Note: Descriptions are shown in the official language in which they were submitted.
ELECTROLUMINESCENT DISPLAY DRIVE_RCUITRY
BACKGROUND ANV SUMMARY OF THE INVENTION
~ --
Electroluminescent display drive circuits (drivers) are traditionally
driven by transformers. Thc transformer driven excitation of ~he
electroluminescent display panel is inheren~ly inefficient due to the capacitive load
from the panel bein~ reflected back into the transformer. This causes an efficiency
drop due to a power factor loss created by the existence of the inductance of the
trans~ormer and the capacitance of the electroluminescent panel.
In addition, electroluminescent display devices have traditionally been
driven by high power devices~ ùsually transformers. Although transformers may benecessary ~o generate high voltage, they are not necessary to drive the
electroluminescent displays.
This discovery has far reaching conseguences on the art due to the
possible reduction of the size and weight of electroluminescent display power
supplies. It also has an additional benefit of making the desi~n less susceptible to
electromagnetic interference (EMI). Now, the electroluminescent display can be
designed to operate at a frequency which will minimize the EMI effects.
Typical of the electroluminescent displays driven by the subject circuitry
can be found by referring to U.S. Patent No. 4,388,554 issued on June 14, 1983, to
Suntola et al.
I ,.
Another type of electroluminescent display device for dashboards of an
automobile is described in U.S. Patent No. 4,323~895 issued on April 6, 1982, toCoste.
Another electroluminescent display device is described in U.S. Patent ¦
No. 4,320,1~9 issued on March 16, 1982, to Yatabe et al.
~ ' ~ I
-1- I
. 1.
It is a feature of the present invention to improve the
efficiency of electroluminescent display drivers.
It is also a feature of the subject invention to improve
the efficiency of electroluminescent display drivers by reducing
or eliminating the effects of the power factor created in trans-
former driven excitation circuits.
It is also a feature of the subject invention to reduce
the si~e and weight of traditional electroluminescent display drive
circuits. And it is still another object of the subject invention
to eliminate the use of a transformer to drive electroluminescent
display panels.
Specifically, the invention relates to an electrolumin-
escent display drive circuit comprising: a high voltage source; an
electroluminescent display panel; two signal transistors~ a source
of two input signals presented to the base of each signal transis-
tor where the input signals to one transistor is 180 degrees out of
phase with the input signals to the second transistor; the electro-
luminescent display panel being connected between the collectors
of each transistor, and the electroluminescent display panel fur-
ther being connected to each end of the high voltage source through
two resistors such that the voltage generated across the electro-
luminescent display panel is in phase with the current passing through
the electroluminescent display panel.
DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present in-
vention will become more fully apparent from the following detailed
description of~the preferred embodiment, the appended claims and in
the accompanying drawings in which:
'.
Fig. 1 shows an elementary embodiment of the electrolum-
inescent display drive circuitry;
--2--
Fig. 2 shows another embodiment of the electrolumin-
escent display drive circuitry;
Fig. 3 shows still another embodiment of the electro-
luminescent display drive circuitry with a masking capacitor used
to reduce the effect of the capacitance changes within the electro-
luminescent display;
Fig. 4 is a timing diagram showing the input signals to
the drive circuit;
Fig. 5 is a schematic diagram showing a prior art
electroluminescent display driver;
-2a-
~27~8
Fig. 6 is a graph showing the voltage across and current through the
electroluminescent display as a function of time; and
Fig. 7 is a graph showing the voltage across and current through the
electroluminescent display as a function of time for the power supply according to
the subject invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Fig. 1, the electroluminescent display panel is shown
connected to a high voltage supply via two resistors R127 and R128.
The circuit is completed by way of two transistors Q105 and Q106, each
connected to one of the resistors R127 and R128. The collectors of each of these
transistors is connected to the electroluminescent display panel, one to each side.
The electroluminescent display panel is depicted as a capacitor connected between
the collectors of the two transistors.
The emitters of each transistor, Qlû5 and Q106, are then connected to
ground.
The bases of each ~ransistor, Q105 and Q106, are connected to a signal
source. This signal source can be an individual oscillating circuit or it can be an
output from a microprocessor. In any event, input A to Q105 is 180 degrees out of
phase with input B to Q106. Both inputs are to the base of the transistors and are
both square wave signals. The frequency of each input signal can be the same, just
180 degrees out of phase. However, it may be desirable to only turn one transistor
on at a time. That is, keep the off time of one input signal slightly less than the on
time for the other input signal. This creates a dead zone. It is designed to never
allow the transistors Q105 or Q106 to be on at the same time. This keeps heat loss
to a minimum and reduces stress on the transistors. There are many ways to
accompllsh this; the ke~ Is never ~o turn them both on at once. This principle is
:'.
