Electromagnetic Interference

25.3.3 Electromagnetic Interference

regulator reduces the field current appropriately. With conven- tional regulator circuits, this takes place on the time scale of Strict limits also exist for the amount of electromagnetic inter- the field winding time constant (typically 100 ms), and results ference (EMI) that an automotive electronic component can in a major transient event. In systems without centralized generate. Limits for both conducted and radiated emissions are

25 Automotive Applications of Power Electronics 647 specified in SAE standards J1113/41 and J1113/42 [4, 9, 10]. of the allowable voltage ripple (in dB µV) appearing across the

Here we will consider the conducted EMI specifications for power leads, since they directly impact the design of EMI

There are a wide range of other technical considerations for filters for automotive power electronics. Meeting the con- EMI testing, including the arrangement of the equipment over ducted specifications is a major step towards achieving overall

a ground plane and the types and settings of the measuring compliance.

devices. One characteristic to consider is that the EMI mea- The conducted EMI specifications in SAE J1113/41 limit surements are done across frequency with a spectrum analyzer the ripple that an electronic circuit can inject onto the voltage having a prespecified receiver bandwidth (RBW). For frequen- bus over the frequency range from 150 KHz to 108 MHz. The cies between 150 kHz and 30 MHz, the receiver bandwidth is amount of ripple injected by a circuit usually depends on the

9 kHz, resulting in spectral components within 9 kHz of one bus impedance. To eliminate any variability due to this, EMI another being lumped together for purposes of the test. A full compliance testing is done using a line impedance stabiliza- test procedure is defined in the SAE specifications, beginning tion network (LISN) between the bus and the device under with narrowband measurements and moving to wideband test, as illustrated in Fig. 25.2. The LISN is also sometimes measurements if necessary. Figure 25.4 illustrates the narrow- referred to as an artificial mains network (AN). Essentially, band conducted EMI limits for power leads in SAE J1113/41. the LISN ensures that the equipment under test receives the It is interesting to note that for the commonly used Class 5 proper dc voltage and current levels and also sees a controlled limits, the allowable ripple current into the LISN at 150 kHz is impedance for the ripple frequencies of interest. Figure 25.3 less than 100 µA! shows the magnitude of the LISN output impedance for a low-

As seen in the previous section, the transient disturbances generated by electrical and electronic equipment are an impor- tant consideration in automotive applications. Because power

impedance of the LISN is typically provided by the measure- electronic circuits typically contain switches and magnetic ele- ment equipment. The EMI specifications are stated in terms ments, they are potential sources for such transients, especially

when powered from the switched ignition line. SAE J1113/42 specifies methods for testing and evaluating the transients generated by automotive electrical components, and proposes transient waveform limits for different severity levels. The

equipment under test is set up in a configuration similar to V IN

that in Fig. 25.2, but with a switching device on one side or the –

Equipment

Under

V LISN

Test

other of the LISN, depending on the application. The equip-

ment under test is then evaluated for transient behavior at turn on, turn off, and across its operating range. The voltage

LISN transients at the input of the equipment are measured and FIGURE 25.2 Conducted EMI test set up with LISN. L LISN =5 µ H,

C LISN = 0.1 µ F, and R LISN

SAE J1113/41 Power Lead Narrow Band Conducted EMI Limits

LISN Output Impedance Magnitude

μ V) 60 Class 4

EMI limit

0 10 5 10 6 10 7 10 8 10 5 10 6 10 7 10 8 Frequency (Hz)

FIGURE 25.4 SAE J1113/41 narrowband conducted EMI limits for FIGURE 25.3 The LISN output impedance magnitude for a low

Frequency (Hz)

power leads. The specification covers the frequency range from 150 kHz impedance input source.

to 108 MHz.

648 D. J. Perreault et al. evaluated with respect to magnitude, duration, and rise and

TABLE 25.4 Automotive temperature extremes by location [3] fall times. Specific limits for such transients are specified by

Vehicle location

Min temp. ( ◦ C) Max temp. ( ◦ C)

the vehicle manufacturer, but SAE J1113/42 proposes a rep- resentative set of limits for four different transient severity

Due to the tight conducted emissions limits, input EMI fil-

Isolated

ter design is an important consideration in automotive power

Near heat source

electronics. Single or multistage low-pass filters are typically

Drive train high temperature

used to attenuate converter ripple to acceptable levels [11–13].

location

When designing such filters, the parasitic behavior of the filter Interior

components, such as capacitor equivalent series resistance and

Rear deck

inductance, and suitable filter damping are important consid-

Instrument panel

erations [14]. One must also ensure that the filter design yields

Instrument panel top

acceptable transients at switch on and off, and does not result

Trunk

in undesired dynamic interactions with the power circuit [13]. Attention to appropriate filter design, coupled with proper cir- Under hood

Near radiator support

cuit layout, grounding, and shielding goes a long way towards

structure

meeting electromagnetic interference specifications [14].

