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Brief introduction of power modules

The circuit that consists of discrete components is called a module, such as a power module. It is essentially the same as IC, except that the general module is suitable for the high-power circuit, which is "semi-integrated circuit" and the inner surface may contain IC, while IC is the complete circuit.

Power modules can be directly mounted on the printed circuit board, whose main characteristic is providing power supply for specific integrated circuit (ASIC), digital signal processor (DSP), microprocessor, memory, field programmable gate array (FPGA) and other digital or analog load.

In general, such modules are called load point (POL) power supply systems or use point power supply systems (PUPS). Because there are a lot of the advantages of the modular structure, module power is widely used in switching equipment, access equipment, mobile communications, microwave communications and optical transmission, routers and other communications and automotive electronics, aerospace, etc.

In recent years, due to the rapid development of data business and the continuous promotion of distributed power supply system, the increase of module power supply has exceeded the primary power source supply. The module power supply has the function of isolation, strong anti-interference ability, built-in protection function, and easy to integrate. With the use of semiconductor technology, encapsulation technology and high frequency soft switch, the power density of the module is increasing, the conversion efficiency is higher and the application becomes simpler.

People in the field of switch power supply technology is doing development of related power electronic components, at the same time developing the switch frequency conversion technology, which boosts the switch power supply with more than two digit growth toward light, small, thin, low noise, high reliable and anti-jamming.

Switching mode power supply can be divided into two major categories of AC/DC and DC/DC, DC/DC converter has been modularized, and the design and production technology at home and abroad have been mature and standardized, and has been recognized by users; AC/DC modular, however, because of its own feature makes it meet complicated manufacturing problems and difficult production process in the process of modularization.

Classification of modules

According to the application field of modern power electronics, we divide the power module as follows:

1. Green power module
The rapid development of computer technology leads the human society into the information society, and also promotes the rapid development of power module technology. In the 1980s, the computer adopted the switching power supply, which was the first to complete computer power generation. Then the switching power supply technology successively entered the field of electronics and electrical equipment.

The development of computer technology, put forward green computer and green power module. Green computer refers to personal computers and related products that harmless to the environment, and green power refers to high efficient and electricity-saving power supply related with the green computer. According to the U.S. environmental protection agency l992 "energy star" on June 17, plan, desktop PC or related peripheral equipment, the power consumption in the sleep state less than 30 watts, in accordance with the requirement of green computer, the basic way to improve the power efficiency is to reduce the power consumption. In the case of a 200-watt switching power supply with an efficiency of 75%, the power supply itself consumes 50 watts of energy.

2. Switching power module
The rapid development of telecommunication industry has greatly promoted the development of communication power. The high frequency miniaturized switching power supply and its technology have become the mainstream of modern communication power supply system. In the field of communication, the rectifier is usually referred to as a power supply, and the DC - DC (DC/DC) converter is referred to as the secondary power supply. The function of a power supply is to convert the single-phase or three-phase ac power grid into a dc power source with a nominal value of 48V. Current in SPC exchange with a power supply, the traditional phased type regulated power supply has been replaced with high frequency switching power supply, high-frequency switching power supply (also known as switch mode rectifier SMR) by MOSFET and IGBT high frequency work, general control switch frequency in 50-100 KHZ range, to achieve high efficiency and miniaturization. In recent years, the power capacity of the switch rectifier has been continuously expanded, and the capacity of the single machine has been expanded from 48V/12.5A, 48V/20A to 48V/200A, 48V/400A.

3. Converter
DC/DC converter convert a fixed DC voltage transformation to a variable DC voltage, this technology has been widely applied to the stepless variable speed and control of trolley buses, subway trains, electric vehicle, and to make the control to obtain smooth acceleration, quick response performance, and at the same time receive the effect of saving energy. Using dc chopper instead of rheostat can save energy (20~30 %). The dc chopper can not only play the function of regulating voltage (switching power supply), but also can effectively suppress the harmonic current noise.

