U.S. patent application number 12/149638 was filed with the patent office on 2008-08-28 for limited current circuit for electro-luminescent lamp inverter.
Invention is credited to Chih-Ping Liang, Chang-Yi Liu, En-Ming Wu.
Application Number | 20080203935 12/149638 |
Document ID | / |
Family ID | 38655998 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080203935 |
Kind Code |
A1 |
Liang; Chih-Ping ; et
al. |
August 28, 2008 |
Limited current circuit for electro-luminescent lamp inverter
Abstract
A current limiting device includes a constant current source
circuit constructed and arranged to be connected to a direct
current input voltage source and to regulate electrical current and
a resonance source circuit receiving current from the constant
current source circuit. The resonance source circuit is constructed
and arranged to be electrically connected with an
electro-luminescent (EL) lamp so that the EL lamp receives
alternating current. The constant current source circuit and the
resonance source circuit are constructed and arranged such that
when current flows from the constant current source circuit to the
resonance source circuit and to the EL lamp, a maximum peak current
received by a user contacting an output of the device or contacting
the EL lamp is less than about 0.7 mA.
Inventors: |
Liang; Chih-Ping; (Hsinchu
City, TW) ; Liu; Chang-Yi; (Changhua County, TW)
; Wu; En-Ming; (Taipei City, TW) |
Correspondence
Address: |
GIBBONS P.C.
ONE GATEWAY CENTER
NEWARK
NJ
07102
US
|
Family ID: |
38655998 |
Appl. No.: |
12/149638 |
Filed: |
May 6, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11727274 |
Mar 26, 2007 |
|
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12149638 |
|
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60787534 |
Mar 31, 2006 |
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Current U.S.
Class: |
315/224 |
Current CPC
Class: |
Y02B 20/30 20130101;
H05B 33/08 20130101 |
Class at
Publication: |
315/224 |
International
Class: |
H05B 37/02 20060101
H05B037/02 |
Claims
1-18. (canceled)
19. A method of compensating for decay of an electro-luminescent
(EL) lamp, the method including: providing an inverter having a
constant current limiting circuit constructed and arranged to be
connected to a direct current input voltage source and to regulate
electrical current, the inverter also including a resonance source
circuit receiving an output voltage from the constant current
source circuit, the resonance source circuit providing an
alternating current output voltage to the EL lamp, and as the EL
lamp ages, increasing the output voltage from the constant current
source circuit and thus the output voltage from the resonance
source circuit, thereby compensating for a decay of the EL
lamp.
20. The method of claim 19, further including changing a current
setting of the constant current source circuit to dim the output
voltage of the resonance source circuit.
21. The method of claim 19, wherein the resonance source circuit
includes a transformer with a separate winding electrically
connected to the EL lamp and output ground,
22. The method of claim 19, wherein the constant current source
circuit includes a transistor, an emitter of the transistor is
connected to the input voltage source, a current sensor is
connected to a collector of the transistor, a first amplifier is
connected with the current sensor to amplify a pass current, a
resistance divider, establishing a reference voltage, is connected
to a positive input of a second amplifier, a negative input of the
second amplifier being connected to an output of the first
amplifier, a base of the transistor is connected to an output of
the second amplifier to complete a constant current feedback loop
defined by the current source circuit.
Description
[0001] This application claims the benefit of the earlier filing
date of U.S. Provisional Application No. 60/787,534, filed on Mar.
31, 2006 which is incorporated by reference herein in its
entirety.
FIELD OF THE INVENTION
[0002] The invention relates to electro-luminescent (EL) lamps and,
more particularly, to a circuit for limiting current of an EL
inverter to prevent or reduce electrical shock to a user.
BACKGROUND OF THE INVENTION
[0003] The use of electro-luminescent (EL) panels, particularly in
portable electronic derives such as keyboards of laptop and desktop
computers, requires meeting various standard safety tests. In order
to produce a high level of backlighting, a high peak to peak
voltage is necessary. To avoid electrical shock, organizations like
UL (Underwriters Laboratories), require that the inverter, used to
power the EL panel(s), provides the power necessary such that the
clearance and creepage distance between the high voltage output of
the EL panel and its surroundings (enclosure, printed circuit board
and connector) do not create a hazardous condition (UL 60950). The
requirement of a safety space usually needs 4-8 mm. For portable
and lightweight devices where space is at a premium, it is
difficult to meet such standards. The alternative method to meet
such standard is to provide limited current circuit (LCC) in the
inverter. The equivalent impedance of a human body is about 2000
ohms. For a 2000 ohm load between outputs or output to ground, a
maximum peak current flow should not exceed 0.7 mA.
