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Automotive
DesignLine  >  Design
Center  >  Infotainment

Component selection, layout
lead to success with Class D audio amps

By Jun Honda & Jonathan Adams,
International Rectifier

Courtesy of
Audio DesignLine
(01/23/2006 8:18 AM
EST)

A
Class D audio amplifier is basically a
switching amplifier or PWM amplifier. There are
a number of different classes of amplifiers. We
will take a look at the definitions for the
main classifications as an introduction:

Class A – In a Class A amplifier, the
output devices are continuously conducting for
the entire cycle, or in other words there is
always bias current flowing in the output
devices. This topology has the least distortion
and is the most linear, but at the same time is
the least efficient at about 20%. The design is
typically not complementary with a high and low
side output devices.

Class B – This type
of amplifier operates in the opposite way to
Class A amplifiers. The output devices only
conduct for half the sinusoidal cycle (one
conducts in the positive region, and one
conducts in the negative region), or in other
words, if there is no input signal then there is
no current flow in the output devices. This
class of amplifier is obviously more efficient
than Class A, at about 50%, but has some issue
with linearity at the crossover point, due to
the time it takes to turn one device off and
turn the other device on.

Class AB –
This type of amplifier is a combination of the
above two types, and is currently one of the
most common types of power amplifier in
existence. Here both devices are allowed to
conduct at the same time, but just a small
amount near the crossover point. Hence each
device is conducting for more than half a cycle
but less than the whole cycle, so the inherent
non-linearity of Class B designs is overcome,
without the inefficiencies of a Class A design.
Efficiencies for Class AB amplifiers is about
50%.

Class D – This class of amplifier
is a switching or PWM amplifier as mentioned
above. This class of amplifier is the main focus
of this application note. In this type of
amplifier, the switches are either fully on or
fully off, significantly reducing the power
losses in the output devices. Efficiencies of
90-95% are possible. The audio signal is used to
modulate a PWM carrier signal which drives the
output devices, with the last stage being a low
pass filter to remove the high frequency PWM
carrier frequency.

Classes A, B and AB
are all what is termed linear amplifiers. We
will discuss the differences between Linear and
Class D amplifiers in the next section. The
block diagram of a linear amplifier is shown
below in figure 1. In a linear amplifier the
signals always remain in the analog domain, and
the output transistors act as linear regulators
to modulate the output voltage. This results in
a voltage drop across the output devices, which
reduces efficiency. Class D amplifiers take on
many different forms, some can have digital
inputs and some can have analog inputs. Here we
will focus on the type which have analog inputs.

Figure 1 above shows the basic block diagram
for a Half Bridge Class D amplifier, with the
waveforms at each stage. This circuit uses
feedback from the output of the half-bridge to
help compensate for variations in the bus
voltages.

So how does a Class D
amplifier work? A Class D amplifier works in
very much the same way as a PWM power supply (we
will show the analogy later). Let’s start with
an assumption that the input signal is a
standard audio line level signal. This audio
line level signal is sinusoidal with a frequency
ranging from 20Hz to 20kHz typically. This
signal is compared with a high frequency
triangle or sawtooth waveform to create the PWM
signal as seen in
figure 2a below.

This PWM signal is then used to drive the
power stage, creating the amplified digital
signal, and finally a low pass filter is applied
to the signal to filter out the PWM carrier
frequency and retrieve the sinusoidal audio
signal (also seen in figure 2b).

Topology Comparison – Linear vs.
Class D
In this section we will discuss
the differences between linear (Class A and
Class AB) amplifiers, and Class D digital power
amplifiers. The primary and main difference
between linear and Class D amplifiers is the
efficiency. This is the whole reason for the
invention of Class D amplifiers. The Linear
amplifier is inherently very linear in terms of
its performance, but it is also very inefficient
at about 50% typically for a Class AB amplifier,
whereas a Class D amplifier is more efficient,
with values in the order of 90% in practical
designs. Figure 3 below shows typical efficiency
curves for linear and Class D
amplifiers.

Gain – With Linear amplifiers the gain
is constant irrespective of bus voltage
variations, however with Class D amplifiers the
gain is proportional to the bus voltage. This
means that the power supply rejection ratio
(PSRR) of a Class D amplifier is 0dB, whereas
the PSRR of a linear amplifier is very good. It
is common in Class D amplifiers to use feedback
to compensate for the bus voltage variations.

Energy Flow – In linear
amplifiers the energy flow is always from supply
to the load, and in Full bridge Class D
amplifiers this is also true. A half-bridge
Class D amplifier however is different, as the
energy flow can be bi-directional, which leads
to the “Bus pumping” phenomena, which causes the
bus capacitors to be charged up by the energy
flow from the load back to the supply. This
occurs mainly at the low audio frequencies i.e.
below 100Hz.

