International Journal of Scientific & Engineering Research, Volume 4, Issue 12, December-2013
ISSN 2229-5518
Power Electronic Modules in Future Vehicles
Vinay Divakar
997
Index Terms— Hybrid Electric Vechicles, HEV’s, Fuel cell vehicles, Power electronic modules, power storage methods,
electric motor drives, future vehicles, plug-in hybrid electric vehicles, PHEV’s
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The implementation of the HEV’s are done using integrat- ed power electronic modules (PEM’s). Power electronic mod- ules are modules that consists of integration of circuits or elec- tronics that is used for conversion of signals e.g. DC to AC, and also consists of active and passive elements. These Power modules are used to control and drive AC motor by converting a DC input from battery to an AC output to drive the motor efficiently. An Inverter is a good example for DC to AC con-
verter. The efficiency of the module depends on the type of semiconductor device used for switching and also on other elements parameters. One such integrated power electronic module is commercially available that is the skai Module de- veloped by the SEMIKRON company Hudson, USA. The skai module uses a semiconductor device of type IGBT with a pow- er rating of 1200V and 800A which can handle up to one Meg- awatt’s of electrical power. Its heat dissipation is approximate- ly 3kw. These power electronic modules are used to control the electrical power by performing tasks such as changing phase, duty cycle, voltage, current etc. Another high frequency power module is the silicon carbide power module CAS100H12AM1 developed by CREE.INC with a power rating of 1200V, 100A. This module is configured by a 50mm half bridge circuit that consists of five 1200V, 80mΩ Z-FET MOSFET’s and five Z- Scotty diodes. The Silicon Carbide (SIC) MOSFET power mod- ule has the capability of switching at exceptionally high fre- quencies allowing better efficiency and total reduction in sys- tem costs compared to the silicon based technologies. These power electronics modules are used for converting signals for AC to DC, DC-DC and in this case, an IGBT based inverter module to convert DC to AC form in order to drive the motor in Hybrid Electric Vehicles or Fuel cell vehicles. The type of switching device used depends on the applications i.e. very high speed applications may use silicon carbide power mod- ules and adequate switching speed applications may go with IGBT based skai module. It has been demonstrated that the switching speed of the SIC MOSFET can go up to 100 KHz and considerably exceed the switching speed of the IGBT based modules.
There are certain issues and several design constraints as- sociated with Power Electronic Modules that must be consid- ered and analyzed for the required application such as a hy- brid electric vehicles or fuel cell vehicles. One of the significant issues with respect to the reliability of the PEM is the process and material uncertainty During the manufacturing process, the components are set or manufactured with a certain toler- ance, all the components have different material properties and this has a significant impact on the reliability of the PEM,
IJSER © 2013
International Journal of Scientific & Engineering Research Volume 4, Issue 12, December-2013
ISSN 2229-5518
998
therefore design validation and verification of parameters is an important stage. Another main factor that decides the flexibil- ity and reliability of the power electronic module is the power loss due to heat dissipation. The junction temperature of the power devices such as IGBT can go up to very high tempera- tures leading to power loss in the form of heat. Then there are diodes and transistors also that have power loss due to heat dissipation. Therefore it is necessary to make sure that the Power module must be designed such that the loss due to heat dissipation is minimum. Let us consider a small transistor with thermal resistance from junction to ambient is 150o C/Watt (W) . If the transistor is dissipating 0.5W of heat , then the temperature of the silicon junction in the transistor will be 75 o C hotter than the ambient air temperature i.e. 0.5W*150 o C
/Watt = 75o C. To prevent the power devices from damaging due to extremely high temperatures or thermal loss, it is neces- sary to use a heat sink which shall absorb the heat dissipated from the power device and prevent the device from damaging, leading to a reliable response. Let us consider a power transis- tor that has a thermal resistance of 0.85 o C /W from the tran- sistors junction to the module outer case, And this transistor is mounted above a heat sink rated at 3 o C /W and including the thermal paste and mica that is a enclosure case of the transistor has a thermal resistance of 1 o C /W. The total thermal re- sistance (TTR) of the transistor junction to the ambient is given by
TTR = 0.85 o C /W+1 o C /W+3 o C /W = 4.85 o C /W
So let us generally consider that the transistor is dissipating 8
Watts as heat, then the temperature of the heat sink will be 3 o
C /W*8W = 24 o C hotter than the ambient air temperature and
the junction of the transistor will be 0.85*8 = 6.8 o C hotter than
the case in which the transistor is enclosed. And finally the case of the transistor will be 1 o C *8 = 8 o C hotter than the heat
sink. The robustness of the power module depends on the thermal resistance offered by the heat sink used and it is neces- sary to use a heat sink with a low thermal resistance which will absorb more heat, thus protecting the power device from being damaged and providing a stable performance. Another im- portant integration issues in the power module is the wire bond failure. The wire bond is the interconnect technology used in the power modules and the reliability of the aluminum (Al) wire bond is dependent on the bond strength between the Al wire and the IGBT chip. The wire bond failure is caused mainly due to thermal expansion co-efficient mismatch be- tween the Al wire and the silicon die at the contact interface. This issue or failure can be overcome by designing wide ther- mal cycling ranges for the power modules. High temperatures of the power devices such as IGBT or MOSFET often forces the designers to reduce the switching frequency, thus reducing the heat dissipation and the switching speed of the device, this in turn will reduce the inverter losses in the PEM and resonant inverters can be used to reduce or eliminate the effects of the hard switching, thus tending to reduction in the overall losses in caused in the Power Modules. And it is important to choose the right power device for the right application to achieve bet- ter response and stability.
