Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
1997-12-19
2001-01-16
Lee, Benny (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S251000
Reexamination Certificate
active
06175272
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to pulse-width modulation (PWM) systems in general, and in particular this invention relates to PWM amplifiers and linear amplifiers using PWM power supplies.
2. Description of Related Art
FIG. 1
illustrates the concept of pulse-width modulation (PWM) using a functional diagram of a conventional PWM amplifier system
10
. An analog waveform
15
, a sine wave for example, is provided to an input of a PWM amplifier
20
. A second input of PWM amplifier
20
receives a periodic signal
30
from an oscillator.
PWM amplifier
20
combines waveform
15
and periodic signal
30
to create a PWM waveform
40
that switches between two voltage levels (e.g., zero and one hundred volts). The durations of the pulses, or positive “swings,” of PWM waveform
40
are selected so that the integrated energy of PWM waveform
40
equals the energy of analog waveform
15
multiplied by a selected gain factor. The gain factor is the gain of PWM amplifier
20
.
A filter
60
filters PWM waveform
40
to produce an analog waveform
50
, a replication of analog waveform
15
multiplied by the gain factor. The filter function is typically accomplished using a mechanical or electrical filter, such as an electrical motor, that is too slow to respond to the square-wave modulation frequency of periodic signal
30
.
PWM amplifier systems are typically noisier, less linear, more complex, and exhibit higher distortion than equivalent analog amplifier systems (e.g., conventional analog amplifiers). Despite these shortcomings, PWM amplifier systems are widely used because they offer superior efficiency. Amplifier efficiency can be approximated by multiplying the difference between the input voltage and the output voltage (i.e., Vout−Vin) by the output current Iout. The voltage difference Vout−Vin can be substantial in analog amplifier systems, especially in high-power applications. In contrast, there is virtually no difference between the input voltage and the output voltage Vout of a PWM amplifier, except during the switching interval when Vout slews between voltage levels. Faster switching speeds reduces the switching interval and therefore improves efficiency. It is not uncommon, for example, to obtain energy transfer efficiencies as high as 90% to 98% in PWM amplifier systems. In contrast, the efficiencies of comparable analog system may be 25% or lower.
High-power PWM amplifiers require that the difference between the input and output voltages (Vout−Vin) be relatively large. Unfortunately, as the voltage difference increases, so too does the difficulty of precisely controlling small-valued output signals. This is because small valued PWM output signals require on times (pulse widths) that are short relative to the total PWM period. Consequently, timing errors, non-linear switching characteristics, and other types of distortion are large relative to the pulse widths for high-power PWM amplifiers operating at low power.
FIG. 2
illustrates the difficulty of precisely controlling small-valued output signals using high-power PWM amplifiers.
FIG. 2
compares an ideal voltage waveform (dashed line) with a waveform distorted as a result of a non-ideal switching time &tgr;. In the first example the ideal energy E is equal to the voltage swing (50V) multiplied by the pulse duration 6 T, or E=300 VT. Also in the first example, the non-ideal energy E′ is somewhat less than the ideal energy E due to the switching time &tgr;. However, the resulting error is relatively small due to the switching time &tgr; being short relative to the pulse width.
The error is far greater in the second example, which illustrates the operation of a PWM system at lower power. Low-power output signals are created by reducing the pulse width: the switching time &tgr; remains the same. Thus, the switching time &tgr; is large relative to the short pulse width of the second example. The non-ideal energy E′ is less than 30% of the ideal energy E. This significant reduction in accuracy at low power is further emphasized by other types of signal distortion, such as noise spikes and ringing. There is therefore a need for a high-power PWM system with improved low-power performance.
SUMMARY
The present invention is directed to a PWM system and associated method that delivers efficient high-power performance without sacrificing accuracy of the PWM waveform at low-power output levels. The system includes a pulse-width modulator configured to modulate the pulse-width of an output signal in response to an analog input signal. The pulse-width modulated signal is then amplified by a high-power amplifier.
The PWM system can be controlled to select from two (or more) switching voltages for the high-power amplifier. Relatively low switching voltages improve accuracy and power-transfer efficiency during low-power operation; higher switching voltages provide better high-power performance. Some embodiments of the PWM system maintain the gain factor during low and high-power operation by adjusting the pulse width of the PWM signal to compensate for changes in the switching voltage. For example, a doubling of the switching voltage may be offset by halving the pulse width of the PWM signal.
The claims, and not this summary, define the scope the invention.
REFERENCES:
patent: 3787730 (1974-01-01), Ray et al.
patent: 4325111 (1982-04-01), Quiogue
patent: 4355277 (1982-10-01), Davis et al.
patent: 4554512 (1985-11-01), Aiello
patent: 4581565 (1986-04-01), Van Pelt et al.
patent: 4683439 (1987-07-01), Atherton et al.
patent: 4841165 (1989-06-01), Bowles
patent: 4851753 (1989-07-01), Hamilton
patent: 4873618 (1989-10-01), Fredrick et al.
patent: 4914399 (1990-04-01), Doany
patent: 5014016 (1991-05-01), Anderson
patent: 5075634 (1991-12-01), French
patent: 5077539 (1991-12-01), Howatt
patent: 5144211 (1992-09-01), Daggett et al.
patent: 5396165 (1995-03-01), Hwang et al.
patent: 5398003 (1995-03-01), Heyl et al.
patent: 5442317 (1995-08-01), Stengel
patent: 5552683 (1996-09-01), Dargent
patent: 5596446 (1997-01-01), Plesko
patent: 5617058 (1997-04-01), Adrian et al.
patent: 5805020 (1998-09-01), Danz et al.
Lee Benny
Nguyen Khanh Van
Nikon Corporation
Skjerven Morrill & MacPherson LLP
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