Switching power supply is to use the circuit control switch tube for high-speed pass and cut. Converting DC 

power to high frequency AC power is supplied to the transformer for voltage transformation to produce one or 

more sets of voltages required! The reason for switching to high-frequency AC is that the efficiency of high-

frequency AC in the transformer transformer circuit is much higher than 50Hz. So the switching transformer can 

be done very small, and it is not very hot when working! The cost is very low. If you do not change 50Hz to high 

frequency, then switching power supply does not make sense.

        The switching power supply is a kind of power supply that uses a modern power electronic technology to 

control the ratio of the time when the switching transistor is turned on and off, and maintains a stable output 

voltage. The switching power supply is generally composed of a pulse width modulation (PWM) control IC and 

a MOSFET. Compared with linear power supplies, switching power supplies increase in cost as output power 

increases, but the growth rates vary. The linear power supply cost is higher than the switching power supply at 

a certain output power point, which is called the cost reversal point. With the development and innovation of 

power electronics technology, switching power supply technology is also constantly innovating. This cost 

reversal point is increasingly moving to the low output power end, which provides a broad space for development 

of switching power supplies.

        The high frequency of switching power supply is the direction of its development. The high frequency 

makes the switching power supply miniaturized, and the switching power supply enters a wider range of 

applications, especially in the high-tech field, which promotes the miniaturization and lightness of high-tech 

products. Turn. In addition, the development and application of switching power supplies are of great 

significance in terms of energy conservation, resource conservation and environmental protection.

        Switching power supplies can be roughly divided into two types: isolated and non-isolated. Isolated 

type must have switching transformers, but not necessarily isolated.

        The working principle of the switching power supply is:
        1. The AC power input is rectified and filtered into DC;
        2. The switching transistor is controlled by a high frequency PWM (Pulse Width Modulation) signal, 

and that DC is applied to the primary of the switching transformer;

        3. The secondary of the switching transformer induces a high-frequency voltage, which is supplied 

to the load through rectification and filtering;

        4. The output part is fed back to the control circuit through a certain circuit to control the PWM duty 

cycle to achieve the purpose of stable output.

        When the AC power input is input, it generally passes through something like the E-flow circle to 

filter out the interference on the power grid and also filter out the interference of the power supply to the 

power grid;

        When the power is the same, the higher the switching frequency, the smaller the volume of the 

switching transformer, but the higher the requirements for the switching tube;

        The secondary of the switching transformer may have multiple windings or one winding with multiple 

taps to obtain the desired output;Generally, some protection circuits should be added, such as no-load, 

short-circuit, etc., otherwise the switching power supply may be burnt.

         The main components of the ATX power supply:
         EMI filter circuit: The main function of the EMI filter circuit is to filter the interference of the high-

frequency pulse of the power grid to the power supply. At the same time, it also reduces the electromagnetic 

interference of the switching power supply itself. In the high-quality power supply, there is generally a two-pole 

EMI filter circuit.First-class EMI circuit: The first-level EMI power filter circuit is soldered on the AC power 

socket. This is a separate circuit board. It is the first group of circuits that pass through the AC input. This is 

composed of a choke coil and a capacitor. The network can filter high frequency clutter and in-phase 

interference signals on the power line, and also shield the internal interference signal of the power supply to 

form the first line of defense against electromagnetic interference.

        Secondary EMI circuit: After the mains enters the power board, it passes through the power fuse, and 

then passes through the second EMI circuit consisting of inductor and capacitor to fully filter out the high-

frequency clutter, and then enters the high-voltage rectification filter circuit through the current limiting resistor. 

.The fuse can be blown to protect the components inside the power supply when the power is too high or the 

component is short-circuited. The current limiting resistor contains a metal oxide component, which can limit 

the instantaneous large current and reduce the current impact of the power supply on the internal components.

        Bridge rectifier and high voltage filtering: After EMI filtering, the mains is converted to high voltage DC 

after full bridge rectification and capacitor filtering. Converting the AC power at the input to pulsed DC power, 

there are currently two forms, one is that the full bridge is to pack four diodes together, the other is to use four 

discrete diodes to form a bridge rectifier circuit, the same effect, the same effect .

        In general, there should be two or more tall barrel-shaped components near the full bridge, namely high-

voltage electrolytic capacitors, which function to filter the pulsating DC power to remove the AC component 

and output a relatively stable DC power. The use of high-voltage electrolytic capacitors is closely related to the 

design of the switching circuit. Its capacity is often the focus of power evaluation in the past, but in fact its 

capacity has nothing to do with the power of the power supply, but increasing its capacity will reduce the power 

supply. Ripple interference improves the current output quality of the power supply.

        PFC circuit: The PFC circuit is called a power factor correction or compensation circuit. The higher the 

power factor, the greater the power utilization.

