DC Transformer and Its Application in Two-Stage Power Conversion

1. Introduction With

the rapid development of information technology, especially in the field of microprocessors, the number of integrated transistors in microprocessors has increased dramatically, as shown in Figure 1, which has put forward higher requirements on the power supply performance of distributed power systems. The development trend of voltage regulator modules (VRM), the core components in distributed power systems, is as follows: 1) The input bus voltage continues to increase. In the future, computer VRM will increase the input bus voltage to 48V, reduce bus loss, improve efficiency, and greatly reduce the size of the input filter.

Improve the transient response speed of voltage regulation. 2) The output voltage is getting lower and lower, and the output current is getting larger and larger, which meets the increasing power supply capacity of computer chips, and the low steady-state operating voltage can increase the speed of microprocessors. 3) The load change rate is getting higher and higher, requiring VRM to have better transient response performance. Figure 2 is the development trend of the working voltage and current of Intel's CPU. The load conversion rate will be as high as 150A/us in the near future [1,2,3].

How to ensure the high reliability of the power supply, how to further improve the power density of the converter, and how to achieve high efficiency at high frequency, and realize a high-quality power supply system with excellent performance such as low voltage, large current, fast dynamic response speed, and high stability output are the key issues of current research. In recent years, scholars led by Fred.C.Lee have proposed the concept of "DC/DC Transformer" and adopted a two-stage power conversion structure in VRM [3].

This article elaborates on the basic concept of DC transformer, summarizes the basic circuit structure of DC transformer, and systematically summarizes the application of DC transformer in three different two-stage power conversion occasions.

2. Basic concept of DC transformer

2.1 Background of the concept of DC transformer

In order to further improve the computing speed of microprocessors, the operating voltage of the next generation of computer microprocessors will drop below 1.0V, while the output power continues to increase. In order to reduce bus losses, computer VRM will increase the input bus voltage to 48V. The high frequency of VRM can greatly reduce the volume of output filter capacitors and filter inductors, improve power density, and reduce costs. However, it is difficult for traditional single-stage 48V VRM converters to achieve high frequency while maintaining high efficiency, and the switching frequency is only about 200-300KHz. The relatively low switching frequency requires larger output filter capacitors and filter inductors for VRM, which not only increases the volume and cost of VRM, but also makes it difficult to integrate into the computer microprocessor. Generally speaking, the output filter capacitor is one of the most expensive components of VRM. To this end, scholars led by Fred.C.Lee from the Virginia Power Electronics Center in the United States proposed a two-stage 48V VRM[3,4], which cascaded the non-isolated voltage regulation module and the DC transformer, greatly improving the switching frequency of the VRM. The DC transformer has a simple circuit structure and works at a constant duty cycle. It plays the role of isolation and voltage reduction. It uses the transformer leakage inductance to achieve energy transmission, does not require an output filter inductor, and achieves soft switching of all switch tubes, which is highly efficient.

2.2 Definition and function of DC transformer

There are two basic types of DC converters[6]: DC/DC converters with output voltage regulation and DC transformers with output voltage regulated according to input. DC transformers are similar to AC transformers, converting one DC voltage into another or more DC voltages; they use high-frequency chopping-transformer isolation-high-frequency rectification to achieve the conversion of one DC voltage into another or more DC voltages proportional to it, which can be used in power transmission and voltage detection.

2.3 Basic requirements of an ideal DC transformer

The basic requirements of an ideal DC transformer are:

① Realize electrical isolation of input and output voltages and proportional relationship between input and output, and realize multi-channel output;

② Use transformer leakage inductance for energy transmission [3,4], no energy consumption, conversion efficiency of 1, and high power density;

③ No output filter inductance is required, which can greatly reduce the volume and weight of the output filter, with good dynamic performance and fast transient response speed;

④ The system bandwidth is wide and can transmit voltage without distortion;

⑤ Adopt open-loop control, the control circuit is simple, and soft switching is easy to implement, which can further increase the switching frequency;

⑥ High reliability and low electromagnetic interference to power supply and electrical equipment.

2. 4 Types of DC transformers

According to the perspective of converter energy transmission capacity, DC transformers can be divided into unidirectional DC transformers and bidirectional DC transformers; in addition, a combined DC transformer can be formed by combining DC transformers in parallel and series.

