Note: Descriptions are shown in the official language in which they were submitted.
CA 02292187 2006-10-26
SWITCHING VOLTAGE REGULATOR
Field of the Invention
The present invention relates to voltage regulator circuits for converting an
input voltage signal to a regulated DC output voltage signal to be applied to
a load,
and more particularly to switching voltage buck regulator circuits.
Background of the Invention
Switching voltage regulators have been developed as an alternative to linear
voltage regulators to provide higher efficiency with smaller size. A switching
voltage regulator includes a switching coritrol device coupled to one or more
switches which are rapidly opened and closed according to a varying duty cycle
to
transfer energy between an input, unregulated voltage source to a load,
through an
output filter. Typically, the output voltage of' the regulator is monitored in
feedback
by an error amplifier comparing thereof to a desired output reference voltage
and
sending an error signal to the switching control device which adjusts the duty
cycle
accordingly.
An example of known DC to DC switching voltage converter is disclosed in
U.S. Patent No. 5,627,460 to Bazinet et al.., which converter includes a
bootstrap
capacitor monitored by a digital logic controller to drive a high side of a
switching
device comprising a series connected pair of N-channel Metal-Oxide-
Semiconductor Field-Effect Transistors (MC)SFET's), and to drive the low side
of
the switching device when the bootstrap voltage decreases below a
predetermined
level to maintain a minimum level of charge on the bootstrap capacitor.
Another
example of a known switching voltage regulator in a battery management
application is disclosed in U.S. Patent 5,774,733 to Nolan et al., which
teaches
making use of a microcontroller and a digital-to-analog converter to provide
reference voltage thresholds to a voltage comparator selectively used through
operation of a logic gate as an error amplifier or a current monitor for
driving an
external power transistor coupled to external inductor and capacitor.
DOCSMTL: 2175621\1
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U.S. Patent 5,773,966 to Steigerwald, U.S. Patent No. 5,903,138 to
Hwang et al. and U.S. Patent No. 5,955,871 to Nguyen disclose other switching
voltage regulators showing complex multi-mode operation features providing
adaptation to low and high power consumption levels of a connected load.
Although the above cited prior art regulator circuits show special
features which may prove to be useful in certain specific applications, the
complex design generally characterizing these known circuits contribute to
increase manufacturing costs thereof. More particularly, although switching
regulators are by far more efficient than linear regulators, they have never
seen
io wide spread use in alarm system panels design because of their greater
costs.
Therefore, there is still a need for simple circuit designs which prove to
reduce
manufacturing cost of switching voltage regulators.
Summaa of the Invention
It is an object of the present invention to provide a switching voltage
regulator circuit of a simple design exhibiting high efficiency and reducing
manufacturing costs.
It is another object of the invention to provide a switching voltage
regulator circuit that can be used for implementing a back-up battery charging
2o device with battery testing feature.
According to the first above object, from a broad aspect, there is
provided a switching voltage regulator circuit for converting an input voltage
signal to a regulated DC output voltage signal to be applied to a load. The
circuit comprises a switching device receiving the input voltage signal and
having an output being coupled to an output filter generating the regulated
output voltage signal, and a current limiting device sensing current drawn by
the load to generate a current error signal whenever the current exceeds a
predetermined current limit value. The circuit further comprises a voltage
comparator amplifier receiving the regulated output voltage signal and the
current error signal and comparing thereof to a reference voltage to generate
a
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voltage error signal, and a switching control device receiving the voltage
error
signal to generate a control signal for the switching device.
According to the second above object, the regulated DC output voltage
signal is to be applied to a back-up battery coupled to the load, the voltage
regulator circuit further comprising a controller having a sensing input being
coupled to the battery and an output being coupled to the input of the voltage
comparator amplifier for causing the regulator to change from a battery
charging mode into a battery test mode allowing the controller to compare
sensed voltage supplied by the battery to a predetermined minimum voltage
io threshold and to generate an alarm signal whenever the sensed battery
voltage
is lower than the minimum voltage threshold.
