Detailed analysis of the working principle of switching power supply

How Much Do You Know About PC Power Supplies?

The power supplies used in personal PCs are all based on a technology called "switching mode", so we often call personal PC power supplies - switching power supplies (SMPS for short), and it also has a nickname - DC-DC converter. In this article, we will explain the working mode and principle of switching power supplies, the introduction of the components inside switching power supplies, and the functions of these components.

●How much do you know about linear power supply

Currently, there are mainly two types of power supplies: linear power supplies and switching power supplies. The working principle of a linear power supply is to first convert 127 V or 220 V mains electricity into low voltage electricity, such as 12V, through a transformer, and the low voltage after conversion is still AC alternating current; then it is corrected and rectified through a series of diodes, and the low voltage AC alternating current is converted into a pulsating voltage ("3" in Figures 1 and 2); the next step is to filter the pulsating voltage, which is completed through a capacitor, and then the filtered low voltage AC is converted into DC direct current ("4" in Figures 1 and 2); the low voltage direct current obtained at this time is still not pure enough, and there will be certain fluctuations (this voltage fluctuation is what we often call ripple), so a voltage regulator diode or a voltage rectifier circuit is needed to correct it. Finally, we can get a pure low-voltage DC direct current output ("5" in Figures 1 and 2)

Figure 1: Standard linear power supply design diagram

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 like PlayStation/Wii/Xbox, etc., they are not sufficient for high-power devices.

For linear power supplies, the size of their internal capacitors and transformers is inversely proportional to the frequency of the AC mains: that is, the lower the frequency of the input mains, the larger the capacitors and transformers required for the linear power supply, and vice versa. Since the current AC mains frequency is 60Hz (50Hz in some countries), which is a relatively low frequency, the size of its transformers and capacitors is often relatively large. In addition, the larger the surge of the AC mains, the larger the size of the transformer of the linear power supply.

It can be seen that for the personal PC field, it would be a crazy move to make a linear power supply, because its size would be very large and its weight would be very heavy. Therefore, linear power supplies are not suitable for personal PC users.

●How much do you know about switching power supplies?

Switching power supplies can solve this problem well through high-frequency switching modes. For high-frequency switching power supplies, the AC input voltage can be boosted before entering the transformer (usually 50-60 KHz before boosting). As the input voltage increases, the size of components such as transformers and capacitors does not need to be as large as linear power supplies. This high-frequency switching power supply is exactly what our personal PCs and devices such as VCR recorders need. It should be noted that the "switching power supply" we often refer to is actually an abbreviation of "high-frequency switching power supply", which has nothing to do with the off and on mode of the power supply itself.

In fact, the power supply of the end user's PC adopts a more optimized solution: a closed loop system - the circuit responsible for controlling the switch tube obtains feedback signals from the output of the power supply, and then increases or decreases the frequency of the voltage within a certain cycle according to the power consumption of the PC to adapt to the power supply transformer (this method is called PWM, Pulse Width Modulation). Therefore, the switching power supply can adjust itself according to the power consumption of the power-consuming equipment connected to it, so that the transformer and other components can take away less energy and reduce heat generation.

On the other hand, the design concept of linear power supply is power first, even if the load circuit does not need a lot of current. The consequence of this is that all components work at full load even when it is not necessary, resulting in much higher heat.

Picture Talk: Switching Power Supply Diagram

Figures 3 and 4 below describe the PWM feedback mechanism of a switching power supply. Figure 3 describes a cheap power supply without a PFC (Power Factor Correction) circuit, and Figure 4 describes a mid- to high-end power supply with an active PFC design.

Figure 3: Power supply without 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 has an active PFC circuit and the other does not. The former does not have a 110/220 V converter and does not have a voltage doubler circuit. Our focus below will be on the active PFC power supply.

In order to help readers better understand the working principle of the power supply, the above diagrams we provide are very basic. The diagrams do not include other additional circuits, such as short-circuit protection, standby circuits, and PG signal generators. Of course, if you want to know more detailed diagrams, please see Figure 5. It doesn't matter if you don't understand it, because this diagram is originally for professional power supply designers.