The DC/DC converter has been commercialized, the module adopts high-frequency PWM technology, the switching frequency is around 500kHz, and the power density is 5W~20W/in3. With the development of large scale integrated circuit for power module to realize miniaturization, therefore to improve switch frequency and adopt a new circuit topology structure, the current existing some companies produced using zero current switch and the secondary power supply module, a zero voltage switching power density are improved greatly.

4. UPS
Uninterruptible power supply (UPS) is a high reliable, high performance power supply that is required by computers, communication systems, and requirements to provide uninterruptible applications. The ac is converted into dc by the rectifier, some of the energy is charged to the battery pack, and the other part of the energy is converted into ac by the inverter, which is sent to the load by the switch. In order to provide energy to the load while the inverter fails, the alternate power supply is realized through the power switch.

Modern UPS generally adopts pulse width modulation technology, power M0SFET, IGBT and other modern power electronic devices, the noise of power supply is reduced, and efficiency and reliability are improved. The introduction of microprocessor and hardware technology can realize intelligent management of UPS, remote maintenance and remote diagnosis.

At present, the maximum capacity of online UPS can be reached to 600kVA. The development of super small UPS is also very rapid. There are already 0.5kva, lVA, 2kVA, 3kVA and other products.

5. Inverter power supply
Inverter power supply is mainly used for inverter speed regulation of ac motor, and its position in electric drive system is becoming more and more important, and it has achieved great energy saving effect. Ac - dc - ac scheme is adopted for main circuit of inverter. Power through the rectifier power frequency into fixed dc voltage, and then made up of high-power transistors or IGBT PWM high frequency converter, dc voltage inverter into ac output voltage and variable frequency, power supply output waveform similar to sine wave, used to drive ac asynchronous motor to realize stepless speed regulation.

The international 400kVA inverter power series products have been developed. In the early 1980s, Toshiba of Japan first applied ac frequency conversion technology to air conditioners. By 1997, its share had reached more than 70% of Japanese household air conditioners. Frequency conversion air conditioning has the advantages of comfort and energy saving. In the early 1990s, we began to study the frequency conversion air conditioner. In 1996, the production line was introduced to produce the variable frequency air conditioner, which gradually formed a hot spot for the development and production of frequency conversion air conditioners. A climax is expected by around 2000. In addition to the frequency converter, the inverter requires a compressor motor suitable for frequency conversion. Optimization control strategy, select functional components, is the development direction of air conditioning frequency conversion power supply.

6. Welding power supply module
The high frequency inverter rectifier welding power supply is a new kind of high performance, high efficiency and material saving, which represents the development direction of welding power supply. Due to the commercial use of IGBT large capacity module, this power supply has a wide application prospect.

The AC-DC-AC-DC (AC-DC- DC) conversion method is used for inverter welding power. 50 hz alternating current through the whole bridge rectifier into dc, IGBT PWM high frequency transformation of dc inverter into 20 KHZ high frequency rectangular wave and the high-frequency transformer coupling, become a stable dc after rectifying filtering, used for arc power supply.

Due to the terrible working conditions of welding power supply, under frequent alternating of short circuit, arc, open change, thus the work reliability of high frequency inverter welding rectifier power supply become the key problem, is also the most concern for the user. Using microprocessor as a pulse width modulation (PWM) controller, through the parameters and the extraction and analysis of information, to predict various working status, the purpose of system, in turn, make adjustments to the system and processing in advance, to solve the current high-power IGBT inverter power supply reliability.

7. DC power module
High-power switching high voltage dc power supply is widely used in electrostatic dust removal, water quality improvement, medical X-ray machine and CT machine. The voltage is up to 50~l59kV, the current reaches 0.5A, and the power can reach 100kW.

Since the 1970s, some Japanese companies have adopted contravariant technology, converting the city's electricity into a mid-frequency conversion of about 3kHz and then boosting it. In the 1980s, high frequency switching power technology developed rapidly. Germany Siemens USES power transistor as the main switching element to increase the switching frequency of power to over 20kHz. The successful application of dry type transformer is applied to the high frequency and high voltage power supply, and the transformer oil tank is eliminated, and the volume of transformer system is further reduced.

