In recent times, as you switch on your TV or browse the web, you're likely to come across numerous articles about the rising trend of mini-houses. The idea that smaller is better has taken hold, and this philosophy extends to other industries—like mobile phone manufacturing, where compact, lightweight devices are increasingly popular.
This same trend is now making its way into the automotive industry, particularly in the field of Class D audio amplifiers. In this blog post, I’ll explore how using a small inductor can enhance the performance of a Class D amplifier within an infotainment system.
**Background**
A Class D audio amplifier designed for vehicles typically requires an output filter to ensure clean audio signals. This filter consists of inductors and capacitors at each output, commonly known as an LC filter. Selecting the right values for these components is essential to meet audio quality, efficiency, EMC/EMI standards, and cost requirements—especially in mainframe and external amplifier applications.
The bridged load configuration shown in Figure 1 is a common setup where the speaker is connected between the two amplifier outputs. For this configuration, a typical vehicle Class D amplifier needs two inductors per channel (or speaker).
**Figure 1:** Class D amplifier bridged load circuit
**Why Are Small Inductors Important?**
As automakers continue to integrate more electronics into vehicles, space becomes a critical constraint. Suppliers must design smaller modules to accommodate this. Additionally, reducing vehicle weight is a priority to improve fuel efficiency. While many components are under review, I want to focus on the size and weight of inductors in car radio designs within infotainment systems.
A standard car radio typically supports at least four channels to drive front and rear speakers. For a Class D amplifier, this means eight inductors are required—two per channel. These inductors significantly contribute to the overall PCB size and system weight.
**Comparing Inductor Size with Amplifier Switching Frequency**
The inductance value needed in the LC filter depends on the switching frequency of the amplifier. In a typical 400kHz Class D amplifier, larger inductors like 10μH or 8.2μH are used. However, with higher switching frequencies, such as 2.1MHz found in devices like TI’s TAS6424-Q1, smaller inductors in the range of 3.3μH–3.6μH can be employed, offering significant size and weight reductions.
**Figure 2:** Comparison of inductor size and switching frequency
**What Increases Inductor Weight?**
The weight difference between an 8.2μH and a 3.3μH inductor mainly comes from the materials used—copper windings and core material. A four-channel car radio requires eight inductors. As shown in Figure 3, the total weight of copper and core material varies greatly between a 400kHz and a 2.1MHz system.
**Figure 3:** Inductance weight difference for a typical four-channel amplifier
**New Metal Alloy Inductors Enable Smaller Amplifiers**
Traditional inductors in 400kHz amplifiers use copper wire and ferrite cores. But with higher frequencies like 2.1MHz, new metal alloy core inductors can be used, allowing for much smaller and lighter components. As seen in Figure 4, these advanced inductors not only reduce weight but also help shrink the overall size of the amplifier.
**Figure 4:** Small Metal Alloy Inductors for Typical Quad Amplifiers
**Conclusion**
Class D amplifiers operating at higher switching frequencies, such as the 2.1MHz TAS6424-Q1, represent the future of automotive infotainment systems. These high-frequency designs not only support smaller, lighter systems but also drive innovation in inductor technology, paving the way for next-generation audio solutions.
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