[0004] Conventional single ended autotransformer type inverters
inhibit the oscillation as the output load is shorted, but do not
take a 2000 ohm load into account. Such an inverter would protect
itself from an output short circuit, but would not prevent an
electrical shock to the human body upon contact with the output of
the inverter under normal conditions.
[0005] Thus, there is a need to provide an inverter circuit with
capability to limit current such that a maximum peak current flow
to ground does not exceed 0.7 mA.
SUMMARY OF THE INVENTION
[0006] An object of the invention is to fulfill the need referred
to above. In accordance with the principles of the present
invention, this objective is obtained by providing a limited
current circuit (LCC) within an inverter of an EL lamp to prevent
or limit the above-mentioned electrical hazard. Thus, the LCC is
configured so that when a human body or a similar resistance (2000
ohm) touches the output of the inverter or touches the EL lamp
under normal conditions, there is a maximum peak current flow to
ground. Present UL requirements specify this at not exceeding 0.7
mA.
[0007] Thus, a current limiting device is provided that includes
current source circuit constructed and arranged to be connected to
a direct current input voltage source and to regulate electrical
current and a resonance source circuit receiving current from the
current source circuit. The resonance source circuit is constructed
and arranged to be electrically connected with an
electro-luminescent (EL) lamp so that the EL lamp receives
alternating current. The current source circuit and the resonance
source circuit are constructed and arranged such that when current
flows from the current source circuit to the resonance source
circuit and to the EL lamp, a maximum peak current received by a
user contacting an output of the device or contacting the EL lamp
is less than about 0.7 mA.
[0008] In accordance with another aspect of the invention, a method
of limiting current of an inverter associated with an
electro-luminescent (EL) lamp is provided. The method provides an
inverter constructed and arranged to convert Direct Current input
voltage to an Alternating Current output voltage to power an EL
lamp. A current limiting circuit is provided within the inverter.
The method ensures that when the EL lamp is powered by the
inverter, the current limiting circuit limits a maximum peak
current received by a user contacting an output of the inverter or
contacting the EL lamp to less than about 0.7 mA.
[0009] In accordance with yet another aspect of the invention, a
method of compensating for decay of an electro-luminescent (EL)
lamp provides an inverter including a constant current limiting
circuit constructed and arranged to be connected to a direct
current input voltage source and to regulate electrical current.
The inverter also includes a resonance source circuit receiving an
output voltage from the constant current source circuit. The
resonance source circuit provides an alternating current output
voltage to the EL lamp. As the EL lamp ages, the output voltage
from the constant current source circuit is increased, thus
increasing the output voltage from the resonance source circuit,
thereby compensating for a decay of the EL lamp.
[0010] Other objects, features and characteristics of the present
invention, as well as the methods of operation and the functions of
the related elements of the structure, the combination of parts and
economics of manufacture will become more apparent upon
consideration of the following detailed description and appended
claims with reference to the accompanying drawings, all of which
form a part of this specification.
BRIEF DESCRIPTION OF THE DRAWING
[0011] The invention will be better understood from the following
detailed description of the preferred embodiments thereof, taken in
conjunction with the accompanying drawing in which:
[0012] FIG. 1 is a schematic of a current limiting device for an
inverter of an EL lamp, provided in accordance with the principles
of the invention.
DESCRIPTION OF THE EXEMPLARY EMBODIMENT
[0013] With reference to FIG. 1, a limited current circuit (LCC)
device for an inverter of an EL lamp is shown, generally indicated
at 10, in accordance with the principles of the present invention.
The LCC device 10 includes a constant current source circuit,
generally indicated at 12, and transformer resonance source
circuit, generally indicated at 14. The LCC 10 is part of a
conventional inverter 11 for an EL lamp 28. The inverter 11
converts DC voltage to AC voltage (preferably 50-80 Vrms) and
frequency (preferably 250-1000 Hz) for operating the AC powered EL
lamp 28.
