Transformerless electroluminescent lamp driver topology

A transformerless driver circuit for an EL device has a battery or other source of dc power, and a microcontroller that is suitably programmed with associated firmware, so that a first switch control signal appears at a first output and a second switch control signal appears at a second output. A flyback circuit is connected to the first microcontroller output and, through a diode, to a charge storage device, e.g., a capacitor. This charges the capacitor stepwise to a relatively high voltage. The flyback circuit includes an inductor having one end connected to a first terminal of the dc power source, and another end coupled to a transistor or similar switch element. The transistor is coupled to the second dc power terminal, and has a base or gate coupled to the first output of the microcomputer. A controlled discharge arrangement bridges across the charge storage device to discharge the same periodically, and create an alternating current to drive the EL device. A second transistor has its base or gate coupled to the microcontroller second output. An isolating capacitor can be positioned in series between the diode and the charge storage device to eliminate the dc offset to the EL element. In one alternative arrangement, two or more flyback circuits are gated on and off sequentially. In a power-saver arrangement, the discharge circuit returns charge back to the battery.

BACKGROUND OF THE INVENTION 
The present invention relates to a drive circuit for illuminating an 
electroluminescent (EL) device, and is more particularly directed to a 
small, efficient, low-power-drain drive circuit which can be 
miniaturizable, e.g., to be contained on a small printed circuit board. 
The invention is more specifically concerned with a microcontroller 
actuated transformerless drive circuit, in which inductor coil(s) are 
switched on and off to charge up a storage element, e.g., capacitor, 
stepwise to a voltage that is many times higher than the dc supply 
voltage, and then the storage element is discharged. The charge and 
discharge cycle produces an ac drive voltage to power the EL device. 
This arrangement is suitable for use with EL lamps and panels for 
illuminating clock faces, for backlighting of liquid crystal displays, or 
for many other applications which are to be dc battery driven, or for 
marine, automotive, or aviation use. 
Several transformerless, i.e., induction flyback driven circuits, have been 
proposed. In Fujita U.S. Pat. No. 5,581,160, a pair of self-oscillating 
signal generators turn charging and discharging circuits on and off. These 
are free-running oscillators and create flyback pulses even during the 
discharge cycle. This renders it impossible to control the output levels 
of the drive circuit. Kimball U.S. Pat. No. 5,483,503 involves a circuit 
for powering an EL lamp in an electronic watch, which employs a 
microprocessor and logic circuit to actuate a push-pull driver circuit. 
Alessio U.S. Pat. No. 5,172,032 shows an EL driver circuit in which 
separate timer circuits actuate the charging and discharging switch 
elements. Other inductor-based EL drive circuits are shown in U.S. Pat. 
Nos. 4,529,322 and 5,502,357. 
Nothing in the prior drive circuits employs a microcontroller, programmed 
with suitable firmware, for producing increasing stepped high voltage 
charge and discharging to create an optimal ac drive waveform. The prior 
drive circuits have not employed ganged or multiple-inductor charge 
stages, actuated alternately or sequentially, to increase the rate of 
charge of the storage capacitor. Also, prior drive circuits have not 
employed any sort of energy conservation or recharge means to dump 
current, on the discharge cycle, back into the battery or other dc power 
source. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a transformerless 
electroluminescent lamp driver circuit that avoids the drawback of the 
prior art. 
It is another object to provide a transformerless electroluminescent lamp 
driver topology capable of producing an ac drive wave with wide ranging 
characteristics. 
It is yet another object to provide a circuit topology that increases power 
through-put and/or permits a decrease in peak input current. 
It is a further object to provide a microcontroller based circuit with 
great flexibility in operation and which can be made as small as possible 
to minimize circuit board space requirements. 
In accordance with an aspect of the present invention, a transformerless 
driver circuit has a battery or other source of dc power, and a 
microcontroller that is suitably programmed with associated firmware, so 
that a first switch control signal appears at a first output and a second 
switch control signal appears at a second output of the microcontroller. A 
flyback circuit is connected to the first microcontroller output and, 
through a diode, to a charge storage device, e.g., a capacitor, for 
charging the same stepwise to a relatively high voltage. The flyback 
circuit includes an inductor having one end connected to a first terminal 
of the dc power source, and another end coupled to a transistor or similar 
switch element. The transistor is coupled to the second dc power terminal, 
and has a control element, e.g., base or gate, coupled to the first output 
of the microcomputer. A controlled discharge arrangement bridges across 
the charge storage device to discharge the same periodically so that an 
alternating current appears thereon to drive the EL device. This 
arrangement can include a second transistor or similar controlled 
switching element, having a control electrode coupled to the 
microcontroller second output. An isolating capacitor can be positioned in 
series between the diode and the charge storage device to eliminate the dc 
offset to the EL element. 
