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Matthew Beardsworth

Jan 29, 2016

Mid/High-Power
Class D Amplifiers

TPA3251D2

audio
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TPA3251D2EVM

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Selecting the right inductor for ultra-low distortion Class-D
audio amplifiers

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Selecting the inductance value for the output filter of a Class-D audio
amplifier is always a critical design decision. With next-generation
ultra-low-distortion Class-D amplifiers, selecting an inductor with poor
electrical properties can severely limit the audio performance. My colleague
Brian talked about how high-definition audio is changing the way we listen in
his December blog post. In this post, I’ll discuss key considerations for
selecting the right inductor to ensure your device is living up to its
high-definition potential.

In higher-power Class-D amplifiers, (generally above 10W of output power),
the passive output filter generally has both an inductor and a capacitor (LC) on
each output terminal, and is thus referred to as an LC filter. The purpose of
the LC filter is to convert the discontinuous pulse-width modulation  (PWM)
pulse-train output of a Class-D amplifier into a continuous smooth analog
sinusoid. The LC filter extracts the audio signal from the PWM representation of
the audio signal.

This filtering process is critical for a couple of reasons:

Electromagnetic Interference (EMI) reduction. The PWM output of a
Class-D amplifier is a high-amplitude voltage signal usually equal to the
output stage or PVDD supply voltage. Filtering these pulses with an LC filter
also filters the high-frequency content associated with the PWM pulses,
reducing offending EMI emissions. With the LC filter placed as close as
possible to the amplifier, long runs of speaker wire will not radiate EMI
throughout the system.

Reduced ripple current. For Class-D amplifiers with AD modulation
schemes, but without an LC filter, there is a ripple current superimposed on
the audio signal. With an LC filter, specifically as the cut-off frequency of
the LC filter is reduced relative to the PWM switching frequency of the
amplifier, the ripple current is also reduced such that only a small residual
ripple will present after the LC filter. The reactance of the LC filter
filters the rest of the ripple and ideally does not dissipate any
power.

Let’s use the TPA3251D2 as an
example. It is a 175W Class-D audio amplifier with total harmonic distortion and
noise (THD +N) capable of approaching 0.001% in the mid power band. For the TPA3251D2, inductor linearity becomes critical to extract
the highest level audio performance.

For this discussion, inductor linearity is defined as inductance vs.
current.

An ideal inductor would maintain the specified inductance value no matter
what current passes through it. However, real-world inductors always have
decreasing inductance with increasing current. At some point, the current level
will saturate the inductor and the inductance will fall off severely. This is
often specified as Isat.

Keep in mind that the inductance change at the Isat current rating
varies between manufacturers and even inductor types. Some manufacturers specify
Isat at a 30% or higher change in inductance. If you expected to use
this inductor all the way to the Isat rating for an LC Class-D
filter, you would observe very poor audio performance.

Table 1 shows data collected from 4 four different inductors that have good
linearity specifications for high-performance Class-D audio amplifiers. I
measured the inductance at 1A of current and again at 20A of current with a
600-KHz test signal, which is the nominal PWM switching frequency of the TPA3251D2 amplifier. The average change of inductance was
calculated for 10 samples of each inductor.

Table 1: Average change in inductance for 10 samples of
four inductor types

From the data above, the 10µH inductor from manufacturer A is more linear
than the 10H inductor from manufacturer B. It is also important to note that the
7µH and 10µH inductors from manufacturer A are wound on the same core. Likewise,
the 7µH and 10µH from manufacturer B are also wound on the same core.

Generally, the higher the inductance (the more turns of wire) for a given
core material, size and geometry, the less linear the inductor.

I then tested these inductors on the TPA3251D2
evaluation module (EVM) and the results were clear, as you can see in Figures 1
and 2.

Figure 1: TPA3251D2EVM plot
of output power and THD+N for measured inductors

Figure 2: TPA3251D2EVM plot
of THD+N and signal frequency

From the data collected from the TPA3251D2EVM, the
10µH inductor from manufacturer A outperforms manufacturer B’s 10µH and 7µH
inductors. If I were to select manufacturer B’s 10µH inductor, the THD
performance of the amplifier would be limited to 0.0045% at 10W, compared to
0.0017% for the manufacturer A’s 10µH inductor.

Also notice that at 20W, the THD vs. frequency performance improves
significantly. If you were designing an LC filter around an amplifier where
higher THD performance is acceptable, or if the native THD of the amplifier was
higher, the 10µH from manufacturer B may be a suitable candidate. In the end,
the designer of the system must make a choice between inductor linearity, cost
and size.

Other considerations such as switching losses or ohmic losses under high
output power levels will determine which inductor is best for a given system.
However, by selecting an inductor with greater linearity, you can improve the
THD performance of a Class-D amplifier significantly, as demonstrated with the
ultra-low THD of the TPA3251D2.

If you have worked with a Class-D amplifier before, log in and leave a
comment below. I’m interested in learning more about which inductor you
chose and the resulting performance.

Additional resources

Search TI’s entire mid-/high-power Class-D amplifiers portfolio.

Read the TPA3251D2
data sheet.

Check out the TPA3251D2D2
PurePath™ ultra-High Definition evaluation module.

See other recent audio blogs.

James Lamb

In Figure 1, the distortion is higher at low power outputs. Why is this?
From that graph the maximum THD over the range 0.5 W -- 100 W is ~0.006%,
regardless of which inductor is used.

Feb 7, 2016

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Matthew Beardsworth

The decreasing slope in figure 1 is caused by the nature of the THD + N
measurement. Since the noise of the amplifier is constant, increasing the
output signal amplitude relative to the noise floor produces a lower THD + N
figure. Once the harmonic content (THD) from the output signal is greater than
the noise floor (N), the THD+N flattens off and begins to increase.

Since the different inductors contribute different amounts of distortion,
the more linear inductors continue to show decreasing THD + N at higher output
power levels.

Hope this helps!

Matt

Feb 8, 2016

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