2. 5 Basic circuit structure of DC transformer

Figure 3 shows the basic circuit structure of DC transformer, where Lr is the transformer leakage inductance (or a small amount of series inductance). The primary high-frequency inverter circuit can be push-pull, half-bridge, full-bridge, push-pull forward, dual-tube forward, active clamp forward, resonant reset forward and asymmetric half-bridge circuit topologies; the secondary rectification and filtering circuit, as shown in Figure 4, can be a half-wave rectification, full-wave rectification, full-bridge rectification and push-pull forward rectification circuit without output filter inductance. The rectifier diode can be replaced with a synchronous rectifier tube to reduce the conduction loss. Replacing the secondary rectifier diode with a bidirectional switch tube can form a bidirectional DC transformer. Figure 5 is a bidirectional half-bridge DC transformer. Flyback, dual-tube flyback and forward-flyback and dual-tube forward-flyback circuits Because the transformer plays the dual role of inductor and transformer, the transformer needs to store energy and cannot directly transmit energy, so it is not suitable for use as a DC transformer. The circuit structure that meets the basic requirements of an ideal DC transformer is:

① Lr is as small as possible; the smaller Lr is, the smaller the line voltage drop is, and the more it can ensure the proportional relationship between the input and output of the DC transformer.

② The DC transformer does not contain large energy storage elements; the small energy storage element is the condition for ensuring the bandwidth, which requires the system duty cycle to be as close to 1 as possible and the system filter element to be small.

③ Achieve zero voltage switching; achieving zero voltage switching helps to improve conversion efficiency. The larger the leakage inductance Lr, the easier it is to achieve zero voltage switching of the switch tube. The parallel capacitor of the switch tube is conducive to the zero voltage shutdown of the switch tube, but at the same time it makes zero voltage switching difficult [3,6].

[page]3. Application of DC transformers in two-stage power conversion

3.1 Application of DC transformers in VRM

The two-stage VRM proposed abroad can be divided into two categories, as shown in Figures 6 and 7 [3]. In Figure 6, the front-stage DC transformer adopts open-loop control and works at a constant duty cycle, playing the role of isolation and voltage transformation, and the back stage is used to achieve output voltage regulation. Figure 7 is the opposite. Since the feedback circuit of the structure of Figure 6 does not require isolation, the VRM transient response is faster, so the two-stage VRM mostly adopts the structure of Figure 6. The DC transformer works at an equivalent duty cycle close to 100%, playing the role of isolation and voltage transformation. The leakage inductance of the transformer is used to achieve energy transmission, and the output does not require a filter inductor, and the conversion efficiency is high; open-loop control is adopted, and the control circuit is simple; it is easy to implement soft switching, which can further increase the switching frequency and power density, and at the same time greatly reduce the size of the filter, improve the dynamic performance of the system, and is suitable for future high-efficiency and high-power density VRMs.

文献[3]提出了一种两级结构的48V VRM，电路由有源箝位正激直流变压器和多相交错并联的同步整流BUCK变换器级联组成，如图8所示。BUCK电路的工作频率为1MHz, 大大减小了输出滤波器的体积，提高了VRM的功率密度和瞬态响应速度。有源箝位正激直流变压器电路结构简单，去除了传统有源箝位正激变换器的输出电感和续流二极管，由于输出电容的箝位，副边整流管上不存在电压尖峰，可以选择电压定额较低的整流管，减小了通态损耗。利用变压器磁化电流实现开关管的零电压开关，同时减小输出滤波电容，和漏感发生谐振，实现零电流关断。满载时，这种两级结构的VRM效率高达96.5%。

文献[5]提出了一种由多相交错并联的同步整流BUCK变换器和推挽直流变压器级联构成的24V VRM，如图9所示。推挽直流变压器恒占空比工作，开关管的占空比为50%。并将这种两级结构的VRM和单级VRM比较，在相同的开关频率下，两级结构的VRM大大提高了动态响应速度和负载变化率，通态损耗小，变换效率高。

3.2 Application of DC transformer in cascaded bidirectional DC/DC converter

In some bidirectional DC/DC converter (BDC) applications that require both voltage regulation and electrical isolation, a non-isolated bidirectional DC converter and a bidirectional DC transformer cascade structure can be used to achieve bidirectional energy transmission, which is a novel BDC solution. Reference [6] uses a Buck/