[0014] The constant current source circuit 12 is a linear
transistor pass type regulator. In order to reduce a drop voltage
of a transistor 16, a PNP type transistor is used. The emitter is
connected to the inverter input voltage source (which is typically
5.0 or 3.3V DC). A current sensor 18 is connected with the
collector of the transistor 16. The sensor 18 is preferably in the
form of a small resistance shunt (preferably 0.2 ohm) that is also
intended to reduce the drop voltage. An operational amplifier 20 is
connected to the shunt resistor 18 to amplify the pass current. A
reference voltage is established from a resistance divider 21
connected to the positive input of another operational amplifier
22. The negative input of amplifier 22 is connected to the output
of amplifier 20. The base of transistor 16 is then connected to the
output of operational amplifier 22 to complete a constant current
feedback loop.
[0015] The resonance source circuit 14 includes a transformer 24
having three sections or phases for resonance windings. N1 is
connected to the output of the constant current source circuit 12,
current limiting resistor 15 and the PNP derive transistor 16. N2
is a step-up winding on N1 which is coupled to the base circuit
through a capacitor 26. N3 is a separate winding connected to the
EL lamp 28 and output ground.
[0016] When an EL panel or lamp 28 is provided power and/or is lit,
power from the constant current source circuit 12 flows to the
transformer 24 and feedback winding N2 forces the derive transistor
16 to switch on and off. The coupled voltage on the N3 winding will
resonate with the EL lamp receiving the AC output. When a user such
as a human body (or similar resistance) touches the high voltage
output of the inverter containing the LCC 10 (or touches the EL
panel or lamp 28), the oscillation is substantially detuned and
insufficient energy is then fed through the capacitor 26 into the
base of transistor 16. The base current of the derive transistor 16
will keep the Direct Current (DC) at a constant value. The constant
current source circuit 12 limits the current flow into winding N1
and the derive transistor 16. The current limiting resistor 15 has
also a voltage drop, and the emitter voltage of transistor 16 will
then drop so that the oscillation stops. Since N3 is a separate
winding, the current will not draw from the constant current source
circuit 12 to the human body (or similar resistance).
Experimentally, this approach shows the peak current is
significantly below UL's current requirements of 0.7 mA and is
typically about 0.2 mA.
[0017] The LCC 10 can be employed in a circuit or an integrated
circuit and/or partially embedded in a semiconductor chip.
[0018] Thus, the LCC 10 can be controlled to levels below the
threshold mandated by UL and enables keyboards and keypads for
computer and related products to be backlit with EL panels and
substantially achieve UL certifications, and Conformite Europeenne
(CE) certifications for Europe. The LCC 10 can be controlled to
levels below the threshold mandated by UL and enable keyboards and
keypads for computer and related products to be backlit with EL
panels and substantially achieve computer related safety
certifications like Certification Body (CB). Control of the LCC 10
provides a much safer backlit environment for EL based products,
and control of the LCC 10 enables EL products to be used into
applications where previously they were unacceptable.
[0019] Another feature of LCC is the fact that it is a constant
power source to the derive EL lamp 28. The life as derived by a
constant power source is much longer than a constant voltage
source. As the EL lamp 28 ages, its capacitance will increase.
Since the autotransformer type inverter has a constant output
voltage to the derive EL lamp 28, the current fed into the
resonance circuit 14 and to the EL lamp 28 will decrease, resulting
in poor life. In order to maintain constant current flow from the
current source 12, the output voltage of the amplifier 22 will
increase and reduce the base current of the pass transistor in the
current source 12. The enhanced conduction of the pass transistor
generates a higher output voltage of the pass transistor. The
increased input voltage to the resonance circuit 14 increases the
output voltage to EL lamp 28. The brightness of the EL lamp depends
on the magnitude of applied voltage. Thus, the (brightness) decay
of the EL lamp 28 can be compensated to extend the life of the EL
lamp 28.
[0020] Furthermore, since the EL lamp 28 is powered by the inverter
11, adjusting the setting of the current of the constant current
source 12 can dim the output of the inverter 11.
[0021] The foregoing preferred embodiments have been shown and
described for the purposes of illustrating the structural and
functional principles of the present invention, as well as
illustrating the methods of employing the preferred embodiments and
are subject to change without departing from such principles.
Therefore, this invention includes all modifications encompassed
within the spirit of the following claims.
* * * * *