Chapter 1 Integrated Power Electronics, an Introduction and the Associated Thermal Challenge

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Chapter 1 - Integrated Power Electronics, an Introduction 1
Chapter 1
Integrated Power Electronics, an
Introduction and the Associated
Thermal Challenge



1.1. Introduction........................................................................................................1
1.2. Developments....................................................................................................2
1.2.1. Integrated Power Electronic Modules (IPEMs).................................................3
1.2.2. Alternative Planar Integration............................................................................6
1.3. Challenges.........................................................................................................7
1.4. Problem Statement............................................................................................8
1.5. Nomenclature.....................................................................................................9
1.6. References.........................................................................................................9

1.1. Introduction

Power electronics refers to control and conversion of electrical power, by power semiconductor
devices wherein these devices operate as switches. The task of power electronics can also be
described as the control of the flow of power by shaping the utility supplied voltage by means of
semiconductor devices.

The four main forms of electric power conversion are:
• Rectification referring to conversion of AC voltage to DC voltage,
• DC-to-AC conversion,
• DC-to DC conversion and
• AC-to-AC conversion.

The market for, and the scope of power electronics applications have expanded in recent years
into numerous areas. Table 1.1 lists various applications of power electronics.










Chapter 1 - Integrated Power Electronics, an Introduction
2
Table 1.1 Power electronics applications [1]
Residential
Refrigeration and freezing
Space heating
Air conditioning
Cooking
Lighting
Electronics
Commercial
Heating, ventilation, and air conditioning
Central refrigeration
Lighting
Computers and office equipment
Uninterruptible power supplies
Elevators
Industrial
Pumps
Compressors
Blowers and fans
Machine tools
Arc furnaces, induction furnaces
Lighting
Industrial lasers
Induction heating
Welding
Transportation
Traction control of electric vehicles
Battery chargers for electric vehicles
Electric locomotives
Street cars, trolley buses
Subways
Automotive electronics including engine
control
Utility systems
High-voltage DC transmission
Static var generation
Supplemental energy sources (wind,
photovoltaic)
Energy storage systems
Induced draft fans and boiler feed water pumps
Aerospace
Space shuttle power supply systems
Aircraft power systems
Telecommunications
Battery chargers
Power supplies


The basic functions in electronic power conversion are:
• Active control of energy transfer by using semiconductor switches and switch assemblies
• Passive energy transfer and storage using transformers, inductors and capacitors
• Control and protection using signal processing circuits

1.2. Developments
Power electronics has the potential of impacting industrial competitiveness. This can be
manifested through the increased energy efficiency of equipment and processes using electrical
power, and through higher industrial productiveness and higher product quality. Industrial firms
are under constant pressure to produce power electronics products that are more powerful,
durable, smaller, lighter, and less costly to the consumer.

It is well recognised within the industrial sector that the performance of power electronics
systems were driven by improvements in semiconductor components in the 1990’s. In around the
same time period, the fundamental approach to power conversion has steadily moved towards
high frequency synthesis, resulting in huge improvements in converter performance, size, weight
and cost. However in some high frequency power conversion technologies, fundamental limits
are being reached that will probably not be overcome without radical change in the design and
implementation of power electronics systems [2].
Chapter 1 - Integrated Power Electronics, an Introduction 3

Recent innovations in power modules have been mostly pushed by semiconductor development
with the help of improved layout and packaging technologies. The power electronics module
development trends are focussed on increasing current and voltage levels, increasing
temperatures, enhancing reliability and functionality as well as reducing size, weight and cost [3].

While semiconductor devices are still one of the dominant barriers for future power system
development, devices do not currently pose the fundamental limitations to power conversion
technology. It is rather packaging, control, thermal management, and system integration issues
that are the dominant technology barriers currently limiting the rapid growth of power conversion
applications [3].

A common attempt at increasing the power density, which is defined as the power output of the
device per unit volume of the device, involves repositioning and reshaping the discrete
components in such a way that they fit better into each other which thus reduces the overall
converter volume. In most converters, typically only 55% of the available volume is utilized [4].
This method of reducing application volume may be effective in the short term, but it does not
render sustainable development in the long run as it still utilizes discrete non-standard
components. Development in this regard is also dampened due to restrictions caused by
electromagnetic influences the discrete components have on each other.

One of the larger obstacles to the development of more compact systems is the lack of
standardisation and the absence of a modular approach to power electronics systems [5]. With
system-orientated strategies, optimisation of the overall system performance, reliability, and cost
of power electronics systems can be approached by standardised power electronics systems and
packaging techniques in the form of integrated power electronic modules (IPEMs).