China has developed high voltage dc power supply for the electrostatic dusting, the city electricity is commuted to dc, adopting full bridge zero current switch series resonant inverter circuit dc voltage inverter for high-frequency voltage, then the high-frequency transformer booster, the rectifier dc high voltage. Under the resistance load condition, the output dc voltage reaches 55kV, the current reaches 15mA, and the working frequency is 25.6khz.

8. Electric filter
When the traditional AC - DC converter put into operation, a large amount of harmonic current will be injected to the grid, which would cause harmonic loss and interference, also, the phenomenon of the device network side deterioration of power factor, the so-called "power pollution", for example, uncontrolled rectifier with capacitor filter, the network side harmonic content can reach three times (70 ~ 80) %, net side power factor is only 0.5 ~ 0.6.

Active power filter (APF) is a new kind of power electronic device which can suppress harmonics dynamically. It can overcome the deficiency of traditional LC filter, and it is a very promising harmonic suppression method. The filter consists of a bridge switching power converter and a specific control circuit. The difference between the traditional switching power supply is :(l) not only feedback of output voltage, but also the input average current; (2) the current loop reference signal is the product of the voltage loop error signal and the full wave rectifier voltage sampling signal.

9. Power supply system
Distributed power supply system using low power module and mass control integrated circuit as the basic components, using the latest theory and technological achievements, modular, intelligent high-power power supply, so that the combined closely with the heavy current and weak current, reduce the development of the high power components, high power device (centralized) pressure, improve production efficiency.

Distributed power supply mode has the advantages of energy saving, reliability, efficiency, economy and maintenance. It has been gradually adopted by large computer, communication equipment, aerospace, industrial control and other systems. It is also the ideal power supply mode for the low voltage power supply (3.3V) of the ultra-high speed integrated circuit. In high power situation, such as electroplating, electrolytic power supply, electric locomotive traction power, medium frequency induction heating power supply, motor power supply and other fields also have broad application prospect.

Power modules design

Power supply design, even in the design of the most common power supply, there are a series of problems in the design of dc to dc switch converter, especially in high-power supply design. In addition to functional considerations, the engineer must ensure the robustness of the design to meet the cost objectives and thermal performance and space limitations, while ensuring the design progress. In addition, due to the consideration of product specification and system performance, EMI must be low enough. However, the electromagnetic interference level of the power supply is the most difficult project to accurately predict. Some people even think it's impossible, and the best designers can do is take full consideration in the design, especially in the layout.

Here the discussed principles is applicable to a wide range of power supply design, but we only focus on the dc to dc converter in this article, because it is of very wide application, almost every hardware engineer will come into contact with the related work, maybe it is necessary to design a power converter for some time. Here we will consider two common tradeoffs associated with low emi design. Thermal performance, electromagnetic interference and scheme size related to PCB layout and EMI. We will use a simple step-down converter as an example, as shown in figure 1.
This is a normal step-down converter.

In the frequency domain, the radiation and conduction electromagnetic interference are measured. This is the Fourier series expansion of known waveforms. In this paper, we focus on the performance of radiation electromagnetic interference. In the synchronous buck converter, the main switch of electromagnetic interference wave is generated by the Q1 and Q2, namely each field effect tube in their respective conduction cycle from the drain to source current di/dt. Current waveform is shown in figure 2 (Q and Q2on) is not very trapezoidal rule, but we are the operation of the degrees of freedom is bigger, because the transition of conductor current relatively slow, so can be applied to Henry Ott classic "noise reduction technology in electronic systems," in the formula 1. We found that for a similar waveform, its rise and fall time directly affect the harmonic amplitude or the Fourier coefficient (In).
Figure 2: Q1 and Q2 waves

In=2IdSin(nπd)/nπd ×Sin(nπtr/T)/nπtr/T (1)

Where, n is the harmonic order, T is the period, I is the peak current intensity of the waveform, d is the duty ratio, and tr is the minimum value of tr or tf.

In practical application, it is very possible to encounter odd and even harmonic emission simultaneously. If only the odd harmonics are produced, the waveform's duty ratio must be exactly 50%. In fact, there is very little accuracy in the actual situation.

