Power electronics in automotive applications

April 8th, 2014, Published in Articles: EngineerIT

 

Over the last few decades, electrical loads in an automotive system have evolved from lighting and battery-charging towards infotainment, sensors and safety, thanks to component miniaturisation. 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 12 V 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.

Fig. 1: A 48 V – 12 V bi-directional power supply and electrical loads.

Fig. 1: A 48 V – 12 V bi-directional power supply and electrical loads.

Power electronics systems exclusively use silicon-based 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.

Higher voltage systems

As mentioned earlier, systems operated by a 12 V battery are becoming more challenging. You might ask, what is the higher voltage of choice compatible with the 12 V battery and electrical loads, primarily in the power train system? It turns out that new architectures are based on a 48 V system for internal combustion engines (ICE) and HEV systems, whereas an additional 400 V – 200 V 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 12 V system [3]. Examples of loads that can be driven from the 48 V system are generator/ starter, electric power steering, electric roll stabilisation, 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 realised 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)
    • Battery management for Lithium-Ion (Li-Ion) batteries
    • 48 V – 12 V bi-directional power supply
  •  400 V Battery (for pure EV only)
    • Bi-directional 400 V – 12 V power supply (DC/DC)
    • 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.

There is a bi-directional power supply converter for both EVs and HEV/ICE, namely 400 V – 12 V and 48 V – 12 V, respectively. Fig. 1 shows the 48 V – 12 V 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 (12 V). An example of this is regenerative braking and during warm cranking. On the other hand, power flow takes place in the boost mode (12 V to higher voltage) during a cold crank or a cold-start mode. This occurs when backup power is required during a situation where the 48 V or 400 V (Li-Ion) battery fails to drive the motor.

Based on power levels that vary from 1 to 3 kW, 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).

Fig. 2: Engine block under high temperature conditions.

Fig. 2: Engine block under high temperature conditions.

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 analogue 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 analogue and digital controllers and gate drivers to turn the power electronic switches on and off.

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°C (Fig. 2).

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.

Properties of silicon and silicon carbide
Property 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 juction temperature (°C)  200 60
Table 1: Properties of SIC and SI.

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.

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.

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: www.ti.com/lsds/ti/apps/

Contact Erich Nast, Avnet Kopp, Tel 011 319-8600, erich.nast@avnet.eu

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