1.2.1. Integrated Power Electronic Modules (IPEMs)

The integration of power electronics systems via IPEMs present several advantages namely [2]:
• A modular approach reduces design and implementation time cycles, as well as simplifies
the integration process.
• The integration of functions in the form of IPEMs leads to improved usage of space,
which ultimately increases the power density and reduces the profile of power electronics
systems.
• The reduction of interconnections at the system level due to integration improves system
reliability.
• The reduction of structural packaging inductances leads to improved electrical
performance.

Various methods have been attempted to achieve integration of the passive power electronics
components.

Wire-bonded components were among the first integration approaches for production of more
compact modules. Refer to Figure 1.1 for an example of a wire-bonded integrated module.
Unfortunately, commercially available leading wire-bond modules have reliability problems
resulting in a lifetime of only a few years. Wire-bonds in power devices and modules are also
prone to resistance, noise and parasitic oscillations. Although the wire bonding technology has
seen many improvements, the approach still limits the possibilities of three-dimensional
integration, and has electromagnetic layout constraints.
Chapter 1 - Integrated Power Electronics, an Introduction
4


Figure 1.1 An integrated power switching stage converter using wire-bonding
Since then, several IPEM packaging technologies and high-density interconnect approaches have
been developed to eliminate wire-bonding [6]. These include technologies such as multi-chip-
modules (MCM), metal interconnect parallel plates structures (MIPPS), and solder bump
connections. With these advances the requirements needed for the construction of cost-efficient,
compact, and reliable IPEMs has not fully yet been met.

Another technology entitled “embedded power” have since also typically been developed for the
integration of active power electronic modules containing power chips as switching devices.
Thin-film power overlay technology for packaging power devices, where thin film deposition is
used followed by electroplating, for device interconnection are used. This process is typically
used for integration of active power electronic modules and is referred to as the embedded power
technique.

Figure 1.2 gives a broken down representation of an active power module used in a DC-DC
converter. A plate with a relatively high thermal conductivity called a heat spreader is used for
both thermal purposes, as well as for providing mechanical stability to the module as a whole.
Above the heat spreader direct bonded copper, comprising of a copper layer, ceramic and etched
copper traces, is mounted. Devices are soldered on top of etched copper traces while a separate
ceramic layer is used to confine the device.


Figure 1.2 Representation of an active planar power electronics module using embedded
power technology

Chapter 1 - Integrated Power Electronics, an Introduction 5
Most of the approaches mentioned up to this point have found more relevance to the integration
of active power electronic components. A large fraction of power electronics systems however is
occupied by passive energy transfer and storage components, which are responsible for
transformative, capacitive and inductive functions.

In order to further increase the power density and reduce the profile of power electronic devices,
there is a demand for the integration of passive components, into a single module. The
electromagnetic structures of any capacitor, C, or inductor, L, already include both types of
electromagnetic energy storage. Instead of attempting to produce pure components, it has been
realised that enhancing the capacitive aspect of inductors can lead to an integrated resonant
component. Planar integration of these functions is a popular approach being used and involves
layered structures. Figure 1.3 shows a typical configuration used to obtain a planar integrated L-
C module. Most topologies require some from of L-C resonance.
In this approach two spiral windings separated by the dielectric material forms a four-terminal
structure containing both distributed inductance (L) and capacitance (C). Through changing the
external interconnections to the structure, a number of different equivalent circuits can be created
without changing the structure itself. This results in a compact multi-function component.

Figure 1.3 Planar integrated L-C structure [7]
Another integrated passive component, such as the planar L-C-T module (T for transformative),
has been the subject of research and it has been proven to be viable at high frequencies. Through
improvements of the constructional technology, the integrated L-L-C-T structure’s power density
has been improved from 8.0 W/cm
3
to 14.6 W/cm
3
[8].
Figure 1.4 shows a representation of an integrated L-L-C-T planar structure while Figure 1.5
shows an approximate circuit model of such a set-up. The shown L-L-C-T structure consists of a
planar magnetic core containing three separate layers. Here L
s
[H] refers to leakage inductance
while L
m
[H] refers to magnetising inductance, and C
s
[F] refers to the equivalent resonance
capacitance.
The core shown is air-gapped to realise the required magnetising conductance. The leakage layer
is constructed from low permeability magnetic material – such as ferrite polymer composites.
Ferrites, due to their low characteristic electric conductance, are used as core material. Low
electric conductivity is required for magnetic materials operated at high frequencies [9].
Windings are created by directly metalising the ceramic. To create the winding pattern in the
conductor layer, electroplating, photo masking and chemical etching is used.