Boost BDC with voltage regulation function and a bidirectional push-pull forward DC transformer with isolation transformer function to form a two-stage BDC, as shown in Figure 10. When energy flows from V1 to V2, switch tubes S3 and S4 work for synchronous rectification; when energy flows from V2 to V1, switch tubes S1 and S2 work for synchronous rectification. The main switch tubes of the cascaded BDC all achieve ZVS, with high conversion efficiency. In the cascaded BDC, since the duty cycle of the DCT stage is close to 1, the inertia is small, and the bandwidth is wide, it does not affect the control model of the cascaded BDC. The cascaded bidirectional DC/DC converter is a new type of BDC structure. The two circuits of the cascaded BDC can be optimized and designed separately, with high power density and suitable for applications with large transformation ratio.

3.3 Application of DC Transformer in Aeronautical Static Inverter

Aeronautical Static Inverter (ASI) is the secondary power supply of aircraft power system. With the improvement of aircraft combat performance and the continuous increase of onboard electrical equipment, higher requirements are put forward for the power supply quality and reliability of aircraft power system. The current medium and high power three-phase ASI generally adopts a two-stage structure. In the case of a small input voltage variation range, a high-frequency isolated DC transformer and a three-phase inverter are cascaded. The structure is simple and conducive to modular design, achieving high power density, conversion efficiency, high reliability and high power quality. The front stage adopts a DC transformer to play the role of isolation and voltage transformation, providing input voltage for the rear stage inverter. The rear stage adopts a single-phase or three-phase inverter for voltage closed-loop control. The inverter has a voltage stabilization function, low output voltage distortion, fast dynamic response speed, and greatly improves the performance of ASI.

Reference [7] proposed a dual forward high-voltage DC transformer as shown in Figure 11. The voltage stress of the switch tube is low and there is no danger of bridge arm direct conduction, so the reliability is high. It works at an equivalent duty cycle close to 100%, and the ZVS switching of the switch tube is realized by using the leakage inductance of the transformer. Moreover, since there is no output filter inductor, the voltage spike of the secondary side rectifier diode is basically eliminated through the clamping effect of the filter capacitor, and the conversion efficiency is high.

4. Conclusion

This paper elaborates on the background, definition, basic structure of the DC transformer and other basic concepts. The DC transformer uses the leakage inductance energy of the transformer to achieve energy transmission, does not require output filter inductance, has a simple circuit structure, is easy to implement soft switching, uses open-loop control, works at a constant duty cycle, plays the role of voltage transformation and isolation, and has high conversion efficiency. It also summarizes and analyzes the application of DC transformers in three different two-stage power conversion occasions: two-stage VRM, cascaded bidirectional DC/DC converter and aviation static converter.


References

[1] Xunwei Zhou, Xingzhu Zhang, Jiangang Liu, Pit-Leong Wong, Jiabin Chen, Ho-Pu Wu, Amoroso. L., Fred C. Lee, Chen, DY "Investigation of candidate VRM topologies for future microprocessors," IEEE, APEC, 1998, pp.145-150;
[2] Laszlo Balogh, "A new cascaded topology optimized for efficient DC/DC conversion with large step-down ratios," Intel Symposium 2000;
[3] Yuancheng Ren, Ming Xu, Kaiwei Yao and Fred C. Lee, "Two-Stage 48V Power Pod Exploration for 64-Bit Microprocessor," IEEE, APEC, 2003, pp.426-431;
[4] Yuancheng Ren and Fred C. Lee, "Energy transfer by leakage inductor," Invention of disclosure Virginia Tech '01;
[5] Alou, P., Cobos, JA, Prieto, R., Garcia, O., Uceda, J, “A two stage voltage regulator module with fast transient response capability,” IEEE, PESC, 2003, pp.138-143;
[6] Zhang Fanghua, Research on bidirectional DC-DC converter, PhD dissertation, Nanjing University of Aeronautics and Astronautics, June 2004;
[7] Dual-channel two-switch forward converter topology, Chinese invention patent application, patent application number: 200310106252.4, Nanjing University of Aeronautics and Astronautics, November 2003.