Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-03-09
2001-06-26
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021140, C363S021180, C363S056070
Reexamination Certificate
active
06252784
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for amplifying a signal, preferably to obtain a sine wave which is suitable for a ringer. More particularly, the invention relates to a low cost, minimal power loss and self-adjustable power amplifier.
In many telephony applications, including fixed wireless access and pair gain systems, it is necessary to generate a ringer voltage with a sinusoidal wave form. Typically the sine wave has a voltage amplitude of between 40 volts RMS (Root Mean Square) and 75 volts RMS with a DC offset. The sine wave frequency is generally 20, 25 or 50 Hertz.
In designing a sine wave generator, key considerations include low cost with standard parts, minimal power loss, and ability of generator to meet specified requirements under varying load and input voltage conditions. Feedback is a preferred method of ensuring correct output in many applications including sine wave generation. Shimizu (U.S. Pat. No. 5,229,929) demonstrates the use of feedback for purposes other than sine wave output generation. Output peak current correction is performed using a feedback signal which indicates the portions of the output waveform that are outside a predetermined range of amplitude.
FIG. 1
shows one type of prior art apparatus
63
for generating a sine wave suitable for ringer. A linear amplifier
64
with output power stage of class B (according to a classification known to those skilled in the art) receives a reference signal
132
being a small accurate sine wave with a DC offset. The amplifier
64
compares the reference signal with a feedback signal
82
of an output
134
. The gain of output
134
, with respect to reference wave
132
, is set by circuit elements
136
and
138
. DC supply voltage outputs
140
and
142
are generated from the conversion of DC input
144
by a DC/DC converter
146
. Chen (U.S. Pat. No. 5,600,713) describes an example of this type of generator. The apparatus
63
does contain a feedback, but it is power inefficient since 25% of its power is lost in the power stage due to the operation in the linear region that is typical for class B amplifiers.
FIG. 2
shows a second type of prior art apparatus for generating a sine wave suitable for a ringer, using a class D switching amplifier
100
. An upper transistor
102
and a lower transistor
104
thereof switch on and off as in a conventional switching power supply, but at a fixed predetermined rate. Upper transistor
102
forms the positive part of the output sine wave
106
, while the lower transistor forms its negative part. The output voltage
106
is filtered from high frequency components by a low pass L-C filter
90
containing inductance
92
and capacitance
94
, to give a final output curve
108
. The frequency and the form of the final output
108
(usually 20, 25 or 50 Hz) is determined by a predetermined timing table block
84
. DC voltages
86
and
88
, which are fed to a class D switching amplifier
100
, determine the amplitude of final output
108
. DC voltages
86
or
88
are generated from the conversion of DC input
96
by DC/DC converter
98
. The apparatus
83
does not include a feedback loop to control the output signal, which is kept approximately sinusoidal by a predetermined timing table block
84
.
Wendt (U.S. Pat. No. 5,307,407) illustrates a variation of the ringer of the second type which does include a feedback loop. The generation of output ringer signal is not, however, coupled to the generation of the power supply voltages, as the feedback loop only impacts the duty cycle of the switches of the ringer output and does not control the magnitude of the power supply voltages which are separately generated. The efficiency of each separately taken unit, namely the high voltage power supply and the ringer circuit, is high, but each unit inevitably contributes to reduce the overall efficiency. Additionally, there is an added expense and complexity in the combination of the two sets of switches.
FIG. 3
illustrates a third type of prior art apparatus
110
for generating a sine wave suitable for a ringer. A switch
114
is turned off and on by a predetermined timing table block
112
, controlling the voltage applied to the transformer
116
. The resulting power supply voltages
118
and
120
are in the form of full wave rectified sine waves generated by DC/rectified sine wave converter
130
from a DC input voltage
113
. The form and frequency of supply voltages
118
and
120
are established by the predetermined timing table block
112
, and the amplitude of these supply voltages is dependent on input voltage
113
and the output load current. Voltages
118
and
120
are unfolded into sine waves by means of an unfold circuit through output transistors
124
and
126
to give the output ringer signal
128
. Apparatus
110
is more cost effective and efficient than apparatus
83
because it uses only one step of power conversion, however it does not include a feedback loop to control the output voltage
128
, and the voltage is kept approximately sinusoidal by a predetermined timing table block
112
.
There is thus a widely recognized need for, and it would be highly advantageous to have, a sine wave generator having minimal power loss, which integrates the generation and correction of the output signal and power supply voltages.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for generating an electrical signal of a required form, including the steps of: (a) generating at least one power supply voltage, (b) producing an output signal by a circuit powered by at least one power supply voltage, (c) producing a first feedback signal from the output signal, for adjusting the mentioned at least one power supply voltage and (d) adjusting the at least one power supply voltage using the first feedback signal.
According to the preferred embodiment, the output signal is generated by amplifying an input reference signal fed to said circuit.
In one particular version, the inventive method is applicable to generating a sine wave, for example with parameters suitable for a ringer in a telecommunication system.
Preferably, the method further includes steps of: (e) producing a second feedback signal from the output signal; (F) comparing the input reference signal with the second feedback signal; and (g) based on said comparison, correcting the output signal.
The output signal generated by amplifying the reference signal is therefore corrected through a second feedback loop. The amplitude of the power supply voltage is adjusted and corrected using a differential voltage feedback signal (the first feedback signal), dependent on a difference between the voltage amplitude of the power supply voltage(s) and the voltage amplitude of the output signal. To produce a correction signal, the differential voltage feedback signal is compared with a pre-selected DC reference level. This correction signal is used to control the duty cycle of a pulse stream from a PWM (Pulse Width Modulation) controller, causing the width of the pulses to vary with the amplitude of the correction signal. The pulse stream, in turn, controls the amplitude of the power supply voltage, generated preferably using a flyback converter, so that there is a minimal loss during the generation of the output ringer signal.
According to the present invention, there is also provided a device for generating an electrical signal of a required form, comprising: (a) an amplifier which produces an output signal, (b) a power supply generator for supplying at least one power supply voltage to the amplifier, and (c) a circuit for producing a first feedback signal based on the output signal, for controlling the at least one power supply voltage.
According to the preferred embodiment of the invention, the device is adapted to generate a sine wave suitable for a ringer in telecommunication systems.
The amplifier is preferably of class B and includes a comparison stage for comparing a reference signal with a (second) feedback signal from the output (
Browdy & Neimark
ECI Telecom Ltd
Vu Bao Q.
Wong Peter S.
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