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Power electronics in automotive applications Introduction Over the last few decades, electrical loads in an auto- motive system have evolved from lighting and battery- charging 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 bat- tery 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 silicon- based power management with power semiconductor switches. These switches are power metal-semicon- ductor 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 electron- ics 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. Nagarajan Sridhar Product Marketing Manager, High-Performance Isolated Power Solutions, Power Management Texas Instruments WHITE PAPER 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 con- verters 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.

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Power electronics in automotive applications

Introduction

Over the last few decades, electrical loads in an auto-motive system have evolved from lighting and battery-charging 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 bat-tery 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 silicon-based power management with power semiconductor switches. These switches are power metal-semicon-ductor 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 electron-ics 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.

Nagarajan SridharProduct Marketing Manager,

High-Performance Isolated Power Solutions, Power Management

Texas Instruments

W H I T E P A P E R

Higher voltage systemsAs 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 conceptSwitched-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 con-

verters 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 systemElectric 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 convertersSeveral 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.

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).

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 componentsRegardless 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 manu-

facturer’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

2 Texas Instruments

12V 48V

GNDGND

12VLoad

12VLoad

12VStarter/

(Cold Crank)

12VBattery

12V/48VDC/DC

Converter

48VBattery

48VStarter/

Generator48V

Load48V

Load

Power electronics in automotive applications November 2013

3Texas Instruments

High temperature requirementAnyone 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) oper-

ates 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 reduc-

ing 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 tem-

peratures, 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 CarbideProperty Si SiC

Band gap energy (eV) 1.12 3.26

Breakdown electric field (V/cm) 2 x 105 2.2 x 106

Thermal conductivity (W/cmK) 1.5 4.56

Maximum junction temperature (°C) 200 600

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.

6 Texas Instruments

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.

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=en7. For more information about automotive and transportation solutions, visit: http://www.ti.com/lsds/ti/apps/ automotive/end_equipment.page

Conclusion

References

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