Intake manifold

Near alternator

Exhaust manifold

25.3.4 Environmental Considerations

Dash panel (normal)

Dash panel (extreme)

The automobile is a very challenging environment for elec- tronics. Environmental factors influencing the design of auto- motive electronics include temperature, humidity, mechanical shock, vibration, immersion, salt spray, and exposure to sand, gravel, oil, and other chemicals. In 1978, the SAE developed

In addition to the temperature extremes in the automobile,

a recommended practice for electronic equipment design to thermal cycling and shock are also important considerations address these environmental considerations [3, 4]. This doc- due to their effect on component reliability. Thermal cycling ument, SAE J1211, provides quantitative information about refers to the cumulative effects of many transitions between the automotive environment to aid the designer in developing temperature extremes, while thermal shock refers to rapid environmental design goals for electronic equipment. Here, we transitions between temperature extremes, as may happen briefly summarize a few of the most important factors affecting when a component operating at high temperature is sud- the design of power electronics for automotive applications. denly cooled by water splash. The damaging effects of thermal For more detailed guidelines, the reader is referred to [3] and cycling and shock include failures caused by thermal expan- the documents cited therein.

sion mismatches between materials. Test methods have been Perhaps the most challenging environmental characteristic developed which are designed to expose such weaknesses is the extreme range of temperatures that can occur in the [3, 16]. The thermal environment in the automobile, including automobile. Table 25.4 summarizes some of the temperature the temperature extremes, cycling, and shock, are challeng- extremes listed in SAE J1211 for different locations in the auto- ing issues that must be addressed in the design of automotive mobile. Ambient temperatures as low as −40 ◦

C may be found power electronics.

during operation, and storage temperatures as low as −50 ◦ C A number of other important environmental factors exist in may be found for components shipped in unheated aircraft. the automobile. Humidity levels as high as 98% at 38 ◦

C can Maximum ambient temperatures vary widely depending on exist in some areas of the automobile, and frost can occur in sit- vehicle location, even for small differences in position. Because uations where the temperature drops rapidly. Salt atmosphere, ambient temperature has a strong impact on the design of a spray, water splash, and immersion are also important fac- power electronic system it is important to work closely with the tors for exterior, chassis, and underhood components. Failure vehicle manufacturer to establish temperature specifications mechanisms resulting from these factors include corrosion and for a particular application. For equipment that is air-cooled, circuit bridging. Dust, sand, and gravel bombardment can one must also consider that the equipment may be operated at also be significant effects depending on equipment location. altitudes up to 12,000 feet above sea level. This results in low Mechanical vibration and shock are also important considera- ambient pressure (down to 9 psia), which can reduce the heat tions in the design of automotive power electronic equipment. transfer efficiency [3]. For equipment utilizing the radiator- Details about the effects of these environmental factors, sam- cooling loop, maximum coolant temperatures in the range of ple recorded data, and recommended test procedures can be 105–120 ◦

C at a pressure of 1.4 bar are possible [15].

found in [3].

25 Automotive Applications of Power Electronics 649

25.4 Functions Enabled by Power

voltage from 12 V to the voltage needed for the steady-state

Electronics

operation of the HID lamp. Any dc–dc converter that can step up the voltage, such as the boost or flyback converter, can be

Over the past 20 years, power electronics has played a major used for this application. An H-bridge is then used to create role in the introduction of new functions such as the antilock the ac voltage that drives the lamp in steady state. The circuit breaking system (ABS), traction control, and active suspension, to initiate the arc can be as simple as a circuit that provides an as well as the electrification of existing functions such as the inductive voltage kick, as shown in Fig. 25.5. engine-cooling fan, in the automobile. This trend is expected to continue, as a large number of new features being considered

25.4.2 Pulse-width Modulated Incandescent

for introduction into automobiles require power electronics.

