Overview of the development of high-frequency link inverter technology

1 Introduction

With the continuous development of high-frequency link inverter technology, its application scope is becoming increasingly wide. First, in the fields of telecommunications, aerospace, military, etc., 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, hybrid electric vehicle drive systems using batteries, solar cells, etc. as energy sources are becoming increasingly research hotspots, and efficiency and volume are its primary considerations; in addition, in the construction industry, vibrating rods are often used for uniform mixing and pouring of concrete, which also requires vibrating rod power supply devices to be small in size, light in weight, safe to use and highly reliable; as well as the growing rise and widespread application of UPS technology... Considering that the safety and matching issues between the above various power supply devices and loads must be resolved, an isolation transformer is often required. In view of the above requirements, it is necessary to study the inverter circuit topology with an isolation transformer. It is under such circumstances that high-frequency link inverter technology has flourished.

The so-called high-frequency link inverter technology is to use high-frequency pulse transformers to replace low-frequency transformers to transmit energy and realize electrical isolation between the primary and secondary power supplies of the converter. From different perspectives, high-frequency link inverters can be divided into different forms. According to the number of load phases, they can be divided into single-phase and three-phase; according to the direction of power flow, they can be divided into two forms: bidirectional and unidirectional; according to the circuit working mechanism, they can be divided into two types: PWM mode and resonance mode; according to the type of power converter, they can be divided into voltage source (Voltage mode or Buck mode) and current source (Current mode or Buck-boost mode); according to the circuit topology, they can be divided into AC/AC conversion type, DC/DC conversion type (DC/HFAC/DC/LFAC) and cycloconverter type. The following discusses the last classification method separately.

2 Classification of high frequency link inverter technology

2.1AC/AC conversion type

2.1.1 Power frequency transformer isolation type

In 1973, Bedford first proposed the idea of ​​HFlink converter [1], which was then further developed by Gyugui and Pelly. As shown in Figure 1, both the input and output sides are isolated by power frequency transformers, and the LC parallel resonant network provides natural switching for the cycloconverter.

Figure 1 Power frequency transformer isolation type

Figure 2 High frequency transformer isolation type

Phase point, can realize AC/AC or DC/AC function, and power can flow in both directions, and the power factor can be adjusted arbitrarily. This conversion type has the following main disadvantages:

1) The power frequency transformer is large and heavy;

2) Having audio noise;

3) When input voltage and load fluctuate, the system responds slowly.

2.1.2 High frequency transformer isolation type

Sood and Lipo experimentally verified the feasibility of using bidirectional GTO in a resonant converter to realize a high-frequency link power distribution system [2], as shown in Figure 2. The main advantages of this conversion type are

1) Adopt high-frequency transformer, which is small in size and light in weight;

2) Resonant soft switching helps reduce switching losses and improve efficiency.

The main disadvantage is

1) The switching device has high current and voltage resistance;

2) Using bidirectional switches, the number of switches is large and the cost is high;

3) The PDM control method requires a strict synchronization relationship.

2.2DC/DC conversion type

This type of high-frequency link inverter is currently the most widely used unidirectional power flow voltage source high-frequency link inverter solution [3][4][5][6], and its classic circuit is shown in Figure 3. This topology inserts a DC/DC converter between the DC side and the inverter, and uses a high-frequency transformer to achieve voltage regulation and electrical isolation. Obviously, it has a three-stage power conversion process: DC/HFAC/DC/LFAC. The main advantages of this conversion type are

1) All switches are unidirectional;

2) The control of DC/DC part and DC/AC part is relatively independent

The two parts are relatively simple to coordinate and basically do not require synchronization.

The main disadvantage is

1) Power flows in one direction;

2) Large conduction loss;

3) Reduced reliability due to more power levels.

2.2.1 Single-ended forward high-frequency link inverter

As shown in Figure 4, the front-end part consists of a DC/DC forward circuit and a magnetic reset circuit, and uses PWM control technology to achieve voltage regulation. The back-end part consists of an absorption circuit, an LC resonant circuit and a single-phase inverter, and uses PDM control technology to achieve ZVS switching conditions in order to reduce switching losses [7].

