[Design Tips] Design Tips for Achieving Extremely Low EMI in Inductorless Class D Audio Applications

导读：功率电感和铁氧体磁环的价格差异显著，这推动了D类音频放大器滤波设计步入无电感时代。但同时，在铁氧体磁珠的作用下，滤波器的截止频率会急剧飙升，从几千赫兹增加到几兆赫兹；从而削弱了滤波器的EMI抑制效果。因此，D类应用亟需降低EMI噪声。在D类音频无电感应用中，要取得良好的EMI结果取决于电路板电平调整与适当的PCB布局。

Ferrite rings equipped with appropriate capacitance can reduce the edge rate of Class D output, but at the same time they will also produce some transient oscillations, which will aggravate the conducted electromagnetic interference. Therefore, a Zobel circuit is needed to reduce transient oscillations.

This article will introduce some circuit board level adjustment techniques, including ferrite bead selection principles to reduce edge rates, Zobel network adjustment methods to reduce transient ringing, and proper PCB layout. These solutions help customers save significant system design costs while achieving excellent audio performance by utilizing TI's latest EMI optimized Class D audio amplifier TPA3140D2.

Inductorless filter

The purpose of inductor-free design is to replace expensive inductors with low-cost ferrite beads, so as to achieve low-cost EBOM (engineering bill of materials) goals at the system level for customers. Ferrite beads are equivalent to multi-layer chip inductors. Limited by the current ferrite ring materials and manufacturing technology, it is difficult for such inductors to withstand high current and high impedance at the same time. Taking Japan's Toko multi-layer chip inductors as an example, if the engineer sets the rated DC current value to >2.5A, most of the inductance values ​​will be less than 1uH. Another product in the industry, the Sunlord ferrite bead series (UPZ2012), also has similar performance: if the maximum rated current is greater than 2.5A, the equivalent inductance value of the ferrite ring bead is less than 0.6uH.

Table 1 shows the impedance of the UPZ2012 series ferrite beads at 100MHz, as well as the maximum rated current and maximum DC resistance of different ferrite beads.

Table 1 Impedance and maximum current of 2012 type chip ferrite magnetic ring

As shown in Figure 1, the equivalent inductance value of the "120Ω@100MHz ferrite bead" is 0.39uH, while the equivalent inductance value of the 600Ω@100MHz ferrite bead is 1.59uH.

Figure 1 Ferrite beads with equal inductance values

When working, ferrite beads are equivalent to a parallel resonant circuit, just like an inductor working in the low frequency domain (<100MHz), a capacitor working in the high frequency domain (>100MHz), and a pure resistor working at its own resonant frequency point. When using ferrite beads to set the output filter, the basis is to use its inductance characteristics. Because each LC filter (passive filter) has its own resonant frequency, at this frequency point, the gain of the filter is very large, resulting in instantaneous oscillation after filtering. R1 and C1 will absorb the oscillation energy caused by the IC itself, usually using a 10Ω resistor and a 330pF capacitor. R2 and C2 will absorb the oscillation energy caused by the filter itself.

Figure 2 Ferrite bead filter design

How to achieve low EMI goals with inductorless filters?

• Recommendation 1: Choose ferrite beads to reduce edge rate

TI devices use some techniques to minimize conducted EMI noise in a 5MHz band (this frequency is usually the cutoff frequency of ferrite bead filters). Spreading the spectrum, phase shifting of the L and R channels (Class D stereo audio), etc. can also help. For EMI bandwidths less than 5MHz, especially when the switching frequency is around 300kHz (for better efficiency), experimental results show that reducing the edge rate is an effective way to reduce EMI.

Figure 3. Edge rate of ferrite rings with different impedances

In Figure 3, higher ferrite bead impedance can achieve lower edge rate Class D output; using 600ohm@100MHz ferrite beads, the lowest edge rate Class D output can be obtained, ultimately achieving the best EMI results in the high frequency band. However, higher impedance means lower rated current. In Table 1, impedance = 600ohm@100MHz, the maximum rated current is 2A. Take a TV customer as an example:

TV application example: PVDD (power supply) = 12V, speaker load = 8Ω, BD mode, ignore the on-resistance and DC resistance of PCB and ferrite beads. Maximum current = 12/8 = 1.5A.

In the case of PVDD = 12V / 8Ω speaker, engineers can use 600ohm@100MHz ferrite beads to design the filter.

Figure 4 shows the effect of ferrite beads on conducted EMI

Figure 4 Effect of ferrite beads on conducted EMI

Figure 5 shows the effect of ferrite beads on radiated EMI

Figure 5 Effect of ferrite beads on radiated EMI

Opinion 2: Use Zobel network to minimize transient oscillation.

Figure 6 is a typical circuit designed to reduce the oscillation effect of the output filter circuit. R1 and C1 will absorb the oscillation energy caused by the IC itself. R2 and C2 are used to absorb the oscillation caused by the filter resonant frequency.

Figure 6 Tuning to reduce oscillation and slow edge rate

In Figure 7.a, in the conducted EMI test noise band, an oscillation with a period of 350ns (about 2.85MHz) is captured. Its energy has been greatly weakened after the Zobel network, and a higher marginal gain is obtained.

Table 2 Filter and Zobel network settings

Figure 7 Adjusting the Zobel network and capacitors (reducing oscillations and obtaining slower edge rates)

However, another problem has arisen. Figure 8 shows that the oscillation exacerbates the noise in the 2MHz~4MHz band (if the Class D output current increases, the oscillation will be more serious). In theory, the higher the harmonic component, the smaller the amplitude should be, but the resonant frequency point of the filter changes this situation. Let's look at Figure 7.a. Compared with setting 4, setting 3 has better noise suppression in the 2MHz~5MHz band. In the end, setting 3 shows the best tuning effect in reducing oscillations, and obtains a lower edge rate and good EMI margin from 2MHz to 5MHz.

Figure 8 Oscillation exacerbates 2MHz~4MHz band noise (setting 4)

PCB Layout

Figure 9 shows the TI inductorless Class D audio reference design board (TPA3140D2). Figure 10 shows a typical output application circuit schematic.

Figure 9 The TPA3140 EVM board (left) saves a lot of filter PCB space

Figure 10 Schematic diagram of TPA3140 typical output application circuit

• Filter PCB layout

To minimize the filter current return path (current returning to GND), make sure the current loop is small.

1) Place the ferrite bead as close to the output pin as possible.

2) Minimize the current loop from filter ground (C8 to Class D ground pin)

3) Try to ensure that the bottom layer of the filter and Class D equipment is a complete ground layer.

4) If you want to add a Zobel network to reduce oscillations, place the Zobel network as close to the filter as possible.

5) Place the buffer circuit as close as possible to the output pin of the device.

Figure 11 Filter layout

•PVCC layout

Figure 12 PVCC layout

in conclusion

TI's latest inductorless Class-D stereo amplifier (TPA3140) enables inductorless design in medium-power Class-D applications. Depending on the different speaker line lengths and output power (current) requirements, audio system engineers can use some of the circuit board level tuning techniques mentioned in this article, including ferrite bead selection principles (reducing edge rate), Zobel network tuning methods (reducing oscillations), and proper PCB layout, etc. Ultimately, in customer system-level testing, the TPA3140 can achieve sufficient EMI margin. Feedback from current user designs shows that TI's TPA3140 is a true inductorless medium-power Class-D audio amplifier that can help customers achieve the best balance in terms of reducing system BOM costs, smaller PCB size, good EMC margin, and stable and good audio performance.

Source: TI

Author: YuQing Yang, Peter Cao, Xiaolin Qin