Electricity: power supply or regulation systems – Input level responsive – Using a linearly acting final control device
Reexamination Certificate
2002-08-12
2004-02-17
Nguyen, Matthew V. (Department: 2838)
Electricity: power supply or regulation systems
Input level responsive
Using a linearly acting final control device
Reexamination Certificate
active
06693414
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power supply apparatuses for regulating a voltage generated by a DC power supply and outputting the regulated voltage.
2. Description of the Related Art
Recently, many electronic apparatuses that use dry cells, such as nickel-cadmium batteries and nickel-metal-hydride batteries, as the power supply source have become generally used. Since these dry cells have relatively low output voltage ranges, the voltage that can be supplied by the dry cell does not necessarily match the voltage used by the electronic apparatus. Thus, the output voltage of the dry cell is converted by a voltage transducer, which is referred to as a DC/DC converter. As a result, the power supply voltage can be stably supplied to an electronic circuit.
FIG. 8
shows an example of the configuration of a known step-up voltage transducer circuit.
A voltage transducer circuit
100
shown in
FIG. 8
is a step-up converter for boosting an input voltage and outputting the boosted voltage. In the voltage transducer circuit
100
, the drain of a transistor Q
21
is connected via a choke coil L
21
to a power supply terminal
12
a
, and the source is grounded. A pulse input terminal
12
b
for receiving a switching pulse from an oscillation circuit (PWM: Pulse Width Modulator) (not shown) is connected to the gate. The anode of a diode (Schottky diode) D
21
is connected to the node between the transistor Q
21
and the choke coil L
21
. A capacitor C
21
is connected to the cathode of the diode D
21
. The other end of the capacitor C
21
is grounded. An output terminal
12
c
(to a load) and a feedback terminal
12
d
(to an error amplifier) are connected to the node between the diode D
21
and the capacitor C
21
.
The transistor Q
21
is an n-channel MOS-FET (Metal Oxide Semiconductor-Field Effect Transistor). In accordance with a switching pulse from the pulse input terminal
12
b
, the transistor Q
21
enters an ON or OFF state, thereby functioning as a switching element. When the transistor Q
21
is changed from ON to OFF in response to a switching pulse, energy excited by the choke coil L
21
is released to the node between the choke coil L
21
and the diode D
21
. As a result, a voltage higher than that of the power supply terminal
12
a
is generated, and the capacitor C
21
is charged. Subsequently, the voltage at the node gradually decreases. When the transistor Q
21
is turned ON the next time, the voltage at the node becomes substantially equal with the ground voltage. Accordingly, a voltage fluctuation in response to the switching pulse occurs at the anode of the diode D
21
. The fluctuation is rectified by the diode D
21
, thus generating a voltage higher than the input voltage. The voltage is smoothed by the capacitor C
21
, the smoothed voltage is removed from the output terminal
12
c
, and the voltage is supplied to the load.
The feedback terminal
12
d
outputs the same voltage as that of the output terminal
12
c
, and the output voltage is supplied to the error amplifier (not shown). At the error amplifier, the voltage output from the feedback terminal
12
d
is compared with a predetermined voltage. In accordance with the comparison signal, a switching pulse output from the oscillation circuit is controlled.
In many cases, a power supply apparatus using a dry cell generally has an output voltage of around 1 V. On the other hand, when a voltage less than or equal to approximately 1 V is input to a power supply apparatus using a known voltage transducer circuit, such as the foregoing voltage transducer circuit
100
, the power supply apparatus having a relatively large load cannot be activated. Even if the power supply apparatus can be activated, the subsequent operation may become unstable.
FIG. 9
is a graph showing an example of the relationship between load power and starting voltage of a known power supply circuit.
FIG. 9
shows the relationship of load power with a starting voltage (Vstart) of the known power supply circuit and the minimum allowable input voltage (Vhold) for stably operating the power supply circuit. According to
FIG. 9
, when the load power is less than or equal to approximately 25 mW, the power supply circuit can be activated by an input voltage ranging from approximately 0.8 to 1.0 V. As the load power increases, the starting voltage also increases. When the load power is greater than or equal to 25 mW, the starting voltage increases substantially in proportion to the load power.
According to
FIG. 9
, if the input voltage slightly decreases relative to the starting voltage subsequent to activation, the power supply circuit can be normally operated. When the load power is less than or equal to approximately 15 mW, the minimum allowable range increases as the load power decreases. When the load power is greater than or equal to 15 mW, the minimum allowable range is smaller and changes at an approximately constant rate.
When the voltage input to the known power supply apparatus slightly falls below 1 V, if the load power is approximately 25 mW, the known power supply apparatus can be activated and operated normally. If the load power is greater than 25 mW, the known power supply apparatus cannot be activated. When the input voltage decreases after activation, the operation of the known power supply apparatus may become unstable since the range in which the power supply apparatus can be normally operated is small. These problems may be caused by the fact that the oscillation circuit for outputting a switching pulse cannot oscillate normally due to a low voltage or the fact that normal switching operation for the choke coil cannot be performed since the gate voltage of a switching element (FET) in the voltage transducer circuit is too small.
In a power supply apparatus which uses the voltage transducer circuit
100
arranged as shown in FIG.
8
and which supplies a constant voltage generated by boosting an input voltage to a load, if the input voltage is less than a predetermined voltage, it is necessary to increase the step-up ratio. If Ton represents time during which the transistor Q
21
in the voltage transducer circuit
100
is turned ON and Toff represents time during which the transistor Q
21
is turned OFF, the relationship between the input voltage Vin from the power supply terminal
12
a
and the output voltage Vout from the output terminal
12
c
can be represented as:
Vout
=
(
Ton
+
Toff
)
×
Vin
Toff
(
1
)
When a dry cell is used as the power supply source for the voltage transducer circuit
100
, Vin may be a value around 1 V. If, for example, Vin is 1 V and Vout is 5 V, then the ratio Ton:Toff=4:1, and the duty ratio in the Ton period is 80%. In this case, if the input voltage becomes smaller than 1 V, the duty ratio may be further biased. When the load is large, activation may become difficult, and the stability during the normal operation may deteriorate.
SUMMARY OF THE INVENTION
In view of the foregoing problems, it is an object of the present invention to provide a power supply apparatus capable of stably operating while having a high load even when an input voltage is low.
According to the present invention, the foregoing objects are achieved through provision of a power supply apparatus for regulating a voltage generated by a DC power supply and for outputting the voltage to a predetermined load. The power supply apparatus includes a first voltage transducer for boosting the voltage generated by the DC power supply; a second voltage transducer for converting the voltage generated by the DC power supply to a predetermined voltage and for outputting the converted voltage to the predetermined load; an operation controller driven by an output voltage from the first voltage transducer, the operation controller outputting a switching pulse in accordance with reception of an activation signal and controlling the operation of the second voltage transducer; and an activation controller for inputting the output voltage from the first voltage transducer to the opera
Frommer William S.
Frommer & Lawrence & Haug LLP
Nguyen Matthew V.
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