U.S. patent application number 10/340809 was filed with the patent office on 2003-07-17 for inverter for driving el lamp and liquid crystal display.
This patent application is currently assigned to Durel Corporation. Invention is credited to Buell, Brian Jeffrey, Kimball, Robert Allen.
Application Number | 20030132715 10/340809 |
Document ID | / |
Family ID | 25149370 |
Filed Date | 2003-07-17 |
United States Patent
Application |
20030132715 |
Kind Code |
A1 |
Buell, Brian Jeffrey ; et
al. |
July 17, 2003 |
INVERTER FOR DRIVING EL LAMP AND LIQUID CRYSTAL DISPLAY
Abstract
An inverter for driving an EL lamp is tapped to provide voltage
or current for driving one or more EL lamps and an LCD. In
accordance with one aspect of the invention, one side of the pump
inductor is tapped for a voltage that drives the LCD.
Inventors: |
Buell, Brian Jeffrey;
(Gilbert, AZ) ; Kimball, Robert Allen; (Gilbert,
AZ) |
Correspondence
Address: |
Paul F. Wille
Durel Corporation
2225 West Chandler Blvd.
Chandler
AZ
85224
US
|
Assignee: |
Durel Corporation
|
Family ID: |
25149370 |
Appl. No.: |
10/340809 |
Filed: |
January 13, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10340809 |
Jan 13, 2003 |
|
|
|
09790007 |
Feb 20, 2001 |
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Current U.S.
Class: |
315/169.3 |
Current CPC
Class: |
H05B 44/00 20220101;
H05B 45/38 20200101; Y02B 20/30 20130101 |
Class at
Publication: |
315/169.3 |
International
Class: |
G09G 003/10 |
Claims
The invention claimed is:
1. In an inverter including an integrated circuit having a
plurality of pins and an external inductor coupled to at least one
of said pins, said inverter producing a first voltage suitable for
driving at least one EL lamp, the improvement comprising: a bias
source coupled to said inductor, said bias source producing a
second voltage suitable for driving an LCD, wherein said second
voltage is derived from the first voltage.
2. The inverter as set forth in claim 1 wherein said bias source
includes a voltage tap coupled to said inductor and a voltage
regulator coupled to said tap.
3. The inverter as set forth in claim 2 wherein said bias source
includes a pass transistor coupling said inductor to said voltage
tap.
4. The inverter as set forth in claim 2 wherein said voltage
regulator includes an SCR coupling said inductor to said voltage
tap.
5. The inverter as set forth in claim 1 wherein said bias source
includes a low pass filter.
6. An inverter for simultaneously driving at least one EL lamp and
an LCD, said inverter comprising: an inductor for producing a high
voltage suitable for driving said EL lamp; and a bias source
coupled to said inductor for providing a bias voltage suitable for
an LCD, wherein said bias voltage is derived from said high
voltage.
7. The inverter as set forth in claim 6 wherein said bias source
includes a low pass filter.
8. The inverter as set forth in claim 7 wherein said bias source
includes an SCR coupling said inductor to said low pass filter.
9. The inverter as set forth in claim 8 wherein said low pass
filter includes a shunt capacitor and a series pass transistor.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a division of Application No.
09/790,007, filed Feb. 20, 2001, now U.S. Pat. No. ______, which is
assigned to the assignee of this invention. The entire contents of
said Application are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to battery operated inverters and, in
particular, to an inverter for driving an electroluminescent (EL)
panel having one or more EL lamps, one or more light emitting
diodes (LEDs), and providing a bias voltage for a liquid crystal
display.
[0003] An EL lamp is essentially a capacitor having a dielectric
layer between two conductive electrodes, one of which is
transparent. The dielectric layer may include a phosphor powder or
there may be a separate layer of phosphor powder adjacent the
dielectric layer. The phosphor powder radiates light in the
presence of a strong electric field, using very little current.
Because an EL lamp is a capacitor, alternating current must be
applied to the electrodes to cause the phosphor to glow, otherwise
the capacitor charges to the applied voltage, the current through
the EL lamp ceases, and the lamp stops producing light.