Analogy to a Synchronous
Buck Converter
A simple analogy can be
made between a Class D amplifier and a
synchronous buck converter. The topologies are
essentially the same as can be seen below in
figure 4.

The main difference between the two circuits
is that the reference signal for the synchronous
buck converter is a slow changing signal from
the feedback circuit (a fixed voltage), in the
case of the Class D amplifier the reference
signal is an audio signal which is continuously
changing. This means that the duty cycle is
relatively fixed in the synch buck converter,
whereas the duty is continuously changing in the
Class D amplifier with an average duty of 50%.
In the synchronous buck converter the load
current direction is always towards the load,
but in Class D the current flows in both
directions.

The final difference is in
the way the MOSFETs are optimized. The
Synchronous buck converter is optimized
differently for the high and low side MOSFETs,
with lower RDS(on) for longer duty and low Qg
for short duty. The Class D amplifier has the
same optimization for both of the MOSFETs, with
the same RDS(on) for high and low
side.

Power Losses in the
MOSFETs
The losses in the power switches
are very different between linear amplifiers and
Class D amplifiers. First lets look at the
losses in a linear Class AB amplifier. The
losses can be defined as:

Where K is the ratio of Vbus to output
voltage. This can then be simplified down to the
following equation for the linear amplifier
Power switch losses:

Note that the power loss is not related to
the output device parameters. Figure 5 below
shows the power loss vs K.

Now lets look at the losses for a Class D
amplifier. The total power loss in the output
devices for a Class D amplifier are given by:

Psw are the switching losses and are given by
the equation:

Pcond are the conduction losses and are given
by the equation:

Pgd are the gate drive losses and are given
by the equation:

As can be seen in a Class D amplifier the
output losses are dependant on the parameters of
the device used, so optimization is needed to
have the most effective device, based on Qg,
RDS(on), COSS, and tf . Figure 6 below shows the
power losses vs K for the Class D
amplifier.

Similar to conventional Class AB amplifiers,
Class D amplifiers can be categorized into two
topologies, half-bridge and full-bridge
configurations. Each topology has pros and
cons. In brief, a half-bridge is potentially
simpler, while a full-bridge is better in audio
performance. The full-bridge topology requires
two half-bridge amplifiers, and thus, more
components. However, the differential output
structure of the bridge topology inherently can
cancel even the order of harmonic distortion
components and DC offsets, as in Class AB
amplifiers. A fullbridge topology allows of the
use of a better PWM modulation scheme, such as
the three level PWM which essentially has fewer
errors due to quantization.

In the
half-bridge topology, the power supply might
suffer from the energy being pumped back from
the amplifier, resulting in severe bus voltage
fluctuations when the amplifier outputs low
frequency audio signals to the load. This
kickback energy to the power supply is a
fundamental characteristic of Class D
amplification. Complementary switching legs in
the full-bridge tend to consume energy from the
other side of the leg, so there is no energy
being pumped back towards the power supply.
Table 1 shows the summary of the
comparison.

An ideal Class D amplifying stage has no
distortion and no noise generation in the
audible band, along with providing 100%
efficiency. However, as shown in
Figure 7, practical Class D amplifiers have
imperfections that generate distortions and
noise.

The imperfections
are caused by the distorted switching waveform
being generated by the Class D stage. The causes
are:

1. Nonlinearity in the PWM signal
from modulator to switching stage due to limited
resolution and/or jitter in timing
2. Timing
errors added by the gate drivers, such as
dead-time, ton/toff, and tr/tf
3. Unwanted
characteristics in the switching devices, such
as finite ON resistance, finite switching speed
or body diode characteristics.
4. Parasitic
components that cause ringing on transient edges

5. Power supply voltage fluctuations due to
its finite output impedance and reactive power
flowing through the DC bus
6. Non-linearity
in the output LPF.

In general, switching
timing error in a gate signal is the primary
cause of the nonlinearity. The timing error due
to dead-time in particular has the most
significant contribution of nonlinearity in a
Class D stage. A small amount of deadtime in
the tens of nano-seconds can easily generate
more than 1% of THD (Total Harmonic Distortion).
Accurate switching timing is always a primary
concern. Let’s take a look at how the dead-time
affects nonlinearity. The operation mode in a
Class D output stage can be categorized into
three different regions based on how the output
waveform follows the input timing. In those
three different operation regions, the output
waveform follows different edges in high side
and low side input signals.