' .i
' I` ~ ', '
, ~
,.,. . , : .' , , .
used for input A and input B with respect to Fig. 1, Fig. 2 and Fig. 3, although each
would work with the signals at the same frequency but 180 degrees out of phase.
These signals are depicted in the timing diagram of Fig. l~, along with the
wave form across the electroluminescent display.
In addition, varying the frequency of the input signal to increase the
dead zone can be used to dirn the output of the display.
It can be seen that the subject invention will drive the
electroluminescent display without the use of a traditional transformer. The
emphasis of traditional designs is toward power and the use of power transistors. In
contrast~ the subject invention utilizes transistors that operate at a much lower
power and emit very little heat.
The design lends itself nicely to surface mounted devices and allows for
a desi~n which lets the desi~ner select the resistor values for speclfic applications.
In other words, the resistor will correspond to the capacitance of the individual
electroluminescent display panel used and the frequency that it is driven at.
There is also evidence that the subject design is less susceptiblc to
electro-ma~netic interference (EMI) because it is run at a constant frequency or run
at a frequency which will not interfere with the radio or other components in anautomotive environment.
All of these factors are of particular importance to almost any
environment, but the subject design was developed for use in an automotive;
environment.
I .
In this environment, large temperature swings are usually present. In
addition, size and wei~ht are of critical importance.
4_
. I
' ' ~' ' " .
-
` '' -' ~ ' ', " ' ' ' .,, '
- ` "' ' ' ', ~ .: .'
.
'~ ' , .
l ~7Z8~ 1
Referring now to Fig. 2, another embodiment of the electroluminescent
display drive circuitry is shown. The particular design calls for the inputs A and B
as in Fig. 1. Both inputs A and B are fed to the base of an NPN transistor, in this
case input A fed to the base of NPN transistor Q109 and input B fed to the base of
NPN transistor QllO. Both transistors Q109 and QllO are connected to ground via
the emitter.
The collectors of each transistor are connected to one end of the
electroluminescent display through resistors R135 and R136. As before, the
electroluminescent display panel is depicted as a capacitance; this is done even
though the display has inherent resistive losses.
However, in Fig. 2, the electroluminescent display is connected to the
high voltage ~hrough two NPN transistors Q107 and Q108. The emitters of the drive
transistors Q107 and Q108 are tied to the high voltage and each collector is tied to
the electroluminescent display panel through a time constant resistor R133 for Q107
and R134 for Q108.
Driving each transistor Q107 and Q108 is a connection between the base
of each transistor anq a resistor. The resistor R131 being connected to the base of
Q107; and resistor R132 being connected to the base of Q108. Each resistor R131
and R132 is connected to the collector of the PNP transistors, Q108 and Q107,
respectively, on the opposite side of the electroluminescent display. Each emitter
of transistors Q107 and Q108 are connected to the high voltage supply and the base
of each transistor is further biased by resistor R129 and R130 being connected
between the bases of each transistor and the high voltage supply.
Referring now to Fig. 3, another embodiment shown in cut-away format
illustrates the use of capacitor C112 hooked in parallel across the
electroluminescent display, still depicted as a capacitor. The cut-away view of Fig.
3 is otherwi5e the sa--e s s`own in Fig. 2.
_5
li
I
,
`
~,
.` ' ' '`: ' '
- :',
~Z~72~
All of the circuits shown in Figs. 1, 2 and 3 genera~e an A/C voltage
across the electroluminescent display panel. For more efficient operation of the
display, the peak-to-peak voltage across the electroluminescent display will be
approximately twice the amGunt of the high voltaKe input. This amount might be
less in Figs. 2 and 3 where the high voltage across the eJectroluminescent display
will be slightly lower due to effect of an RC network.
The RC network is made up of the series combination of resistors R133
and R134 in series with the electroluminescent display panel shown in Fig. 2.
The RC network Is Fig. 3 comprises the resistors R13~ and R134 in series
with the parallel combination of the eiectroluminescent display9 along with the
capacitor C112.
The efficiency of the circu;ts shown in Figs. 1, 2 and 3 is very good ~ue
to the fact that a power factor of one is ~enerated.
