W H I T E
PA P E R
Nagarajan Sridhar
Product Marketing Manager,
High-Performance Isolated Power Solutions,
Power Management
Texas Instruments
Introduction
Over the last few decades, electrical loads in an automotive system have evolved from lighting and batterycharging towards infotainment, sensors and safety,
thanks to component miniaturization. This has made the
car smarter and more fun to drive! While the trend of
electronics in infotainment systems continues, a future
trend is electronics in power train systems for better
engine propulsion, like the engine blocks, transmission
and controls. Incorporating electrical loads and replacing
the conventional mechanical and hydraulic loads in the
power train improves efficiency. This trend is seen with
more and more focus on electric vehicle concepts –
hybrid (HEV) and pure (EV). This trend is also expected
to lower C02 emission standard requirements.
This increasing need and demand makes the
conventional 12V power system more challenging [1],
[2]. As such, it is critical to have higher voltages in
order to handle power train loads more efficiently – and
with flexibility. Switched-mode power supplies (SMPS)
provide the basis to do so. This is made possible due to
advances in power electronics. In other words, having
different power-conditioning systems between the battery and loads is a way to accomplish this. Additionally,
higher fuel efficiency standards have been mandated
in several countries across the globe. To be clear, fuel
efficiency is measured in terms of miles per gallon
(MPG) for fuel injection vehicles, and miles per charge
for electric and hybrid electric vehicles. Implementation
of power electronic circuits makes the system smaller
and lighter and, therefore, provides the basis to improve
the fuel efficiency as well.
Power electronics systems exclusively use siliconbased power management with power semiconductor
switches. These switches are power metal-semiconductor field-effect transistors (MOSFETs), insulated
gate bipolar transistors (IGBTs), and a variety of diodes
that have made significant improvements in their
performances.
This paper presents a review of power electronics systems in electric vehicles with emphasis on the
power train system. The paper also addresses the high
temperature requirements in the power train system
and the advances in power electronics to handle them.
Power electronics in
automotive applications
Higher voltage systems
As mentioned earlier, systems operated by a 12V battery is becoming more challenging. You might
ask, what is the higher voltage of choice compatible with the 12V battery and electrical loads,
primarily in the power train system? It turns out that new architectures are based on a 48V system
for internal combustion engines (ICE) and HEV systems, whereas an additional 400V-200V system
is used in EVs.
Incorporating high voltages makes system wiring less complex and lighter. This has a direct
impact on lowering the vehicle’s overall weight, in addition to overcoming other disadvantages
in the 12V system [3]. Examples of loads that can be driven from the 48V system are generator/
starter, electric power steering, electric roll stabilization, electric AC compressor, electric heating,
and electric pumps (water, oil, and vacuum).
The SMPS concept
Switched-mode power supply (SMPS) is based on semiconductor devices that operate in ON and
OFF states. Operating in these states, in theory, implies no power loss at either of these states
(zero current during off state and zero voltage during off state). Theoretically, this implies 100%
efficiency. The switches are turned on or off using a technique known as pulse-width modulation
(PWM). Also, these switches can operate under high switching frequencies, making the power converters less bulky and smaller in size. Based on this information, three types of power conditioners
can be realized in automotive power train systems: AC/DC (rectifier); DC/DC (converter); and AC/DC
(inverter).
SMPS applications in the power train system
Electric vehicles, HEVs and ICE primarily need the following SMPS conditioners in their power train
systems:
• Regenerative braking (AC/DC)
• Onboard charger (AC/DC)
• Dual-battery system (DC/DC)
o Battery management for Lithium-Ion (Li-Ion) batteries
o 48V-12V bi-directional power supply
• 400V Battery (for pure EV only)
o Bi-directional 400V-12V power supply (DC/DC)
o Traction motor (DC/AC)
DC/DC converters
Several basic topologies are available for these power-conditioning systems based on the electrical
load safety requirements – classified as isolated and non-isolated topologies [4]. In general, both
types of topologies are adopted in power train systems. This depends on the loads and standards
requirements. Regardless of type, market adoption is trending towards a soft-switching concept
using an LLC or resonant mode. Soft switching means that the switches are subjected to lower
stress, which implies a longer life and higher reliability of these converters, very vital for automotive
markets.
2
Texas Instruments
There is a bi-directional power supply converter for both EVs and HEV/ICE, namely 400V-12V and 48V-12V,
respectively. Figure 1 shows the 48V-12V network with different associated loads. Normal operation of these
converters is in the buck mode. For instance, the power flows from the higher battery voltage to the lower
voltage (12V). An example of this is regenerative braking and during warm cranking. On the other hand, power
flow takes place in the boost mode (12V to higher voltage) during a cold crank or a cold-start mode. This occurs
when backup power is required during a situation where the 48V or 400V (Li-Ion) battery fails to drive the motor.
Based on power levels that vary from 1 to 3kW, full-bridge topology is a good choice when isolation is
required with a buck-boost topology for non-isolated cases. These converters are based on power MOSFET
switches that switch at very high frequencies (typically 50-500 kHz).
12V
48V
12V
Load
12V
Starter/
(Cold Crank)
12V
Load
12V
Battery
12V/48V
DC/DC
Converter
48V
Battery
GND
48V
Starter/
Generator
48V
Load
48V
Load
GND
Figure 1. A 48V-12V bi-directional power supply and electrical loads.
