U.S. patent application number 10/377559 was filed with the patent office on 2004-09-02 for class d switching audio amplifier.
Invention is credited to Butler, Joel.
Application Number | 20040169552 10/377559 |
Document ID | / |
Family ID | 25520813 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040169552 |
Kind Code |
A1 |
Butler, Joel |
September 2, 2004 |
CLASS D SWITCHING AUDIO AMPLIFIER
Abstract
A Class D switching audio amplifier incorporating four state
modulation, input-to-output drive and feedback signal isolation,
dual topology output filtration, and a low inductance board layout.
The four state modulation results in a common mode voltage in the
absence of audio. The input-to-output isolation of drive and
feedback signals allows for elimination of large power transformers
in applications without user-accessible outputs. Such isolation may
make use of optical isolators. The output filter includes common
mode and differential topology filter stages. The low inductance
board layout treats the amplifier and power supply boards as
modules, and utilizes both sides of the amplifier board in order to
minimize trace length.
Inventors: |
Butler, Joel; (Gardner,
KS) |
Correspondence
Address: |
THOMAS B. LUEBBERING
HOVEY, WILLIAMS, TIMMONS & COLLINS
Suite 400
2405 Grand
Kansas City
MO
64108
US
|
Family ID: |
25520813 |
Appl. No.: |
10/377559 |
Filed: |
February 27, 2003 |
Current U.S.
Class: |
330/10 ;
330/59 |
Current CPC
Class: |
H03F 3/217 20130101;
H03F 3/2173 20130101; H03F 3/2171 20130101; H03F 3/38 20130101;
H03F 3/085 20130101 |
Class at
Publication: |
330/010 ;
330/059 |
International
Class: |
H03F 003/38 |
Claims
Having thus described the preferred embodiment of the invention,
what is claimed as new and desired to be protected by Letters
Patent includes the following:
1. An amplifier comprising: a signal generator circuitry adapted to
generate a first pulse width modulated signal and a second pulse
width modulated signal; an output stage having a first input and a
second input; a first optoisolator interposed between and adapted
to electrically isolate the signal generator circuitry from the
first input of the output stage while transmitting the first pulse
width modulated signal therebetween; and a second optoisolator
interposed between and adapted to electrically isolate the signal
generator circuitry from the second input of the output stage while
transmitting the second pulse width modulated signal
therebetween.
2. The amplifier as set forth in claim 1, wherein the amplifier is
a Class-D switching amplifier.
3. The amplifier as set forth in claim 1, wherein the amplifier is
incorporated into a loudspeaker.
4. The amplifier as set forth in claim 1, wherein the output stage
is an H-bridge output stage.
5. A loudspeaker comprising: a Class-D switching amplifier
including--a signal generator circuitry adapted to generate a first
pulse width modulated signal and a second pulse width modulated
signal, an output stage having a first input and a second input, a
first optoisolator interposed between and adapted to electrically
isolate the signal generator circuitry from the first input of the
output stage while transmitting the first pulse width modulated
signal therebetween, and a second optoisolator interposed between
and adapted to electrically isolate the signal generator circuitry
from the second input of the output stage while transmitting the
second pulse width modulated signal therebetween.
6. A method of providing electrical isolation within an amplifier,
wherein the amplifier includes an output stage having a first input
and a second input, the method comprising the steps of: (a)
generating a first pulse width modulated signal and a second pulse
width modulated signal (b) optoisolating the first pulse width
modulated signal; (c) optoisolating the second pulse width
modulated signal; (d) using the optoisolated first pulse width
modulated signal to drive the first input of the output stage; and
(e) using the optoisolated second pulse width modulated signal to
drive the second input of the output stage.
7. The amplifier as set forth in claim 6, wherein the amplifier is
a Class-D switching amplifier.
8. The amplifier as set forth in claim 6, wherein the amplifier is
incorporated into a loudspeaker.
9. The amplifier as set forth in claim 6, wherein the output stage
is an H-bridge output stage.
10. A method of providing electrical isolation within a Class-D
switching amplifier, wherein the amplifier includes an output stage
having a first input and a second input, the method comprising the
steps of: (a) generating a first pulse width modulated signal and a
second pulse width modulated signal; (b) optoisolating the first
pulse width modulated signal; (c) optoisolating the second pulse
width modulated signal; (d) inverting the first pulse width
modulated signal to provide a third pulse width modulated signal;
(e) inverting the second pulse width modulated signal to provide a
fourth pulse width modulated signal; (f) using the first pulse
width modulated signal and the third pulse width modulated signal
to drive the first input of the output stage; and (g) using the
second pulse width modulated signal and the fourth pulse width
modulated signal to drive the second input of the output stage.
11. The amplifier as set forth in claim 10, wherein the amplifier
is incorporated into a loudspeaker.
