active PFC circuit and the other does not have, the former is not 110/220 V converter, but also no voltage doubler circuit. Below we will focus on active PFC power explain.

In order to allow readers to better understand the power of the working principle, the above we provide is a very basic illustration, the figure does not include other additional circuits, such as short circuit protection, standby circuit and PG signal generator and so on. Of course, if you want to know a more detailed illustration, look at Figure 5. If you do not understand it does not matter, because this picture was originally for those professional power designer to see.

Typical low-end ATX power supply design

Figure 5: Typical low-end ATX power supply design

You may ask, why is there no voltage rectifier circuit in the design of Figure 5? In fact, the PWM circuit has shouldered voltage rectification work. The input voltage will be recalibrated before it passes through the open-tube and the voltage into the transformer has become a square wave. Therefore, the transformer output waveform is square wave, rather than a sine wave. Since the waveform is already a square wave at this time, the voltage can easily be converted into a DC voltage by a transformer. That is, after the voltage is recalibrated by the transformer, the output voltage has become a DC voltage. This is why many times the switching power supply is often referred to as a DC-DC converter.

The loop feeding the PWM control circuit is responsible for all the required tuning functions. If the output voltage is wrong, the PWM control circuit will change the duty cycle of the control signal to adapt to the transformer, the final output voltage correction. This situation often occurs when the PC power consumption increases, when the output voltage tends to decline, or when the PC power consumption decreases, the output voltage tends to rise.

What we need to know

All circuits and modules before the transformer are called 'primary' (primary side) and all circuits and modules behind the transformer are called 'secondary'

  • Active PFC power supply design does not have 110 V / 220 V converter, but also no voltage doubler.

  • For power supply without PFC circuit, if 110 V / 220 V is set to 110 V, the current will use the voltage doubler to raise 110 V to 220 V before entering the rectifier bridge.

  • PC power on the open tube by a pair of power MOSFET, of course, there are other combinations, after which we will explain in detail.

  • Transformer waveform required for the square wave, so after the transformer voltage waveform is a square wave, rather than a sine wave.

  • The PWM control current is often an integrated circuit, usually isolated from the primary side by a small transformer, and sometimes through a coupling chip (a small IC chip with LEDs and phototransistors) and a primary side isolation.

  • PWM control circuit is based on the power output load conditions to control the power switch tube closed. If the output voltage is too high or too low, the PWM control circuit will change the voltage waveform to adapt to open the light tube, so as to achieve the purpose of the school positive output voltage.

Next we will be through the picture to study the power of each module and circuit, through the physical image to tell you where in the power can find them.

When you turn on a power for the first time (make sure the power cord is not connected to the mains, or it will be powered), you may be disoriented by the weird components inside, but there are two things you know for sure : Power supply fan and heat sink.

Switching power supply internal

However, you should be able to easily identify which components within the power supply belong to the primary side and which belong to the secondary side. In general, if you see a large filter capacitor (power supply with active PFC circuit) or two (power supply without PFC circuit), that side is the primary side.

Under normal circumstances, there are three transformers arranged between the two heat sinks of the power supply. For example, as shown in FIG. 7, the main transformer is the largest one. the medium 'body' is usually responsible for the output of + 5VSB and the minimum Is generally used for PWM control circuits and is used mainly to isolate the primary and secondary side parts (which is why the 'isolator' tag is attached to the transformer in Figures 3 and 4 above). Instead of using the transformer as an 'isolator' some power supplies use one or more optocouplers (which look like IC-integrated chips), meaning that the power supply using this design has only two transformers - Main transformer and auxiliary transformer.

The power supply usually has two heat sinks inside, one on the primary side and the other on the secondary side. If it is an active PFC power supply, then on the primary side of the heat sink, you can see the switch, PFC transistors and diodes. This is not absolute, as some vendors may choose to install active PFC components on separate heatsinks, with two heatsinks on one side.

On the secondary side of the heat sink, you will find there are some rectifiers, they look a little like the transistor, but in fact, they are two power diodes combined.

Next to the secondary heatsink, you'll also see a lot of capacitors and inductors that together make up the low-voltage filter module - find them and find the secondary side.

The simplest way to differentiate the primary and secondary sides is to follow the power line. In general, the output line is often connected to the secondary side, while the input line is connected to the primary side (input line from the mains). As shown in Figure 7.

Differentiate primary and secondary sides

Above, we give a general introduction of the internal modules of a power supply from a macro point of view. Below we refine, the topic transferred to the power of the various components of the module ...


VIII.Transient filter circuit analysis

When connected to PC switching power supply, the electricity goes into the transient filter circuit (Transient Filtering), we often say that the EMI circuit. Figure 8 below shows a circuit diagram of a recommended transient filter circuit for a PC power supply.