Brief Description of the Drawings
A preferred embodiment of the present invention is illustrated in the
appended drawings, in which:
Fig. 1 is a schematic block diagram of the embodiment;
Figs. 2a and 2b represent a detailed schematic of the regulator of the
embodiment;
Fig. 3a is a detailed schematic of a microcontroller as part of a battery
charge control circuit of the embodiment;
Fig. 3b is a detailed schematic of a battery coupling circuit and a
microcontroller power supply circuit of the embodiment;
Fig. 3c is a detailed schematic of a voltage adjusting circuit for battery
test mode of the embodiment;
Detailed Description of the Ppreferred Embodiment
Referring now to Fig. 1, the switching voltage regulator circuit
according to the present invention and generally designated at 10 is used to
convert a typically higher input voltage signal applied to an input terminal
12
into a typically lower regulated DC output voltage signal generated between
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main and grounded output terminals 14 and 14' across parallel resistors R140
and R142 as better shown in Fig. 2b, to which terminals a load is to be
connected. The basic regulator circuit 10 comprises a switching device 16
having an input terminal 19 being coupled to input terminal 12 through main
line 17' for receiving the input voltage signal, the switching device 16
further
having an output 18 being coupled to an output filter 20 generating the
regulated output voltage signal at an output 22 thereof with reference to
corresponding terminal 22'. The switching device 16 preferably makes use of a
N-channel MOSFET 210, as shown in Fig. 2b. Although any other suitable
lo electronic switch may be used to implement the switching device 16, the
regulator circuit as shown is designed to accommodate the reverse polarity of
the N-channel MOSFET. This reduces cost, since an N-channel FET is less
expensive than a P-channel MOSFET. The filter 20 typically comprises one or
more input inductors 27 coupled to one or more output capacitors 41. The high-
energy current pulses from the switching device 16 are stored and filtered by
inductors 27 and capacitors 41. A schottky diode 43 is also provided to clamp
the back EMF of inductor 27 when the switching device opens. The regulator
circuit 10 further includes a current limiting device 24 for sensing current
drawn by the load. The current limiting device 24 preferably makes use of a
first comparator amplifier 26 having a sensing input 28 preferably being
coupled to an input terminal 30 provided on the MOSFET as shown in Fig. 2b,
to receive a signal representative of the current drawn by the load. It is to
be
understood that other connecting configuration for the purpose of current
sensing may be employed, such as an appropriate coupling to the output
terminal 14.. The regulator circuit 10 further comprises a second voltage
comparator amplifier 32 receiving the regulated output voltage signal at a
first
input 34 thereof through a first node 39 connected to a feedback line 36, for
comparison to a reference voltage applied to input 35 from reference voltage
source terminal 37. As better shown in Fig. 2b, the current limiting device 24
is
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configured to compare the signal entering at input 28 with a further reference
voltage applied to input 29 from reference voltage source terminal 31
corresponding to a predetermined current limit value, to generate at an output
33 a current error signal whenever the current exceeds the set current limit
5 value. The current error signal is fed through a line 35 to a second input
node
38 connected to a transistor 41 having its output linked to first input node
39.
The second voltage comparator 32 receives the current error signal for
comparing thereof, in combination with the regulated output voltage signal, to
the reference voltage applied to source terminal 37 to generate a voltage
error
io signal at an output 40 provided on voltage comparator 32. The reference
voltage level is set in accordance with the desired regulated output level.
The
regulator circuit 10 is further provided with a switching control device
preferably of a Pulse Width Modulator (PWM) type and generally designated at
42, which receives at a first input terminal 44 thereof the voltage error
signal
through a line 46 to generate a control signal for the MOSFET 210 of the
switching device 16. Basically, the PWM switching control device 42 makes
the decisions to turn on and off the switch and for how long in generating
pulses for controlling the duty cycle of switching device 16. The PWM
switching control device 16 comprises a third voltage comparator amplifier 48
2o arranged in an oscillator configuration to generate a triangular ramp
signal at an
output 50 thereof, the frequency of oscillation being set an appropriate value
selection for resistor R156 and capacitor C109. The PWM switching device 16
further comprises a fourth voltage comparator 52 receiving the voltage error
signal at an input 45 for comparing thereof to the triangular ramp signal
received from third comparator 48 at an input 54, to generate a Pulse Width
Modulated output signal generated at an output 56 provided on fourth
comparator amplifier 52. Since the output signal at 56 is not of a sufficient
level
to adequately control the MOSFET 210 of the switching device 16, the
switching control device 42 further comprises a driving stage 58 receiving at
an
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input 57 thereof the signal from comparator output 56 for generating at an
output 59 thereof a switching control signal of a sufficient level that is fed
to an
input termina160 provided on the switching device 16. In the example shown in
Fig. 2b, the driving stage 58 conveniently makes use of only four (4)
transistors
59, 61, 63 and 64 to keep manufacturing costs low, and the switching device
input 60 is coupled to the gate terminal of MOSFET 210 through a resistor
R132. It is to be understood that the switching device 16 may be implemented
according to other designs, such as a pair of N-Channel MOSFET's alternately
driven by an appropriate switching control circuit providing two different
lo driving voltage levels for separately controlling the duty cycle of each
MOSFET. It is pointed out that the preferred embodiment shown in Figs. 2a
and 2b makes use of only four (4) independent comparator amplifiers 26, 32, 48
and 53, which can be advantageously provided by a single package integrated
circuit readily available on the marketplace, such as the Quad comparator
LM339 IC manufactured by National Semiconductor Corp. Since the reduced
number of comparators required to implement a regulator circuit according to
the present invention allows the use of such a single package IC, the cost of
the
regulator circuit may be reduced accordingly as compared to prior designs
requiring a higher number of comparators or an expensive special function
switching controller IC, while maintaining high efficiency.
As shown in Fig. 2a, the regulator circuit 10 is typically adapted to
receive a rectified DC signal generated by a rectifier bridge circuit 11 being
coupled with capacitors 13 between a pair of input terminals 15 connected to
an
alternating voltage source and the input terminal 12 through input line 17 .
It is
to be understood that the regulator circuit according to he present invention
can
be adapted to receive DC input signals exhibiting other types of waveforms.
