Control circuit for LED and corresponding operating method

Details Control circuit for LED and corresponding operating method Control circuit for LED and corresponding operating method

2001-06-06

2003-02-04

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S18500S, C315S224000, C362S800000

Reexamination Certificate

active

06515434

ABSTRACT:

The invention is based on a drive circuit for LEDs and an associated operating method. This relates in particular to reducing the drive power losses in light-emitting diodes (LEDs) by means of a pulsed LED drive circuit.
As a rule, series resistors are used for current limiting when driving light-emitting diodes (LEDs), see, for example, U.S. Pat. No. 5,907,569. A typical voltage drop across light-emitting diodes (U
F
) is a few volts (for example, for Power TOPLED U
F
=2.1 V). The known resistor R
v
in series with the LED (see FIG. 1 in U.S. Pat. No. 6,400,101) produces a particularly high power loss, particularly if the battery voltage U
Batt
is subject to major voltage fluctuations (as is normal in motor vehicles). The voltage drop across the LED still remains constant even when such voltage fluctuations occur, that is to say the residual voltage across the series resistor R
v
falls. R
v
is thus alternately loaded to a greater or lesser extent. In practice, a number of LEDs are generally connected in series (cluster) in order to achieve better drive efficiency (FIG. 2 in U.S. Pat. No. 6,400,101). Depending on the vehicle power supply system (12 V or 42 V), an appropriate number of LEDs can be combined to form a cluster. With a 12 V vehicle power supply system, there is a lower limit on the battery voltage U
Batt
down to which legally specified safety devices (for example the hazard warning system) must be functional. This is 9 volts. This means that, in this case, up to 4 Power TOPLEDs can be combined to form a cluster (4×2.1 V=8.4 V).
Furthermore, U.S. Pat. No. 6,400,101 describes the use of a pulsed LED drive to reduce the drive power loss in light-emitting diodes (LEDs). When the LEDs are driven in this way, the current is regulated autonomously in each individual LED cluster. This leads to the maximum possible reliability (intelligent LED driver module, see FIG. 4 in U.S. Pat. No. 6,400,101). However, a relatively large number of LED driver modules are required to illuminate a relatively large area—as is the case with motor vehicle tail lights—using this technique, and this results in considerable costs. The number of LED driver modules depends on two factors.
The smaller the available supply voltage U
v
for the LED cluster, the greater is the number of clusters required.
The greater the forward voltage drop U
F
across the LEDs which are intended to be used for an LED cluster, the smaller is the number of LEDs which can be used in one cluster.
The greater the number of LED driver modules required, the greater are the costs.
The requirements to which an LED drive as described in U.S. Pat. No. 6,400,101 is subject are, of course, complied with, for example temperature regulation and fault identification are possible in the LED cluster.
One object of the present invention is to provide a drive circuit for LEDs which is simpler and thus considerably more cost-effective.
This object is achieved by the distinguishing features of claim
1
. Particularly advantageous refinements can be found in the dependent claims.
The fundamental principle of the present invention is to cascade the already known drive system. An LED drive based on the concept in U.S. Pat. No. 6,400,101 is simplified in that the basic drive clock is governed by the clock (CLK) of the transistor T
1
in a first higher-level LED cluster, which is referred to as the “master” in the following text, at the output of the comparator of the associated control loop, and this clock is also made available to other lower-level LED clusters (slaves) (FIG.
1
). The resistors R
SSn
connected in series with the LEDs represent optional additional shunt resistors R
Shunt
. These are not absolutely essential in operation, since their values are chosen to be very small (for example, R
SS1
=1&OHgr;) and they do not adversely affect the setting of the forward current I
F
. However, if interruption identification is intended to be carried out in the slave clusters (see below) they are essential.
This concept in principle allows an area of any desired size to be illuminated using a single control loop. This is so-called master/slave current regulation, with the master cluster predetermining the clock (CLK) and all the additional LED clusters (slave clusters) being driven by the master clock.
In the simplest embodiment, only one master cluster is used. This solution is particularly cost-effective. However, in this case, it is advantageous to ensure that the number of LEDs in the slave clusters is the same as the number in the single master cluster. Otherwise, brightness and light intensity differences would occur.
In one particularly preferred embodiment, the number of LEDs in the clusters can be varied. In this case, there are two different embodiments for driving the LED clusters which have different numbers of LEDs (for example if the number of LEDs in the master cluster is greater):
In a first variant, a series load (resistor, zener diode or the like), in particular a series resistor R
xn
, is connected in each n-th slave cluster, replacing the missing light-emitting diode (or light-emitting diodes). In this case, the master clock is the drive clock of that LED cluster which is driving the greatest number of light-emitting diodes (FIG.
2
). The number of LEDs in the other clusters (slave clusters) may in this case be chosen to be the same, or less. It is advantageously not more than 30% less than the number in the master cluster, in order to avoid the losses becoming too large.
In a second variant, one or more additional LED control loops (second or third, etc. master control loop) is or are integrated in the LED drive module (IC), whose LED cluster operates with a smaller number of LEDs than the first master cluster. The battery voltage U
Batt
is in this case connected to all the transistors T. The drive clock of the second master control loop (CLK
2
) is then made available to the group of slave clusters having the corresponding, lesser number of LEDs (FIG.
3
). This embodiment is suitable for groups of LED clusters which each have the same number of LEDs.
The failure rate of a master cluster is preferably also improved. This is because the master clock is no longer available to any of the connected slaves if the master fails, for whatever reason (for example interruption) The slaves are no longer driven, and must then likewise be switched off to protect them against destruction.
One of the slave clusters is thus advantageously in the form of a reserve master cluster. This means that a means is provided for identifying an interruption in the control loop of the master cluster and this means, for its part, is connected to a changeover switch in the control loop, with the changeover switch being connected on the one hand to the master cluster and on the other hand to the reserve master cluster. If the master fails, the switch is then changed to the reserve master cluster (FIG.
4
). This redundancy ensures that, if the master is operating incorrectly (interruption), an immediate change is made to the reserve master. This ensures that the connected slaves (not shown) continue to receive their drive clock (master clock) for operation.
The reserve master is preferably used as a slave as long as the master
1
is operating normally.
Fault identification (malfunction) in the LED cluster is also preferably improved. The commonest fault types are interruptions and short-circuits. An LED failure always means an interruption. In this case, this means an interruption in the LED cluster, that is to say at least one LED has failed.
Nevertheless, it is possible for a short-circuit to occur, to be precise in the sense of an interruption in the electrical connecting line between the LED anode (A
1
) and ground (GND), see FIG.
5
. Especially for use in motor vehicles (for example a tail light composed of LEDs), this type of fault must be detected so that appropriate countermeasures can be initiated in the event of a fault, such as immediately switching off the LED driver module or switching off only the sh

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