Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-01-09
2003-04-01
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S242000, C315S307000
Reexamination Certificate
active
06541920
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a self-luminous display apparatus which includes a plurality of self-luminous pixels and a DC/DC (DC to DC: direct current to direct current) converter which can be used as a power supply for a self-luminous display apparatus of the type described and particularly to a DC/DC converter of the chopper type with which the switching loss is comparatively low.
DESCRIPTION OF THE RELATED ART
A DC/DC converter is conventionally used for various electronic equipments because it is convenient to step down or step up the voltage of a DC power supply. Particularly, a DC/DC converter of the chopper type wherein a switching element is controlled on and off to obtain a predetermined voltage is spread widely because it is advantageous in that it includes a comparatively small number of parts and is small in size, light in weight and low in cost.
Meanwhile, it is demanded that electronic apparatus in recent years be small in size and convenient for carrying. Therefore, a battery is used frequently as a power supply for the apparatus. Therefore, a DC/DC converter which can step up the voltage is used frequently for a power supply circuit provided in the apparatus. Also where a self-luminous display unit is used in a terminal of the battery driven type such as, for example, a portable telephone set, usually a DC/DC converter of the voltage step-up type is used in order to supply a sufficient voltage to a driver circuit.
Here, an example of a DC/DC converter conventionally used as a power supply circuit for stepping up the voltage is shown in a circuit diagram of FIG.
3
. Referring to
FIG. 3
, a choke coil L
1
′ and a switching transistor Q
1
′ are connected in series between an input terminal
1
′ of the DC/DC converter and the ground, and a capacitor C
1
′ for filtering is connected between the input terminal
1
′ and the ground. A node between the choke coil L
1
′ and the collector of the switching transistor Q
1
′ is connected to an output terminal
2
′ of the DC/DC converter through a rectifying diode D
1
′, and a smoothing capacitor C
2
′ is connected between the output terminal
2
′ and the ground. A pair of resistors R
1
′ and R
2
′ for voltage detection are connected in series between the output terminal
2
′ and the ground, and a node between the resistors R
1
′ and R
2
′ is connected to an input terminal of a control circuit
3
′. An output (drive signal) of the control circuit
3
′ is connected to the base of the switching transistor Q
1
′.
In the circuit having the configuration described above, if an input voltage is first supplied from a DC power supply such as an external battery (not shown) to the input terminal
1
′, then the control circuit
3
′ supplies a drive signal to the switching transistor Q
1
′. Consequently, the switching transistor Q
1
′ repeats on and off operations to interrupt the current between the collector and the emitter of the switching transistor Q
1
′. While the switching transistor Q
1
′ is in an on state, current flows through the choke coil L
1
′ and the choke coil L
1
′ stores energy, but if the switching transistor Q
1
′ is placed into an off state, then the choke coil L
1
′ releases the energy in the form of a fly-back voltage. Thereupon, a high voltage formed from the fly-back voltage superposed on the input voltage appears at the node between the choke coil L
1
′ and the switching transistor Q
1
′. The superposed voltage of the two voltages passes through the rectifying diode D
1
′ and charges the smoothing capacitor C
2
′. A voltage appearing between the terminals of the smoothing capacitor C
2
′ in this manner servers as an output voltage of the power supply circuit and is supplied to an external load through the output terminal
2
′. The output voltage is kept substantially fixed because the control circuit
3
′ supervises the divided voltage appearing at the node between the resistors R
1
′ and R
2
′ and varies the on-duty of the switching transistor Q
1
′ in response to the divided voltage.
It is known that the conversion efficiency of the DC/DC converter relies much upon the inductance value of the choke coil L
1
′.
FIG. 4
is a graph illustrating a relationship between the magnitude of the inductance value of the choke coil L
1
′ which is a component of the DC/DC converter and the efficiency of the DC/DC converter. When the output current of the DC/DC converter has a predetermined value Ic, the efficiency varies in response to the inductance value of the coil and exhibits a maximum efficiency at an inductance value Lc in FIG.
4
. When a DC/DC converter is designed, a single choke coil having an inductance value with which the conversion efficiency is in the maximum with regard to predetermined output current corresponding to a steady state of the apparatus is determined.
A DC/DC converter developed from the circuit configuration of the type described is disclosed, for example, in Japanese Patent Laid-Open No. 252901/1999, Japanese Patent Laid-Open No. 262250/1999, Japanese Patent Laid-Open No. 2000-152611 and so forth.
FIG. 5
shows an example of a conventional DC/DC converter having a configuration different from those of the documents mentioned above. Referring to
FIG. 5
, one of terminals of a channel of a switching field effect transistor (FET)
1
A is connected to an input terminal Vin of the DC/DC converter, and the cathode of a rectifying diode
2
A and a choke coil
3
A are connected to the other terminal of the switching FET
1
A. The anode of the diode
2
A is grounded, and the other terminal of the choke coil
3
A is connected to an output terminal Vout of the DC/DC converter which in turn is connected to a load (not shown). The output terminal Vout is grounded through a smoothing capacitor
4
A and connected to an input of an oscillation control circuit
5
A. An output of the oscillation control circuit
5
A is inputted to an oscillation circuit
6
A, and an output (drive signal) of the oscillation circuit
6
A which is controlled with the input from the oscillation control circuit
5
A is connected to the gate terminal of the switching FET
1
A.
The oscillation circuit
6
A switches the switching FET
1
A in a predetermined period. For such switching, the oscillation control circuit
5
A controls the on-time of the switching FET
1
A, that is, the pulse width to be applied to the gate of the switching FET
1
A, so that the output voltage of the DC/DC converter may be fixed. During the on time of the FET
1
A, power is stored into the choke coil
3
A, but when the switching FET
1
A is off, the power stored in the choke coil
3
A is transmitted to the load through the rectifying diode
2
A.
The output voltage of the DC/DC converter is kept fixed by controlling the on-time t of the switching FET
1
A in this manner. The inductance value of the choke coil
3
A is set to a value with which a stabilized oscillation operation of the oscillation circuit
6
A can be achieved. A DC/DC converter developed from the circuit configuration described above is disclosed, for example, in Japanese Patent Laid-Open No. 294269/1996, Japanese Patent Laid-Open No. 121534/1997 and so forth.
Japanese Patent Laid-Open No. 121534/1997 mentioned above, particularly in
FIG. 1
thereof, discloses a DC/DC converter wherein, in order to prevent such a disadvantage that, when the load current is low, a switching FET repeats intermittent oscillations, the load current is detected and the inductance value of a choke coil is varied based on the load current. FIG.
6
shows a circuit diagram of the DC/DC converter disclosed in the document mentioned above. In
FIG. 6
, as reference characters, reference characters used in the document are used with the suffix B added thereto so as to facilitate recognition of a corresponding relationship.
The DC/DC converter of
NEC Corporation
Scully Scott Murphy & Presser
Vu David
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