The electromagnetic interference amplitude of the harmonic series is affected by the pass - off of Q1 and Q2. It can be seen clearly when measuring the rise time tr and down time tf of the drain voltage VDS, or the current rising rate di/dt through Q1 and Q2. This also means that we can simply reduce the level of emi by slowing down the velocity of Q1 or Q2. It is true that the extended switching time does have a great effect on the harmonics of the frequency over f=1/ PI tr. However, there must be a compromise between increasing heat dissipation and reducing wastage. Still, controlling these parameters is a good way to balance electromagnetic interference and thermal performance. Concrete by adding a small value resistors (usually less than 5 Ω) implementation, the resistance with the Q1 and Q2 grid series can control the tr and tf, you can also give grid resistance in series a diode "off" to independent control transition time tr or tf (see figure 3). This is an iterative process that even the most experienced power designers use. Our ultimate goal is to reduce the speed of the transistor by slowing the transistor to an acceptable level, while ensuring that its temperature is low enough to ensure stability.
Figure 3: Use an associated diode to control the transition time.

The physical circuit area of the switching node is also very important for controlling electromagnetic interference. Usually, the designer wants to be as tight as possible because of the PCB area, but many designers do not know which part of the layout has the greatest impact on electromagnetic interference. Back to the previous example of voltage regulator, there are two loop nodes in this example (as shown in figure 4 and figure 5), and their size will directly affect the level of electromagnetic interference.
Figure 4: Step-down voltage stabilizer model 1
Figure 5: Step-down voltage stabilizer model 2

Ott's formula for the level of electromagnetic interference of different modes (2) indicates the direct linear effect of the loop area on the electromagnetic interference level of the circuit.

E=263×10-16(f2AI)(1/r) (2)

The radiation field is proportional to the following parameters: the harmonic frequencies involved (f, unit Hz), loop area (A, unit m2), current (I), and measurement distance (r, unit m).

This concept can be extended to all applications using trapezoidal waveform for circuit design, but this paper only discusses power design. Refer to the communication model in figure 4, study the loop current flows: starting point for the input capacitor, and then in Q1 conduction flow during Q1, again through L1 into the output capacitor, finally returned to the input capacitors.

The second circuit is formed when Q1 is switched off and Q2 is connected. The energy stored in L1 flows through the output capacitor and Q2, as shown in figure 5. These loop area control is very important to reduce the electromagnetic interference, and the layout of the device should be considered in advance when PCB routing wiring. Of course, there are practical limitations to how small the loop area can be.

It can be seen from formula 2 that reducing the circuit area of the switching node can effectively reduce the electromagnetic interference level. If the area of the loop is reduced to three times, the electromagnetic interference will decrease by 9.5dB, and if it is reduced to 10 times, it will be reduced by 20 dB. At the time of design, it is better to start from minimizing the loop area of the two loop nodes shown in FIG. 4 and FIG. 5, carefully considering the layout of the device, and paying attention to the copper wire connection problem. Try to avoid both sides of PCB at the same time, because the hole will heighten the inductance, which will lead to other problems.

Here is a design example of an off-line switch with an integrated ballast: the designer wants to reduce the electromagnetic interference in the final power level. I simply moved the hf output capacitor closer to the output level, and the loop area was about half the size of the original, and the electromagnetic interference decreased by about 6dB. The designer obviously didn't know much about it. He called the capacitor "magic hat," and in fact we just reduced the area of the circuit.

There is also the point that the new improved circuit may be more problematic than it originally was. In other words, although prolonged transition time can reduce electromagnetic interference, the thermal effect caused by it becomes an important problem. One way to control emi is to replace the traditional dc to dc converter with a fully integrated power module. The power module is a switching regulator with a full set success rate transistor and inductance, which can be easily incorporated into the system design as a linear regulator. Module of the switch node similar loop area is far less than the size of the voltage regulator or controller, power module is not a new thing, it's already has a period of time, but until now, due to a series of problems, can not effective heat dissipation module, and once after installation will not be able to change.
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