In one alternative arrangement, there are two or more flyback circuits, 
each with a respective inductor and switching transistor, and each being 
coupled to a respective output of the microcontroller. The multiple 
flyback circuits can be driven alternately, i.e., sequentially. In another 
alternative arrangement, the discharge circuit is configured as a 
power-saver arrangement, so that the charge that has accumulated in the 
charge storage device is dumped back to the positive (first) terminal of 
the battery or other power source. 
The code or firmware present in the microcontroller can vary the ac voltage 
and waveform frequency, so as to achieve a desired level of illumination 
from the associated EL device. 
The above and many other objects, features, and advantages of this 
invention will become apparent to persons skilled in the art from the 
ensuing description of a preferred embodiment, which is to be read in 
conjunction with the accompanying Drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to the Drawing, and initially to FIG. 1, a first embodiment 
of the EL driver circuit 10 of this invention has a microcontroller 12 
whose memory carries a program or microcode as firmware, for generating at 
its output terminals P.sub.0 and P.sub.1 suitable charge and discharge 
control pulses, as described later in respect to FIGS. 4A, 4B, and 4C. A 
flyback charging circuit 14 is formed of an inductor 16 of value L1 having 
a first end coupled to a source of dc (i.e., battery) power at a voltage 
+V, here for example between about +2 volts and +6.5 volts, and a second 
end coupled to the collector of a switching transistor 18. This transistor 
has its emitter connected to the other power terminal, i.e., ground, and 
its base or control electrode coupled, via a resistor R1, to 
microcontroller output P.sub.0. The transistor 18 and inductor 16 define a 
junction which is coupled by a diode 20 to a charge accumulator, formed of 
a first capacitor 22 and a second capacitor 24. One plate of the capacitor 
24 is grounded. A discharge circuit 26 is connected between the diode end 
of the capacitor 22 and ground for discharging electrical charge that 
accumulates on the pair of capacitors 22 and 24. The discharge circuit 26 
is formed of a switching transistor 28 whose collector is coupled through 
a resistor R2 to the capacitor 22, and whose emitter is grounded. The base 
of the transistor 28 is coupled through a base resistor R3 to the 
microcontroller output P.sub.1. 
An electroluminescent lamp 30 is coupled between the two leads of the 
capacitor 24 and is driven into luminescence by the alternating current 
produced by the drive circuit 10. In this case a phantom capacitor 30a 
represents the inherent capacitance of the device 30. In practice, it is 
possible to omit the discrete capacitor 24, since it is used only for 
enhancement of storage capacity or for waveform improvement. 
An alternative embodiment is shown in FIG. 2 and the same elements as shown 
in FIG. 1 are identified with like reference numbers. Here, there is a 
second flyback circuit 114, formed of switching transistor 118 and a 
second inductor 116 connected between the positive terminal of the dc 
supply and the collector of the transistor 116. The emitter of this 
transistor 116 is grounded, and its base electrode is coupled through a 
base resistor R4 to an output terminal P2 of the microcontroller 12. A 
second diode 120 has its anode connected to the junction of the inductor 
116 and the collector of the transistor 118, and has its cathode feeding 
flyback voltage to the capacitors 22 and 24. In this case, the output 
signals at outputs P.sub.0 and P.sub.2 can be pulses that appear 
alternately, so that the two flyback circuits 14 and 114 operate 
sequentially to charge up the capacitors 22 and 24. This will be described 
later in respect to FIGS. 5A, 5B, 5C, and 5D. Alternatively, the two 
flyback circuits can be actuated simultaneously to double the amount of 
current pulsed into the capacitors for each step. 
A third embodiment of the invention is shown if FIG. 3, which is generally 
similar to that of FIG. 1. In this view, similar elements to those in FIG. 
1 are identified with the same reference numbers, and a detailed 
description will not be repeated. In this embodiment, the capacitor 22 is 
omitted, as the latter is optional and serves a DC offset function, 
described below with reference to FIG. 4A. Also, the storage capacitor 24 
is omitted as a discrete element, and its function is achieved by the 
inherent capacity of the EL device 30. 
Here, the discharge circuit 126 is configured as a regenerative or 
power-saver configuration. That is, in place of the transistor 28 of FIG. 
1, there is a PNP switching transistor 128 having its emitter connected to 
the high end of the capacitor and a collector connected through a resistor 
R5 to the positive terminal of the power source (here shown as a battery 
11). The base of transistor 128 is coupled through a resistor R6 to the 
collector of an NPN switching transistor 129, whose base is coupled 
through a resistor R7 to the microcontroller output P.sub.1. 
