[0001] The present invention relates to LED arrays.
[0002] It is a common practice to arrange a set of LEDs in some form of array, in order
to provide a required level of total optical power. For example bulbs or lamps using
LEDs have in recent years been adopted as replacements for more traditional light
sources such as incandescent bulbs. An array of several LEDs is typically needed to
provide illumination equivalent to that of one conventional bulb. The LEDs are typically
placed side-by-side upon some form of carrier such as a printed circuit board. An
early example of an LED "bulb" constructed in this manner is found in FR 2586844 (Sofrela)
but such devices are by now very widely known.
[0003] In designing an LED array to be driven from a supply with a given voltage, the necessary
number of LEDs is typically split between a set of series-connected "strings" of LEDs,
each string being connected across the supply. The number of LEDs in a string is chosen
such that their total forward voltage is slightly lower than the supply voltage. For
example in an array to be run from a 28V DC supply, the string length could be chosen
to give a 26 V forward voltage across the string, e.g. by using 10 LEDs each with
a forward voltage (V
f) of 2.6. This arrangement permits power loss in an associated LED driver/control
circuit to be kept low whilst effecting enough control to protect the LEDs from overcurrent
and enabling intensity reduction if required.
[0004] Problems arise, however, if the LED array is required to work over a range of temperatures
and supply voltages, particularly if it is also necessary to meet a tight specification
concerning the array's light output. The present invention was devised in connection
with lights for use at the exterior of an aircraft, which is a particularly demanding
environment in these respects, although it is applicable in other fields.
[0005] Temperature variations are problematic because LED forward voltage V
f is temperature dependent. Specifically, V
f increases as temperature reduces. In the example given above, if ambient temperature
is reduced from 25°C to -40°C the string's forward voltage would increase from 26V
to 28.8V, exceeding the supply. This range of operating temperatures is not excessive
for military and aeronautical applications. The result could be that at extremely
low temperatures so little current would pass that the LEDs would not illuminate at
all, and at more moderate low temperatures LED output would be very low.
[0006] Supply voltage variations can also prevent the LED array from operating correctly
given that the minimum voltage of a nominal 28V supply array may be 22V or even less.
[0007] One way to address such problems is to select the length of the LED strings to give
acceptable performance at the lower temperatures and voltages. In the example above
this might mean using a string with a forward voltage of 22V at room temperature.
However this results in excess power dissipation at room temperature and normal voltage.
Higher power must therefore be dissipated, typically necessitating use of a large
and heavy heat sink. The problem is exacerbated by the fact that as the temperature
rises the LED efficiency falls, creating a vicious circle.
[0008] Another way to address the above problems is to use some form of switched mode power
supply, which can generate different voltages with very high efficiency and thus match
the LED's optimum drive voltage over ranges of temperature and supply voltage. However
such supplies are complex but worse - and often crucially - they generate electromagnetic
interference which makes them unacceptable e.g. in many aerospace applications.
[0009] European patent application 1006506 (Hewlett-Packard Company) concerns an optical
vehicle display having a set of LEDs arranged in a matrix. A circuit diagram is shown
in which each of the LEDs is provided with a parallel-connected switch by which an
LED which fails can be short-circuited enabling the remaining LEDs to continue to
function. The LED matrix is driven from a current source. The problems posed by changes
in LED forward voltage and supply voltage is not addressed in this document.
[0010] European patent application EP 1318701 (Audi AG) concerns a method and apparatus
for driving a plurality of LEDs. The circuit includes a series-connected set of light
emitting diodes, and a current source and switch connected in series with the diodes.
A topped bridge across some, but not all, of the diodes contains a second current
source and a second switch.
[0011] US 6320322 (Rohm Co,. Ltd.) discloses an arrangement having red, green and blue LEDs.
By adjusting the luminance of the three elements, a range of colours can be created.
Driving circuitry for the three LEDs is disclosed, but this does not address the problems
associated with changes in LED forward voltage and supply voltage.
