Integrated Power Electronic

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INTRODUCTION

In power electronics, solid-state electronics is used for the control

and conversion of electric power .The goal of power electronics is to realize

power conversion from electrical source to an electrical load in a highly

efficient, highly reliable and cost effective way. Power electronics modules

are key units in a power electronics system. These modules contain

integration of power switches and associated electronic circuitry for drive

control and protection and other passive components.

During the past decades, power devices underwent generation-by-

generation improvements and can now handle significant power density. On

the other hand power electronics packaging has not kept pace with the

development of semiconductor devices. This is due to the limitations of

power electronics circuits. The integration of power electronics circuit is

quite different from that of other electronics circuits. The objective of power

electronics circuits is electronics energy processing and hence require high

power handling capability and proper thermal management.

Most of the currently used power electronic modules are made by

using wire-bonding technology [1,2]. In these packages power semi

conductor dies are mounted on a common substrate and interconnected with

wire bonds. Other associated electronic circuitries are mounted on a multi

layer PCB and connected to the power devices by vertical pins. These wirebonds are prone to resistance, parasitic and fatigue failure. Due to its two

dimensional structure the package has large size. Another disadvantage is

the ringing produced by parasitic associated with the wire bonds.




To improve the performance and reliability of power electronics

packages, wire bonds must be replaced. The researches in power electronic

packaging have resulted in the development of an advanced packaging

technique that can replace wire bonds. This new generation package is

termed as ‘Integrated Power Electronics Module’ (IPEM) [1]. In this,

planar metalization is used instead of conventional wire bonds. It uses a

three-dimensional integration technique that can provide low profile high-

density systems. It offers high frequency operation and improved

performance. It also reduces the size, weight and cost of the power modules.




FEATURES OF IPEMS

The basic structure of an IPEM contains power semi conductor

devices, control/drive/protection electronics and passive components. Power

devices and their drive and protection circuit is called the active IPEM and

the remaining part is called passive IPEM. The drive and protection circuits

are realized in the form of hybrid integrated circuit and packaged together

with power devices. Passive components include inductors, capacitors,

transformers etc.

The commonly used power switching devices are MOSFETs and

IGBTs [3]. This is mainly due to their high frequency operation and low on

time losses. Another advantage is their inherent vertical structure in which

the metalization electrode pads are on two sides. Usually the gate source

pads are on the top surface with non-solderable thin film metal Al contact.

The drain metalization using Ag or Au is deposited on the bottom of chip

and is solderable. This vertical structure of power chips offers advantage to

build sand witch type 3-D integration constructions.

In IPEM integration of active part is done by using ‘Embedded

Power Technology’ [4] and that of passive part is done by using ‘spiral

Integration Technology’ [5]. Embedded power technology provides a high-

density integration of active components with negligible parasitic effects,

and spiral integration of passive components with a large amount of volume

reduction.




To describe these advanced integration techniques we can use a

typical power electronics system. A Distributed Power Supply (DPS)

system can be used for this purpose

DPS SYSTEM

The complete circuit diagram of DPS system is shown in figure 1. In

this DC-DC converter, the primary dc supply is split in with the help of

capacitors and then inverted to high frequency ac by MOSFET by half

bridge inverter. The resonant capacitor voltage is transformer coupled,diode rectified and then filtered to get the output dc voltage [6].

Fig 1: DPS System




The active part of the DPS system contains a power switching stage

with two MOSFETs and their drive circuit. The drive circuit contains high

and low drivers with over current protection and the drive control logic. The

passive part contains capacitors, transformer and current doubling inductors.

The active and passive parts of DPS system can be implemented

using the above mentioned techniques and are discussed in the following

part.




EMBEDDED POWER TECHNOLOGY

Embedded power technology is a three-dimensional, multilayer

integrated packaging technology. Figure 2 shows the conceptual structure of

an embedded power packaged module.

FIG 2: Conceptual structure of embedded power module

It consists of three parts: embedded power stage, electronics circuitry

and base substrate, which are soldered together to build a module. The

electronics circuitry (components in figure) includes gate drive, control and

protection components. A hybrid (thick film) drive circuitry with high

density interconnect is employed to shrink the module size.

Base substrate

The base substrate provides electrical interconnection and thermal

path of power chips. An Al2O3 or AlN direct bonded copper (DBC)




substrate with 25mil ceramic and 10mil thick copper is used as the base

substrate. It is bonded to a 3mm thick heat spreader to improve thermal

management and to provide suitable mechanical stability.