Referring again to Fig.l, the embodiment shown includes further
components enabling the regulator circuit 10 to be used as a back-up battery
charging device with battery testing feature which is useful is many
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applications, such as for the management of back-up battery found in the
control panel of electronic alarm systems. There is provided a controller 62,
which is preferably an integrated microcontroller circuit having a sensing
input
66 being coupled through a voltage divider formed by resistors R162 and R164
to the positive terminal 64 of a battery having its negative terminal 64'
being
coupled to ground through parallel protections 65 and 65' and parallel
resistors
R212 and R214, as better shown in Fig. 3b. The microcontroller 62 is
preferably powered by a linear low power regulator 67, which is a LM78L05
SOT-89 integrated regulator package also manufactured by National
io Semiconductor Corp, in the example shown in Fig. 3b which is self-
explanatory. The microcontroller 62 further has an output port 68 being
coupled
through line 68 and resistor R208 to a comparator voltage adjusting circuit 70
being coupled through lines 74, 36 and node 39 to input 34 of comparator 32,
as better shown in Fig. 2a. In the example shown in Fig. 3c, the comparator
voltage adjusting circuit 70 makes use of series connected voltage op-
amplifiers
76 and 78. The interruption control signal generated by microcontroller 62 at
its
output 68 is sent through resistor R208 to a first input 80 of op-amplifier 76
having a second input 82 being coupled to ground through resistors R218 and
R220. The output 84 of op-amplifier 76 is coupled to an electroluminescent
2o diode 86 indicating a testing mode of operation and having a cathode
terminal
being coupled to ground through resistor R219. The op-amplifier output 84,
which is coupled in feedback to input 82 through resistor R216, is also
coupled
to a second input 88 of op-amplifier 78 having its first input 90 receiving a
reference voltage signal. The voltage adjust signal generated at output 92 of
op-
amplifier 78 is coupled, through diode 94 and resistor R206 connected to
grounded capacitor C130, to comparator 32 of the regulator 10, as described
before with reference to Figs. 1 and 2a. It is to be understood that different
configurations for providing a voltage adjusting circuit operating in
accordance
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with other control principles, such as a control over the set reference
voltage,
may be used in accordance with the present invention.
In operation as a basic voltage regulator, referring again to Fig. 1, the
circuit 10 is fed by an input voltage signal applied to input terminal 12, and
directed to input terminal 19 of switching device which generates a power
signal sent to the filter 22 for generating a regulated output voltage signal
at an
output 22 thereof, which signal is supplied to a load. Switching control for
providing a desired regulation is performed by the voltage comparator 32 in
combination with the current limiting device 24, which receives from the
io switching device 16 a signal representative of the current drawn by the
load, for
comparing thereof with the reference voltage corresponding to a predetermined
current limit value, to generate at output 33 a current error signal whenever
the
current exceeds the set current limit value. The voltage comparator 32
receives
the regulated output voltage signal and the current error signal at inputs 34
and
38, for comparison thereof to the reference voltage applied to input 35 from
source terminal 37, to generate a voltage error signal at comparator output 40
when the compared voltages are different. The value of reference voltage is
set
and maintained stable in accordance with the desired regulated output level.
Therefore, the voltage comparator 32 makes a decision as to if the regulated
voltage is at the correct level. When it is not, it generates an error signal
to the
switching control device 42. The switching control device 42 receives at input
44 the voltage error signal to generate a control signal for the switching
device
16, the duty cycle of which is being characterized by the control signal
accordingly. More specifically, the comparator 52 causes the error signal to
intersect the triangle waveform at different points; these different points
corresponding to different duty cycles, the top of the waveform being very
narrow and the bottom very wide. The error signal cause comparator 52 to
select a corresponding intersection. Hence, when load conditions are severe
the
duty cycles are long and when load conditions are light duty cycles are low
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accordingly. The voltage comparator 48 of the switching control device
generates a triangular ramp signal at output 50, which is applied to input 54
of
voltage comparator 52 also receiving the voltage error signal at input 44, to
generate a pulse-width-modulated output signal generated at output 56. From
the PWM signal, the driving stage 58 generates at output 59 a switching
control
signal of a sufficient level that is fed to input terminal 60 of switching
device
16, the duty cycle of which allows the generation of an output signal being
characterized by the desired voltage level.
In operation as a battery charging device with battery testing feature, the
lo controller 62 is operated to receive at input 66 a battery check signal
from
positive battery terminal 64 through the voltage divider formed by resistors
R162 and R164. The microcontroller 62 sends through output 68 a test signal to
the voltage adjusting circuit 70, which in turn generates a corresponding
voltage adjust signal to the comparator input 34 for causing regulator
interruption , thereby causing the regulator to change from a battery charging
mode into a battery test mode allowing microcontroller 62 to compare sensed
voltage supplied by the battery to a predetermined minimum voltage threshold.
If the voltage supplied by the battery is stable, the test is passed and the
regulator is caused to return in the battery charging operation mode. However,
if the sensed battery voltage is lower than the minimum voltage threshold, the
microcontroller generates an alarm signal before causing the regulator to
return
to the battery charging operation mode.
