EP1246511A1 - Driver circuit for a LED-array - Google Patents

Abstract

The control circuit has a respective switch (S1,S2,S3) connected in series with each column (40,42,44) of the LED array and controlled for allowing a current to flow through the LED column. A regulating stage (46) controls the switch for a first LED column to maintain a constant mean current through the latter, with corresponding control of the switches for the remaining LED columns and measurement of the overall sum current through the remaining LED columns, for comparison with a required value, e.g. to allow an information signal to be provided. An Independent claim for an operating method for a LED array is also included.

Description

Technical field

The present invention relates to a control circuit for an LED array, which comprises a first LED strand and at least a second LED strand, whereby a switch is arranged in series with each LED string and each LED strand has a supply connection via which it is connected to a supply voltage can be connected, each switch being controllable in this way is that a current flow in the associated LED string is made possible with a control circuit that is designed, the switch of the first LED string to be controlled such that a constant mean value of the first LED strand flowing current is achieved, the control loop for control at least one switch of a second LED string is designed. It affects moreover, a method for operating an LED array, the first LED strand and comprises at least a second LED strand, wherein A switch is arranged in series with each LED strand and each LED strand has a supply connection via which it is connected to a supply voltage is connectable.

State of the art

The invention is concerned with the control of LEDs. To do this Typically series resistors or current sources are used to carry the current limit or regulate with the LED. The LEDs are usually in one String interconnected, that is, a string comprises a series connection several LEDs. Depending on the size of the one to be illuminated or backlit Several LED strings must be connected in parallel, so can be combined into an array. This is basically the case here Problem that a status connection of the control circuit is corresponding Display should deliver as soon as one or more LED strands an error has occurred.

A first solution to this problem known from the prior art from ST Microelectronics AG is that the entire LED array is connected to a single LED string. adversely of this solution is that such an LED strand has a much higher Supply voltage required to the LED string voltage, that is, the Sum of all LED forward voltages. As soon as an error occurs the entire LED array is de-energized, which means it is no longer lit.

A second solution known from the prior art from Infineon Technologies AG consists of the fact that every single LED string is from one own LED driver module is regulated and monitored. Because an LED array Usually consists of several LED strings, this invention goes with the disadvantage that several LED driver modules are required for this are. All LED driver modules are on a single status connection interconnected so that it cannot be determined exactly how many LED strands have failed. The use of several LED driver modules is undesirable as this is disadvantageous in cost reflected.

Another solution known from the prior art to the above problem is provided by the applicant of the present invention (DE19930174; Biebl) and works as follows:

The principle of a clocked current control is first explained with reference to FIG. 1: A series circuit comprising several LEDs, D1 to D4, is connected on the one hand to a supply voltage U _Batt via a switch S1, and on the other hand to ground via a measuring resistor R _Shunt . The voltage U _Shunt dropping across the resistor is fed to an integrator 10, which provides at its output a mean value of the voltage present at the input. This voltage is fed to a regulator 12, which also receives as input signal a reference voltage U _Ref , which corresponds to an average target value of the current I _Led through the LEDs, D1 to D4. The control voltage U _Regel provided at its output by the regulator 12 is applied to the positive input of a comparator 14, a triangular voltage U _{D being} applied to its negative input, which is provided by a delta generator 16. The output signal of the comparator 14 is used to control the switch S1. As can be seen from the graphic in the right half of FIG. 1, the signal which drives the switch S1 is a clocked signal, which can be recognized by the rectangular function of the LED current I _LED . This arrangement ensures that the current I _LED flowing through the LEDs is regulated to a value correlated with the voltage U _Ref .