Embedded Power stage

The core element in this structure is the embedded power stage that

comprises of ceramic frame, power chips (Si in figure), isolation dielectric

and metalization circuit. Inside the power stage, multiple bare power semi

conductor dies, featuring vertical semi conductor structures with topside and

backside electrode pads, are buried in a ceramic frame.

Planar metallization

In this module, wire bonds are replaced b metalization using copper

layers. It is shown in figure 3. Here the deposited Cu pattern layers connect

the Al pads of the power chips with external circuitry.

.

FIG 3: contact scheme in embedded power connection

To accomplish this metallurgical interconnect the under bump

metalization (UBM) schemes are employed in our approach. These UBM

schemes such as Ti-Ni-Cu or Cr-Ni-Cu deposited layers provide low film

Si

Plated Cu

Sputtered Cu

Sputtered Ni

Sputtered Cr/Ni

Al pad on chip

IGBT or MOSFET chip

Drain contact

Dielectric

and

passivation




stress with good adhesive and electrical/ thermal conduction. For carrying

high current an electro plated Cu layer is added to this thin Cu layer of

UBM. The process flow chart of embedded power module is given in figure

4. Table 1 summarizes the fabrication steps. They are ceramic cutting,

device mounting, dielectric printing and metalization [6]. One of the

features of this technology is its mask based processing. The metalized base

substrate is patterned using photolithography, the dielectric polymer is

applied with a screen-printing method, and the chip-carrier ceramic frame is

fabricated by computer controlled laser machining.

Figure 4: Process flow chart of embedded power

Si

Ceramic

cutting

Dielectric

pattern

Device

mount

Metalization

pattern




Steps Description

Ceramicframe

Openings in flat Al2O3 or AlN plate by laser cutting

Die mount Dispense dielectric

Dielectric

pattern

Void free precision dielectric pattern, good adhesion

by screen - printing or/and photolithography.

Metalization Adhesion, barrier, low stress, low resistance by

sputtering of Ti/Cr-Cu thin film. Thicker (>5mil),

low stress, low resistance, solderable, precision

pattern by electroplating of Cu, etching

Table1: processing step for embedded power stage.

By assembling the three parts, i.e. the substrate, the power stage and the gate

driver, we get the active module. It has a substrate area of 28.5 × 27.3mm. The

exploded view of active IPEM is shown in figure 5.

Fig.5: Three Dimensional View




SPIRAL INTEGRATION TECHNOLOGY

In order to integrate the electromagnetic power passive components used

in power electronic converters in to modules, we use spiral integration technology.

This integration technology for power passives can best be described by first

considering a simple bifilar spiral winding as shown in figure 6.

Fig:6 Spiral Integrated LC structure

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 best described as an electro magnetically integrated LC

resonant structure for which equivalent circuit characteristics depends on theexternal connections. Even more complex integrated structures can be realized by

adding more winding layers.

Design of these structures requires deliberate increase and modification of

naturally existing structural impedances, like intra winding capacitance, to realize

a particular equivalent circuit function. These models will provide power densities

of 29W/cm3 at frequencies up to 1MHz. In our example of DPS system the

passive part contains decoupling capacitor, current doubler inductors and isolation

transformer. Because of the current doubler configuration, passive IPEM can be

realized by stacking two transformers and using only one DC blocking capacitor

as illustrated in figure 7.




Fig:7 Components of passive IPEM (a) equivalent circuit

( b) Exploded view of passive IPEM (c) prototype

In this circuit, the two magnetic structures i.e. inductors and transformers,

can be integrated in to one physical structure through integrated magnetics

technology. The equivalent magnetizing inductance is used to realize the current

doubler inductors.

The transformers are built with two planar E-cores that share a common I-

core. The ac flux is partially cancelled in the shared I-core. The D C blocking

capacitor is now implemented in only transformer T1 using the hybrid winding

technology. This technology is implemented using Cu traces on both sides of the

winding and a dielectric layer placed in the middle to enhance the capacitive

component of the winding. For this we use a high permittivity ceramic [Er

>12000]. The Transformer T2 is a conventional planar low-profile transformer.

To get the complete IPEM, we mount the active and passive IPEMs on a

single ceramic chip carrier with metalization and then bonded to the heat spreader.

Figure 8 shows an IPEM based DPS system and its wire bonded version.