Traction inverter (DC/AC)
To convert electrical to mechanical energy in order to run the vehicle, motors are required. Previously, DC motors
were implemented for their simplicity and ease of control. However, DC motors are unreliable and show lower
efficiency compared to AC motors. Over the last couple of decades, tremendous progress has been made in
building controllers for AC motors. Moreover, AC motors are physically smaller with fewer parts that significantly
improve its reliability.
Therefore, the power stored in the battery (EV, HEV or ICE) must be converted from DC to AC in order to run
AC motors. These inverters, called traction inverters, usually transfer power in the tens of kW range. Hence, the
switches used in these topologies (full bridge) are IGBTs (individual or intelligent power modules, depending on
the current requirement).
Power electronic components
Regardless of power conditioning system type (DC/DC, AC/DC or DC/AC), all require controllers and gate drivers.
Currently, the choice of analog or digital controllers is highly dependent on the vehicle or power supply manufacturer’s requirements. This includes cost, flexibility, integration, reliability and availability to write firmware (for
digital controllers). Likewise, the choice of gate drivers is dependent on the drive current that in turn is dependent
on several factors. This includes the semiconductor switch it needs to operate, reduced component count (single
channel versus bridge drivers), features such as dead-time control, and adaptive delay to avoid shoot-through
between high-side and low-side switches and isolation, to name a few. Texas Instruments has several automotive
grade analog and digital controllers and gate drivers to turn the power electronic switches on and off.
Power electronics in automotive applications
November 2013
Texas Instruments
High temperature requirement
Anyone opening the hood of a car can feel heat radiating, especially after the car has been driven for a while. This
is because the power train system with the engine as one of the sub-systems (internal combustion or motor) operates at temperatures exceeding 125 degrees Celsius (Figure 2).
Figure 2. Engine block under high temperature conditions
We discussed the value of power electronic systems that transfers power very efficiently and occupies much
less space. This makes the car lighter, and improves vehicle reliability due to lack of moving parts.
As space or size comes down, power density gets higher. This is true especially for higher power transfer in the
kW range, typical of power train systems. The problem, however, is heat dissipation. Therefore, in addition to reducing the overall size, managing the thermal issues is another key factor towards improving fuel efficiency. Using
commercially available power electronic switches, such as silicon-based MOSFETs and IGBTs, there are limitations
to overcome.
For example, silicon-based power switches suffer from reverse recovery and breakdown issues at higher
temperatures (Table 1). If using silicon-based switches for the power electronic circuits subjected to high temperatures, cooling systems such as large copper blocks with water jackets need to be added. However, this affects
vehicle size, weight and cost.
Properties of Silicon and Silicon Carbide
Property
Band gap energy (eV)
Si
SiC
1.12
3.26
2 x 105
2.2 x 106
Thermal conductivity (W/cmK)
1.5
4.56
Maximum junction temperature (°C)
200
600
Breakdown electric field (V/cm)
Table 1. Properties of SiC and Si
Alternatively, wide-band gap semiconductors such as silicon carbide have much higher operating
temperature (known as the junction temperature), thermal conductivity is two to three times higher than Si,
higher breakdown voltage, and has a capability of switching at much higher frequencies with negligible power loss.
The higher operating temperature of the silicon carbide allows the circuit to be placed close to the location where
the temperatures are high. High thermal conductivity of silicon carbide eliminates the need for big copper blocks
and water jackets. Higher switching speed in the MHz enables the overall power circuitry to become smaller in size.
To address these concerns, TI offers the UCC27531, a gate driver developed to drive these SiC power FETs very
efficiently.
Power electronics in automotive applications
November 2013
3
6
Texas Instruments
Conclusion
The value of SMPS using power electronics, particularly with the requirement for higher voltage systems in the
automotive power train system, was discussed. This was followed by a review of the power conditioners involved
to drive various loads in the power train system, followed by the type of topologies being designed into these
systems. The type of semiconductor switches, controller and gate driver requirements were discussed for DC/
DC bi-directional power supplies and DC/AC traction inverters. Finally, the value of implementing wide-band gap
semiconductors such as SiC was discussed for high-temperature applications in the power train.
References
1. K. K. Afridi, R. D. Tabors, and J. G. Kassakian, “Alternative electrical distribution system architectures for
automobiles,” Proc. of ZEEE Workshop on Power Electronics in Transportation, Dearborn, Oct. 1994, pp. 33-38.
2. S. W. Anderson, R. W. Erickson, and R. A. Martin, “An improved automotive power distribution system using
nonlinear resonant switch converters,” ZEEE Trans. on Power Electronics, vol. 6, no. 1, Jan. 1991, pp. 48-54
3. J. M. Miller, A. Emadi, A. V. Rajarathnam, and M. Ehsani, “Current status and future trends in more electric car
power systems,” in Proc. IEEE Vehicular Technology Conference, Houston, Texas, May 1999.
4. N. Mohan, T. M. Undeland, and W. P. Robbins Power Electronics: Converters, Applications, and Design, 3rd Ed,
John Wiley & Sons, 2003.
5. Download a datasheet for the UCC27531: www.ti.com/product/ucc27531.
6. Check out our latest video on how to overcome temperature challenges in engine blocks: http://focus.ti.com/
general/docs/video/Portal.tsp?entryid=0_tgi6wxfv&lang=en
7. For more information about automotive and transportation solutions, visit: http://www.ti.com/lsds/ti/apps/
automotive/end_equipment.page
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