Chapter 1 - Integrated Power Electronics, an Introduction
6


Figure 1.4 Exploded view of a planar integrated L-L-C-T structure [8]

Figure 1.5 First order approximate lumped circuit model of a L-L-C-T integrated module [10]

1.2.2. Alternative Planar Integration
When considering Figure 1.4, it might be seen that not the entire surface of the winding planes are
encapsulated by ferrite, and that these planar metalised structures are protruding on both sides of
the structure. An alternative packaging and orientation approach was developed to utilise volume
even better than in the planar passive structures by eliminating the geometric restriction that
causes these protrusions. Refer to Figure 1.6 for a representation of an alternative proposed
physical L-C structure.

This structure has a copper-plated ceramic substrate enclosed in a planar ferrite magnetic core
with interconnections essentially in the third dimension by side-straps. As with the planar L-C
integrated structure, the function of the alternative L-C cell can be changed, either being a series
resonator or a parallel resonator, depending on the interconnection configuration which is used.

Chapter 1 - Integrated Power Electronics, an Introduction 7

Figure 1.6 Proposed structure for an integrated L-C reactive component [11]

1.3. Challenges

Electrical and thermal issues are by nature conflicting entities, which mean that improving
electrical performance by bringing components closer to each other, thermal performance is
deteriorated. As a result, for power electronics systems of compact design, the power dissipation
density, and thus the absolute temperature in the components increases. In addition, the surface
exposed to the surroundings becomes smaller, leading to a reduced ability to facilitate heat
exchange because the heat has to be dissipated in a smaller area.

For high power densities, good thermal management is needed to aid in overcoming such
problems. Thermal management can be defined as controlling the packaging issues that
determine and limit the amount of heat that can be removed from a heat source and conducted to
the environment. The primary function of the thermal management system is to ensure that the
maximum temperature of the different materials is never exceeded.

Currently, the conventional cooling strategy involves power electronic components being
mounted on a heat sink. Refer to Figure 1.7 for an example. Heat paths are thus created from the
source of heat inside the components to the heat sink to assist the cooling of the components.

For a typical bare assembly, the heat path contains three major thermal barriers:
• Adhesive/solder between the die and substrate
• Substrate
• Adhesive between substrate and heat sink

The interface between substrate and heat sink represents the main barrier along the thermal path.
For mechanical reasons, the substrate is bonded to the heat sink by an adhesive that is able to
compensate for a mismatch between the coefficients of thermal expansion of the two materials.

Chapter 1 - Integrated Power Electronics, an Introduction
8


Figure 1.7 Representation of conventional configuration used for cooling indicating major
thermal paths [2].
When referring to planar structural orientations mentioned earlier, the presence of metalic
layering aid in the transportation of heat via conduction and promote heat-spreading within
modules. Even though temperature distribution is aided by this, the absence of thermal corridors
towards lower temperature regions such as heat sinks, result in large temperature differences
within modules in lateral directions.

The bad thermal conductivity of commonly used insulation materials and ferrite materials
separating internal regions from heat sink surfaces, inhibit the flow of heat from internal regions,
where it is being generated, to module surfaces. The use of surface mounted heat-sinks on its
own may thus not be sufficient to improve heat transfer to the ambient. The thermal problem
becomes more acute when higher levels of integration are pursued.

1.4. Problem Statement
From studies conducted by Strydom et.al. [12,13], it is clear that the thermal limitations of the
structure have to be addressed in order to accommodate further increase in power density. In
order to satisfy future thermal demands associated with integrated power electronic devices, the
focus is starting to shift toward innovative design of the internal structure of power modules to
assist in heat-extraction, while maintaining high levels of electromagnetic performance and
efficiency.

Although the loss density in passive energy transfer and storage components such as capacitors,
inductors and transformers, is much lower in than found in power semiconductor devices, the
corresponding lower thermal conductivities of the ceramics used in these structures lead to
thermal limitations. Due to such relative low thermal conductivities, surface cooling as is
currently common in electronic cooling, would not be sufficient to support increased power and
heat-generation densities. In addition to power electronic integration, internal cooling system
integration as a possible solution to thermal problems needs to be considered.

