Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2003-01-16
2004-02-24
Vu, Bao Q. (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C323S284000
Reexamination Certificate
active
06696821
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from each of the prior Japanese Patent Application No. 2002-36639 filed on Feb. 14, 2002, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC—DC converter, an electric appliance and a duty-ratio setting circuit.
2. Description of Related Art
A DC—DC converter capable of converting the direct current voltage into a predetermined level has been miniaturized and its efficiency has been highly increased, so that the DC—DC converter has been used for a power supply device of various kinds of electric appliances and its range of use has been increasingly enlarged. In such electric appliances, particularly such as a notebook type of personal computer and portable electric appliances such as a portable phone terminal, the DC—DC converter is an essential device since an IC, an electric circuit, motor and a liquid crystal display device in a main body of the above electric appliances are operated by means of a battery, for example, a primary battery such as an alkaline cell or a secondary battery such as a lithium-ionic cell and a nickel-hydrogen cell as a power supply.
The DC—DC converter, generally, comprises a converter circuit for converting an input voltage into an output voltage having a voltage value different from that of the input voltage in accordance with a switching element turned on and off by a rectangular-wave signal, as well as an output voltage detection circuit for detecting a value of the output voltage to output the detection voltage and a duty-ratio setting circuit for feedback-controlling a duty ratio of the rectangular-wave signal on the basis of the detection voltage so that the output voltage would be controlled at a predetermined value. This structure allows the output voltage of the DC—DC converter to be controlled at a constant value.
Recently, there is a need for the long use of electric appliances, particularly portable electric appliances. It means that there is a requirement of providing a DC—DC converter capable of operating and of supplying a main body of an electric appliance with a predetermined value of output voltage not only in the case that a between-terminal voltage, that is, an input voltage is high since a battery has enough energy, but also in the case that the between-terminal voltage (the input voltage) decreases since the energy is used to be discharged. In other words, a DC—DC converter having a wide operable range of the input voltage is required.
Furthermore, there is also a requirement of a DC—DC converter having a wider setting range of the output voltage so as to be applicable to devices having various specifications.
In a DC—DC converter, the voltage conversion is impossible when the on-duty ratio of the rectangular-wave signal is 100%. As a result, the output voltage in a step-up type of DC—DC converter would decrease to the earth potential, while that of a step-down type would rise up to the input voltage (the voltage of the battery). It is impossible, in practice, that the on-duty ratio exceeds 100%.
In the DC—DC converter, however, the on-duty ratio of the rectangular-wave signal is raised to maintain control in order to set the output voltage at a predetermined value, when the input voltage decreases due to consumption of a battery or the like. Therefore, the on-duty ratio of the rectangular-wave signal finally reaches nearly 100%.
In such case, slight fluctuation of the output voltage due to noise or fluctuation in voltage sometimes causes the on-duty ratio of the rectangular-wave signal, which is set in the duty-ratio setting circuit, to be 100% in calculation. Then, the DC—DC converter cannot output an appropriate voltage, so that the output voltage would decrease (in the case of the step-up type) or rise (in the case of the step-down type). This makes the difference larger between the actual value and the predetermined value of the output voltage and causes the on-duty ratio set in the duty-ratio setting circuit to be increased much more. In the above situation, feedback control is impossible for the DC—DC converter and the output voltage rapidly decreases to the earth voltage or rises to the input voltage. This sometimes makes it impossible to completely carry out a necessary sheltering operation by the time when the power supply of an electric appliance is cut.
In order to prevent the above problem, the following way is taken in some cases. That is, the maximum on-duty ratio possible to be set in the duty-ratio setting circuit is set at a value lower than 100% (80%, for example) in view of noise, fluctuation in voltage and the like. Then, after the duty ratio of the rectangular-wave signal reaches the maximum on-duty ratio to make the feedback control impossible, the output voltage to be generated is determined in accordance with the maximum duty ratio, so that a necessary sheltering operation can be carried out.
In such DC—DC converter, however, the on-duty ratio of the rectangular-wave signal cannot be set in more than the maximum on-duty ratio. Therefore, the operative range of the input voltage is made narrow or the range of the output voltage possible to be outputted is made narrow.
In view of the above, on the basis of the detection voltage of the output voltage detection circuit, a second pulse signal having a fixed on-duty ratio for determining the maximum on-duty ratio is generated separately from generation of a first pulse signal having an on-duty ratio appropriate for feedback control. A DC—DC converter is provided in which a signal having the smaller on-duty ratio is selected to be outputted by a logic process of the first and second pulse signals (see FIG.
1
). The above DC—DC converter
100
is a DC—DC converter for raising an input voltage Vin such as a battery, which is inputted to an input terminal Pi, and for outputting an output voltage Vout from an output terminal Po. The DC—DC converter
100
has a converter circuit
110
for converting the input voltage Vin into the output voltage Vout, an output voltage detection circuit
120
for outputting a detection voltage Vd corresponding to the output voltage Vout and a duty-ratio setting circuit
130
for setting an on-duty ratio Don of a rectangular-wave signal PS applied to a switching element M
1
on the basis of the detection voltage Vd.
The converter circuit
110
comprises an N-channel of MOS transistor M
1
, which is a switching element, a coil L
1
, a capacitor C
1
and a diode D
1
for preventing a reverse current. The DC input voltage Vin is applied through the coil L
1
to a drain of the transistor M
1
. A source of the transistor M
1
is connected to the ground. An anode of the diode D
1
is connected to the drain of the transistor M
1
while a cathode of the diode D
1
is connected to the output terminal Po. The capacitor C
1
is connected between the output terminal Po and the ground.
The output voltage Vout outputted from the output terminal Po is made higher than the input voltage Vin in accordance with ON/OFF control of the transistor M
1
, that is, raised to be outputted. Changing the ratio between ON time Ton and OFF time Toff of the transistor M
1
can control the output voltage Vout at a predetermined value.
The output voltage Vout is, concretely, given by an equation of Vout={(Ton+Toff)/Toff} Vin=Vin/Doff. The off-duty ratio Doff is here expressed by an equation of Doff=Toff/(Ton+Toff), while the on-duty ratio Don is expressed by an equation of Don=Ton/(Ton+Toff)=1−Doff. Therefore, Doff+Don=1, and thereby, Vout=Vin/Doff=Vin/(1−Don).
In the output voltage detection circuit
120
, the output voltage Vout at the output terminal Po is resistance-divided by resisters R
1
and R
2
to input the detection voltage Vd into the duty-ratio setting circuit
130
.
The duty-ratio setting circuit
130
comprises an error amplifier
131
, a tria
Haraguchi Akira
Matsumoto Takashi
Takimoto Kyuichi
Arent Fox Kintner & Plotkin & Kahn, PLLC
Vu Bao Q.
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