In FIG. 2, three such circuits with clocked current regulation, shown in FIG. 1, are shown schematically and by way of example in the right half, namely the blocks 18, 20, 22. The supply voltage, the individual LEDs and the resistors R _shunt are omitted for reasons of clarity , If each of the blocks 18, 20, 22 comprises an LED strand, an LED array can be realized by such an interconnection. A triangle generator 24 applies a clock signal 26 to a counter 28 which acts on a multiplexer 30. As a result of the clock signal, the multiplexer 30 is caused to scan the control voltages of the three blocks 18, 20, 22 one after the other and to supply an error detection logic with a comparator 32 and a flip-flop 34. As soon as one of the control _voltages U control ₁ , U _{control 2} , U _{control 3 is} less than a predetermined threshold voltage U _SW , the comparator 32 generates a signal to the flip-flop 34, so that a signal is generated at the Q output of the flip-flop 34 which contains an error in indicates one of the LED strings of blocks 18 to 22. As an example, only the blocks 18 to 22 were mentioned here, it being understood that further blocks, as indicated by the dash-dotted line, can of course be part of the same LED array.

The problem with this solution is firstly the additional effort for one Counter 28 and a multiplexer 30, on the other hand the fact that at larger LED arrays, several LED driver modules are necessary because of the number current control circuits per LED driver module is limited, for example on eight. The use of several LED driver modules is worthwhile again disadvantageously reflected in the price.

In addition to the disadvantages mentioned, the solutions mentioned also exist another disadvantage is that an error signal is emitted immediately, as soon as an error has occurred. However, this only makes sense if, as with In the first solution, the entire LED strand failed. in the With regard to certain areas of application of LED arrays, for example in the vehicle area as a rear light, this would again require the use of a Justify light bulb. In the case of a light bulb, there are only two states Light bulb intact, light bulb not intact. The advantage of using it of LEDs in this area, however, is that if one LED strands the lamp, provided there are enough other working LED strings are available, can continue to be operated - with something reduced luminance - but, if dimensioned appropriately, still above a statutory limit.

Presentation of the invention

The object of the present invention is therefore that of the beginning to further develop the mentioned control circuit for an LED array such that at Cost-effective implementation of continued operation of the LED array ensured provided the total luminance of the LED array is above a predeterminable Value.

This problem is solved by a control circuit with the features of claim 1 and a method for operating an LED array with the features of claim 17.

The starting point is a patent application by the present applicant with the title "Control circuit for LED and associated operating method" (Registration number DE19950135.1; Biebl; not yet disclosed), in which several LED strings are operated in a cascade arrangement. there a higher-level LED strand is referred to as the master strand, the average current is fed to a control loop, the control signal of the master line also for controlling several subordinate ones LED strings, so-called slave strings, is used. Starting from the teaching of the application just mentioned is the knowledge of Invention in that the above object can be achieved if the total current is measured by all slave strands and this current against one predeterminable setpoint is compared. As long as individual failure Slave strings the total current is above the threshold, no error message generated. Only when the specified threshold is undershot is synonymous, for example, with the fact that the reduced luminance now no longer meets the legal requirements, a Error signal generated.

This implementation has the advantage that the entire LED array despite failure individual LED strings can be operated that the error detection logic can be kept very simple, in particular provided only once must be, and finally that the LED array with any number LED strings are monitored by the fault detection device can. The only limiting factor is the driver for generating the control signal for the switch of each LED string.

In a particularly preferred embodiment, the target size is one User adjustable. This measure allows a user to determine how many LED strings may fail before the error signal is generated and thus signals a failure to the user. Furthermore is preferably to design the comparison unit in such a way that it falls short an information signal is output from the target variable by the actual variable. This information signal can then also be used, for example be informing a user to switch to another array, Etc.

Furthermore, the control circuit according to the invention preferably comprises one Monitoring unit with which the current flows through the first LED strand can be monitored. This is particularly advantageous because the control also the slave strands with regard to the so-called master strand he follows. The current flow through the master strand serves as an input signal of the control loop, which also controls the slave lines. At a Failure of the master strand would therefore run the risk that all of them Slave lines would be destroyed by incorrect control. on the other hand if it detects that an error has occurred in the master line, to a replacement control loop, for example one for a slave line provided control loop are switched, so that then this slave line becomes the master strand.

The monitoring unit is therefore preferably designed so that when detected a current flow in the LED string outside of a predetermined Tolerance range, for example with no current flow, the control loop is switched off.