12. The amplifier as set forth in claim 10, wherein the output
stage is an H-bridge output stage.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to Class D switching audio
amplifiers. More particularly, the present invention relates to a
Class D switching audio amplifier making use of four state
modulation, input-to-output drive and feedback signal isolation, a
dual topology output filter, and a low inductance board layout.
[0003] 2. Description of the Prior Art
[0004] It is often desirable to amplify audio signals using a Class
D switching audio amplifier. Basic circuit layout of the Class D
amplifier is substantially similar to that of linear amplifiers,
such as Class A, B, and AB, with a major difference being in the
signals provided to an output stage. Rather than feeding an audio
waveform directly to the output stage, as is done in linear
amplifiers, the Class D amplifier first feeds the audio waveform
into a Pulse Width Modulator (PWM) circuit which feeds modulated
pulses to the output stage. By quickly switching the output stage
completely on and completely off with varying pulse widths, the
Class D amplifier is able to recreate waveforms of almost any
shape, and, by filtering the switching output, sound is produced by
a loudspeaker connected thereto. In practice, the pulses are fed to
the output stages at a frequency between 100 and 300 kHz, or 100 to
300 thousand pulses per second, which is required to produce a
smooth waveform at the loudspeaker.
[0005] An advantage of the Class D amplifier is that the output
stage transistors are switched either completely on or completely
off. Amplifier topologies that operate in a partially on state,
such as Class A and AB, act like resistors and produce heat,
thereby wasting energy. Thus, Class D amplifiers are substantially
more efficient than non-switching linear amplifiers. Higher
efficiency and less waste heat allows the Class D amplifier to
utilize a smaller power supply and to be offered in a more compact
package than a comparable linear amplifier.
[0006] Unfortunately, existing Class D amplifier designs suffer
several disadvantages, including disadvantages related to
modulation, isolation, feedback, and board layout. Existing Class D
amplifier designs incorporate a full H-bridge output stage and use
a single PWM signal to derive four FET gate drive signals providing
two H-bridge switch states. Both H-bridge switch states result in a
differential voltage across the outputs leading to current flow
through the load. These two-state Class D amplifiers typically
compare a reference triangle waveform to an audio error waveform
(audio feedback) using a single comparator. The output of the
comparator is a single PWM signal with the same frequency as the
reference triangle waveform. The PWM signal is then passed through
a logic circuit that generates four drive signals used to drive the
H-bridge, resulting in a 180.degree. phase difference between
output.sub.NEG and output.sub.POS. Thus, a differential voltage is
always present at the output causing power to be lost via the
loudspeaker or low pass filter even in the absence of an audio
input to the amplifier.
[0007] Existing Class D amplifiers typically require large power
transformers to accommodate a relatively inefficient output stage
and to meet government regulations requiring high voltage isolation
between AC mains and all user-accessible inputs and outputs. This
isolation is typically achieved by incorporating one or more power
transformers between the AC mains and the input and output stages.
Unfortunately, such power transformers are large and expensive.
Furthermore, because 99% of any incoming power is required to drive
the output stage and the loudspeakers connected thereto, a power
transformer isolating the output stage must be substantially larger
than a power transformer isolating the input stage.
[0008] Even in applications where the outputs are not
user-accessible, no effort is typically made to isolate the input
stage from the output stage. Where input-to-output isolation is
attempted, small-signal audio transformers are typically used.
Unfortunately, these transformers suffer from limited frequency
response, making implementation difficult.
[0009] Typical output efficiencies for prior art linear amplifiers
are approximately 60%, with the remaining 40% of supplied power
being dissipated as heat. Consequently, expensive heat-sinking is
required, and large, expensive power transformers are needed to
deliver 66% more power than the desired output power of the
amplifier. With the development of Class D amplifiers, output
efficiencies increased to 85%, thereby reducing power supply
requirements and waste heat. Unfortunately, expected theoretical
efficiencies of 90+% for the Class D amplifier have not been
achieved, due primarily to the many problems and disadvantages set
forth herein.
[0010] Existing high-power Class-D amplifier designs incorporate a
control or feedback loop to minimize distortion. Conventional
control theory requires filtering, attenuating, and summing the
output signal with the input signal. This typically involves a
feedback loop comprising a differential RC low pass filter,
followed by an attenuating differential amplifier, and then a
summing amplifier to combine the feedback signal with the input
signal. For high power applications where common-mode voltages can
exceed 70Vdc, precision matching of feedback resistors is a
critical concern. Resistor tolerances greater than 1% in the
differential amplifier and the RC low pass filter sections result
in reduced common-mode rejection, potentially damaging voltages at
the differential amplifier, and degraded product reliability. The
RC low pass filter is required to attenuate the PWM switching
energy and to pass the audio signal to the differential amplifier.
This can result in decreased efficiency as power is lost in the RC
low pass filter even in the absence of an audio input signal. High
power applications require the use of high power resistors (>1
W) that can effectively dissipate the switching energy.
