Meeting Car Audio Design Challenges with Class D Amplifiers

Automotive audio designers are looking for high-performance solutions at a low cost. As in-vehicle infotainment systems continue to add more features and subsystems, the audio power budget for the main unit and relay unit is approaching its limit. Many people believe that the appropriate use of ultra-efficient Class D amplifiers is becoming the most sensible option available.

Unlike audio amplifiers used in home theater systems, design engineers cannot simply increase power while finding a way to control audio quality to achieve these goals. The head unit under the dashboard of a car has very strict heat dissipation and space requirements. The power supply voltage is also limited and is often subject to interference such as voltage spikes and other electronic and mechanical systems in the car. At the same time, as the number of speakers and channels increases, the power requirements are higher, and the space to accommodate the audio drive system is smaller.

As a result, audio power requirements are bound to increase. There are two main ways to deal with this situation. One is the traditional approach, which is to add more channels driven by standard audio amplifiers. This solution has been used in active audio systems, where each amplifier drives a speaker. However, due to the large number of channels, this approach is becoming increasingly complex and difficult to handle as a complete solution.

Another approach is to increase the power output by reducing the speaker resistance or using a DC/DC converter to increase the supply voltage. With this solution, a single amplifier can drive two or three speakers and still maintain high audio performance.

However, both methods have a common drawback: they increase power dissipation. Therefore, to achieve the power dissipation target, a more efficient amplifier is needed.

Compared to Class AB amplifiers, Class D amplifiers have efficiencies up to 95%, which allows power budgets to be controlled while producing superb sound. This also means that they require smaller heat sinks and more electronic components can be installed 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 certain output power range.

It’s important to note that these two approaches are not mutually exclusive. In fact, innovative engineering often uses hybrid solutions, and automotive audio is no exception. Design engineers often make decisions based on the following key factors:

Head unit size, power requirements, and power dissipation performance

Cost of audio system

Audio Performance

Eliminate interference from other electronic and electromechanical equipment

Class D and Class AB Amplifier Comparison

Class AB amplifiers have become the standard amplifier in the field of automotive audio applications due to their many advantages. The relevant technology is also relatively mature, and it is relatively easy to develop various applications without adjustment. Compared with early products of Class D amplifiers, Class AB amplifiers have the natural advantage of not generating electromagnetic interference (EMI).

However, the 50% operating efficiency of the Class AB amplifier results in relatively high power and heat dissipation, which is important in extremely sophisticated audio systems. For the audio body, the Class AB amplifier's 18V or higher supply voltage does not produce higher output power due to increased power dissipation.

In contrast, Class D amplifiers operate at 90% efficiency and can be designed with digital connections to the digital signal processor (DSP) that processes the audio, saving the cost of integrating analog/digital signal converters for the DSP. Class D amplifiers can also be integrated into the 60 V distribution mains.

Six-channel case

Today, most production cars are equipped with four audio channels connected to eight speakers. In addition, the amplifier must support the entire audio frequency range, and the bass and mid-range speakers usually share a channel and power amplifier. This final adjustment of the four-channel configuration can cause echoes near the doors.

Adding two channels solves a number of problems. First, it allows the powerful subwoofer to be driven individually through the two additional channels to the speakers under the front seats of the car, eliminating door echo. It also allows for higher sound fidelity because all speakers do not have to operate over the entire frequency range. See Figure 2 for a comparison of four-channel and six-channel audio architectures.

But every car audio designer will tell you that space and cooling requirements limit the power dissipation of the head unit to less than 20W. The traditional way to solve this problem is to connect some of the speaker lines to an external amplifier distribution box in the relay unit. Although this solution works, it makes the system more complex overall and increases the cost.

Appropriate application of Class D amplifiers is a cost-effective solution. First, look at the traditional amplifier values. The 55% efficient Class AB amplifier dissipates 100 Ω, while the 94% efficient Class D amplifier only dissipates 100 Ω.

Using six Class AB amplifier channels results in a total power dissipation of 27W, which is more than the head unit would normally dissipate at maximum. However, if a mix of the two amplifiers is used, the power budget can be met, even with just two Class D amplifiers (most likely for the woofer). Figure 3 compares the use of only two Class D amplifiers in a six-channel audio system with other solutions, with the last row showing the difference between 20W and the total power dissipation for that particular configuration.

The cost of Class D amplifiers may make Option B the best choice for mid-range cars. But looking forward, especially in the future "premium audio system" market (and higher voltage rail market), Class D amplifiers are likely to expand their market penetration.

A top-of-the-line car audio system may support at least eight and as many as 22 channels, many of which are routed to the repeater unit. If the system does not use Class D amplifiers, supporting that many channels would be a nearly impossible task.

In the process of constantly balancing cost and sound quality goals, design engineers can study various combinations of Class AB and Class D amplifiers. The primary application area of ​​Class D amplifiers is applications that require low power dissipation and high output power. These applications include systems above 90W. Specific application categories can be divided into the following four categories:

Top audio system: 8 to 22 channels driven by a mix of Class AB and Class D amplifiers, with an output power greater than 28W per channel.

Mid-range audio system, optimized for low power dissipation: 4 to 6 channels driven by Class D amplifiers with an output power greater than 25W per channel.

Mid-range audio system, optimized for cost: 4 to 6 channels driven by a mix of Class AB and Class D amplifiers.

Primary audio system: 2 to 4 channels driven by Class AB amplifiers with an output power of less than 28W per channel.

Optimizing Class D Amplifiers in Automotive Audio Systems

The automotive environment is extremely challenging for the application of Class D amplifiers. For example, the output voltage of a Class D amplifier is affected by the power supply voltage, and the supply voltage in a car is not constant. Therefore, measures must be taken to suppress the ripple voltage of the power supply in practice, and this suppression effect can be achieved 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 through phase interleaving, frequency hopping and AD/BD modulation. NXP has further designed and developed a patented solution that integrates EMI suppression functions into the amplifier itself.

The current spikes that cause EMI interference are caused by the dead time between transistors when the amplifier is switching. When not loaded, charge accumulates in the diode and is released in the form of a current spike as shown in Figure 4, where the red line represents the current spike.

Obviously, the solution to EMI interference is to eliminate dead time. NXP's semiconductor manufacturing experts point out that silicon-on-insulator (SOI) technology is an ideal choice because all components can be isolated by oxide. When the output is below ground, the substrate of the device does not accumulate charge, reducing the reverse recovery time and there is no cross-interference with other channels.

NXP uses SOI Advanced Bipolar-CMOS-DMOS (ABCD) technology in its Class D amplifiers. In addition to suppressing EMI interference, this process has another advantage over bulk Bipolar-CMOS-DMOS (BCD) processes: it does not experience latch-up effects that could damage the device.

Summarize

Class D amplifiers are increasingly popular in car audio applications. Perhaps by 2015, they will account for 300% of the car audio amplifier market. NXP has accumulated a lot of knowledge about Class D amplifiers. This experience in the consumer field will lead to trend-setting products and applications as Class D amplifiers gradually enter the automotive field.