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EMI filters for switch mode power supplies

EMI filters for switch mode power supplies


1.1. EMI filters for switch mode power supplies

1.1.1. Electromagnetic compatibility (EMC)

E​nvironmental electromagnetic pollution has been a serious problem for electronic and electrical equipment for years. Any electrical or electronic device is a potential noise source to its environment. High-level electromagnetic disturbances may cause electrical and electronic devices and systems to malfunction in a common electromagnetic environment. A piece of equipment is considered electromagnetically compatible only if its effects are tolerable to all other equipment operating in its environment. To ensure this compatibility, electromagnetic compatibility (EMC) becomes an important engineering discipline. In order to achieve EMC, disturbances should be considered from two distinct points of view: electromagnetic emission (EME) and electromagnetic susceptibility (EMS) [1] – [5].

Although it first emerged as a serious issue in telecommunications, electromagnetic interference (EMI) problems are also found in other applications. With the rapid growth and spread of power semiconductors and power electronics systems, interference levels on power systems have increased significantly in intensity and frequency of occurrence. Fig. 1-1 shows the assorted high frequency (HF) disturbances by frequency content. The EMI noise frequency range of highest concern for power electronics systems is conducted radio frequency disturbance, gauged from 150 KHz to 30 MHz.

 Classification of electromagnetic disturbance by frequency

Over the last decade, power electronics systems have steadily moved toward integration, modularization, standardization and planarization to improve the electric and thermal performance with reduced size, weight and cost. These are driven by improved semiconductor components and circuit topologies, increased switching frequency, and advanced packaging and integration technologies [6] – [11]. Though higher switching frequency helps to reduce converter size, weight and cost to some extent, it also increases EMI concerns [12]. Advanced packaging and integration technologies [13][14] make it possible to squeeze more components into a small space, while more considerations regarding EMI are required for power electronics circuit design [15][16].

1.1.2. EMI for power electronics converters

T​he basic operation of electronic power processing is the switching function to control the flow of electromagnetic energy through the converters. The switching function, however, is also the major mechanism of electromagnetic noise generation, which implies that a power electronics converter is potentially a large noise source to its vicinity. So far, there has been a lot of research on reducing EMI noise during switching transitions. Quite a number of them have focused on improving circuit topologies by applying soft-switching technologies, and active or passive snubbers [17]-[22] to suppress the HF EMI noise. Recently there has been more research work concentrating on developing advanced semiconductor devices, such as SiC diode and cool-MOS, with improved switching characteristics. For example, because the SiC shottcky diode has almost no reverse recovery problem, the switching noise is greatly reduced even without using complicated circuit topologies [23][24]. However, the noise at switching frequency and its harmonics are inherent to the switching function, which cannot be alleviated by soft-switching techniques or advanced semiconductor devices. Therefore, research on modeling and improving the EMI characteristics of power electronics converters by using appropriate circuit layout and input filter design is necessary [25] – [31].

Schematic of EMI filterEMI filters are placed between the power line and the input of the converter to attenuate common mode (CM) and differential mode (DM) switching noise [32], as illustrated in a typical distributed power system (DPS) structure, shown in Fig. 1-2. The DPS front-end converter consists of a power factor correction (PFC) converter followed

by a DC/DC converter. The PFC converter converts the input AC voltage to 400 V DC output, while the DC/DC converter steps this 400 V DC down to 48 V DC to supply power to all the load converters [33][34]. To meet the EMI standard (EN55022 Class B), there is always an EMI filter in front of the PFC converter to prevent CM and DM noise propagating from the PFC side to the AC line. Fig. 1-3 shows a typical EMI filter structure for this application. Normally the EMI filter occupies about 15% – 20% of the overall system size. As the volume and profile of the front-end converter gets smaller, the physical size of the EMI filter also needs to be reduced.

1.1.3. Issues regarding conventional discrete EMI filters

Modeling, characterization, design and optimization of EMI filters are always challenging tasks for power electronics engineers. A lot of work has been done regarding EMI filter design and optimization [35] – [48]. Conventionally, EMI filters are implemented by using discrete components, which raises some issues. First, because of the existence of parasitics of the discrete components, such as the equivalent parallel capacitance (EPC) of the inductors and the equivalent series inductance (ESL) of the capacitors, the effective filter frequency range is normally below a few MHz. Some analysis shows clearly that the HF characteristic of EMI filters is mainly determined by parasitics. Second, the parasitics caused by the filter layout further impairs the filter performance at high frequencies. Hence, the design of the EMI filter layout requires extreme care and special expertise. Third, this type of EMI filter consists of a fairly large number of components, each of which involves different processing techniques. Some of them may require labor-intensive processing steps. These components are functionally and structurally separated. This requires excessive material and manufacturing time. Last, because the components of a discrete EMI filter vary in type, value, size and form factor, considerable space is taken by interconnections between components, which leads to inefficient utilization of space. In order to improve HF characteristics with a compact size and low profile, and to achieve structural, functional, processing and mechanical integration to reduce manufacturing time and cost, planar electromagnetic integration technology is proposed for EMI filter design.

1.2. Principle of electromagnetic passive integration

In most power electronics converters, the total size and profile of the system are largely determined by that of the passive components. Electromagnetic integration technology has been a topic of research to increase power density in the past few decades, which can be best described by first considering a simple bifilar spiral winding as shown in Fig. 1-4(a). This structure consists of two windings (A-C and B-D), separated by a dielectric material. This resultant structure has distributed inductance and capacitance, and is an electromagnetically integrated LC-resonant structure, for which equivalent circuit characteristics depend on the external connections (Fig. 1-4(a)). More complex integrated structures can be realized by adding more winding layers, which is illustrated with an integrated resonant transformer structure (L-L-C-T) in Fig. 1-4(b) and (c). the design of these structures requires a deliberate increase and modification of naturally existing structural impedances to realize a particular equivalent circuit function – for example the increase of the intra-winding capacitance to form the LC resonant structure. The classical term “parasitics” therefore no longer applies, and all the higher-order impedances are rather referred to as “structural impedances” [14].