The operation of the EL driver circuitry of this invention can be described 
with reference to FIG. 1 and also to FIGS. 4A, 4B and 4C. The capacitor 24 
is charged to a relatively high voltage (over N steps--FIG. 4A) based on 
the value of the inductance L2 and the time rate of change of current 
(dI/dt) when the transistor 18 is cut off. The capacitor 24 is charged and 
discharged over an EL lamp cycle (FIG. 4C), which has a repetition rate on 
the order of a few hundred Hz to a few KHz. This range is favorably 
between 200 Hz and 1 KHz, and typically this can be about 800 Hz. The 
charge portion, i.e., EL charge cycle, as shown, need not be exactly 
one-half the entire EL lamp cycle. During the EL charge cycle, the charge 
circuit 14 is cycled N times, and at each charge cycle, here shown as 
inductor charge/discharge cycle, flyback voltage is pumped from the 
inductor 16 through the diode 20 to the capacitor(s) 22, 24. 
In this embodiment, during the EL charge cycle, switching pulses (FIG. 4B) 
appear at the output P.sub.0 of the microcontroller 12, and these cut the 
transistor 18 on and hard off. This causes the current to pulse N times 
through the inductor 16, generating the relatively high flyback voltage 
that charges up the capacitor 24. During the EL discharge cycle, when the 
output P.sub.1 is low, the pulses do not appear at the output P.sub.0. The 
value L1 of the inductor 16, the capacitance of the capacitor 24, and the 
current requirement of the EL device 30 will determine the number of 
charging steps N and the duty cycle of the output pulses at outputs 
P.sub.0 and P.sub.1. Favorably, the pulses at output P.sub.0 can have a 
rate of between 10 times and 40 times the pulse rate of the pulses at the 
output P.sub.1, and preferably about 20 times that pulse rate. 
As mentioned above, the capacitor 22 is an optional circuit element. In the 
absence of this element, the EL lamp device 30 charges up to a positive 
peak voltage, and then returns to ground potential at the discharge 
portion of the EL lamp cycle. When the capacitor 22 is present, it serves 
to block the dc portion (i.e., voltage V.sub.dcoffset --FIG. 4A) of the ac 
voltage appearing across the device 30. The value of the capacitor 22 is 
chosen to be approximately twenty times the capacitance value of the EL 
lamp 30 (or, as a rule of thumb, about twenty times the combined 
capacitance of the lamp 30 and the capacitor 24 plus any coupling 
capacitance). This means that the commensurate reactance of the capacitor 
22 is only about one-twentieth that of the EL lamp 30, so there is only an 
insignificant voltage loss across the capacitor 22. However, with the 
capacitor 22 in circuit, the relative polarity that the EL lamp device 30 
sees will alternate with each half cycle. That is, the capacitor 
ac-couples the drive circuit to the lamp device 30. This provides the 
benefit of reducing the effective peak value of the voltage impressed 
across the EL lamp, by allowing it to center itself about a zero dc 
offset. The duty cycle of the signal at output P.sub.1 is then selected to 
achieve a symmetrical EL lamp drive signal. An optional resistor R6, in 
series with transistor 128, is for peak discharge current control. 
In the embodiments having two or more flyback circuits, e.g., FIG. 2, the 
operation is basically the same as that of FIG. 1, except that two, or 
more, flyback circuits can be gated on and off using time-phased pulse 
trains, as shown in FIGS. 5B and 5C. The pulse signals appearing at 
outputs P.sub.0 and P.sub.2 gate the transistors 16, 116, etc., 
sequentially. In this embodiment, this permits twice as many charge steps 
in each EL lamp cycle (FIG. 5D), i.e., a multiple depending on the number 
of flyback circuits. Alternatively, all the flyback circuits can be pulsed 
on together, to increase the flyback current for each of the N steps. 
All of the above-described circuits can be implemented using appropriate 
power MOSFETs to reduce the component count and to enhance circuit 
performance. Also, as the system is completely under firmware control, 
additional features can be implemented, such as external synchronization, 
multiple brightness levels, programmed on-time, flash mode ambient light 
sensing, sleep mode, etc. The circuit as described could also be arranged 
for negative battery operation, substituting PNP for NPN transistors and 
reversing the diode polarities. The various transistors could also be 
integrated, rather than discrete devices. 
While the invention has been described in detail with respect to one 
preferred embodiment, it should be recognized that there are many 
alternative embodiments that would become apparent to persons of skill in 
the art. Many modifications and variations are possible which would not 
depart from the scope and spirit of this invention, as defined in the 
appended claims.