[0004] In portable electronic devices, automotive displays, and
other applications where the power source is a low voltage battery,
an EL lamp is powered by an inverter that converts direct current
into alternating current. In order for an EL lamp to glow
sufficiently, a peak-to-peak voltage in excess of about one hundred
and twenty volts is necessary. The actual voltage depends on the
construction of the lamp and, in particular, the field strength
within the phosphor powder. The frequency of the alternating
current through an EL lamp affects the life of the lamp, with
frequencies between 200 hertz and 1000 hertz being preferred. Ionic
migration occurs in the phosphor at frequencies below 200 hertz.
Above 1000 hertz, the life of the phosphor is inversely
proportional to frequency.
[0005] The prior art discloses several types of inverters in which
the energy stored in an inductor is supplied to an EL lamp as a
small current at high voltage as the inductor is discharged either
through the lamp or into a storage capacitor. The voltage on a
storage capacitor is pumped up by a series of high frequency pulses
from the inverter. The direct current produced by inverter must be
converted into an alternating current in order to power an EL lamp.
U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge
for this purpose. The bridge acts as a double pole double throw
switch to alternate current through the EL lamp at low frequency.
U.S. Pat. No. 5,313,141 (Kimball) discloses an inverter that
produces AC voltage directly. A plurality of inverters are
commercially available using either technology.
[0006] Many portable electronic devices, such as personal digital
assistants and cellular telephones, use LEDs for illuminating a
liquid crystal display (LCD) and an EL lamp for illuminating a
keypad. The LEDs are typically powered by a driver, the liquid
crystal display has its own driver, and the EL lamps are powered by
an inverter. A driver is required for the LEDs because portable
electronic devices use a 1.5-3 volt battery and LEDs have a minimum
forward voltage drop of 1.8 volts. If LEDs are connected in series
to minimize current, then the voltage requirement increases
accordingly. If the LED is switched by a transistor, then the
forward voltage drop of the transistor further increases the
required supply voltage. In addition, LEDs require a ballast or
current limiting device, such as a resistor or inductor, which
dissipates power. Inductors are more efficient than resistors but
are relatively expensive components and it is desired to minimize
the number of inductors required by a circuit. The liquid crystal
display requires bias voltages.
[0007] It would provide a significant cost savings if LEDs, LCDs,
and EL lamps could be driven from the same driver. The problem is
that EL lamps need 100 volts or more AC, LEDs need from 5-12 volts
DC, and LCDs typically need a bias of 5-30 volts.
[0008] In view of the foregoing, it is therefore an object of the
invention to provide an inverter for providing plural voltages for
a personal electronic device.
[0009] Another object of the invention is to provide an inverter
for driving LEDs, LCDs, and EL lamps by adapting an existing,
commercially available inverter.
[0010] A further object of the invention is to provide an inverter
for driving LEDs, LCDs, and EL lamps by modification to either type
of existing inverter for EL lamps.
SUMMARY OF THE INVENTION
[0011] The foregoing objects are achieved in this invention in
which an inverter for producing plural voltages is tapped to
provide voltage or current for driving one or more EL lamps, LEDs,
and an LCD. One side of a pump inductor is tapped for a voltage
that drives one or more EL lamps and the LCD.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A more complete understanding of the invention can be
obtained by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0013] FIG. 1 is a schematic based upon U.S. Pat. No.
4,527,096;
[0014] FIG. 2 is a schematic based upon U.S. Pat. No.
5,313,141;
[0015] FIG. 3 is a schematic based upon U.S. Pat. No.
4,529,322;
[0016] FIG. 4 is a schematic of a preferred embodiment of a bias
source for an LCD; and
[0017] FIG. 5 is a schematic of an alternative embodiment of a bias
source for an LCD.
DETAILED DESCRIPTION
[0018] In FIG. 1, pulse generator 11 provides high frequency pulses
to transistor 12, which is coupled in series with inductor 13
between supply and common. The pulses are coupled through diode 14
to storage capacitor 15, which is coupled across the DC diagonal of
a switching bridge including transistors 21, 22, 23, and 24. Diode
14 prevents storage capacitor 15 from discharging through
transistor 12. EL lamp 27 is coupled across the AC diagonal of the
switching bridge. Because neither end of EL lamp 27 is continuously
grounded, the configuration illustrated in FIG. 1 is sometimes
referred to as a floating lamp configuration. A low frequency
signal is coupled to transistors 23 and 24 through flip-flop 29,
which causes opposite legs of the bridge to conduct alternately,
thereby producing an alternating current through lamp 27. The
apparatus of FIG. 1 is known in the art, e.g. U.S. Pat. No.