[0012] German Grebrauchsmusterschrift 20101418 (Insta Elektro GmbH & Co. KG) concerns a
switching control for multiple lighting units with a microcomputer controller, in
which individual LEDs can be bridged by respective switches
[0013] It is an object of the present invention to provide an improved LED array. More specifically,
it is intended to provide an LED array capable of improved operation over a range
of temperatures and/or supply voltages.
[0014] In accordance with the present invention, there is an array of light emitting diodes
(LEDs) comprising at least one string of LEDs within which the LEDs are connected
in series with one another, the string being connectable across a potential difference
to drive the LEDs, and at least one switching means being provided for selectively
reducing the effective length of the string by removing at least one LED from the
current path through the string.
[0015] By shortening the effective length of the string, its forward voltage can be reduced.
This provides a very straightforward way to compensate for changes in the forward
voltage of individual LEDs and for supply voltage variability.
[0016] While the "array" in question could have just one LED string, in the more typical
case it comprises a plurality of strings in parallel with one another. The strings
typically all comprise the same number of LEDs.
[0017] In the preferred embodiment the switching means serves to selectively bridge at least
one LED in order to effectively remove it from the string.
[0018] A further preferred embodiment comprises two or more switching means for selectively
removing different LEDs or sets of LEDs from the string, providing three or more possible
string lengths.
[0019] A transistor is the preferred form of switching means and has the advantage of creating
no appreciable electromagnetic interference. It can also be used to regulate LED current.
[0020] Preferably, the switching means is controlled by an electronic controller responsive
to one or more of (I) a measured temperature; (ii) a measured LED current; and (iii)
a measured supply voltage.
[0021] A specific embodiment of the present invention will now be described, by way of example
only, with reference to the accompanying drawing, which is a circuit diagram of an
LED array.
[0022] The illustrated array 2 contains a total of fifty LEDs 4 arranged in five series
strings 6. Of course the total number of LEDs is chosen according to requirements
for optical power output. The total length of each string is chosen with reference
to the intended supply voltage and the LEDs' forward voltage. The supply is connected
across high rail 8 and ground 10. The strings are connected in parallel with each
other.
[0023] A microprocessor/controller 12 receives, in this particular embodiment, two inputs.
The first input is indicative of the supply voltage, and is obtained through a connection
14 to the mid point of a potential divider formed by two resistors R1, R2 which are
connected in series across the supply. The second of the two inputs is indicative
of the sum of the currents passing through the LED strings 6 and is obtained by detecting
the voltage across a resistor R3 through which all of the strings are connected to
ground. Based upon these two inputs, the controller selects the states of control
outputs O1, 02, 03 to respective switching transistors T1, T2, T3. In this way, the
controller determines the length of the LED strings.
[0024] T1 serves, when in its closed circuit or "on" state, to connect the LED 4' at the
end of each string to resistor R3 and so to ground. If T1 is on and the other transistors
are "off" (open circuit) then all ten of the LEDs in each string are illuminated.
If T2 is then switched on and T1 is switched off then the second LED 4" in each string
is connected to ground via T2 and R3. End LEDs 4' are effectively disconnected and
no longer illuminated. Each LED string 6 is thus shortened to contain nine illuminated
LEDs. Switching on T3 and switching off T2 then connects the third LED 4'" in each
string via T3 and R3 to ground. Both end LEDs 4' and second LEDs 4" are then disconnected
and each string contains only eight illuminated LEDs.
[0025] By adjusting string length the strings' forward voltage is correspondingly adjusted
and in this way compensation is effected both for the temperature dependent changes
in forward voltage of individual LEDs and for supply voltage variation.
[0026] In the specific exemplary embodiment illustrated in the drawing, shortening of the
strings entails reducing the total number of LEDs which are illuminated. In the case
where compensation is effected for low temperature, this need not necessarily reduce
total light output, since LED efficiency is higher at low temperatures. Where compensation
is being made for low supply voltage, LED current is of course increased and even
if this is insufficient to maintain light output there will at least be significant
intensity, which is acceptable in some programs where LED lights are replacing incandescent
bulbs whose output would in any case drop to 40% of normal if supply voltage falls
from 28V to 22V.