2.2.2 Bridge-type high-frequency link inverter[8][9]

1) Control scheme 1 is shown in Figure 5, and its main circuit includes DC voltage - PWM high-frequency inverter - high-frequency transformer - fast recovery diode rectification - large capacitor 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 - (sinusoidally modulated high-frequency AC with a sinusoidal envelope) - fast recovery diode rectification - small capacitor filtering - industrial frequency voltage full-wave rectification - 50Hz square wave drive - 50Hz sine wave output.

As shown in Figure 5 and Figure 6, 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 are independent of each other, but the switching loss is large. In Scheme 2, the 50Hz square wave drive is equivalent to the ZVS condition, the switching loss is small, but strict synchronization is required. In addition, Scheme 2 can achieve three-phase

Figure 3 DC/DC conversion type

Figure 4 Single-ended forward high-frequency link inverter

Figure 5 Control scheme 1

Figure 6 Control scheme 2

Figure 7 Bidirectional cycloconversion high frequency link inverter

Figure 8 Hard switch PWM control method

Figure 9 LC resonance mode

Output load, but requires three sets of identical single-phase circuits, the structure is more complicated, and the phases need to be strictly synchronized.

2.3 Cyclic conversion type

It is a common solution for achieving bidirectional power transmission. This topology is generally composed of a cascaded inverter and a cycloconverter, as shown in Figure 7, which eliminates the DC link in the DC/DC conversion type high-frequency link inverter, so only two-stage power conversion (DC/HFAC/LFAC) is required, reducing the conduction loss of the inverter and improving system efficiency and reliability.

2.3.1 Hard Switching PWM Control Method

As shown in Figure 8 [10], its three-phase output uses a cycloconverter to convert high-frequency voltage into three-phase industrial frequency voltage, which is mainly used for small and medium-capacity UPS. Using a cycloconverter to directly convert high-frequency AC into industrial frequency AC has the following characteristics compared to DC conversion:

1) Fewer power conversion stages can improve efficiency;

2) The high-frequency part does not require a DC capacitor, so the overall system cost is low and the structure is simple;

3) Hard switching PWM control;

4) When the secondary side of the high-frequency transformer is open, a large voltage spike is generated due to the transformer leakage inductance energy storage without a discharge circuit.

In order to solve the problem of the circuit in Figure 8, in the literature [11], the switching control of the cycloconverter is synchronized with the primary high-frequency inverter and is performed under zero voltage conditions. At the same time, a pulse distribution method for outputting multiple voltage vectors within one sampling period is proposed. In the literature [12], the commutation overlap method is used to suppress the secondary voltage overshoot problem caused by the transformer leakage inductance, and the ZCS effect is obtained.

2.3.2LC resonance mode

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

1) Quasi-ZCS can be achieved without detecting the zero-crossing moment of the HFlink current;

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

3) The HFlink current amplitude varies with the output current.

2.3.3 DC link quasi-resonance mode

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

3 Development Trends

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

Figure 10 DC link quasi-resonant mode

Overview of the development of high-frequency link inverter technology

1) The DC/DC conversion type requires three-stage power conversion, with high conduction loss and complex control;

2) The frequency conversion type uses a large number of bidirectional switches, which increases circuit cost and loss;

3) There is a voltage overshoot problem during current commutation;

4) When the load is not purely resistive, it is difficult to continue the current;

5) Most circuits are designed for CVCF systems, and are relatively complex to control for VVVF systems;

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

1) Use turn-off devices and soft switching technology to increase the operating frequency, so as to achieve device miniaturization, low cost, no audio noise, high reliability and high efficiency;

2) Research new combined topology structures, analyze complex working processes and establish mathematical models to solve the shortcomings of current high-frequency link inverters;

3) Study various control methods, including PFM, SPWM, SVPWM, DPWM, PDM and difference frequency control.

4 Conclusion

The high-frequency link converter is a flexible and versatile topology, and its common features are compact circuit structure, high power density and efficiency, and fast response speed. In addition, the system can operate above 20kHz, has no audio noise, is relatively easy to filter, and has a power of more than kW level. Therefore, it has great practical value in both the field of constant voltage and constant frequency (CVCF) and the field of frequency and voltage modulation (VVVF), and it is an important topic for future research and development.