The fuel efficiency and the overall response of the hybrid electric vehicles are highly dependent on the energy or power storage system of the vehicle. Batteries have been employed for ground electric vehicles due to its high energy density, compact size and reliability. The batteries of the electric vehicle should be designed such that it meets the superior or desired performance. Some constraints considered while designing a battery for electric vehicles are energy density (Wh/L) , power density (W/L) etc, and eventually the most important and sig- nificant parameter is the cost ($ /KWh). Some of the batteries presently used are Lead-Acid batteries, Nickel Metal Hydride, Lithium Ion Batteries, Nickel Cadmium and Nickel Zinc Bat- teries. Each of these batteries differs in their cost, power densi- ty, weight and size. Another energy storage element deter- mined was the Flywheel for several applications and presently this method of power storage has been used as propulsion source in some experimental vehicles. In an HEV, the running flywheel stores mechanical energy and then it is converted into electrical energy by the power module, thus driving the transmission system of the electric vehicles or the Motor of the electric vehicles. Fuel cells have been a viable source of power for HEV’s such as transport vehicles. Fuel cells undergo chem- ical reaction and converts the chemical energy of the fuel i.e. hydrogen, directly into electrical energy without combustion and achieve high efficiencies, about two to three times more than gasoline engines. Then there is Ultra-Capacitors (UC) that shows interest in applications of HEV. The power storage den- sity of UC is two times in order higher than the magnitude of the traditional electrolytic capacitors. In UC, energy is stored in two plates placed in parallel and separated by an insulator of certain di-electric constant. But however the energy density of UC is very low compared to that of batteries. It has been known that the combination of both fuel cell and UC’s are be- ing experimented for obtaining better efficiency in the HEV. As we know that for any electric vehicle propulsion system, the DC Motor is one of the most important parts of the system along with the converter. DC motor drives have been promi- nently used in electric vehicles, since they offer simple speed control. If we replace the field windings and poles with high energy permanent magnets, then we obtain a permanent mag- net Dc motor which leads to reduction in size of the stator di- ameter improving the commutation. Advancements in tech- nology has made AC motor drives much more preferred over DC drives, since the AC motor drives offers better efficiency, greater reliability, higher power density and also requires less- er maintenance than that of DC motor drives. Induction mo- tors again are motors that offer a mature technology for drive applications in electric vehicles. Induction motors are widely used commutatorless motor type drive used for electric vehicle propulsion system. Then there are permanent magnet brush- less ac motor drives that are used in electric vehicles which give a good competition to the induction type motors and the reason for this is that its magnetic field is excited by using high energy permanent magnets and this leads to increase in power density. And then there is switch reluctance motor drive , that
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International Journal of Scientific & Engineering Research Volume 4, Issue 12, December-2013
ISSN 2229-5518
999
has been identified to have tremendous potential in this field due to its simple construction, low manufacturing cost and outstanding.