[0027] The controller 12 sets the state of transistors T1 - T3, and hence the LED string
length, based upon its sensor inputs. In the illustrated example this is done on the
basis of supply voltage and LED current. However there are other operating parameters
which could be sensed and used in setting string length. For example ambient temperature
or LED operating temperature could be measured and taken account of in this regard.
[0028] A current regulating function is also carried out by whichever transistor is conducting.
The microprocessor 12 monitors voltage across resistor R3, which corresponds to total
LED current. In response, the microprocessor controls the conductive transistor T1,
T2 or T3 and thereby regulates the current.
[0029] The LED array may of course be incorporated into any form of LED lamp, light or bulb.
However the low component count makes the present invention particularly suited to
use in units designed to substitute directly for incandescent bulbs (e.g. having a
conventional bayonet or screw bulb fitting). The fact that no appreciable electromagnetic
interference is created also makes the invention suitable for sensitive applications
e.g. on aircraft.
1. An array of light emitting diodes (LEDs) comprising at least one string of LEDs within
which the LEDs are connected in series with one another to form a current path through
the string, the string being connectable across a potential difference to drive the
LEDs, the array being provided with an arrangement for controlling forward voltage
of the string, comprising at least one switching device connected to the LEDs such
that by switching it serves to remove at least one LED from the current path through
the string.
2. An array of LEDs as claimed in claim 1 which comprises a plurality of strings in parallel
with each other.
3. An array of LEDs as claimed in claim 2 wherein the switching means is connected to
each string for selectively reducing the effective length of each.
4. An array of LEDs as claimed in any preceding claim wherein the switching means serves
to selectively bridge at least one LED in order to effectively remove it from the
string.
5. An array of LEDs as claimed in any preceding claim comprising at least first and second
switching means, the first switching means serving when "on" to complete a circuit
through the complete LED string and the second switching means serving when "on" to
complete a circuit through a shortened LED string.
6. An array of LEDs as claimed in claim 5 wherein only one of the switching means is
"on" at any time.
7. An array of LEDs as claimed in claim any preceding claim, comprising two or more switching
means for selectively removing different LEDs or sets of LEDs from the string, providing
three or more possible string lengths.
8. An array of LEDs as claimed in any preceding claim wherein the switching means comprises
a transistor.
9. An array of LEDs as claimed in any preceding claim wherein the switching means is
controlled by an electronic controller which responsive to one or more of (I) a measured
temperature; (ii) a measured LED current; and (iii) said potential difference.
10. An array of LEDs as claimed in claim 8 comprising an electronic controller which senses
LED current and controls it by means of the transistor.
11. An LED lamp, light or bulb comprising an array of LEDs as claimed in any preceding
claim.
12. A method of controlling the forward voltage of a set of light emitting diodes (LEDs),
comprising connecting the diodes in a series string to provide a current path through
them, connecting the string of LEDs across a potential difference to cause a current
to flow through them, and selectively removing at least one LED from said current
path when necessary to reduce the total forward voltage of the string.
13. A method as claimed in claim 12 wherein selective removal of the at least one LED
is performed as necessary to prevent the total forward voltage of the string from
exceeding a supply voltage.
14. A method as claimed in claim 12 or claim 13, comprising monitoring at least one of
(I) temperature; (ii) LED current; and (iii) said potential difference, and controlling
said selective removal of at least one LED in dependence upon said monitoring.
15. A method as claimed in any of claims 12 to 14, comprising removing said at least one
LED from the string in response to a fall in the potential difference applied across
the string.
16. A method as claimed in any of claims 12 to 15, comprising removing said at least one
LED from the string in response to a fall in current through the LEDs.
17. A method as claimed in any of claims 12 to 16, which comprises connecting a plurality
of strings in parallel with each other across the potential difference, and wherein
said selective removal involves concurrently removing at least one LED from each of
said strings.