As car entertainment and infotainment systems add more features andsubsystems, the audio power budgets of the head and trunk units arebeing pushed to the limit. Automotive audio designers are looking for ahigh-performance, cost-effective solution. For many, the judicious useof ultra-efficient Class D amplifiers is emerging as the best possiblechoice.
In particular, multichannel and multispeaker systems are becomingcommon in high-end cars. The design challenge for automotive engineersis to maintain – or even to improve – the high audio amplificationlevels and low distortion that customers have been expecting.
A specific instance of the need for higher power is the trend towardhigh-power, two- or even three-way speaker systems and subwoofers.
Unlike audio amplifiers in home entertainment systems, designengineers can't simply crank up the power and simultaneously findclever ways to control the audio quality to achieve these goals. Heatdissipation and space constraints in the head unit under the dashboardare quite stringent.
The power supply voltage is also restricted and is frequentlydisrupted by events such as voltage spikes and interference from otherelectronic and mechanical systems in the car.
Every new model year brings a new subsystem – such as video or evennavigation and GPS – into the audio design space: more speakers, morechannels, higher power requirements and typically less space to housethe audio drive system.
Audio power requirements will certainly increase. There are twoprimary ways to meet those needs. The conventional approach is to addmore channels driven by standard audio amplifiers. This solution isalready being used in active systems in which each amplifier drives asingle speaker. But it is becoming complex and increasingly untenableas a complete solution because of the sheer number of channels.
Another approach is to raise power outputs by either lowering thespeaker impedance or raising the supply voltage using DC/DC converters.With this solution, a single amplifier can drive two or three speakersand still produce high performance audio.
Although the second solution is less complex, both methods havesomething in common: They both increase dissipated power. Thus, to meetpower dissipation goals, using more efficient amplifiers become acritical part of the solution.
|Figure1: Class D amplifiers provide better efficiency over a wider range thanClass AB amps.|
This need for more efficient amplifiers has made the discussion ofClass D audio amplifiers a hot topic among audio engineers. Withefficiencies as high as 95 percent (comparedwith around 50 percent for Class AB amplifiers ), Class D ampscan get the power budget under control and still produce superiorsound.
Their superior power efficiency means they need a smaller heat sink,which means more space available for electronics in the tight space ofa head unit. However, Class D amps are more expensive than Class AB andthey have special design considerations. Figure 1 above shows the relativeefficiencies of Class AB and Class D amplifiers over a range of outputpowers.
Keep in mind that the two approaches are not mutually exclusive. Infact, innovative engineering often uses hybrid solutions.
Automobile audio power is no exception. Design engineers will maketheir decisions based on several key considerations: size, powerrequirements and power dissipation capability of the head unit; cost ofthe audio system; audio performance; and mitigating interference fromother electronic and electromechanical equipment.
|Table1. Power dissipation values for several combinations of Class AB andClass D amplifiers are compared.|
To fully understand the benefits and drawbacks of Class D amplifiers, areview of different amplifier types is helpful.
* The output devices used in Class A amplifiers conductcontinuously during the entire cycle. In other words, a bias current isalways flowing in the output devices. Class A amplifiers deliver themost linear output and thus create the least distortion. The downsideis that they are inefficient; they are typically about 20 percentefficient.
* Output devices of Class B amps conduct for half thesinusoidal cycle (one in the positive region, the other in thenegative). If there is no input signal, there is no current flow in theoutput devices.
Class B amplifiers have maximum efficiencies of 78.5 percent atmaximum output power. However, the interval between the time one deviceturns off and the other turns on creates linearity problems at thecrossover point.
* Class AB amps combine the two types.Both devices conduct at the same time (although minimally) near thecrossover point. Each device conducts for more than half but less thanthe whole cycle, and this overcomes the nonlinearity of Class Bdesigns.
Class AB amplifiers have efficiencies of about 50 percent and arepresently one of the most common types of power amplifiers.
* Class D amps are switching orpulse-width modulation (PWM) amplifiers. Because the switches areeither fully on or fully off, losses in the output devices aredrastically reduced. Efficiencies of 90-95 percent have been reported.
The audio signal is used to modulate a PWM carrier signal, whichdrives the output devices. Since Class D amps are switchers, however,they create switching noise. The last stage is a low pass filter thatremoves the high frequency PWM carrier frequency.
Class D vs. Class AB
Class AB amplifiers are today's standard in automotive audioapplications for a good reason. The technology is mature and wellunderstood, so applications are relatively easy to develop and do notrequire tweaks or re-spins.
High volume production and keen competition between several ICmanufacturers make prices reasonable. BOM cost is further reducedbecause Class AB amps require very few external components.
|Figure2: Four channel and six channel audio architectures are compared.Adding two channels allows the bass speakers to be drivenindependently, limits door resonance and makes higher sound fidelitypossible.|
And when comparing them with the initial product offerings of ClassD amps, AB amplifiers have the inherent advantage of not creating EMI.
The shortcomings of Class AB amps (comparatively high powerconsumption and heat dissipation caused by 50 percent operatingefficiency) are only becoming important as audio systems become moresophisticated.
