You can’t turn on the TV or open a website lately without seeing or reading about the tiny home craze. Smaller is supposedly better, right? Similarly, mobile phone manufacturers tout how small, slim or lightweight their next-generation phone is.
It just seems natural that this trend would spill over into the automotive audio Class-D amplifier space. So in this post, I’ll talk about how tiny inductors are used with Class-D amplifiers in infotainment systems.
Automotive Class-D audio amplifier designs require a filter on the output of the amplifier. This filter uses both an inductor and a capacitor on each output terminal, usually referred to as an inductor-capacitor (LC) filter. Proper selection of the LC filter values is critical to meet the desired audio performance, efficiency, electromagnetic compatibility (EMC)/electromagnetic interference (EMI) requirements and cost, especially in automobile head unit and external amplifier applications.
The Class-D amplifier bridge-tied load circuit shown in Figure 1 is an output configuration where the speaker (or load) is connected between two amplifier outputs, bridging the two output terminals. Automotive Class-D audio amplifiers therefore typically require two inductors per channel (speaker).
Figure 1: Class-D amplifier bridge-tied load circuit
Why are tiny inductors important?
Automobile manufacturers are adding more electronic subsystems in today’s vehicles. Yet space is at a premium, so electronic module suppliers are under pressure to make their subsystems smaller. A secondary effect is that automobile manufacturers are continuously striving to make vehicles lighter weight to increase fuel efficiency. While there are many subsystems to evaluate, today I’d like to focus on comparing inductor sizes and weight as part of car radio designs within an infotainment system.
A typical car radio design has at least four channels to drive two front speakers and two rear speakers. This simple configuration would require eight total inductors for a Class-D automotive audio amplifier, since each channel requires two inductors, as shown in Figure 1. Thus, the size of each inductor is multiplied by eight, which is a significant contribution to overall printed circuit board (PCB) size and design weight.
Inductor size vs. amplifier switching frequency
For Class-D automotive audio amplifiers, the value of the inductor required in the LC filter to ensure the proper pulse-width modulation (PWM) demodulation filter characteristic depends on the switching frequency. In Figure 2, the inductor size used in LC filters for a typical Class-D audio amplifier design that switches around 400kHz are quite a bit larger and bulkier than those required for a newer Class-D amplifier design switching at a much higher frequency, such as 2.1MHz on TI’s TAS6424-Q1 device.
A 400kHz automotive audio amplifier typically uses either a 10µH or 8.2µH inductor value, while the 2.1MHz higher-switching-frequency amplifier design can take advantage of a much smaller and lighter-weight inductor in the range of 3.3µH to 3.6µH (assuming that each amplifier provides equivalent output power).
Figure 2: Comparison of inductor size vs. switching frequency
What drives inductor weight?
The weight difference between 8.2µH and 3.3µH inductors is mainly caused by the sheer difference in the weight of the copper windings and core material required to achieve the LC filter’s required inductance value and current-handling capability. As previously mentioned, a simple four-channel car radio design requires eight total inductors. In Figure 3, you can see the difference in weight (the total weight of copper and core material) of a four-channel radio design based on a 400kHz switching frequency versus one based on a 2.1MHz switching frequency.
Figure 3: Inductor weight differences for a typical four-channel amplifier
Newer metal-alloy inductors enable very small amplifier size
Inductors used in 400kHz switching frequency amplifier designs are traditionally made using copper wire and ferrite materials for the core. With the implementation of the higher 2.1MHz switching frequency, it’s possible to develop inductors with copper wire and a new metal-alloy core technology, which enables incredibly tiny inductor sizes. Figure 4 shows that the newer metal-alloy technology not only provides the lightest weight solution, but helps greatly reduce the overall solution size for a four-channel amplifier.
Figure 4: Tiny metal-alloy inductors for a typical four-channel amplifier
Higher-switching-frequency automotive Class-D audio amplifier designs, like the 2.1MHz TAS6424-Q1, are the way of the future to fulfill industry demands for next generation car radios and external amplifiers. The higher-switching-frequency amplifiers also help to drive technology advancement in the inductor industry, thereby helping to provide overall smaller and lighter weight system solutions. Want to know more about LC filter design? Download this application report for in-depth design information.
This piece, as well as the referenced application report, characterize the speaker itself as a resistor. While this gross over-simplification can be a time-saver for a quick analysis, in the case of Class-D amplifiers, it hides some important problems. I would remind the authors that speaker voice coils have both resistance and inductance. Of particular note is that the inductance is far from loss-less. The magnetic properties of the pole piece and surrounding magnetic structure are designed without consideration for their losses at MHz frequencies. Overlooking this is especially problematic in the highly-promoted filter-less class-D amplifiers made by TI, Maxim, and others. As a circuit design consultant, I recently worked on a project where my client complained that the speaker in his prototype ran hot and battery drain was unexpectedly high - even with no audio signal present! It wasn't a DC offset problem but "core loss" in the loudspeaker itself - and, of course, easily solved by using a proper filter. It seems ironic that papers about such filters are willing to "split hairs" over relatively insignificant THD differences among filter inductors (Wurth vs Coilcraft, for example in the application report) while overlooking something as important as real-world characterization of the loudspeaker itself.
Bill Whitlock, AES Life Fellow and owner/chief engineer of Jensen Transformers 1989-2014
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