Figure 5: Typical low-end ATX power supply design

You may ask, why is there no voltage rectification circuit in the design diagram of Figure 5? In fact, the PWM circuit has already taken on the work of voltage rectification. The input voltage will be rectified again before passing through the switch tube, and the voltage entering the transformer has become a square wave. Therefore, the waveform output by the transformer is also a square wave, not a sine wave. Since the waveform is already a square wave at this time, the voltage can be easily converted into a DC voltage by the transformer. In other words, when the voltage is re-corrected by the transformer, the output voltage has become a DC voltage. This is why switching power supplies are often referred to as DC-DC converters.

The loop feeding the PWM control circuit is responsible for all the necessary regulation functions. If the output voltage is wrong, the PWM control circuit will change the control signal of the duty cycle to adapt to the transformer and finally correct the output voltage. This situation often happens when the PC power consumption increases, when the output voltage tends to decrease, or when the PC power consumption decreases, when the output voltage tends to increase.

Before looking at the next page, we need to understand the following information:

★All circuits and modules before the transformer are called "primary" (primary side), and all circuits and modules after the transformer are called "secondary" (secondary side);

★Power supplies with active PFC design do not have a 110 V/220 V converter and no voltage doubler;

★For power supplies without PFC circuits, if 110 V / 220 V is set to 110 V, the power supply itself will use a voltage doubler to increase 110 V to around 220 V before the current enters the rectifier bridge;

The switch tube on the PC power supply is composed of a pair of power MOSFET tubes. Of course, there are other combinations, which we will explain in detail later;

★The waveform required by the transformer is a square wave, so the voltage waveform after passing through the transformer is a square wave, not a sine wave;

★PWM control circuits are often integrated circuits, usually isolated from the primary side by a small transformer, but sometimes they may be isolated from the primary side by a coupling chip (a very small IC chip with an LED and a phototransistor);

★The PWM control circuit controls the closing of the power switch according to the output load of the power supply. If the output voltage is too high or too low, the PWM control circuit will change the voltage waveform to adapt to the switch, thereby achieving the purpose of correcting the output voltage;

On the next page we will study each module and circuit of the power supply through pictures, and tell you where to find them in the power supply through physical pictures.

Picture and story: The secrets inside the power supply

When you turn on a power supply for the first time (make sure the power cord is not connected to the mains, otherwise you will get an electric shock), you may be confused by the strange components inside, but there are two things you must recognize: the power supply fan and the heat sink.

Inside a switching power supply

But you should be able to easily tell which components belong to the primary side and which belong to the secondary side. Generally speaking, if you see one (power supply with active PFC circuit) or two (power supply without PFC circuit) large filter capacitors, that side is the primary side.

Generally, there are three transformers between the two heat sinks of the power supply, such as shown in Figure 7. The main transformer is the largest one; the medium-sized one is often responsible for the +5VSB output, and the smallest one is generally used for the PWM control circuit, mainly used to isolate the primary and secondary sides (this is why the transformers in Figures 3 and 4 above are labeled "isolator"). Some power supplies do not use transformers as "isolators", but use one or more optocouplers (which look like IC integrated chips), which means that the power supply using this design has only two transformers - the main transformer and the auxiliary transformer.

There are usually two heat sinks inside the power supply, one for the primary side and the other for the secondary side. If it is an active PFC power supply, then on the heat sink on the primary side, you can see the switch tube, PFC transistor and diode. This is not absolute, because some manufacturers may choose to install the active PFC components on a separate heat sink, in which case there will be two heat sinks on the primary side.

On the heat sink on the secondary side, you will find some rectifiers. They look a bit like triodes, but in fact, they are composed of two power diodes.

Next to the heat sink on the secondary side, you will also see many capacitors and inductors, which together form a low-voltage filtering module - finding them means finding the secondary side.

The easiest way to distinguish the primary side from the secondary side is to follow the power supply line. Generally speaking, the secondary side is often connected to the output line, and the primary side is connected to the input line (the input line connected from the mains). See Figure 7.

Distinguishing between primary and secondary sides

Above we have briefly introduced the various modules inside a power supply from a macro perspective. Now let's go into more detail and move the topic to the components of each module of the power supply...