Besides a smaller and more efficient engine, In future many other tricks or methods can be used to increase fuel effi- ciency, these developments squeeze every last mile out of gas- oline. One development is the use of advanced aerodynamics to reduce drag, in this process, a car being driven on a freeway, and the work of the engine is to push the car through the air, this pushing force generated is known as the aerodynamic drag. One way to reduce drag is by reducing the frontal area of the vehicle. It is believed by many, that the future HEV’s may be the fuel cell type HEV’s in which hydrogen is used as a fuel to combine with oxygen and generate power to the on-board electric motor, in order to increase the efficiency and the out- put of the fuel cells, a DC-DC Buck-Boost converter in order to regulate the output voltage of the fuel cell or just a DC-DC boost converter to boost the output of the fuel cell if it is too low. Another technique that can be used to improve the effi- ciency of the power systems is by using the combination of fuel cell and ultra capacitors along with a DC-DC Boost con- verter to improve the overall output and the response of the system. Another development that should be focussed on is the use of light weight materials to build the PEM’s along with the batteries and a smaller engine; this reduces the overall weight of the hybrid car improving the mileage i.e. A lighter vehicle requires lesser power or energy to be accelerated. Composite materials like carbon fibre or light weight materials like aluminium and magnesium can be used. It is recommend- ed and believed that the power devices used in the power elec- tronic modules shall be replaced by silicon based to silicon- carbide (SIC) based power devices in the coming future and thus SIC based devices having the potential to work efficiently at higher temperatures and has lesser loss and reverse recov- ery time compared to that of silicon (Si) based power devices. The future electric cars can be plug-in hybrids which evolutes from today’s “full hybrid” electric vehicles. It is known that a full hybrid vehicle has the ability to accelerate to lower speeds without using the gasoline engine and battery is exclusively charged from the on-board in internal combustion engine and by regenerative breaking, whereas, a plug-in hybrid electric vehicles (PHEVS) operates the same way as the full hybrid vehicles, but it’s just that, the PHEV’s have a larger battery pack and gives the driver the option to charge the battery from an household outlet (Grid electricity) and then running their vehicle on the grid electricity instead of the petrol or diesel. The PHEV’s have an advantage over pure battery electric vehi- cles is that the driver does not need to worry about the battery running down on charge, because PHEV’s will also operate like the conventional HEV’s.
The two commercially available power modules are the skai module which is Silicon (Si) IGBT based and then there is CAS100 Silicon Carbide (SIC)-MOSFET based power module. Among this, high speed switching frequency and high temperature withstanding capability is offered by the SIC MOSFET, whereas adequate switching speed and lower tem- perature withstanding capability is offered by the skai Silicon IGBT based power module. A significant issue in the power electronic module is the power devices reaching extremely high temperatures due to the hard switching and this problem can be tackled by using resonant inverter (soft switching), us- ing appropriate heat sinks and by thermal derating. The focus for fuel cell vehicles will be high in future and especially hy- drogen fuel cells will be used. And a combination of both hy- drogen fuel cells along with the ultra capacitors will provide better efficiency and better response. In case of hybrid electric vehicles, One of the efficient batteries that can be used is the Nickel Metal Hydride (NiMH), as it can be designed for short- battery driving distance. In terms of efficiency, the most effi- cient motor drives are the permanent magnet brushless motor followed by the induction motor and the switch reluctance motor drives which have identical efficiency. Amongst all the motors used to drive the electric vehicle, the least efficient are the DC motors, but in terms of maturity of technology used for propulsion systems, induction motors and DC motors score the highest points. A significant development in the future hybrid electric vehicles will be the PHEV’s, which will run the electric vehicle on the grid electricity and has a capability of charging through the household supply outlet. The PHEV’s will be probably using NiMH batteries initially in the near fu- ture.
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[2] Alireza Khaligh, Zhihao Li (2010), “Battery, Ultracapacitor, Fuel Cell, and Hybrid Energy Storage Systems for Electric, Hybrid Electric, Fuel Cell, and Plug-In Hybrid Electric Vehicles: State of the Art” IEEE TRANSACTIONS ON VEHICULAR TECHNOLO- GY, VOL. 59, NO. 6, July.
[3] Gaurav Nanda (2006), “A SURVEY AND COMPARISON OF CHARACTERISTICS OF MOTOR DRIVES USED IN ELEC- TRIC VEHICLES” University of Windsor, Canada, May.
[4] Leon M. Tolbert, Burak Ozpineci ,“Impact of SiC Power Elec- tronic Devices for Hybrid Electric Vehicles” The University of Tennessee, Michigan.
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Vinay Divakar is currently pursuing M.sc (Engg) degree program in Electronic System Design Engineering in M.S Ramaiah School of Advanced Studies Affliat- ed to Conventry University, U.K. PH- +91-9449835011.
E-mail: vinaydivakar@hotmail.com
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