Unfortunately, precision matching and increased power handling
requirements for the RC low pass filter resistors result in
increased cost and size. For example, surface mount 1 W 1%
resistors are 7.5 times larger and 18 times more expensive than
standard 1/4 W 5% surface mount resistors.
[0011] Existing Class-D amplifier designs incorporate pairs of
multi-pole differential LC low pass filters to filter the
ever-present differential switching output voltage. Typical
multi-pole differential LC filter designs dissipate a majority of
attenuated energy in the first LC low pass filter pair. No
advantage is gained from common-mode filtering because the output
of the H-bridge continues to be a differential voltage. As a
result, high power designs are required to incorporate expensive
high power inductors that can dissipate the switching energy even
when no audio input signal is present.
[0012] Existing Class D amplifiers typically exhibit high harmonic
distortion above 1 kHz as a result of pulse transient damping
issues and poor triangle waveform damping generation. Excessive
pulse undershoot and overshoot result from high inductance board
layouts and power supplies. Some existing designs attempt to reduce
pulse overshoot and undershoot on H-bridge outputs by incorporating
large, expensive RC snubbers. Such undershoot and overshoot can
degrade reliability for many standard FET driver ICs such as
Harris' HIP4080A. Additionally, pulse transient damping issues also
lead to increased EMI emissions that increase the cost of shielding
the amplifier.
[0013] Triangle waveform generation has always been a source of
distortion in Class D amplifier designs. Triangle waves are
typically generated using RC oscillators made of operational
amplifiers or logic gates. These Class D amplifier designs suffer
from high frequency noise superimposed on the triangle waveform; in
turn, the high frequency noise results in increased harmonic
distortion. Thus, existing Class D amplifiers typically exhibit
undesirable harmonic distortion much greater than 0.5%.
[0014] Due to the above-identified and other problems and
disadvantages in the art, a need exists for an improved Class-D
audio switching amplifier.
SUMMARY OF THE INVENTION
[0015] The present invention overcomes the above-identified as well
as other problems and disadvantages in the art of Class D and
linear audio amplifiers by providing a Class D switching amplifier
operable to provide increased efficiency, increased reliability,
and reduced distortion through use of four state modulation,
input-to-output driver and feedback signal isolation, dual topology
output filtration, and a low inductance board layout. Though not
limited thereto, the amplifier is particularly ideal for
applications without user-accessible outputs, such as powered
loudspeakers, wherein isolation of input-to-output drive and
feedback signals allows for the elimination of large expensive
power transformers required by the prior art. Furthermore, though
not limited thereto, the amplifier is particularly ideal for
high-power applications involving, for example, 50 W or more.
[0016] The preferred Class D switching amplifier broadly comprises
an input stage; a triangle stage; a gate drive stage; an output
stage; a filter stage; and a feedback stage. The input stage is
operable to receive first and second feedback signals, FDBK_P and
FDBK_N, and an audio input signal, AUDIO_IN, and to therefrom
derive first and second error signals, ERROR and ERROR_INV. The
error signals represent the combined audio input and error for both
positive and negative swings.
[0017] The triangle stage is operable to derive a low noise
triangle waveform, TRIANGLE, having reduced high frequency noise
that might otherwise lead to excessive distortion.
[0018] The gate drive stage is operable to generate four optically
isolated gate drive signals, DRV_Q1, DRV_Q2, DRV_Q3, and DRV_Q4.
Within the gate drive stage, the ERROR signal and the TRIANGLE
waveform are also compared to produce an output signal, PWM_A; and
the ERROR_INV signal and the TRIANGLE waveform are compared to
produce an output signal, PWM_B. The PWM_A and PWM_B signals are
then input, respectively, to first and second optoisolators. The
optoisolators preferably allow isolated pulse transmission with
minimal delay and pulse width distortion. The outputs of the
optoisolators are taken directly to produce, respectively, the
DRV_Q2 and DRV_Q3 gate drive signals; and inverted to produce,
respectively, the DRV_Q1 and DRV_Q4 gate signals.
[0019] The output stage is operable to receive the gate drive
signals and to derive therefrom intermediate output signals, OUT_HP
and OUT_HN, and broadly comprises first and second H-bridge halves
which combine to form a full H-bridge.
[0020] The filter stage is operable to reduce EMI emissions by
attenuating switching energy, and is essential for series
connecting, or "daisy chaining", the floating output stages of
multiple instances of the Class D switching audio amplifier. The
filter stage receives as input the OUT_HP and OUT_HN signals, and
broadly comprises a four-pole LC low pass filter combining
common-mode filter topology for lowering inductor current in the
absence of an audio input signal, and differential filter topology
for attenuating high frequency differential signals. The filter
stage provides final output signals, OUT_P and OUT_N, to drive the
loudspeaker or other load.