A new shortcoming in the head unit will be that AB amplifiers arenot useful with supply voltages above 18V for higher output power dueto their increased power dissipation.
In addition to the benefits derived from its 90 percent operatingefficiency, Class D amps can be designed with a digital interconnect tothe DSP, which processes the audio and thus saving the DSP the cost ofintegrating ADC. Class AB amps primarily have an analog link, but it isa misnomer to call Class D “digital” amplification. Finally, Class Dscan be integrated into 60V power distribution mains.
The case for six channels
Most high volume cars manufactured today have four audio channels thatfeed eight speakers. Moreover, the amplifier must support the fullaudio frequency range, and the bass and midtone speakers typicallyshare the same channel and power amplifier. This last accommodation tothe four-channel configuration can create resonances in the door (Figure 2 above ).
Adding two channels solves several problems. First, it allows thepower-hungry bass speakers to be driven independently over the two newchannels to speakers under the front seats of the car. Door resonanceis eliminated. Higher sound fidelity is also possible because all ofthe speakers are not obligated to operate over the entire frequencyrange.
As any automotive audio designer would tell you, however, space andheat dissipation restrictions limit the power dissipation of the headunit to 20W. The conventional way around this problem is to route someof the speakers to an external amplifier box in the trunk unit. Whilethis solution is feasible, it increases overall system complexity andcost.
The use of Class D amps provides a cost-effective answer. Startingwith conventional amplifier values, a 55-percent efficient AB amplifierwould dissipate 4.5W. A 94-percent efficient Class D amp woulddissipate 0.6W.
Using six Class AB amp channels would result in a total of 27W powerdissipation – 7W more than the value typically considered the maximumfor a head unit, as shown in Case A in the table.
But mixing the two types of amplifiers would meet the power budgeteven if only two Class Ds were used, which would most likely be for thebass speakers. The bottom row of the table shows the difference between20W and the total power dissipation of the particular configuration.
The cost of Class D amps probably makes Case B the most likelychoice for a mid-range vehicle. But looking into the future(particularly the prospect of the “premium audio sound system” marketand higher voltage power rails), Class D amps are likely to expandtheir market penetration.
The audio systems of a premium vehicle may support at least eightand as many as 22 channels— and many of these would be in the trunkunit. Without incorporating Class D amplifiers into the system,supporting large numbers of channels would be a nearly impossible task.
In their never-ending balancing act between cost and quality goals,design engineers will find many combinations of Class AB and Class Damps. Class Ds will find their initial niches where low powerdissipation is critical and also (somewhat surprisingly) inapplications where very high power output is required. Theseapplications include greater than 90W systems where stereo Class Ds area good fit. The options, however, are likely to fall into fourcategories:
Premium: 8to 22 channels driven by a combination of Class AB and Class D withover 28W/channel goal;
Midrange soundoptimized for low power dissipation: four to six channels alldriven by Class D with a goal of greater than 25W/channel;
Midrange soundoptimized for cost: Four to six channels all driven by acombination of Class AB and Class D amps;
Basic sound: twoto four channels with all AB driven with a goal of less than 28Wchannel.
Dealing with EMI
The automotive environment is challenging for Class D applications. Allthe knowledge and skill of a semiconductor vendor experienced withClass D amps and automotive applications must be applied to design anoutstanding product.
|Figure3: During the dead time between transistors in the amplifier switching,a charge builds up in the body diode and this charge is released as acurrent spike (shown in red).|
For starters, I2C control has to be included because automotivedesigns require it. Beyond that, the challenges become more difficult.The output voltage of a Class D is influenced by the supply voltage,for example, and the supply voltage in a car is not constant.
Measures have to be taken for supply ripple voltage rejection. Thebest way to accomplish this is to use a negative feedback loop. Using asecond-order feedback loop provides superior ripple rejection.
As previously mentioned, EMI caused by switching is one of the mostimportant Class D problems and a very difficult one to solve. At thedesign level, EMI can be mitigated by phase staggering, frequencyhopping and AD/BD modulation.
Current spikes that contribute to EMI are created as a result of thedead time between the transistors in the amplifier switching. Duringthe dead time, a charge builds up in the body diode and this charge isreleased as a current spike (as shown in Figure 3 above , where the red lineindicates the spike).
The obvious solution is to eliminate dead time. Silicon-on-insulator(SOI) technology is ideal because all components are isolated by oxide.When an output falls below ground, no charge is built up in thesubstrate of the device, which reduces the reverse recovery time, andthere is no crosstalk to other channels.
(NXP uses an SOI Advanced BipolarCMOS-DMOS (ABCD) technology to fabricate its Class D amplifiers. Inaddition to taming EMI, this process has another advantage over bulkBipolar CMOSDMOS (BCD) processes – it is not subject to latch-up, whichcan potentially destroy the device. )
Class D amps are increasingly finding their way into automotive audioapplications and will continue to win market share. By 2015, they mayhave 30 percent of the audio amplifier automotive market.
Michael Kaufmann is Product MarketingManager, Business Unit Automotive & Identification, Business LineCar; and Kees van der Wolf is Application Engineer, Car AudioAmplifiers, at NXP Semiconductors.