        At present, there are two ways of PFC circuit, one is passive PFC, also known as passive PFC, and the 

other is active PFC, also known as active PFC. The passive PFC compensates the phase difference between

the fundamental current and the voltage of the AC input through a power frequency inductor, forcing the 

current and voltage phases to be consistent. The passive PFC is inefficient, generally only 65%-70%, and the 

work used. The frequency inductance is large and cumbersome, but due to the low cost, there are still many 

ATX power supplies in this way. The active PFC is composed of electronic components, small in size and light 

in weight. The phase of the current waveform is adjusted by a dedicated IC, and the efficiency is greatly 

improved, up to 95%, but due to the high cost, it is usually only in advanced applications. Can you see it.

        Switching triode and switching transformer: As the name implies, the switching power supply is the 

switch word. The switching transistor and the switching transformer are the core components of the switching 

power supply. The self-excited or other excitation mode causes the switching tube to operate in a saturated, 

cut-off (ie, open, closed) state, thereby inducing a high-frequency voltage on the secondary winding of the 

switching transformer. After rectification, filtering and voltage regulation, various DC voltages are output. 

Switching triode and switching transformer are the core components of ATX power supply. Its quality directly 

affects the quality and service life of the power supply. Especially the switching transistor, working under high 

back pressure, there is not enough protection circuit, it is easy to breakdown and burn. The quality of the 

switch directly determines the stability of the power supply. It is also the main heating element in the power 

supply. The two transistors on the main heat sink seen after the power is removed are the switch tubes.

        Factors affecting the performance of high-frequency switching transformers include the efficiency of 

ferrite, the cross-sectional area of the core and the width of the magnetic gap. Transformers with too small 

cross-sectional area are prone to magnetic saturation and cannot output large power, and the number of 

turns of each winding Directly affect the output voltage, usually we can not specifically grasp these 

parameters, so we can not accurately determine how much power the transformer can output, only through 

the electronic load machine measurement can be known, in addition, the output of the switching transformer, 

although many, but Some outputs use the same windings, such as +3.3VDC and +5VDC, so when +3.3VDC 

outputs the maximum current, +5VDC can't output a large current, so we can't put the output of each power 

supply. The power is simply accumulated.

        In addition to the main transformer, there should be two small transformers in the general power supply. 

One of them will amplify the switching circuit control signal to drive the switching tube to work. At the same time, 

it can also operate the high voltage area of the switching tube and the low voltage area of the integrated circuit. 

Physical isolation. The other is a completely independent small switching power supply. This is what we call the 

standby circuit. The output voltage is the main circuit of the power supply. At the same time, the +5VStandBy 

output is output to the main board to realize the wake-up function.

        Low-voltage rectification and filtering circuit: After the high-frequency transformer is stepped down, the 

ripple voltage is also rectified and filtered by diodes and capacitors. However, the operating frequency during 

rectification is very high, and Schottky rectifier diodes with fast recovery function must be used. The ordinary 

rectifier diode is difficult to do this, and the capacitor used in the rectification part can not have too much AC 

impedance, otherwise the high-frequency AC component cannot be filtered out, so the selected capacitor not 

only has a large capacity but also has a lower capacity. The AC resistance is also available. In addition, one or 

two large-sized inductors with a magnetic core can be seen, and the high-frequency AC component is filtered 

together with the filter capacitor to ensure the output of pure DC power.

        Since the low-voltage rectifier terminal needs to output a large current, the rectifier diode also generates 

a large amount of heat. These diodes and the front switch tube require a separate heat sink for heat dissipation, 

and these components are fixed on the other heat sink in the power supply. Output from these components is 

the output current of various voltages.

        Voltage regulation and protection circuit: The voltage stabilization circuit usually samples a part of the 

voltage from the output voltage of the power supply terminal and compares it with the standard voltage. The 

compared difference is amplified to drive the switching transistor and adjust the duty cycle of the switching tube. 

The voltage is stable. The function of the protection circuit is to detect the change of the output voltage or current 

at each end. When the output terminal is short-circuited, over-voltage, over-current, overload, under-voltage, etc., 

the protection circuit operates, and the excitation signal of the switch tube is cut off, so that the switch tube 

Stopping the vibration, the output voltage and current are zero, and it plays a protective role.

        Classification of switching power supplies:
        In the field of switching power supply technology, people develop related power electronic devices and 

develop switching frequency conversion technology. The two promote each other to promote the switching power 

supply to a lighter, smaller, thinner, lower noise and high reliability. The direction of anti-interference is developing. 

Switching power supply can be divided into two major categories: AC/DC and DC/DC. The DC/DC converter has 

been modularized, and the design technology and production process have been matured and standardized at 

home and abroad, and have been recognized by users, but The modularity of AC/DC, due to its own characteristics, 

leads to more complex technical and process manufacturing problems in the process of modularization. The 

structure and characteristics of the two types of switching power supplies are described below.

        2.1DC/DC conversion
        The DC/DC conversion converts a fixed DC voltage into a variable DC voltage, also known as DC chopping. 