[0021] The feedback stage is operable to provide the processed
feedback signals, FDBK_P and FDBK_N, to the input stage, and
broadly comprises first and second optoisolators 84,86 and first
and second RC low pass filters 88,90. Within the feedback stage,
the OUT_HP and OUT_HN signals produced by the output stage are
optoisolated and filtered through RC low pass filters to result in
the FDBK_P and FDBK_N signals.
[0022] As mentioned, the present invention introduces a unique four
state modulation scheme that advantageously increases efficiency
and allows for common-mode filtering to reduce loss during no-audio
conditions. Using the four state modulation scheme of the present
invention, in the absence of an audio input signal the H-bridge
outputs are common-mode (in phase) and no current is delivered to
the load. In the presence of an audio input signal, the H-bridge
outputs differentially drive current through the load at double the
frequency of the triangle waveform.
[0023] The input stage is isolated from the output stage using
optoisolators. Alternatively, small signal transformers may be used
in place of the optoisolators; however, the optoisolators, being
more cost and space effective, are preferred. Many available
optoisolators provide fast data transmission while minimizing pulse
distortion effects. By isolating the input from the output,
applications without user-accessible outputs can advantageously
eliminate expensive high power transformers commonly found in
existing amplifiers, thereby resulting in an estimated 75% weight
savings and 40% cost savings over typical prior art amplifiers.
[0024] Furthermore, isolating the input stage from the output stage
advantageously allows the output stage to float with respect to the
chassis or input ground, which, in turn, allows for series
connecting or "daisy chaining" multiple amplifiers to increase
power delivered to the loudspeaker. Another benefit of floating the
output stage is reduction of typical Class D chassis referenced DC
voltage present at the amplifier output.
[0025] Additionally, the present invention improves upon prior art
Class D feedback topology by isolating feedback signals and
referencing the RC low pass filter to the input stage ground. This
improvement eliminates potentially damaging differential and
common-mode voltages present in the feedback circuit. As a result,
precision resistor matching is no longer required, and less power
is lost in the RC low pass filter. Thus, isolating the feedback
signals substantially reduces costs and increases efficiency and
design reliability.
[0026] The filter stage includes an LC low pass output filter
operable to attenuate the high frequency switching, pass the
amplified audio signal, reduce radiated emissions, and smooth the
output current. In prior art Class D amplifiers, differential LC
low pass filter designs are used with 3 dB cutoffs at no less than
25 kHz. Regardless of whether audio is present at the output or
not, the filter is absorbing energy at the switching frequency.
With prior art modulation schemes no advantage was gained from
common-mode filtering because the output of the H-bridge was a
differential voltage waveform. As a result, high power designs were
required to incorporate expensive high current, low resistance
inductors in the LC low pass filters that could absorb the
switching energy with or without an audio signal present.
[0027] The modulation scheme of the present invention results in a
common-mode voltage in the absence of audio that allows for use of
a combination common-mode and differential LC low pass filter
constructed with inexpensive 5022 series surface-mounted inductors.
The first two-pole LC low pass filter combination is arranged in a
common-mode topology; the second two-pole LC low pass filter
combination is arranged in a differential topology. Through use of
a four pole combined common mode and differential LC output filter,
inductor current is reduced in the absence of an audio signal. With
the first two-pole combination typically absorbing more power, a
common-mode topology results in less power dissipation by reducing
inductor current 37% over prior art filters. By incorporating a
differential second two-pole combination, the filter maintains
beneficial rejection of high frequency differential signal
components.
[0028] The present invention utilizes a unique low inductance board
layout and modularization that advantageously lowers pulse
overshoot and undershoot, leading to reduced distortion, reduced
radiated emissions, and increased efficiency. The low inductance
design allows for elimination of expensive RC snubbers common in
prior art Class D amplifiers. With the improved board layout,
harmonic distortion has been reduced to less than 0.2% typical at
200W.sub.rms. Furthermore, the unique board layout reduces overall
size and allows for small lightweight construction.