Such integrated power electronics system requires advances in technologies, which depend upon
finding solutions to deal with the multi-disciplinary issues in materials, electromagnetic
compatibility and thermal management. In this investigation it was attempted to contribute to this
drive by exploring the possibilities available in reducing operating temperatures at a given power
Chapter 1 - Integrated Power Electronics, an Introduction 9
and heat-generation density, or visa versa in increasing the power and heat-generation density at a
specified maximum operating temperature.


1.5. Nomenclature
Symbol Unit Description
C F capacitance
C
s
F equivalent resonance capacitance
L H inductance
L
s
H leakage inductance
L
m
H magnetising inductance

1.6. References

[1] Mohan, N., Undeland, T.M., and Robbins, W.P., Power electronics: converters, applicaton
and design, 1st edition, Wiley, 1989
[2] Barbosa, P., Lee, F.C., Van Wyk, J.D., Boroyevich, D., Scott, E., Thole, K., Odendaal,
H., Liang, Z., Pang, Y., Sewell, E., Chen, J., and Yang, B., An overview of IPEM-based
modular implementation for distributed power systems, Center for Power Electronics
Systems (CPES) Power Electronics Seminar, Virginia Tech, 674 Whittemore Hall, Campus
Mail Code 0179, Blacksburg, VA 24061, 2002, pp. 70-76
[3] Lee, F.C., Van Wyk, J.D., Boroyevich, D., Jahns, T., Chow, T.P., and Babosa, P.,
Modularization and integration as a future approach to power electronic systems,
Proceedings of the 2nd International Conference on Integrated Power Systems (CIPS),
2002, pp. 9-18
[4] Ferreira, J.A., and Gerber, M., Three dimensional integration based on power module
technology, Proceedings of the 2nd International Conference on Integrated Power Systems
(CIPS), 2002, pp. 35-41
[5] Liang, Z., Lee, F.C., Lu, G.Q., and Borojevic, S., Embedded power - a multilayer
integration technology for packaging of IPEMs and PEBBs, International Workshop on
Integrated Power Packaging, IWIPPL, 14-15 July 2000, pp. 41-45
[6] Liu, X., Jing, X., and Lu, G., Chip-scale packaging of power devices and its applications in
integrated power electronics modules, IEEE Transactions on Advanced Packaging, Vol. 24,
No. 2, 2001, pp. 206-215
[7] Strydom, J.T., and Van Wyk, J.D., Experimental investigation of failure mechanisms in
integrated spiral planar power passives, Center for Power Electronics Systems (CPES)
Power Electronics Seminar, Virginia Tech, 674 Whittemore Hall, Campus Mail Code 0179,
Blacksburg, VA 24061, 2002, pp. 106-110
[8] Strydom, J.T., and Van Wyk, J.D., Improved loss determination for planar integrated
power passive modules, Center for Power Electronics Systems (CPES) Power Electronics
Seminar, Virginia Tech, 674 Whittemore Hall, Campus Mail Code 0179, Blacksburg, VA
24061, 2002, pp. 415-421
[9] Chen, R., Strydom, J.T., and Van Wyk, J.D., Second order approximation lumped
parameter model for planar integrated L-L-C-T module, Center for Power Electronics
Systems (CPES) Power Electronics Seminar, Virginia Tech, 674 Whittemore Hall, Campus
Mail Code 0179, Blacksburg, VA 24061, 2002, pp. 450-455
Chapter 1 - Integrated Power Electronics, an Introduction
10
[10]Popovic, B.D., Introductory Engineering Electromagnetics, Addison-Wesley Publishing
Company, 1971
[11]Van Wyk, J.D., Strydom, J.T., Zhao, L., and Chen, R., Review of the development of
high density integrated technology for electromagnetic power passives, Proceedings of the
2nd International Conference on Integrated Power Systems (CIPS) 2002, pp. 25-34
[12] Strydom, J.T., and Van Wyk, J.D., Electromagnetic design optimisation of planar
integreted passive modules, Proceedings of the 33rd I.E.E.E. Power Electronics Specialist
Conference (PESC), Australia, Vol. 2, June 2002, pp. 573-578
[13] Strydom, J.T., Van Wyk, J.D., and Ferreira, J.A., Volumetric limits of planar integrated
resonant transformers: a 1 MHz case study, Proceedings of the 32
nd
annual Power
Electronics Specialist Conference, 2001, PESC.2001, Vol. 4, 17-21 June 2001, pp. 1944-
1948