There are two ways to operate the chopper. One is that the pulse width modulation mode Ts is unchanged, the 

ton is changed, and the other is the frequency modulation mode. The ton is unchanged and the Ts is changed 

(prone to interference). Its specific circuit consists of the following categories:

(1) Buck circuit - step-down chopper, its output average voltage
U0 is smaller than the input voltage Ui and has the same polarity.
(2) Boost circuit - boost chopper, its output average voltage
U0 is greater than the input voltage Ui and has the same polarity.
(3) Buck-Boost circuit - buck or boost chopper, its
The output average voltage U0 is greater than or less than the input voltage Ui, the polarity is opposite, and the 

inductance is transmitted.

(4) Cuk circuit - buck or boost chopper, its output average
        The voltage U0 is greater than or less than the input voltage Ui, the polarity is reversed, and the capacitance 

is transmitted.Today's soft switching technology makes a qualitative leap in DC/DC. The various ECI soft-switching 

DC/DC converters designed and manufactured by VICOR in the United States have a maximum output power of 

300W, 600W, 800W, etc., and the corresponding power density is (6.2). , 10, 17) W / cm3, the efficiency is 

(80 ~ 90)%. Japan's NemicLambda company recently introduced a high-frequency switching power supply module 

RM series using soft switching technology, its switching frequency is (200 ~ 300) kHz, power density has reached 

27W / cm3, using synchronous rectifier (MOSFET instead of Schottky diode ), the overall circuit efficiency is 

increased to 90%.

        2.2AC/DC conversion
        AC/DC conversion is to convert AC to DC, and its power flow direction can be bidirectional. The power flow 

from the power supply to the load is called “rectification”, and the power flow is returned to the power supply by 

the load called “active inverter”. AC/DC converter inputis 50/60Hz AC, because it must be rectified and filtered, 

so the relatively large size of the filter capacitor is essential, and at the same time due to safety standards 

(such as UL, CCEE, etc.) and EMC directives The restrictions (such as IEC, FCC, CSA), the AC input side must 

be EMC filtered and use components that meet safety standards, thus limiting the size of the AC / DC power supply.

        In addition, due to the internal high-frequency, high-voltage, high-current switching action, it is difficult to solve 

the EMC electromagnetic compatibility problem, which puts high requirements on the internal high-density

installation circuit design. For the same reason, high voltage and large The current switch increases the operating 

loss of the power supply and limits the modularization process of the AC/DC converter. Therefore, the power system 

optimization design method must be adopted to achieve a certain degree of satisfaction in theworking efficiency.

        AC/DC conversion can be divided into half-wave circuits and full-wave circuits according to the wiring mode of 

the circuit. According to the number of power phases can be divided into single phase, three phase, multi phase. 

According to the working quadrant of the circuit, it can be divided into one quadrant, two quadrants, three quadrants 

and four quadrants.

        Switching power supply selection:

        Switching power supply has anti-interference performance. Due to its own circuit structure (multi-stage series), 

general input interference such as surge voltage is difficult to pass. Compared with linear power supply, the output 

voltage stability is compared with the linear power supply. The larger advantage is that the output voltage stability 

can reach (0.5~1)%. Switching power supply module as a power electronic integrated device, should pay attention to 

the following points in the selection:

        3.1 Output current selection
        Because the switching power supply has high working efficiency, it can generally reach more than 80%. Therefore, 

in the selection of its output current, the maximum absorption current of the electrical equipment should be accurately 

measured or calculated, so that the selected switching power supply has a high performance-price ratio. Usually the 

output calculation formula is:

Is=KIf
Where: Is—the rated output current of the switching power supply;
If—the maximum absorbed current of the powered device;
K-margin coefficient, generally taking 1.5 to 1.8;
        3.2 Grounding
        Switching power supply will generate more interference than linear power supply. Grounding and shielding 

measures should be taken for electrical equipment sensitive to common mode interference. According to EMC 

limits such as ICE1000, EN61000, FCC, etc., the switching power supply adopts EMC electromagnetic 

compatibility measures, so the switch The power supply should normally have an EMC EMC filter. For example, 

Lid Huafu Technology's HA series switching power supply, the FG terminal is connected to the earth or the user's 

casing to meet the above electromagnetic compatibility requirements.

        3.3 protection circuit
        The switching power supply must have overcurrent, overheat, short circuit and other protection functions in 

the design. Therefore, the switching power supply module with complete protection function should be preferred 

in design, and the technical parameters of the protection circuit should match the working characteristics of the 

electrical equipment. Avoid damage to electrical equipment or switching power supplies.Development trend of 

switching power supply technology:

        The development direction of switching power supply is high frequency, high reliability, low consumption, 

low noise, anti-interference andmodularization. Because the key technology of switching power supply is light, 

small and thin, it is high frequency, so the foreign major switching power supply system.

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