Analysis of transient filter circuit

After the mains is connected to the PC switching power supply, it first enters the transient filtering circuit, which is what we often call the EMI circuit. Figure 8 below describes the circuit diagram of the "recommended" transient filtering circuit of a PC power supply.

Circuit diagram of transient filter circuit

Why do we emphasize "recommended"? Because many power supplies on the market, especially low-end power supplies, often omit some components in Figure 8. So by checking whether the EMI circuit is reduced, you can judge the quality of your power supply.

The main component of the EMI circuit is MOV (l Oxide Varistor), or varistor (shown as RV1 in Figure 8), which is responsible for suppressing spikes in the mains transient. MOV components are also used in surge suppressors. However, many low-end power supplies often cut off important MOV components to save costs. For power supplies equipped with MOV components, the presence or absence of surge suppressors is no longer important, because the power supply already has the function of suppressing surges.

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

X capacitors can be any capacitors connected in parallel with the mains; Y capacitors are usually paired in pairs, and need to be connected in series between the hot and the neutral, and the midpoint of the two capacitors is grounded through the chassis. In other words, they are connected in parallel with the mains.

The transient filter circuit can not only filter the AC power, but also prevent the noise generated by the switching tube from interfering with other electronic equipment on the same AC power.

Let's look at some real examples. As shown in Figure 9, can you see something strange? This power supply does not have a transient filter circuit! This is a cheap "copycat" power supply. Please note that if you look at the markings on the circuit board, the transient filter circuit should have been there, but it was brought to the market by the unscrupulous JS who lost their conscience.

Switching tube, power diode

Next we will focus on the switch tube...

Switching tube

●Switch tube

The switching inverter stage of the switching power supply can have multiple modes. We summarize several situations:

Of course, we are only analyzing how many components are needed in a certain mode. In fact, engineers are constrained by many factors when considering which mode to adopt.

The two most popular modes are the two-transistor forward and full-bridge (push-pull) designs, both of which use two switching transistors. We have already introduced these switching transistors placed on the primary side heat sink on the previous page, so I will not go into details here.

The following are the designs of these five modes:

Single-transistor forward configuration

Two-transistor forward configuration

Half bridge configuration

Full bridge configuration

Push-pull configuration

Transformer and PWM control circuit

●Transformer and PWM control circuit

As we have mentioned before, a PC power supply is generally equipped with three transformers: the largest one is the main transformer marked in Figures 3, 4 and 19-23. Its primary side is connected to the switching tube, and its secondary side is connected to the rectifier circuit and the filter circuit, which can provide the power supply's low-voltage DC output (+12V, +5V, +3.3V, -12V, -5V).

The smallest transformer load +5VSB output is usually also called the standby transformer, which is always in "standby mode" because this part of the output is always on, even when the PC power is turned off.

The third transformer room isolator connects the PWM control circuit and the switch tube. Not all power supplies are equipped with this transformer, because some power supplies are often equipped with an optocoupler integrated circuit with the same function.

transformer

This power supply uses an optocoupler integrated circuit instead of a transformer.

The PWM control circuit is based on an integrated circuit. Generally, power supplies without active PFC will use the TL494 integrated circuit (Figure 26 below uses a compatible DBL494 integrated chip). Power supplies with active PFC circuits sometimes use a chip that replaces the PWM chip and PFC control circuit. The CM6800 chip is a good example, which can well integrate all the functions of the PWM chip and PFC control circuit.

PWM control circuit

Secondary side (I)

Secondary side

The last part to be introduced is the secondary side. In the secondary side, the output of the main transformer will be rectified and filtered, and then the voltage required by the PC will be output. The rectification of -5 V and –12 V can be completed with ordinary diodes because they do not require high power and high current. However, the rectification of positive voltages such as +3.3 V, +5 V and +12 V requires a high-power Schottky rectifier bridge. This type of Schottky has three pins and looks similar to a power diode, but they have two high-power diodes integrated inside. Whether the secondary side rectification work can be completed is determined by the power circuit structure. Generally, there may be two rectifier circuit structures, as shown in Figure 27:

Rectification mode

Mode A is more often used in low-end entry-level power supplies, and this mode requires three pins from the transformer. Mode B is more often used in high-end power supplies, and this mode generally only requires two transformers, but the ferrite inductor must be large enough, so this mode is more expensive, which is the main reason why low-end power supplies do not use this mode.