4,527,096 (Kindlmann).
[0019] In FIG. 2, pulse generator 31 provides low frequency pulses
to one of transistors 32 and 33 and low frequency pulses to the
other of the transistors. Inductor 34 is coupled in series between
transistors 32 and 33. While transistor 32 is conducting, high
frequency pulses coupled to the base of transistor 33 cause
inductor 34 to produce a series of high frequency, high voltage,
positive pulses that are coupled through transistor 37 and diode 38
to EL lamp 27. After a predetermined period, or after a
predetermined number of high frequency pulses, the signals to
transistors 32 and 33 are reversed. While transistor 33 is
conducting, high frequency pulses coupled to the base of transistor
32 cause inductor 34 to produce a series of high frequency, high
voltage, negative pulses that are coupled through transistor 41 and
diode 42 to EL lamp 27. The process repeats, producing a low
frequency alternating current through EL lamp 27. The apparatus of
FIG. 2 is known in the art, e.g. U.S. Pat. No. 5,313,141 (Kimball).
Because neither end of inductor 34 is continuously grounded, the
configuration illustrated in FIG. 2 is sometimes referred to as a
floating inductor configuration.
[0020] FIG. 3 is the schematic of an inverter for driving two
capacitive loads as disclosed as prior art in U.S. Pat. No.
4,529,322 (Ueda). In this inverter, transistor 41 is switched on
and off at about eight kilohertz. When transistor 41 turns on,
current flows through inductor 42, storing energy in the magnetic
field generated by the inductor. When transistor 41 shuts off, the
magnetic field collapses at a rate determined by the turn-off
characteristics of the transistor. The voltage across inductor 42
is proportional to the rate at which the field collapses. A large
positive voltage is developed at node 43, charging EL lamp 44 above
supply voltage, and at node 45, charging piezoelectric buzzer 46
above supply voltage. Transistors 41, 47 and 48 are active in
different combinations to operate either buzzer 46 or EL lamp
44.
[0021] In FIG. 4, a commercially available inverter is modified in
accordance with the invention. Inverter 51 is known as a type
D372A, available from Durel Corporation, Chandler, Ariz. The
inverter includes an inductive boost circuit and an H-bridge
output; i.e. an EL lamp (not shown) is coupled across the AC
diagonal (pins 8 and 9) of a switching bridge within the inverter.
Inductor 52 is part of the inductive boost circuit and has a first
end coupled to supply 53 and an output end coupled to pin 6 of
inverter 51.
[0022] Bias source 50 includes a voltage tap and a voltage
regulator to reduce ripple. The anode of SCR 54 is coupled to the
output end of inductor 52, the gate of the SCR is coupled to
comparator 55, and the cathode of the SCR is coupled through diode
56 to capacitor 57. When SCR 54 conducts, pulses from inductor 52
(caused by circuitry in integrated circuit 51) charge capacitor 57.
The voltage on capacitor 57 is sampled by a voltage divider
including series resistors 61 and 62 and the tap voltage is coupled
to one input of comparator 55. The voltage divider, comparator, and
SCR provide a simple regulator for controlling the bias voltage
applied to liquid crystal display 68. Capacitor 57 and resistor 65
are a low frequency filter.
[0023] FIG. 5 is a schematic of a preferred embodiment of the
invention in which a series pass transistor couples the inductor to
a liquid crystal display. In bias source 60, elements common to
source 50 have the same reference number. Transistor 66 is
controlled by comparator 67, which has one input coupled to the
junction of resistors 61 and 61 and the other input coupled to a
reference voltage. The reference signals coupled to comparators 55
and 67 need not have the same voltage. Transistor 66 provides
additional smoothing of the bias signal to liquid crystal display
68.
[0024] The invention thus provides an inverter for driving LCDs and
EL lamps by adapting an existing, commercially available inverter.
The inverter can have either an H-bridge (floating EL lamp) output
or a non-floating (one end grounded) output for an EL lamp.
[0025] Having thus described the invention, it will be apparent to
those of skill in the art that various modifications can be made
within the scope of the invention. For example, only a single bias
source is illustrated but it is understood that two bias sources
would be used to provide positive bias and negative bias or high
bias and low bias, depending upon whether or not a split power
supply was used for the liquid crystal display.
* * * * *