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As in-vehicle infotainment systems continue to add more functionality and systems, the audio power budget of the body and relay unit is approaching the limit. Car audio designers are looking for low-cost, high-performance solutions. Many people believe that the appropriate use of high efficiency class D amplifiers is gradually becoming the most sensible choice.Unlike audio amplifiers used in home theater systems, it is not possible for design engineers to find the right way to control audio quality while simply increasing power to achieve these goals. The sound body under the dashboard of the car has strict requirements on heat dissipation and space. Power supply voltages are also limited and are often subject to interference such as voltage spikes and other electronic and mechanical systems in the vehicle. At the same time, as the number of speakers and channels increases, the power requirements are higher and the space for the audio drive system is smaller.
In this way, the audio power demand will inevitably increase. There are two main ways to deal with this situation. One is the traditional approach of adding more channels driven by standard audio amplifiers. This solution has been applied to active audio systems where each amplifier drives a speaker. However, due to the large number of channels, such solutions are becoming more and more complex and increasingly difficult to handle as a total solution.
Another method is to increase the power output by lowering the speaker resistance or by using a DC/DC converter to increase the power supply voltage. With this solution, a single amplifier can drive two or three speakers while still maintaining the high performance of the audio.
But both methods have a common flaw: they increase the power dissipated. Therefore, in order to achieve the power dissipation goal, it is necessary to utilize a more efficient amplifier.
Class D amplifiers are up to 95% efficient compared to Class AB amplifiers, allowing power budgets to be controlled while generating super-sound effects. This also means that they require smaller heat sinks and can accommodate more electronic components in a compact space. However, it cannot be ignored that Class D amplifiers are more expensive than Class AB and have special design requirements. Figure 1 shows the efficiency comparison of class AB amplifiers and class D amplifiers over a range of output powers.
It is worth noting that the two methods are not mutually exclusive. In fact, innovative engineering often uses hybrid solutions, and in-vehicle audio is no exception. Design engineers often make decisions based on the following key factors:
Comparison of Class D and Class AB amplifiers
Due to its many advantages, Class AB amplifiers have become the standard amplifiers for automotive audio applications. The related technology is also relatively mature, and it is relatively easy to develop various applications without adjustment. Class AB amplifiers have the natural advantage of not generating electromagnetic interference (EMI) compared to earlier products in Class D amplifiers.
However, the 50% efficiency of the Class AB amplifier results in relatively high power and heat dissipation, which is important in the case of extremely sophisticated audio systems. For the audio body, the power supply voltage of the Class AB amplifier of 18V or higher does not produce higher output power due to increased power dissipation.
In contrast, class D amplifiers operate at up to 90% efficiency and can be designed to interface with digital signal processors (DSPs) that process audio, saving the DSP the cost of integrating analog/digital signal converters. Class D amplifiers can also be integrated into 60 V distribution trunks.
Six-channel case
Today, most production cars are equipped with four audio channels that connect eight speakers. In addition, the amplifier must support the entire audio frequency range, and the bass and midrange speakers typically share a single channel and power amplifier. This final adjustment of the four-channel configuration creates an echo near the door.
Adding two channels can solve many problems. First of all, this allows the powerful woofer to be driven separately through two new channels to transmit sound to the speakers under the front seat of the car, eliminating door echo. Higher sound fidelity can also be achieved because all speakers do not have to operate over the entire frequency range. A comparison of the four-channel and six-channel audio structures is shown in Figure 2.
But every car audio designer will tell you that space and heat dissipation requirements will limit the power dissipation of the audio body to less than 20W. The traditional way to solve this problem is to connect some of the speaker lines to the external amplifier power distribution box of the relay unit. This solution, while feasible, can make the system more complex overall and increase costs.
Proper application of Class D amplifiers is a cost-effective solution. First look at the traditional amplifier values, Class AB amplifiers with an efficiency of 55% will dissipate 4.5W, while Class D amplifiers with an efficiency of 94% will only dissipate 0.6W.
Using six Class AB amplifier channels, a total of 27W of power dissipation is produced, which is more than the usual maximum dissipation of the audio body. But if you mix two amplifiers, you can achieve the power budget, even if you only use two Class D amplifiers (most likely for woofer). Figure 3 compares the scheme using only two Class D amplifiers in a six-channel audio system with other schemes. The last row shows the difference between the total power dissipation of 20W and a particular configuration.
The cost of a Class D amplifier may make Option B the best choice for mid-range cars. But looking ahead, especially in the future "top audio system" market (and higher voltage rail market), Class D amplifiers are likely to expand their market penetration.
The audio system of a top-class car may support at least 8 and a maximum of 22 channels, many of which will be placed in the relay unit. If the system does not use a Class D amplifier, supporting a large number of sound channels can be an almost impossible task.
Design engineers can develop multiple combinations of Class AB and Class D amplifiers while constantly weighing the cost and sound quality goals. The primary application area for Class D amplifiers is in applications requiring low power dissipation and high output power. These applications include systems above 90W. Specific application categories can be divided into the following four categories:
Optimize Class D amplifiers in automotive audio systems
The automotive environment is extremely challenging for Class D amplifier applications. For example, the output voltage of a class D amplifier is affected by the supply voltage, while the supply voltage in the car is not constant. Therefore, in practice, measures must be taken to suppress the ripple voltage of the power supply, and this suppression effect can be achieved by using a second-order feedback loop.
As mentioned earlier, EMI interference caused by switching is one of the most important issues for Class D amplifiers. At the design level, EMI interference can be mitigated by phase interleaving, frequency jitter, and AD/BD modulation. NXP further designed and developed a patented solution that incorporates EMI suppression into the amplifier itself.
The cause of the current spike causing EMI interference is the dead time between the transistors when the amplifier is switched. At no load, the charge accumulates in the diode and is released as a current spike as shown in Figure 4. The red line in the figure represents the current spike.
Obviously, the way to solve EMI interference is to eliminate the dead time. NXP's semiconductor manufacturing experts point out that silicon-on-insulator (SOI) technology is ideal because all components are insulated by oxide. When the output is below ground, the device's substrate does not accumulate charge, reducing the reverse recovery time and without cross-interference with other channels.
NXP uses SOI Advanced Bipolar-CMOS-DMOS (ABCD) technology in Class D amplifiers. In addition to suppressing EMI interference, this process has another advantage over the bulk Bipolar-CMOS-DMOS (BCD) process, that is, there is no latch-up that can damage the device.
to sum up
Class D amplifiers are increasingly favored by automotive audio applications. Maybe by 2015, they will account for 300% of the automotive audio amplifier market. NXP has accumulated a large amount of knowledge of Class D amplifiers. These consumer experience will gradually lead to trending products and applications as Class D amplifiers enter the automotive arena.
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