[0029] These and other important features of the present invention
are more fully described in the section titled DETAILED DESCRIPTION
OF A PREFERRED EMBODIMENT, below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] A preferred embodiment of the present invention is described
in detail below with reference to the attached drawing figures,
wherein:
[0031] FIG. 1 is a circuit diagram of a preferred embodiment of the
present invention;
[0032] FIG. 2 is a circuit diagram of an alternate circuit topology
for an input stage portion of the present invention;
[0033] FIG. 3 is a circuit diagram of an alternate circuit topology
for a gate drive stage portion of the present invention;
[0034] FIG. 4 is a circuit diagram of a first alternate circuit
topology for a feedback stage portion of the present invention;
[0035] FIG. 5 is a circuit diagram of a second alternate circuit
topology for the feedback stage portion of the present
invention;
[0036] FIG. 6 is a depiction of a switching portion of the output
stage in a configuration corresponding to a first switch state;
[0037] FIG. 7 is a depiction of the switching portion of the output
stage in a configuration corresponding to a second switch
state;
[0038] FIG. 8 is a depiction of the switching portion of the output
stage in a configuration corresponding to a third switch state;
[0039] FIG. 9 is a depiction of the switching portion of the output
stage in a configuration corresponding to a fourth switch
state;
[0040] FIG. 10 is an side elevation view of a preferred hardware
layout scheme of the present invention;
[0041] FIG. 11 is a front elevation view of the preferred hardware
layout scheme shown in FIG. 10;
[0042] FIG. 12 is a top plan view of a topside amplifier board
hardware mounting scheme; and
[0043] FIG. 13 is a top plan view of a bottomside amplifier board
hardware mounting scheme.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0044] Referring to FIG. 1, a Class D switching audio amplifier 20
is shown constructed in accordance with a preferred first
embodiment of the present invention. The amplifier 20 is operable
to provide increased efficiency, increased reliability, and reduced
distortion through use of four state modulation, input-to-output
drive and feedback signal isolation, dual topology output
filtration, and a low inductance board layout. Though not limited
thereto, the amplifier 20 is particularly ideal for applications
without user-accessible outputs, such as powered loudspeakers,
wherein isolation of input-to-output drive and feedback signals
allows for the elimination of large expensive power transformers
required by the prior art. Furthermore, though not limited thereto,
the amplifier is particularly ideal for high-power applications
involving, for example, 50 W or more. The preferred Class D
switching amplifier 20 broadly comprises an input stage 22; a
triangle stage 24; a gate drive stage 26; an output stage 28; a
filter stage 30; and a feedback stage 32.
[0045] The input stage 22 is operable to derive first and second
error signals, ERROR and ERROR_INV, and broadly comprises a
differential amplifier 40; an error amplifier 42; and an inverting
amplifier 44. The differential amplifier 40 receives from the
feedback stage 32 first and second feedback signals, FDBK_P and
FDBK_N, and combines said feedback signals into one single-ended
signal. The single-ended signal is input to the error amplifier 42
where it is negatively summed with an audio input signal, AUDIO_IN.
Thus is the ERROR signal derived, representing the combined audio
input and error for both positive and negative swings. The
ERROR_INV signal is derived by inverting the ERROR signal using the
inverting amplifier 44.
[0046] It will be appreciated by those with ordinary skill in the
electrical arts that alternative circuit topologies may be devised
substantially equivalent in function to those described herein and
shown in the figures. Referring also to FIG. 2, for example, an
alternative topology for the input stage 122 is shown wherein two
error amplifiers 141,143 and an inverting amplifier 145 are used in
a substantially similar manner to generate the ERROR and ERROR_INV
signals.
[0047] The triangle stage 24 is operable to derive a low noise
triangle waveform, TRIANGLE, and broadly comprises an ultra-low
noise voltage regulator 46; a ramp capacitor 48; and a low ESR
capacitor 50; and a switched current source IC 52. The ultra-low
noise voltage regulator 46 reduces high frequency noise that might
otherwise lead to excessive distortion, and provides the ramp
capacitor 48 with a clean supply rail with minimized high frequency
transients. The low ESR capacitor 50 provides a low impedance path
to ground for high frequency transients. The switched current
source IC 52 provides a switched current capacitive charging
circuit; a suitable switched current source IC 52 is available from
Unitrode in the model UC3637. By incorporating such a switched
current source IC, a majority of the components required for
modulation signal derivation are self-contained, which results in
reduced cost and minimizes board real estate usage. The TRIANGLE
waveform is taken from the positive terminal of the ramp capacitor
48.
[0048] The gate drive stage 26 is operable to generate four
optically isolated gate drive signals, DRV_Q1, DRV_Q2, DRV_Q3, and
DRV_Q4, and broadly comprises a first comparator 54; a second
comparator 56; a first optoisolator 58; a second optoisolator 60; a
first invertor 62; and a second invertor 64. The ERROR signal and
the TRIANGLE waveform are input to the first comparator 54 to
produce an output signal, PWM_A; the ERROR_INV signal and the
TRIANGLE waveform are input to the second comparator 56 to produce
an output signal, PWM_B. The PWM_A and PWM_B signals are then
input, respectively, to the first and second optoisolators
58,60.
[0049] The optoisolators 58,60 preferably allow isolated pulse
transmission with minimal delay and pulse-width distortion. A
suitable optoisolator is available from NEC Corporation as model
number PS9701, which allows for fast data transmission with a
standard LED input drive and CMOS/TTL logic output. The outputs of
the optoisolators 58,60 are taken directly to produce,
respectively, the DRV_Q2 and DRV_Q3 gate drive signals; and
inverted by the invertors 62,64 to produce, respectively, the
DRV_Q1 and DRV_Q4 gate signals.
