Overview of the development of the hottest high fr

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Summary of the development of high-frequency link inverter technology

Abstract: This paper summarizes and classifies the high-frequency link inverter technology, analyzes the characteristics of their respective circuit topologies, and puts forward its development trend in the future. Keywords: high frequency link; Inverter technology; Development trend

Figure 1 power frequency transformer isolation type

1 introduction

with the continuous development of high-frequency link inverter technology, its application range is increasingly extensive. First of all, in the fields of telecommunications, aerospace, military and so on, power supply devices are often required to be light in weight, small in size, high in power density and high in reliability; Secondly, with the continuous consumption of mineral energy such as oil, coal and natural gas and environmental pollution, the hybrid electric vehicle drive system using batteries, solar cells and other energy sources has increasingly become a research hotspot, and efficiency and volume are its primary considerations; In addition, in the construction industry, vibrating rods are often used for evenly mixing and pouring concrete, which also requires that the vibrating rod power supply device is small, light, safe and reliable; And the rising and wide application of UPS Technology. Considering the combination with other cutting-edge research units to solve the safety and matching problems between the above power supply devices and loads, it is often necessary to add isolation transformers. In view of the above requirements, it is necessary to study the inverter circuit topology with isolation transformer. High frequency link inverter technology is booming in this case

the so-called high-frequency link inverter technology is to use high-frequency pulse transformer to replace low-frequency transformer to transmit energy, and realize the electrical isolation between the primary and secondary power supplies of the converter. From different perspectives, high-frequency link inverter can be divided into different forms. According to the number of load phases, it can be divided into single-phase and three-phase; According to the direction of power flow, it can be divided into two forms: bidirectional and unidirectional; According to the working mechanism of the circuit, it can be divided into PWM mode and resonance mode; According to the type of power converter, it can be divided into voltage source (voltagemode or buckmode) and current source (currentmode or buck  boostmode); According to the circuit topology, it can be divided into ac/ac conversion type, dc/dc conversion type (dc/hfac/dc/lfac) and cycloconverter type. The last division method is discussed below

2 classification of high frequency link inverter technology

2.1ac/ac conversion type

2.1.1 power frequency transformer isolation type

1973, Bedford first proposed the idea of high frequency link converter [1], and then gyugui and Pelly made in-depth development. As shown in Figure 1, power frequency transformers are used to isolate the input and output sides, and the LC parallel resonant network provides natural commutation for the cycloconverter


overview of the development of high-frequency link inverter technology

figure 4 single ended forward high-frequency link inverter

Figure 5 control scheme 1

Figure 6 control scheme 2

Figure 2 high-frequency transformer isolated

phase points, which can realize ac/ac or dc/ac functions, and the power can flow in both directions, And arbitrary adjustment of power factor. This type of transformation has the following main disadvantages:

1) it adopts power frequency transformer, which is large and bulky

2) it is unnecessary to adjust the audio noise of the machine

3) when the input voltage and load fluctuate, the system response speed is slow

2.1.2 isolation type of high-frequency transformer

sood and lipo have verified the feasibility of using bidirectional GTO in resonant converter to realize high-frequency chain power distribution system [2], as shown in Figure 2. The main advantages of this type of transformation are

1) the use of high-frequency transformers, small size, light weight

2) resonant soft switching is beneficial to reduce switching loss and improve efficiency

the main disadvantages are

1) the current resistance and voltage resistance of switching devices are large

2) adopt two-way switch, with a large number of switches and high cost

3) adopting PDM control mode requires strict synchronization relationship

2.2dc/dc conversion type

this type of high-frequency link inverter is the most widely used one-way hot innovation at present, and Yongqi smells business opportunities from it. The power flow voltage source high-frequency link inverter scheme [3][4][5][6], its classic circuit is shown in Figure 3. In this topology, a primary dc/dc converter is inserted between the DC side and the inverter, and a high-frequency transformer is used to realize voltage regulation and electrical isolation. Obviously, it has a three-stage power conversion process: dc/hfac/dc/lfac. The main advantages of this transformation type are

1) all switches are unidirectional

2) the control of dc/dc part and dc/ac part of 309 insulation pipe is relatively independent, and the coordination of the two parts is relatively simple, and basically there is no need for synchronization

the main disadvantages are

1) one-way flow of power

2) large on state loss

3) due to more power levels, the reliability is reduced

2.2.1 single ended forward high-frequency link inverter

as shown in Figure 4, the front part is composed of dc/dc forward circuit and magnetic reset circuit, which adopts PWM control technology to realize voltage regulation, and the rear part is composed of absorption circuit, LC resonance circuit and single-phase inverter, which adopts PDM control technology to realize ZVS switching conditions, so as to reduce switching loss [7]

2.2.2 bridge high-frequency link inverter [8][9]

1) control scheme 1 is shown in Figure 5. Its main circuit includes DC voltage PWM high-frequency inverter high-frequency transformer fast recovery diode rectification large capacitance filtering SPWM inverter single-phase 50Hz sine wave output