1.3. Previous work on passive integration in Power Electronics There have been applications of passive integration technologies in power electronics, such as the original bifilar structure [66] – [69], the cascaded transmission lines structure [72], the planar spiral winding integrated LC structure [49] – [58], as well as the more advanced multi-cell [59] and stacking structures [60]. Among these integration structures, a lot of attention has been devoted to low-profile structures with high power density, such as the planar spiral winding integrated LC structure. These structures have been applied to resonant converters, PWM converters and other applications such as snubbers and low pass filters, as will be described shortly.

Spiral integrated LC structure with distributed

1.3.1. Applications in resonant converters (L-L-C-T)

The developed technologies for integrated passive components have mostly been implemented for resonant converter applications [49]-[54]. The 500 W 1 MHz full-bridge LLC resonant converter shown in Fig. 1-5 is an example [49], in which the LC resonant tank and the high frequency power transformer have been integrated into a planar module, as illustrated in Fig. 1-6. It can achieve about 90% efficiency at 1 MHz switching frequency, and the power density is about 30W/cm3.

1.3.2. Applications in PWM converters (Passive IPEM)Schematic of 1MHz LLC resonant

The developed passive integration technologies can also be applied to non-resonant PWM converters [55] – [58]. In this application, the planar passive integration and planar

Applications in PWM converters

integrated magnetics technology were combined to integrate all of the high frequency passive components in a 1 kW asymmetrical half-bridge DC/DC converter (AHBC) for a DPS system. The circuit diagram of AHBC is shown in Fig. 1-7 (a). The current-doubler inductors and the isolation transformer are not magnetically coupled, but can be integrated into two separate structures by splitting the isolation transformer, and utilizing the equivalent magnetizing inductances to realize the current doubler inductors (reflected to the secondary side). These two magnetic structures can in turn be integrated into one physical structure through integrated magnetics technology. A cross-section, reluctance diagram and exploded view of this resultant first generation spiral integrated passive module are shown in Fig. 1-7(b-c). The I-core shares the flux for both of the integrated modules LLCT1 and LLCT2, and the AC flux is partially cancelled in the shared I-core as shown in Fig. 1-7(b). The relatively large DC decoupling capacitor is integrated into both the primary windings of the structure, using very high permittivity (εr > 12000) ceramics (Fig. 1-7(c)). This integrated module has a much smaller size and profile, much better thermal characteristics and a comparable electrical performance as the discrete baseline.

1.3.3. Other applications

Other applications of planar passive integration technology in power electronics include output filters and RC snubbers [61] – [65]. S. J. Marais studied the lowpass filter configuration of integrated LC structures in [61] and [62]. Another example given in [65] is the planar passive integration technology used for output filter and RC snubber integration. The output stage of the converter is shown in Fig. 1-8. The 3D assembly of the integrated output filter, RC snubber and the output diode bridge are shown in Fig. 1-9. Promising experimental results showed that voltage stress and parasitic ringing can be greatly reduced, as illustrated in the compared voltage waveforms shown in Fig. 1-10 and Fig. 1-11.

1.4. Aim of this study

1.4.1. Electromagnetic integration of EMI filters

While the electromagnetic passive integration technology has been investigated and applied in power electronics for years, EMI filters are still implemented using discrete components. To bridge this gap, this study aims to integrate EMI filters using the

Electromagnetic integration of EMI filters

available passive integration technologies. As will be discussed later, with these established technologies, the electromagnetic characteristics obtained from the aforementioned integrated power passive modules are not suitable for the integration of EMI filters, because these modules are optimized for storing and processing electromagnetic energy at switching frequency. On the other hand, EMI filters have to attenuate electromagnetic energy at switching frequency. Thus, in this research work, special integration technologies, modeling and design methodologies will be developed for the integrated EMI filters to achieve smaller size and profile and better highfrequency characteristics.

1.4.2. Research work covered in this thesis

The rest of this dissertation will be organized as follows:

Chapter 2 will give a brief analysis of the issues of discrete passive components and discrete EMI filters. The effects of different parasitic parameters on filters’ high frequency characteristics will be discussed.

Chapter 3 will focus on the integrated EMI filter implementation. Implementation requirements are categorized, and the appropriate integration technologies are explored.

In Chapter 4, modeling of fundamental integrated LC structures will be discussed since HF modeling is essential in order to understand and design integrated EMI filters. An improved frequency-domain model using multi-conductor lossy transmission-line theory is studied. To calculate the impedance and admittance matrices, detailed electromagnetic modeling will be provided.

In Chapter 5, the experimental assessment of two integrated EMI filter prototypes shows the achievements of smaller size and lower profile, but the high frequency performance yet to be improved.

Chapter 6 will introduce the developed structure winding capacitance cancellation method, followed by the detailed modeling and parametric study. Applying this technology, an improved integrated EMI filter structure will be presented and verified experimentally.

In Chapter 7, frequency domain modeling of RF EMI filter will be discussed, and a correlation between the calculated and measured results will be shown. Based on this study, a new integrated EMI filter structure is proposed combining all the developed technologies. It will be shown that it has the smallest size and best performance of all available technologies.


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