[0050] Referring also to FIG. 3, an alternative topology for the
gate drive stage 126 is shown wherein a high-speed inverting
amplifier 153 and a unity gain amplifier 155 precede, respectively,
the first and second comparators 154,156. In this topology, rather
than compare the TRIANGLE signal to the ERROR and ERROR_INV
signals, the ERROR signal is compared to the TRIANGLE and a
TRIANGLE_INV signal. The high-speed inverting amplifier 153
receives the TRIANGLE signal and produces the TRIANGLE_INV signal.
The unity gain amplifier 155 buffers the TRIANGLE signal.
[0051] It will be appreciated that the gate drive stage 26,
regardless of which circuit topology is used, includes a first
internal branch and a second internal branch. In the illustrated
embodiment, for example, the first internal branch includes the
first comparator 54, the first optoisolator 58 and the first
inverter 62, while the second internal branch includes the second
comparator 56, the second optoisolator 60, and the second inverter
64. Also regardless of which topology is used, separate reference
or triangle signals, whether TRIANGLE or TRIANGLE_INV, and separate
error signals, whether ERROR or ERROR_INV, must be applied to each
of the branches. Thus, as a matter of convenience, one may refer to
a first triangle or error signal (applied to the first internal
branch) and a second triangle or error signal (applied to the
second internal branch) to encompass both situations where the
first and second signals are identical and situations where the
first and second signals are different in some way (e.g., one is
inverted relative to the other). Use of the terms "first signal"
and "second signal" should not be interpreted to preclude these
signals from being identical signals branching from a single source
signal.
[0052] The output stage 28 is operable to provide intermediate
output signals, OUT_HP and OUT_HN, and broadly comprises a first
half-bridge 67, including a first half-bridge FET driver IC
(HVIC_1) 68 and first and second MOSFETs 70,72, and a second
half-bridge 77, including a second half-bridge FET driver IC
(HVIC_2) 78 and third and fourth MOSFETs 74,76. The HVIC_1 68 is
operable to level shift the DRV_Q2 and DRV_Q1 signals prior to
inputting said drive signals to the first and second MOSFETs 70,72,
with said MOSFETs 70,72 combining to produce an output signal,
OUT_HP. The HVIC_2 78 is operable to level shift the DRV_Q3 and
DRV_Q4 signals prior to inputting said drive signals to the third
and fourth MOSFETs 76,78, with said MOSFETs 76,78 combining to
produce an output signal, OUT_HN. The HVIC_1 and HVIC_2 68.78 are
required to properly drive the gates of each MOSFET 70,72,74,76 at
potentially high voltages.
[0053] The filter stage 30 is operable to reduce EMI emissions by
attenuating switching energy, and is essential for series
connecting, or "daisy chaining", the floating output stages 28 of
multiple instances of the Class D switching audio amplifier 20. The
filter stage 30 receives as input the OUT_HP and OUT_HN signals,
and broadly comprises a four-pole LC low pass filter combining
common-mode filter topology 80 for lowering inductor current in the
absence of an audio input signal, and differential filter topology
82 for attenuating high frequency differential signals. The filter
stage 30 provides final output signals, OUT_P and OUT_N, to drive
the loudspeaker or other load.
[0054] The feedback stage 32 is operable to provide processed
feedback signals, FDBK_P and FDBK_N, to the input stage 22, and
broadly comprises first and second optoisolators 84,86 and first
and second differential RC low pass filters 88,90. The OUT_HP and
OUT_HN PWM output signals produced by the output stage 28 directly
drive LEDs of, respectively, the first and second optoisolators
84,86. Passing the PWM output signals through optoisolators 84,86
effectively attenuates the output signals. The differential RC low
pass filters 88,90 are referenced to the input stage ground, and
operate to further attenuate switching energy and pass the desired
audio feedback signals. Driving the RC low pass filters 88,90 with
the low voltage outputs of the optoisolators 86,88 lowers current
in said RC low pass filters 88,90 and eliminates high common mode
voltages at the differential amplifier 40. The result is increased
efficiency, low resistor power handling requirements and reduced
precision matching requirements. Isolating the feedback signals
prior to the differential amplifier 40 and RC low pass filters
88,90 is necessary for complete input-to-output isolation and
improves product reliability and lowers cost.
[0055] Referring also to FIG. 4, a first alternative topology for
the feedback stage 132 is shown wherein isolation is accomplished
using a high-frequency small signal transformer 185 rather than
optoisolators. A disadvantage of this topology is pulse distortion
and poor performance as the pulse width approaches 100% duty cycle.
Nevertheless, this first alternative topology may be desirable for
some applications.
[0056] Referring also to FIG. 5, a second alternative topology for
the feedback stage 232 is shown wherein isolation is accomplished
using an audio small signal transformer 285 following the first and
second RC low pass filters 288,290. A disadvantage of this topology
is that it cannot servo out DC error, and that significant power is
lost in said RC low pass filters 288,290. Nevertheless, this second
alternative topology may be desirable for some applications.
