As car entertainment and infotainment systems add more features and
subsystems, the audio power budgets of the head and trunk units are
being pushed to the limit. Automotive audio designers are looking for a
high-performance, cost-effective solution. For many, the judicious use
of ultra-efficient Class D amplifiers is emerging as the best possible
choice.
In particular, multichannel and multispeaker systems are becoming
common in high-end cars. The design challenge for automotive engineers
is to maintain - or even to improve - the high audio amplification
levels and low distortion that customers have been expecting.
A specific instance of the need for higher power is the trend toward
high-power, two- or even three-way speaker systems and subwoofers.
Unlike audio amplifiers in home entertainment systems, design
engineers can't simply crank up the power and simultaneously find
clever ways to control the audio quality to achieve these goals. Heat
dissipation and space constraints in the head unit under the dashboard
are quite stringent.
The power supply voltage is also restricted and is frequently
disrupted by events such as voltage spikes and interference from other
electronic and mechanical systems in the car.
Every new model year brings a new subsystem - such as video or even
navigation and GPS - into the audio design space: more speakers, more
channels, higher power requirements and typically less space to house
the audio drive system.
Audio power requirements will certainly increase. There are two
primary ways to meet those needs. The conventional approach is to add
more channels driven by standard audio amplifiers. This solution is
already being used in active systems in which each amplifier drives a
single speaker. But it is becoming complex and increasingly untenable
as a complete solution because of the sheer number of channels.
Another approach is to raise power outputs by either lowering the
speaker impedance or raising the supply voltage using DC/DC converters.
With this solution, a single amplifier can drive two or three speakers
and still produce high performance audio.
Although the second solution is less complex, both methods have
something in common: They both increase dissipated power. Thus, to meet
power dissipation goals, using more efficient amplifiers become a
critical part of the solution.
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| Figure
1: Class D amplifiers provide better efficiency over a wider range than
Class AB amps. |
Efficiency
This need for more efficient amplifiers has made the discussion of
Class D audio amplifiers a hot topic among audio engineers. With
efficiencies as high as 95 percent (compared
with around 50 percent for Class AB amplifiers), Class D amps
can get the power budget under control and still produce superior
sound.
Their superior power efficiency means they need a smaller heat sink,
which means more space available for electronics in the tight space of
a head unit. However, Class D amps are more expensive than Class AB and
they have special design considerations. Figure 1 above shows the relative
efficiencies of Class AB and Class D amplifiers over a range of output
powers.
Keep in mind that the two approaches are not mutually exclusive. In
fact, innovative engineering often uses hybrid solutions.
Automobile audio power is no exception. Design engineers will make
their decisions based on several key considerations: size, power
requirements and power dissipation capability of the head unit; cost of
the audio system; audio performance; and mitigating interference from
other electronic and electromechanical equipment.
 |
| Table
1. Power dissipation values for several combinations of Class AB and
Class D amplifiers are compared. |
Amplifier basics
To fully understand the benefits and drawbacks of Class D amplifiers, a
review of different amplifier types is helpful.
*The output devices used in Class A amplifiers conduct
continuously during the entire cycle. In other words, a bias current is
always flowing in the output devices. Class A amplifiers deliver the
most linear output and thus create the least distortion. The downside
is that they are inefficient; they are typically about 20 percent
efficient.
*Output devices of Class B amps conduct for half the
sinusoidal cycle (one in the positive region, the other in the
negative). If there is no input signal, there is no current flow in the
output devices.
Class B amplifiers have maximum efficiencies of 78.5 percent at
maximum output power. However, the interval between the time one device
turns off and the other turns on creates linearity problems at the
crossover point.
*Class AB amps combine the two types.
Both devices conduct at the same time (although minimally) near the
crossover point. Each device conducts for more than half but less than
the whole cycle, and this overcomes the nonlinearity of Class B
designs.
Class AB amplifiers have efficiencies of about 50 percent and are
presently one of the most common types of power amplifiers.
*Class D amps are switching or
pulse-width modulation (PWM) amplifiers. Because the switches are
either fully on or fully off, losses in the output devices are
drastically reduced. Efficiencies of 90-95 percent have been reported.
The audio signal is used to modulate a PWM carrier signal, which
drives the output devices. Since Class D amps are switchers, however,
they create switching noise. The last stage is a low pass filter that
removes the high frequency PWM carrier frequency.