Consider the
first operating region where the output current
flows from the Class D stage to the load when
the amount of the current is larger than the
inductor ripple current. At the instant of high
side turn-off and prior to low side turn-on, the
output node is driven to the negative DC bus.
This action is automatically caused by the
commutation current from the demodulation
inductor, regardless of low side turn-on timing.
Therefore the timing in the output waveform is
not influenced by the dead-time inserted into
the turn-on edge of low side, and always follows
the high side input timing. Consequently, the
PWM waveform is shortened only by the dead-time
inserted into the high side gate signal,
resulting in slightly lower voltage gain as
expected from the input duty cycle. A similar
situation happens to the negative operation
region where the output current flows from the
load to the Class D stage. The amount of the
current is larger than the inductor ripple
current. In this case, the timing in the output
waveform is not influenced by the dead-time
inserted into the turn-on edge of the high
side, and always follows the low side input
timing. Consequently, the PWM waveform is
shortened only by the dead-time inserted into
the low side gate signal.

There is a
region between the two operation modes described
earlier where the output timing is independent
of the dead-time. When the output current is
smaller than the inductor ripple current, the
output timing follows the turn-off edge of each
input because, in this region, turnon is made by
ZVS (Zero Voltage Switching) operation. Hence,
there is no distortion in this middle region. As
the output current varies according to the audio
input signal, the Class D stage changes its
operation regions, which each have a slightly
different gain. The output waveform will be
distorted by these three different gain regions
in a cycle of the audio signal.

Figure 8 shows how significantly dead time
affects THD performance. A 40nS dead time can
create 2% THD. This can be improved to 0.2% by
tightening the dead time down to 15nS. This
punctuates the significance of seamless high
side and low side switching for better
linearity.

Audio Performance Measurement
Audio
measuring equipment with an AES17 brick wall
filter, such as the Audio Precision AP2, are
necessary. However a classic audio analyzer like
the HP8903B can be used with appropriate
pre-stage low pass filter is applied. The
important consideration here is that the output
signal of a Class D amplifier still contains
substantial amount of switching frequency
carrier on its waveform, which causes a wrong
reading, and those analyzers might not be immune
enough to the carrier leak from a Class D
amplifier.
Figure 9 shows an example of a
filter.

Narrow Deadtime Risk
A narrow
dead-time can be very risky in mass production.
Once both high and low side MOSFETs are turned
on simultaneously, the DC bus voltage will be
short circuited by the MOSFETs. A huge amount of
shoot-through current starts to flow, which will
result in device destruction. It should be
noticed that the effective deadtime can be vary
from unit to unit due to variation of component
values and the MOSFET die temperature.

Figure 10 shows the relationship between the
length of the dead time and the amount of
shoot-through charge. It is extremely important
for a reliable design of a Class D amplifier to
ensure that the dead-time is always positive and
never negative to prevent MOSFETs from entering
the shoot through condition.

Another marked cause of degradation in Class
D amplifiers is bus pumping, which can be seen
when the half bridge topology is powering a low
frequency output to the load. Always keep in
mind that the gain of a Class D amplifier stage
is directly proportional to the bus voltage.
Therefore, bus fluctuation creates distortion.
Since the energy flowing in the Class D
switching stage is bi-directional, there is a
period where the Class D amplifier feeds energy
back to the power supply. The majority of the
energy flowing back to the supply is from the
energy stored in the inductor in the output LPF.
Usually, the power supply has no way to absorb
the energy coming back from the load.
Consequently the bus voltage is pumped up,
creating bus voltage fluctuations. Bus pumping
does not occur in full bridge topologies because
the energy kicked back to the power supply from
one side of the switching leg will be consumed
in the other side of the switching
leg.

EMI in Class D Amplifiers

EMI (Electro-Magnetic Interference) in Class D
amplifier design is troublesome like in other
switching applications. One of the major sources
of EMI comes from the reverse recovery charge of
the MOSFET body diode flowing from the top rail
to the bottom, similar to the shoot-through
current. During the dead-time inserted to
prevent shoot through current, the inductor
current in the output LPF turns on the body
diode. In the next phase when the other side of
the MOSFET starts to turn on at the end of the
dead-time, the body diode stays in a conducting
state unless the stored minority carrier is
fully discharged. This reverse recovery current
tends to have a
sharp spiky shape and leads to unwanted
ringing from stray inductances in PCB traces and
the package. Therefore, PCB layout is crucial
for both ruggedness of the design and reduction
of EMI.

Conclusion
Highly efficient Class D
amplifiers now provide performance similar to
conventional Class AB amplifiers if key
components are carefully selected and the layout
takes into account the subtle, yet significant
impact of parasitic components. Greater
efficiency, increased power density and better
audio performance are driving the increased use
of Class D amplifiers with even more
improvements on the horizon.

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