AN1042D 반도체 회로 부품 판매점
High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs
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 ON Semiconductor |
AN1042/D
High Fidelity Switching
Audio Amplifiers Using
TMOS Power MOSFETs
Prepared by: Donald E. Pauly
ON Semiconductor
Special Consultant
http://onsemi.com
APPLICATION NOTE
Almost all switching amplifiers operate by generating a
high frequency square wave of variable duty cycle. This
square wave can be generated much more efficiently than
an analog waveform. By varying the duty cycle from 0 to
100%, a net dc component is created that ranges between
the negative and positive supply voltages. A low pass filter
delivers this dc component to the speaker. The square wave
must be generated at a frequency well above the range of
hearing in order to be able to cover the full audio spectrum
from dc to 20 kHz. Figure 1 shows a square wave
generating a sine wave of one–ninth its frequency as its
duty cycle is varied.
1.0
0.75
0.5
0.25
0
–0.25
–0.5
–0.75
–1.0
Input
Output
Switching Frequency =
9X Modulation Frequency
+1
0
–1
0°
90°
180°
270°
360° 420°
Figure 1. Switching Amplifier Basic Waveforms
The concept of switching amplifiers has been around for
about 50 years but they were impractical before the advent
of complementary TMOS power MOSFETs. Vacuum tubes
were fast enough but they were rather poor switches. A
totem pole circuit with supply voltages of ±250 volts would
drop about 50 volts when switching a current of 200
milliamps. The efficiency of a tube switching amp could
therefore not exceed 80%. The transformer needed to
match the high plate impedance to the low impedance
speaker filter was impractical as well.
This document may contain references to devices which are no
longer offered. Please contact your ON Semiconductor represen-
tative for information on possible replacement devices.
With the introduction of complementary bipolar power
transistors in the late 1960s, switching amplifiers became
theoretically practical. At low frequencies, bipolar transistors
have switching efficiencies of 99% and will directly drive
a low impedance speaker filter. The requirement for
switching frequencies above 100 kHz resulted in excessive
losses however. Bipolar drive circuitry was also complex
because of its large base current requirement.
With the advent of complementary (voltage/current
ratings) TMOS power MOSFETs, gate drive circuitry has
been simplified. These MOS devices are very efficient as
switches and they can operate at higher frequencies.
A block diagram of the amplifier is shown in Figure 2.
An output switch connects either +44 or –44 volts to the
input of the low pass filter. This switch operates at a carrier
frequency of 120 kHz. Its duty cycle can vary from 5% to
95% which allows the speaker voltage to reach 90% of
either the positive or negative supplies. The filter has a
response in the audio frequency range that is as flat as
possible, with high attenuation of the carrier frequency and
its harmonics. A 0.05 ohm current sense resistor (R27) is
used in the ground return of the filter and speaker to provide
short circuit protection.
The negative feedback loop is closed before the filter to
prevent instabilities. Feedback cannot be taken from the
speaker because of the phase shift of the output filter, which
varies from 0° at dc to nearly 360° at 120 kHz. Since the
filter is linear, feedback may be taken from the filter input,
which has no phase shift. Unfortunately, this point is a high
frequency square wave which must be integrated to
determine its average voltage. The input is mixed with the
square wave output by resistors R4 and R5 shown in Figure
2. The resultant signal is integrated, which accurately
simulates the effect of the output filter. The output of the
integrator will be zero only if the filter input is an accurate
inverted reproduction of the amplifier input. If the output
is higher or lower than desired, the integrator will generate
a negative or positive error voltage. This error voltage is
applied to the input of the switch controller, which makes
the required correction. The integrator introduces a 90°
phase shift at high frequencies which leaves a phase margin
of nearly 90°.
© Semiconductor Components Industries, LLC, 2002
August, 2002 – Rev. 3
1
Publication Order Number:
AN1042/D
Free Datasheet http://www.datasheet4u.com/
AN1042/D
Input
R5
R4
+44 V
Integrator
Switch
Controller
±2 V
Error
Current
Voltage
Sense
–44 V
Output
Switch
Low Pass
Filter
8Ω
Speaker
R27
Figure 2. Block Diagram of Class D Amplifier
The switch controller has three main functions. First, it
insures that the output duty cycle is never less than 5% or
greater than 95%. This is made necessary by the use of ac
coupling for the drive. Second, it controls the output duty
cycle in response to the error voltage input. This duty cycle
is a linear function of the error voltage input. Third, it
provides short circuit protection to the amplifier in
response to the current sense input. If overcurrent is
detected, the error voltage input will be overridden and the
amplifier output voltage reduced as necessary to bring the
current back within limits.
A class B analog amplifier has a theoretical efficiency of
78.5% when producing a sine wave at the point of clipping.
A switching amplifier, or so called class D amplifier, must do
much better to justify its extra complexity. The switching
amplifier described in this paper achieves an efficiency of
92% at its rated power of 72 watts. Its efficiency peaks at
95% for 30 watts output and falls to 50% for 1.5 watts
output. These efficiencies result from the good performance
of TMOS power MOSFETs at high switching frequencies
and the simplicity of complementary drive circuitry.
Above the 100 watt level, a switching amplifier costs less
than a conventional amplifier although it is slightly more
complex. The heatsink size is about one–tenth and the
weight is about one–fourth that of a class B amplifier.
1.0
0.8
–3 dB
0.6
0.4
0.2
0
Frequency (kHz)
0 12 24 36 48 60
Figure 3. 20 kHz Butterworth Filter Frequency
Response
A switching amplifier must switch at a frequency well
above the highest frequency to be reproduced. A low pass
filter must follow the switching stage to eliminate the high
frequency square waves and pass the audio to the speaker.
High switching frequencies can simplify filter design, but
cause excessive losses in the switching devices. Low
switching frequencies limit the upper frequency response
of the amplifier and complicate filter design. The amplifier
described in this paper operates at a switching frequency of
120 kHz. Its response extends down to dc, with an upper
–3 dB point of 20 kHz.
The filter chosen here is a 4 pole Butterworth Low Pass
which is maximally flat in the passband. It is designed to
be driven by a voltage source and loaded into 8 ohms. This
type of filter has a transfer function of
ǸE + 1
1
)
ǒ f Ǔ8
fc
where f is the frequency of interest and fc is the cutoff
frequency. At the 120 kHz switching frequency, this filter
has a voltage attenuation of 62 dB. With a ±44 volt square
wave into the filter at 120 kHz, the maximum residue is a
sine wave of about 30 millivolts rms. The filter is only 0.1 dB
down at 12.5 kHz and 1 dB down at 17 kHz as shown in
Figure 3. The –3 dB point is 20 kHz.
The frequency response of the filter will be flat only if it
is properly loaded into 8 ohms. A 16 ohm speaker load will
cause high frequency peaking and a 4 ohm speaker will
cause high frequency loss. The output impedance of the
filter changes across the band as shown in Figure 4. It
exhibits a parallel resonance at 11.4 kHz and 35.2 kHz, and
a series resonance at 20 kHz. In practice, these resonances
cause no difficulty with typical speakers and crossover
networks.
This amplifier and a high quality conventional amplifier
were both fed pink noise while driving full range speakers.
A broad band audio spectrum analyzer with a calibrated
microphone was used to measure sound pressure level. The
difference in sound pressure level between the two, if any,
was well under 1 dB from 60 Hz to 16 kHz.
http://onsemi.com
2
Free Datasheet http://www.datasheet4u.com/
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AN1042
High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs - ON Semiconductor
AN1042D
High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs - ON Semiconductor