[0057] As mentioned, the Class D switching audio amplifier 10 is
operable to provide increased efficiency, increased reliability,
and reduced distortion through use of four state modulation,
input-to-output optical isolation, feedback isolation, dual
topology output filtration, and a low inductance board layout.
[0058] Referring also to FIGS. 6, 7, 8, and 9, the four state
modulation scheme increases efficiency and allows for common-mode
filtering to reduce loss during no-audio conditions.
[0059] Existing Class-D amplifier designs typically use one
comparator to compare a reference triangle waveform to an error
signal. The output of the comparator is a single PWM signal with
the same frequency but different duty cycle as the reference
triangle waveform. The PWM signal is used to drive the four MOSFET
output switches in a full H-bridge configuration, wherein the four
drive signals are derived from the single PWM signal by
inversion.
[0060] The four state modulation scheme of the present invention,
when used to drive the full H-bridge output stage 28, increases
efficiency by reducing loss when no AUDIO_IN signal is present at
the input stage 22, and, furthermore, allows for a more efficient
filter stage 30 design. The modulating signals, DRV_Q1, DRV_Q2,
DRV_Q3, and DRV_Q4, used to drive the output MOSFETs 70,72,74,76
are derived in the gate drive stage 26 by comparing the ERROR
signal to the TRIANGLE waveform to produce the PWM_A signal; and
comparing the ERROR_INV signal to the TRIANGLE waveform to produce
the PWM_B signal. By comparing both the non-inverted and inverted
error signals, the audio is effectively sampled at two points. This
results in an output switching frequency two times higher than that
of the TRIANGLE and zero current drive when no AUDIO signal is
present.
[0061] The DRV_Q2 and DRV_Q3 drive signals are taken directly from
the PWM_A and PWM_B signals, while the DRV_Q1 and DRV_Q4 drive
signals are derived by inversion. In the presence of an AUDIO_IN
signal, the output signals, OUT_HP and OUT_HN, differentially drive
current through the loudspeaker at double the frequency of the
TRIANGLE waveform, as shown in FIGS. 6 and 7. In the absence of an
AUDIO_IN signal, however, the OUT_HP and OUT_HN signals are common
mode (in phase) and no current is delivered to the loudspeaker, as
shown in FIGS. 8 and 9.
[0062] In the gate drive stage 26, the optoisolators 58,60 are
incorporated to isolate the input from the output. Alternatively,
small signal transformers may be used in place of the
optoisolators; however, the optoisolators, being more cost and
space effective, are preferred. The optoisolation of the present
invention provides a minimum 2500V isolation barrier. Many
available optoisolators provide fast data transmission and isolated
pulse transmission while minimizing data throughput delays and
pulse width distortion effects. By isolating the input from the
output, applications without user-accessible outputs can
advantageously eliminate expensive high power transformers commonly
found in existing amplifiers, thereby resulting in an estimated 75%
weight savings and 40% cost savings over a typical prior art
amplifier.
[0063] Furthermore, isolating the input stage from the output stage
advantageously allows the output stage to float with respect to the
chassis or input ground, which, in turn, allows for series
connecting or "daisy chaining" multiple amplifiers to increase
power delivered to the loudspeaker. Another benefit of floating the
output stage is reduction of typical Class D chassis referenced DC
voltage present at the amplifier output.
[0064] Additionally, the present invention improves upon prior art
Class D feedback topology by isolating feedback signals and
referencing the RC low pass filter to the input stage ground. This
improvement eliminates potentially damaging differential and
common-mode voltages present in the feedback circuit. As a result,
precision resistor matching is no longer required, and less power
is lost in the RC low pass filter. Thus, isolating the feedback
signals substantially reduces costs and increases efficiency and
design reliability.
[0065] The filter stage 30 includes an LC low pass output filter
operable to attenuate the high frequency switching and pass the
amplified audio signal, and are required to reduce radiated
emissions and smooth the output current. In prior art Class D
amplifiers, differential LC low pass filter designs are used with 3
dB cutoffs at no less than 25 kHz. Regardless of whether audio is
present at the output or not, the filter is absorbing energy at the
switching frequency. With prior art modulation schemes no advantage
was gained from common-mode filtering because the output of the
H-bridge was a differential voltage waveform. As a result, high
power designs were required to incorporate expensive high current,
low resistance inductors in the LC low pass filters that could
absorb the switching energy with or without an audio signal
present.
[0066] The modulation scheme of the present invention results in a
common-mode voltage in the absence of audio that allows for use of
a combination common-mode and differential LC low pass filter
constructed with inexpensive 5022 series surface-mounted inductors.
