In smart cockpits, in‑vehicle power amplifiers, and active speakers, one or two power inductors are often placed between the amplifier chip and the speaker. Though small in size, they can directly affect the system’s… Output ripple, power loss, temperature rise, and EMI performance 。

Image | The Class-D amplifier in-vehicle audio system—this inductor is… Class-D amplifier inductor . Today, First, clearly explain “what it is and what it does.” I. What is a Class-D amplifier?

When checking the technical specifications of speakers or car amplifiers, you’ll often see the “Class-D” designation.

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Here, “Class” denotes the operating class of the power amplifier; “Class-D” refers to a Class D amplifier. Common solution passed. Pulse Width Modulation (PWM) A switching-mode amplifier that achieves power amplification through technological means.

The letter “D” denotes only the topological sequence and does not indicate a sound quality grade.

Unlike Class A, Class B, and Class AB amplifiers, Class-D amplifiers… MOSFET The main work is in Fully turned on and Completely shut down With two operating modes, losses in the power device’s linear region are reduced, resulting in higher conversion efficiency.

Its efficiency typically exceeds 90%. Suitable for in-vehicle audio systems, powered speakers, and other devices with stringent requirements for power consumption, size, and heat dissipation.

1 Why Class-D Amplifiers? The output is PWM. Signal?

Class-D amplifiers do not directly linearly amplify the audio waveform; instead, they first use a modulation circuit to convert the audio signal into High-speed PWM pulse 。

The duty cycle of the PWM pulse varies with the input audio signal, so the audio information is effectively encoded in the pulse width.

After MOSFET Power stage Zoom in Subsequently, the output signal contains two components simultaneously:

To be retained Low-frequency audio signal Breath;

Need Inhibition of the High-frequency switching carrier and its harmonics.

Therefore, between the power-stage output and the speaker load, it is usually necessary to insert a Low-pass filter (LPF) 。2 LC filter: from a square wave Audio “extraction” in Chinese

The PWM signal output by a Class-D amplifier contains both audio information and a high-frequency switching carrier; therefore, it must be filtered through… LC low-pass filter Extract the audio component.

LC filter Its wiring method is: An inductor (L) is connected in series at the power amplifier’s output, while a capacitor (C) is connected in parallel across the load (the speaker). 。 f₀ = 1 / (2π√LC) Frequencies above the cutoff are attenuated, while frequencies below the cutoff are preserved; the actual cutoff characteristics are further influenced by the speaker’s impedance. , damping and output topology effects.

Figure | Connection configuration of the LC low-pass filter in a Class-D amplifier (single-ended equivalent schematic) 3 How does an LC filter “filter” the audio signal back?

The inductive reactance is:

The higher the frequency, the greater the impedance of the inductor (Z = 2πfL); therefore, it presents a higher impedance to high-frequency currents. It’s very good at blocking. ; whereas for low-frequency (audio) currents, it offers almost no resistance. Capacitors, by contrast, exhibit the opposite behavior: their impedance decreases with increasing frequency, allowing high-frequency currents to pass through… “Leak” to the ground . Inductor “blocks,” capacitor “leaks” », thus forming a low Bandpass filter: Audio below the cutoff frequency passes through. ， Switching carrier frequencies above the cutoff frequency are significantly attenuated. .4 The role of inductors in LC filters The inductor in a Class-D amplifier is connected in series within the power output path. In this context, it primarily undertakes four tasks: Retain Audio signal; attenuates the PWM switching carrier; reduces high-frequency ripple; improves system EMI.

From a functional standpoint, it must both carry the power amplifier’s output current and contribute to high-frequency filtering.

Overall,

Its inductance, saturation current, DC resistance, core losses, and temperature rise directly affect the power amplifier’s output capability, efficiency, and reliability.

Therefore, automotive and medium-to-high-power Class-D amplifiers typically require Power-type output filter inductor 。

Figure | Schematic of the operating principle of a Class-D amplifier II. Detailed explanation of the “Class-D” inductor

Medium- and High-Power Class-D Power Amplifier Commonly used It has good magnetic shielding. Integrated molded inductor 。 Used for Filtering at the output.

1 What does this inductor look like? Essentially, it is still an inductor: Magnetic core + winding However, in Class-D audio applications, the power‑amplifier inductor is typically Molded inductor — Press the alloy magnetic powder and the winding together into the package. This approach offers three advantages: Magnetic shielding Good—low external radiation helps improve the system’s EMI performance. Anti-saturation High performance, with more stable inductance under high current conditions; Surface-mount package , small in size, suitable for high-density deployment. 2 Common package types

Independent Molded Inductor:

Figure | SPMA Molded Inductor

Figure | TSPM Molded Inductor

Two-in-one molded inductor.

Figure | SPFDA1009S 2-in-1 Molded Power Inductor 3 Five-Step Method for Selecting “Class-D” Inductors

4 “Class-D” Inductor Application Scenarios Class-D amplifier inductors are primarily used in smart cockpits, automotive external amplifiers, active speakers, televisions, soundbars, and home audio systems.

Having understood the role of the inductor in a Class-D amplifier, the next question arises: In a BTL amplifier, why are two inductors used for a single channel?