Lighting

This section discusses some of the new functions that have been enabled by power electronics, and some existing ones Future automobiles may utilize a 42 V electrical system in place that benefit from it.

of today’s 14 V electrical system (see Section 25.7). Because HID lamps are driven through a power electronic ballast, HID lighting systems operable from a 42 V bus can be easily devel-

25.4.1 High Intensity Discharge Lamps

oped. However, the high cost of HID lighting – as much as an order of magnitude more expensive than incandescent light-

High intensity discharge (HID) lamps have started to appear in ing – largely limits its usefulness to headlight applications. automobiles as low-beam headlights and fog lights. The HID Incandescent lamps compatible with 42 V systems can also be lamps offer higher luminous efficacy, higher reliability, longer implemented. However, because a much longer, thinner fila- life, and greater styling flexibility than the traditional halo- ment must be employed at the higher voltage, lamp lifetime gen lamps [17, 18]. The luminous efficacy of an HID lamp is suffers greatly. An alternative to this approach is to use pulse- over three times that of a halogen lamp and its life is about width modulation to operate 12 V incandescent lamps from a 2000 hours, compared to 300–700 hours for a halogen lamp.

42 V bus [20].

Therefore, HID lamps provide substantially higher road illumi- In a pulse-width modulated (PWM) lighting system, a semi- nation while consuming the same amount of electrical power conductor switch is modulated to apply a periodic pulsed and, in most cases, should last the life of the automobile. The voltage to the lamp filament. Because of its resistive nature, HID lamps also produce a whiter light than halogen lamps the power delivered to the filament depends on the rms of the since their color spectrum is closer to that of the sun.

applied voltage waveform. The thermal mass of the system fil- High intensity discharge lamps do not have a filament. ters the power pulsations so that the filament temperature and Instead, light is generated by discharging an arc through a light production are similar to that generated by a dc voltage pressurized mixture of mercury, xenon, and vaporized metal with the same rms value. The PWM frequency is selected low halides – mercury produces most of the light, the metal halides enough to avoid lamp mechanical resonances and the need for determine the color spectrum, and xenon helps reduce the EMI filtering, while being high enough to limit visible flicker; start-up time of the lamp [17, 19]. Unlike halogen lamps that PWM frequencies in the range of 90–250 Hz are typical [20]. can be powered directly from the 12-V electrical system, HID

Ideally, a 11.1% duty ratio is needed to generate 14 V rms lamps require power electronic ballasts for their operation. across a lamp from a 42 V nominal voltage source. In practice, Initially, a high voltage pulse of 10–30 kV is needed to ignite deviations from this duty ratio are needed to adjust for input the arc between the electrodes and a voltage of about 85 V is voltage variations and device drops. In some proposed systems, needed to sustain the arc [4.3]. Figure 25.5 shows a simplified multiple lamps are operated within a single lighting module power electronic circuit that can be used to start and drive with phase staggered (interleaved) PWM waveforms to reduce an HID lamp. A step-up dc–dc converter is used to boost the the input rms current of the module.

12 V HID lamp

Starter

Boost converter

H-bridge

FIGURE 25.5 Simplified power electronic circuit for an HID lamp ballast.

650 D. J. Perreault et al. Another issue with PWM lighting relates to startup. Even

with operation from a 12 V dc source, incandescent lamps have an inrush current that is 6–8 times higher than the steady-

state value, because of how filament resistance changes with rotor

metal ring

lining material

temperature; this inrush impacts lamp durability. The addi- tional increase in peak inrush current due to operating from neutral plane

metal ring piezoelectric

stator

a 42 V source can be sufficient to cause destruction of the

ceramic

filament, even when using conventional PWM soft-start tech-

r niques (a ramping up of duty ratio). Means for limiting the peak inrush current – such as operating the controlling MOS-

FET in current limiting mode during startup – are needed to

segment

segment

make practical use of PWM lighting control. While PWM incandescent lighting technology is still in the

poling direction

early stages of development, it offers a number of promis-

C (b)

ing advantages in future 42 V vehicles. These include low-cost FIGURE 25.6 (a) Basic structure of a traveling wave piezoelectric ultra- adaptation of incandescent lighting to high-voltage systems, sonic motor and (b) structure of the piezoceramic ring and electrode for a control of lighting intensity independent of bus voltage, the four-wavelength motor. Arrows indicate direction of polarization. Dashed ability to implement multiple intensities, flashing, dimming, lines indicate segments etched in the electrode for poling but electrically etc. through PWM control, and the potential improvement of connected during motor operation. lamp durability through more precise inrush and operating control [20].

When a positive voltage is applied between terminals A and C, the downwards poled segment elongates and the