2) control scheme 2 is shown in Figure 6. Its main circuit includes DC voltage SPWM inverter high-frequency transformer (sinusoidal modulated high-frequency AC with sinusoidal envelope), fast recovery diode rectification, small capacitance filtering, power frequency voltage full wave rectification, 50Hz square wave driving, 50Hz sine wave output

it can be seen from figures 5 and 6 that the main circuit structures of the two control schemes are basically the same, but the control methods are different. In scheme 1, the front and rear circuits do not need to be synchronized and independent of each other, but the switching loss is large. In scheme 2, 50Hz square wave drive is equivalent to ZVS condition, with small switching loss, but strict synchronization is required. In addition, scheme 2 can realize three-phase

figure 3dc/dc conversion type

Figure 7 bidirectional cycle converter high-frequency link inverter

figure 8 hard switching PWM control mode

figure 9lc resonance mode

output load, but three sets of the same single-phase circuit are required, the structure is complex, and the phase needs to be strictly synchronized

2.3 cycle variable current mode

it is a common scheme to realize bidirectional power transmission at present. This topology is generally formed by cascading an inverter and a cycle converter, as shown in Figure 7, thus eliminating the DC link in the dc/dc conversion high-frequency link inverter. Therefore, only two-stage power conversion (dc/hfac/lfac) is required, which reduces the on state loss of the inverter and improves the efficiency and reliability of the system

2.3.1 the hard switch PWM control mode

is shown in Figure 8 [10], and its three-phase output adopts the form of cycle converter to convert the high-frequency voltage into three-phase power frequency voltage, which is mainly used for small and medium-sized ups. The cycle converter is used to directly convert high-frequency AC into power frequency AC. compared with DC conversion, it has the following characteristics:

1) the power conversion stages are less, which can improve efficiency

2) DC capacitor is not needed in the rear stage of high-frequency part, so the overall cost of the system is low and the structure is simple

3) hard switch PWM control

4) when the secondary side of high-frequency transformer is open circuit, there is a large voltage spike due to no discharge circuit of transformer leakage inductance energy storage

in order to solve the problems of the circuit in Figure 8, in reference [11], the switching control of the cycloconverter is synchronized with the primary side high-frequency inverter and carried out under the condition of zero voltage. At the same time, a pulse distribution method of outputting multiple voltage vectors in a sampling period is proposed. Reference [12] adopts commutation overlap method to suppress the secondary side voltage overshoot caused by transformer leakage inductance, and obtains ZCS effect

2.3.2lc resonance mode

the primary side of the high-frequency transformer adopts two power switches and LC series resonance mode, and the secondary side adopts the form of cycloconverter [13], as shown in Figure 9. The quasi zero current ZCS condition is used to reduce the switching loss, and the real-time feedback control method is used to make the output voltage sine wave. Its main characteristics are

1) quasi ZCS is realized without detecting the zero crossing time of hflink current

2) it is easy to realize real-time control of output voltage

3) hflink current amplitude changes with output current

2.3.3 DC link quasi resonant mode

the front part of the high-frequency transformer adopts the DC link quasi resonant inverter circuit (qrdcli for short), and the rear part adopts the form of cycloconverter [14], as shown in Figure 10. At the same time, the improved PDM control strategy and digital control method are also proposed. The system does not need buffer circuit and can work in four quadrants

3 development trend

since the 1980s, high-frequency link inverter technology has attracted great attention, and a large number of relevant literatures have been published. The existing high-frequency link inverter topology generally has the following characteristics:

Figure 10 quasi resonant mode of DC link


overview of the development of high-frequency link inverter technology

1) dc/dc conversion type requires three-stage power conversion, high on state loss and complex control

2) the cycloconverter uses a lot of bidirectional switches, which increases the circuit cost and loss

3) there is voltage overshoot during current commutation

4) when the load is not pure resistive, the freewheeling is difficult

5) most circuits are designed for CVCF system, and the control of VVVF system is relatively complex

in the single-phase high-frequency link inverter circuit, there have been some relatively mature schemes, but the three-phase high-frequency link inverter circuit is still very immature and needs further research. Generally speaking, it mainly involves three aspects:

1) using turn-off devices and soft switching technology to improve the working frequency, so as to achieve miniaturization, low cost, no audio noise, and high reliability and efficiency

2) study the new combined topology, analyze the complex working process and establish a mathematical model to solve the shortcomings of the current high-frequency link inverter

3) study various control modes, including PFM, SPWM, SVPWM, DPWM, PDM and differential frequency control

4 conclusion

high frequency link converter is a flexible topology. Its common characteristics are compact circuit structure, high power density and efficiency, and fast response speed. In addition, the system can work above 20kHz, without audio noise, filtering is relatively easy, and the power can reach more than kW level. Therefore, it has great practical value both in the field of constant voltage and constant frequency (CVCF) and in the field of frequency modulation and voltage regulation (VVVF). It is an important topic for further research and development in the future

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