As high-definition audio systems become larger and more powerful, the redesign of Class D audio amplifiers will meet the special challenges of car audio design. For automotive information and infotainment systems, the increasing number of functions and subsystems have placed limits on the audio power budget of the front and body of the car. Car audio designers are looking for high-performance, low-cost solutions. For many applications, the use of ultra-high efficiency Class D amplifiers may be the best choice.
Especially for high-end cars, multi-channel, multi-speaker systems have become standard equipment. The design challenges faced by car audio engineers include: maintaining or even surpassing customer expectations for extremely high audio amplifier levels and ultra-low distortion; and in response to the trend towards dual or even three-channel speaker systems and subwoofers, higher power design.
Unlike audio amplifiers in home entertainment systems, design engineers cannot simply increase power and find a way to control audio quality to meet these goals. The space under the driving dashboard is very limited and does not allow a lot of heat. In addition, the power supply voltage in the car is also limited, and there are often voltage abnormalities caused by voltage jumps and interference from other electronic and mechanical systems in the car.
Each new model will add new subsystems to the sound design, such as video or even navigation and global positioning system (GPS). As a result, the audio system is faced with requirements for more speakers, more channels, and higher power. Usually, the space left for the audio drive system is more cramped.
The requirements for audio power have been increasing. There are two basic ways to meet these needs. The traditional way is to add more channels driven by standard audio amplifiers. This scheme is used in an active system where each amplifier drives a speaker. But because of the increase in the number of channels, this method becomes more complicated and more and more unsustainable.
Another method is to increase the power output by reducing the impedance of the speaker or using a DC / DC converter to increase the power supply voltage. With this method, a single amplifier can drive two or three speakers and still get high-definition audio output.
Although the second scheme is less complicated, the two schemes still have something in common: they both increase dissipative power consumption. Therefore, in order to meet the power consumption index, the use of a more efficient system becomes the key to the specific solution.
The need for more efficient amplifiers has made the discussion of Class D audio amplifiers a hot topic among audio designers.
The efficiency of Class D audio can be as high as 95%, while the efficiency of Class AB audio amplifier is only about 50%; therefore, Class D audio amplifier can control power consumption while providing excellent sound quality. The good power efficiency of Class D audio amplifiers means that they require only a smaller heat sink, saving space for the narrow front unit and accommodating more electronic systems. However, Class D amplifiers are much more expensive than Class AB amplifiers and require special design considerations.
Figure 1 shows the relative efficiency of Class AB amplifiers (Figure 1a) and Class D audio amplifiers (Figure 1b) in the output power range.
Figure 1 In a wider range, Class D audio amplifiers have higher efficiency. It is important to remember that these two methods are not incompatible. In fact, innovative engineering designs often use mixed solutions. Car audio is no exception. Design engineers will make decisions based on the following key considerations:
The size, power requirements and heat dissipation capacity of the front unit The cost of the audio system The audio performance How to reduce interference from other electronic and electromechanical equipment
1 Technical foundation of the amplifier In order to fully understand the class D audio amplifier, we first briefly introduce the technical foundation of the amplifier.
The output element of the Class A amplifier continues to conduct throughout the cycle. In other words, the bias current always flows through the output element. Class A amplifiers have the best linear output with minimal distortion. But the disadvantage is that the efficiency is too low, only about 20%.
The output elements of the Class B amplifier are turned on during the half cycle of the sinusoidal curve (one in the positive half and the other in the negative half cycle). If there is no input signal, no current flows through the output element. At maximum output power, Class B amplifiers have the highest efficiency, at 78.5%. However, the gap between one output element turning off and the other output element turning on can cause linearity problems at the intersection.
Class AB amplifiers are a combination of the above two types. Both components conduct simultaneously near the intersection (although very close). The turn-on time of each element is longer than half but shorter than one cycle, which overcomes the non-linearity of class B amplifier design. Class AB amplifier efficiency is about 50%. It is the most commonly used power amplifier.
Class D audio amplifiers are switching and pulse width modulation (PWM) amplifiers. Because the switch is either fully open or fully closed, the loss on the output element is greatly reduced. It is said that its efficiency can reach 90 ~ 95%. The audio signal modulation can drive the PWM carrier signal of the output device. However, because the Class D audio amplifier is a switching type, it will generate switching noise. The last stage is a low-pass filter that filters out high-frequency PWM carrier frequencies.
2 Comparison of Class D and Class AB amplifiers Class AB amplifiers are currently the standard in car audio applications. The technology is very mature, so it is relatively easy to develop products using it, and there is no need to adjust and start again. The fierce competition among many IC manufacturers also makes the price of Class AB amplifiers more reasonable. Because Class AB amplifiers only need a few external components, the raw material cost is further reduced. In addition, when compared to the original Class D audio amplifier, Class AB amplifiers have the advantage of not generating electromagnetic interference (EMI).
The disadvantages of class AB amplifiers include relatively high power consumption and heat generation caused by 50% operating efficiency, but these disadvantages become serious problems only when the audio system becomes more complicated. However, in the application of the front unit, the class AB amplifier has caused a new problem: due to the increasing power consumption, when the power supply voltage is higher than 18V, the class AB amplifier cannot be used to generate higher output power.
In addition to 90% efficiency, Class D audio amplifiers can also be designed by interconnecting with a digital signal processor (DSP) that processes audio, which saves the cost of integrating an analog / digital converter in the DSP (Class AB amplifiers have a basic analog connection; but it is not appropriate to refer to Class D audio amplifiers as 'digital' amplification.) Finally, Class D audio amplifiers can be integrated into 60V power lines.
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