The LC low pass filter reduces EMI emissions by attenuating
switching energy, and is essential for series connecting, or "daisy
chaining", the floating output stages 28 of multiple Class D
switching amplifiers 20. The first two-pole LC low pass filter
combination is arranged in a common-mode topology; the second
two-pole LC low pass filter combination is arranged in a
differential topology. Through use of a four pole combined common
mode and differential LC output filter, inductor current is reduced
in the absence of an audio signal. With the first two-pole
combination typically absorbing more power, a common-mode topology
results in less power dissipation by reducing inductor current 37%
over prior art filters. By incorporating a differential second
two-pole combination, the filter maintains beneficial rejection of
high frequency differential signal components.
[0067] The low inductance board layout and modularization scheme
advantageously lowers pulse overshoot and undershoot, leading to
reduced distortion, reduced radiated emissions, and increased
efficiency. The low inductance design also allows for elimination
of expensive RC snubbers commonly found in prior art Class D
amplifiers, and for simpler EMI shielding. With the improved board
layout, harmonic distortion has been reduced to 0.2% typical at
200W.sub.rms. Furthermore, the unique board layout reduces overall
size and allows for small lightweight construction.
[0068] Referring also to FIGS. 10 and 11, the board layout calls
for a power supply board 92 and an amplifier board 94
interconnected as a module or "power brick" to minimize inductance.
The power supply board 92 includes two high value filter capacitors
95,96 and provides AC voltage rectification. The amplifier board 94
includes the amplifier circuit 10, described above, with the layout
of said amplifier circuit 10 being such that the H-bridge of the
output stage 28 resides directly above the high value filter
capacitors 95,96. Interconnecting the power supply and amplifier
boards 92,94 is a 12 gauge copper buss bar 97 which minimizes
lumped inductance between the power supply circuit board 92 and the
output stage 28. Standoffs 100 separating the power supply board 92
and amplifier boards 94 also provide low inductance paths to
ground. The full H-bridge 65 of the output stage 28 is arranged
with each half bridge 67,77 in parallel, and opposing MOSFET pairs
70,72 and 74,76 spaced 0.25 inches apart as shown in.
[0069] FIGS. 12 and 13 illustrate low inductance board routing of a
topside layer 98 and a bottomside layer 99 of the amplifier board
94. This arrangement minimizes lumped inductances between the
H-bridge 65 and the filter stage 30. Spacing the half bridges 67,77
0.25 inches apart also allows for use of standard 0.25 inch
aluminum bar stock heat sinking for minimized cost and weight.
[0070] Because HVIC_1 68 and HVIC_2 78 are off-the-shelf
components, their pinouts are identical, meaning that if one is
positioned for shortest lead length, the other must be positioned
to have an undesirably long lead length. Those with ordinary skill
in the art will appreciate that longer lead lengths result in
undesirable disadvantages, including increased ringing and higher
inductance. Thus, the present invention makes use of both the
topside 98 and the bottomside 99 of the amplifier board 94 to
shorten lead lengths.
[0071] By using both the topside 98 and bottomside 99 of the
amplifier board 94, the positioning of each HVIC 68,78 immediately
adjacent a respective half-bridge 67,77 minimizes inductances. By
selecting an HVIC that has an optimal pinout, routing distances
from the MOSFET gates 70,72,74,76 and sources can be minimized. One
such HVIC is available from International Rectifier as the IR2113S
half bridge driver. Thus, for example, by facing opposing MOSFETs
70,72,74,76 and placing HVIC_2 78 on the topside 98 of the
amplifier board 94, and HVIC_1 68 on the bottomside 99 of the
amplifier board 94, the pinouts of the HVICs 68,78 are made to
appear identical, thus minimizing routing distances between the
MOSFET gates 70,72,74,76. This HVIC placement advantageously
reduces pulse overshoot and undershoot on the floating high side
supply and gate drive pins of the HVICs reduces inductance between
output nodes and the HVICs 68,78 high side floating supplies,
increases reliability, and allows for higher power.
[0072] Thus, from the preceding description, it can be seen that
the Class D switching audio amplifier of the present invention, in
its various described embodiments, provides a Class D switching
amplifier operable to provide increased efficiency, increased
reliability, and reduced distortion through use of four state
modulation, input-to-output optical isolation, feedback isolation,
dual topology output filtration, and a low inductance board
layout.
[0073] Although the invention has been described with reference to
the preferred embodiments illustrated in the attached drawings, it
is noted that equivalents may be employed and substitutions made
herein without departing from the scope of the invention as recited
in the claims. For example, as mentioned, circuit topologies
alternative to those shown are easily devised but which do not
depart from the inventive concepts disclosed herein and which,
therefore, are considered within the scope of the present
invention. Thus, by way of illustration, is the alternative input
stage circuit topology shown in FIG. 2. Therefore, the concepts of
the present invention should not be viewed as narrowly limited to
the implementing circuit topologies described herein for purposes
of illustrative disclosure.
* * * * *