Fig:8 Comparison of wire bond and IPEM

PERFORMANCE OF IPEM

The performance of IPEM can be evaluated using various parameters. A

comparison of IPEM and wire bonding technology is given in table 2. As given inthe table, IPEM has achieved 35% reduction of foot print area as compared to the

wire-bonding version. The planar interconnects in IPEM reduces the structural

inductance by a factor of three when compared to the wire bonding. But the

structural capacitance is increased by a factor of five.




Table:2 Comparison of wire bond and IPEM

Parameters Wire bond IPEMSubstrate area (mm2) 40 x 30 28.5 x 27.3

Inductance (nH) 10 3

Capacitance (pF) 4 20

No. of passive

components

6 1

Volume of passives 173 82

No. of terminals 15 5

Volume of terminals 170 5

Total passive volume 343 87

System profile 20 10

System power density 1 X3.6

In IPEM, the volume of passive components is reduced to half of that in

wire bonding and total passive volume is reduced to 1/3rd

of that in wire bonding.

Also the system power density is increased by a factor of 3.6.




ADVANTAGES AND DISADVANTAGES

There are several advantages for integrating power electronics system

using the IPEM concept. The main advantages are

1. Modular approach:

This modular approach reduces the design and implementation time cycles as

well as simplifies the assembly process.

2. Improved usage of space

The reduction in volume increases the power density and reduces the profile

of the system.

3. Reduction of components and inter connects.

This improves the system reliability and also increases the speed.

4. Reduction in structural packaging inductance.

It leads to improved electrical performance, which in turn leads to reduced

voltage ringing across the power switches. It increases the switching

frequency.

The main disadvantage of this system is that it is very complex comparedto other 2-D modules. Also it requires efficient combination of a large number of

different technologies.

APPLICATIONS

IPEM can be used for most of the power electronic circuits. Hence it has a

wide range of applications. It includes

Motor drives

UPS systems

Power supplies

Inverters

Converters etc.




CONCLUSION

In power electronics modules further improvements in performance,

reliability and cost can be achieved by using IPEMs. Various experiments have

proved its manufacturability and other features of this technology. The impacts of

system integration via IPEM will enable a rapid growth of power electronics that

can be compared to the impacts in computer applications brought about by VLSI

technology.




REFERENCES

[1] F.C.Lee, J.D.Van Wyk, D. Boroyevich, G.Q. Lu, Z. Liang and P. Barbosa

“Technology trends towards system in a module in power electronics ”,

IEEE circuits &systems, Vol. 2, No 4, Vth quarter, pp 6-21, 2002.

[2] X. Liu, and G. Q. Lu “Power chip inter connection: from wire bonding to

area bonding,” Advancing microelectronics, Vol. 28.No 4, July/August

2001.

[3] B.K.Bose, “ Modern power electronics “, Jaico Books, pp 8-32

[4] Z. Liang, F.C. Lee, “Embedded power technology for IPEM packaging

applications”, IEEE proceedings on APEC 2001 pp 1057-1061

[5] http:// www.scholar.lib.vt.edu/thesis/avalable

[6] B. G. Streetman, S. Banerjee, ”Solid state electronic devices, PHI,pp 150-160




ABSTRACT

IPEM is an improved power processing technology through advanced

integration of power electronics components. It provides high frequency synthesis,

resulting in important improvements in performance, size, and cost.

Currently, assemblies of power semiconductor switches and their

associated drive circuitry are available in modules. Though the module contains a

small size power switching part, the associated control, sensing, electro magnetic

power passives and inter connect structures are very bulky. In IPEM, the reduction

in size and weight is provided by planar metalization that allows 3-D integration

of power devices and power passives to increase the power density.

This paper addresses the improvements of power processing technology

through advanced integration of power electronics. The fundamental functions in

electronic power processing, the materials, processes, and integration approaches

and future concepts are explained.




CONTENTS

Introduction 1

Features of IPEM 3

DPS system 4

Embedded power technology 6

Spiral integration technology 10

Performance of IPEM 12

Advantages and disadvantages 14

Applications 14

Conclusion 15

References 16




ACKNOWLEDGEMENT

I extend my sincere gratitude towards Prof . P.Sukumaran Head of

Department for giving us his invaluable knowledge and wonderful technical

guidance

I express my thanks to Mr. Muhammed kutty our group tutor and

also to our staff advisor Ms. Biji Paul for their kind co-operation and

guidance for preparing and presenting this seminar.

I also thank all the other faculty members of AEI department and my

friends for their help and support.

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Integrated Power Electronic