Circuit diagram of transient filter circuit

Why I emphasize 'recommended'? Because a lot of power on the market, especially low-end power supply, often will save some components in Fig. 8. So by checking whether there is shrinking EMI circuit can determine the pros and cons of your power quality.

The main components of EMI circuit circuits are MOVs (Metal Oxide Varistors) or varistors (shown as RV1 in Figure 8), responsible for suppressing spikes in mains transients. MOV elements are also used on surge suppressors. However, many low-end power supplies often cut off important MOV components in order to save costs. Surge suppressors are no longer important for MOV-equipped power supplies because the power supply already has a surge suppression feature.

L1 and L2 in Figure 8 are ferrite coils. C1 and C2 are disc capacitors, usually blue, these capacitors are often called Y capacitors. and C3 is a metallized polyester capacitor with a typical capacity of 100nF, 470nF or 680nF, also called the X capacitor. Some power supplies are equipped with two X capacitors connected in parallel with the mains, as shown in RV1 in Figure 8.

X capacitors can be any of the parallel capacitors and mains. Y capacitors are generally pairs of pairs, the need to be connected in series to the fire, zero and the midpoint of the two capacitors through the chassis ground. In other words, they are connected in parallel with the mains.

Transient filter circuit can not only play a role in filtering the mains, but also to prevent the noise generated by the open tube interfere with the same in a mains on the other electronic devices.

Let's look at a few practical examples. Can you see some weirdness as shown in Figure 9? This power is actually no transient filter circuit! This is a cheap cottage power supply. Note that looking at the markings on the board, the transient filter circuit should have been talented, but it was brought to the market by the conscience-riddled JS.

This inexpensive cottage power supply has no transient filter

As you can see in Figure 10, this is a low-side power supply with a transient filter, but as we can see, this power supply's transient filter eliminates the need for an important MOV varistor and only A ferrite coil. however, this power supply is equipped with an extra X capacitor.

Low-end power supply EMI circuit

Transient filter circuit is divided into first-level EMI and second-level EMI, a lot of power supply EMI will often be placed in a separate PCB board, near the mains interface section, two EMI is placed in the power of the main PCB As shown in Figures 11 and 12 below.

Level 1 EMI is equipped with an X capacitor and a ferrite inductor

Look at the second level of this power EMI. Here we can see the MOV varistor, although its placement is somewhat weird, behind the second ferrite. Overall, it should be said that this power supply EMI circuit is very complete.

Complete secondary EMI

It is worth mentioning that the above MOV varistor power supply is yellow, but in fact most of the MOV are dark blue.

In addition, this power transient filter circuit is also equipped with fuses (F1 in Figure 8). Note that if you find that the fuses in the fuse have been blown, you can be sure that one or some of the components inside the power supply are defective. If you replace the fuse at this time is useless, when you boot is likely to be burned again.

Switch Mode Power Supply Measurements and Analysis video site:   

This video shows that switch mode power supply measurements and analysis

Book Recommendation

  • Switching Power Supplies A - Z, Second Edition 2nd Edition

This book is the most comprehensive study available of the theoretical and practical aspects of controlling and measuring Electromagnetic Interference in switching power supplies, including input filter instability considerations.The new edition is thoroughly revised with six completely new chapters, while the existing EMI chapters are expanded to include many more step-by-step numerical examples and key derivations and EMI mitigation techniques. New topics cover the length and breadth of modern switching power conversion techniques, lucidly explained in simple but thorough terms, now with uniquely detailed "wall-reference charts" providing easy access to even complex topics.

  1. Step-by-step and iterative approach for calculating high-frequency losses in forward converter transformers, including Proximity losses based on Dowell's equations

  2. Thorough, yet uniquely simple design flow-chart for building DC-DC converters and their magnetic components under typical wide-input supply conditions

  3. Step-by-step, solved examples for stabilizing control loops of all three major topologies, using either transconductance or conventional operational amplifiers, and either current-mode or voltage-mode control

--Sanjaya Maniktala  (Author)

  • Switching Power Supply Design and Optimization, Second Edition

Extensively revised throughout, Switching Power Supply Design & Optimization, Second Edition, explains how to design reliable, high-performance switching power supplies for today's cutting-edge electronics. The book covers modern topologies and converters and features new information on designing or selecting bandgap references, transformer design using detailed new design charts for proximity effects, Buck efficiency loss teardown diagrams, active reset techniques, topology morphology, and a meticulous AC-DC front-end design procedure.This updated resource contains design charts and numerical examples for comprehensive feedback loop design, including TL431, plus the world’s first top-down simplified design methodology for wide-input resonant (LLC) converters. A step-by-step comparative design procedure for Forward and Flyback converters is also included in this practical guide.

--Sanjaya Maniktala  (Author)

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