Electroluminescent displays are generally discussed in an article "D.C.
Electroluminescence for Automobile Instruments" published by the Institute of
Electrical Engineers on July 6-9, 1976, and written by B. ~hepherd, R. N. Thomas
and P. :1. ith.
Also discussing electrolurninescent displays is SAE Paper 810076,
"Electroluminescent Instrumentation" by ~. Shepherd dated February 1981.
. I
. I
Referring to Fig. 5, shown is a typical excitation circuit for an
electroluminescent display. Notice that a transformer is utilized such that the
secondary of the transformer is connected directly across electroluminescent
display. A power factor other then 1 is created due to the interaction between ~he
inductance of the transformer and the capacitance of the electroluminescent
display. In addition, the bulk and weight of a transformer is eliminated.
-6-
I .
~ 72~
Prior art power supplies to drive an electroluminescent display are run at
low frequency since the displays run at low frequency. This means that large
transformers must be used to generate the low frequency drive.
This is shown in Fig. 5 with a +V voltage supplied to the primary of
transformer T. The collector of transistor Q is connected to the primary of T; the
emitter to ground; the base to an input signal 1. The electroluminescent display
panel is depicted as a capacitor connected across the secondary of transformer Q.
The circuits described in the subject invention run at high frequencies
and create a voltage without a power factor to contend with.
Examples of waveforms in prior art power supplies and in the subject
invention are shown in Fig. 6. Examples of waveforms for the power supplies of the
present invention are shown in Fig. 7. Both Fig. 6 and Fig. 7 graph voltage and
current as a function of time. It can be seen that the power factor is not an issue in
Fig. 7 as the current through the electroluminescent display is in phase with the
voltage across it.
The subject circuit does not introduce a transformer to the direct
excitation of the display and, therefore, reduces the size and weight of the design.
In addition, the subject invention eliminates or reduces the effect of the power
factor as no inductance is present in the direct excitation of the electroluminescent
display drive circuit; this is because current and voltage are in phase.
Referring again to Fig. 4, capacitor C112 is shown connected in parallel
across the capacitive electroluminescent display panel. The capacitor C112 is used
to mask or reduce the effect of capacitance changes of the electroluminescent
display panel.
.
. : , . ~ ,: , : -
.. . . .
,- ~
., ' ,
~7~
Over a period oi time, the electroluminescent display panel will change
in its inherent capacitance due to age. In general, the electroluminescent display
will decrease in capacitance over a period of time.
~ Ihen the capacitance of the electroluminescent display panel changes,
the voltage across the paneJ increases due to the new and shorter RC time constant
created by the changed capacitance; i.e., the RC charging time effects of resistors
R133, R134 in series with the parallel combination of the electroluminescent display
and capacitor C112. This shorter RC time constant provides for a constant output
from the electroluminescent display panel. Therefore, the panel can be used for a
longer period of time since the affects of aging are masked by the higher voltage
input.
Typically, the capacitance of the electroluminescent display drops to
about one-half of the capacitance of its original capacitance between 500 and 1,000
hours of operation. Therefore, if capacitor C112 is chosen to be approximately
equal to the capacitance of the electroluminescent display when new, the above- j
described increases in the voltage across the electroluminescent display will
increase the peaks of the electroluminescent voltage waveform shown in Fig. 7.
If capacitor C112 is chosen to be much larger than the
electroluminescent capacitance, i.e., such that the capacitance of Cl 12 is ten times
or more greater than the capacitance of the electroluminescent display, then the
effect of the capacitance change in the electrolurninescent display will be felt as
the voltage across the panel will remain relatively constant due to the lack of
change in the RC time constant.
Alternatively, if the capacitance of capacitor C112 is much smaller than
the capacitance of the electroluminescent display, then the aging effects of the
electroluminescent disl lay on its inherent capacitance will be exaggerated.
~ -8-
. .
, :.
' , '' ';,
,
,' . . , -, ' ~
: . - .
~L272~
While the present invention has been disclosed in connection with the
preferred embodiment thereof, it should be understood that there may be other ¦
embodiments which fall within the spirit and scope of the invention and that theinvention is susceptible to modification, variation and chan~e without departin~l~ol the proper scope or lai~ meanin6 ol the lollowine claims.
: :
_9_ 1
, '.
~ -
- " ' . . . , ~ ' ' ' ' ' ' . , ' ,,