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

Whether it is a high-end or low-end power supply, its +12 V and +5 V outputs are equipped with complete rectification and filtering circuits, so all power supplies require at least 2 sets of rectification circuits as shown in Figure 27.

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

☆Add a 3.3V voltage regulator to the +5V output part. Many low-end power supplies adopt this design.

☆Add a complete rectifier circuit and filter circuit as shown in Figure 27 for the 3.3 V output, but it needs to share a transformer with the 5 V rectifier circuit. This is a common design solution for high-end power supplies.

☆Use a complete independent 3.3V rectifier circuit and filter circuit. This solution is very rare and can only be found in a few top-level power supplies, such as Antec's Galaxy 1000W.

Since the 3.3V output usually fully shares the 5V rectifier circuit (common in low-end power supplies) or partially shares it (common in high-end power supplies), the 3.3V output is often limited by the 5V output. This is why many power supplies have "3.3V and 5V combined output" on their nameplates.

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. Usually, low-end power supplies use the LM339 integrated circuit.

Secondary side

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

In order to observe this power supply more clearly, we remove all the flying leads and filter coils on the power supply, as shown in Figure 29. Here we can see some small diodes, which are mainly used for rectification of -12 V and –5 V, and the current passing through is very small (this power supply only needs 0.5A). The current of other voltage outputs must be at least 1A, which requires power diodes to be responsible for rectification.

Rectifier diodes for –12 V and –5 V negative voltage circuits

Secondary side (two)

●Secondary side (2)

Figure 30 below describes the components on the secondary side heat sink of a low-end power supply:

Components on the secondary side heat sink

From left to right:

☆ Voltage regulator IC chip - Although it has three pins and looks very similar to a transistor, it is a voltage regulator IC chip. This power supply uses a 7805 voltage regulator (5V voltage regulator) to regulate the +5VSB voltage. As we have mentioned before, +5VSB uses an independent output circuit because it still needs to provide +5 V output to +5VSB even when the PC is powered off. This is why the +5VSB output is often referred to as the "standby output". The 7805 IC can provide a maximum current output of 1A.

☆Power MOSFET transistor, mainly responsible for 3.3V output. The MOSFET model of this power supply is PHP45N03LT, which can allow a maximum current of 45A. We have mentioned on the previous page that only low-end power supplies will use a 3.3V regulator shared with 5V.

☆Power Schottky rectifier, integrated with two diodes. The Schottky model of this power supply is STPR1620CT, and each diode can allow a maximum current of 8A to pass through (a total of 16A). This power Schottky rectifier is usually used for 12V output.

☆Another power Schottky rectifier. The model used in this power supply is E83-004, which allows a maximum current of 60A to pass through. This power rectifier is often used for +5 V and + 3.3 V outputs. Because the +5 V and + 3.3 V outputs use the same rectifier, their sum cannot exceed the current limit of the rectifier. This is what we often call the concept of combined output. In other words, the 3.3V output comes from the 5V output. Unlike other outputs, the transformer does not have a 3.3V output. This design is often used for low-end power supplies. High-end power supplies generally use independent +3.3 V and +5 V outputs.

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

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

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

Here we can see:

Two parallel-connected Schottky rectifiers are responsible for the 12V output. Low-end power supplies often only have one such rectifier. This design naturally doubles the maximum current output of the rectifier. This power supply uses two STPS6045CW Schottky rectifiers, each of which can run a maximum current of 60A.

☆A Schottky rectifier responsible for 5V output. This power supply uses the STPS60L30CW rectifier, which can allow a maximum current of 60A to pass through.

☆A Schottky rectifier responsible for 3.3V output, which is the main difference between high-end power supplies and low-end power supplies (low-end power supplies often do not have a separate 3.3V output). This power supply uses STPS30L30CT Schottky, which can allow a maximum current of 30A to pass through.

☆A voltage regulator for power protection circuit. This is also a symbol of high-end power supply.

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