The control circuit preferably further comprises an undervoltage detection device, which is designed to emit an undervoltage warning signal, if the supply voltage drops below a predeterminable value. If the supply voltage of the circuit, for example in the automobile the on-board voltage, to the string voltage of the LEDs, i.e. the sum of all LED forward voltages, can do so uncontrolled processes occur. In particular, the specified one Target size to be modified inadvertently, so that the comparison unit incorrectly outputs an information signal. Prefers this is achieved in that the supply voltage with a reference voltage is compared, which is preferably equal to or greater than the sum of the forward voltages through all LEDs of a string. As long as the The supply voltage is greater than the reference voltage Output of an undervoltage warning signal. This measure enables that the control circuit for non-critical drops in the supply voltage can stay active.

The control circuit is preferably designed such that it can also be predetermined The value can be set manually or predefined.

According to a particularly preferred embodiment, the control circuit comprises further an output unit to which the information signal and / or the undervoltage warning signal can be transmitted. This opens the Possibility that the output unit when receiving the information signal this forwards only, for example to a user as an error signal Provides if no undervoltage warning signal has been transmitted.

The output unit is therefore preferably designed so that it is Receive the undervoltage warning signal even for a predetermined time or deactivated for the duration of receipt of the undervoltage warning signal, so that during a period during which the supply voltage has dropped below a critical value, none of the output unit wrong results are produced.

The output unit preferably has at least one transistor which is in the open collector circuit and its basis for transmission of the information signal with the comparison unit and / or for transmission the undervoltage warning signal with the undervoltage detection device connected is. An open collector circuit has the advantage that at Occurrence of the information signal and / or the undervoltage detection signal the collector of the transistor is pulled to ground. In this manner and way the signal applied to the collector can easily be sent to anyone Realizations of an error evaluation circuit can be adapted. For example this opens up the possibility of other output units that also implemented in open collector circuit, via the respective collectors connect with each other. As soon as a collector is pulled to ground, that means that there is a signal at the base of the respective transistor which Switching transistor to the conductive state is one for all output units joint display can be activated.

A particularly preferred embodiment of the invention comprises the control circuit furthermore a switch-on time delay device, which is designed to output the dispensing unit for a predetermined time after the Deactivate switching on of the control circuit. Such a switch-on time delay device has an advantageous effect against uncontrolled switching operations, that are related to switching on, especially within the Control loop.

The output unit can comprise a flip-flop, the base of the transistor with the output of the flip-flop and the set input of the flip-flop with the undervoltage detection device for transmitting the undervoltage warning signal and / or with the comparison unit for transmitting the Information signal can be connected. By using a flip-flop a sporadic error signal, for example in the event of contact problems, prevented. This means that once an error signal has been set, it remains as long received as long as the control circuit is switched on or is activated.

In this context, it is particularly advantageous if the switch-on time delay device is designed for the duration of the switch-on delay a switch-on delay signal to the reset input of the flip-flop of the output unit. That way it is very easily possible to also use the flip-flop to output an Error signal via the output unit for a predetermined period of time after switching on the control circuit.

The control circuit according to the invention is not only based on the clocked Operation of the LED control is limited, but is also suitable for DC operation of LEDs. In the former case, the actual size is one Average of the sum of the currents through at least two, in particular determined by all second LED strands to compare against the target size to become.

The above object is also achieved by a method for operating a LED arrays that have a first LED strand and at least a second LED strand comprises, with a switch arranged in series with each LED strand and each LED string has a supply connection, via which it can be connected to a supply voltage. With this procedure the switch of the first LED string is controlled with a control signal, so that a constant mean value of the one flowing through the first LED strand Current is achieved, with at least a second LED strand with the same control signal is driven. The sum is the actual size the currents through at least two, in particular through all second LED strands measured, after which the actual size with a specifiable Target size is compared.

Description of the drawings

Figur 1

eine aus dem Stand der Technik bekannte Schaltungsanordnung zur Ansteuerung eines LED-Strangs mit getakteter Stromregelung;

Figur 2

eine aus dem Stand der Technik bekannte Ansteuerschaltung für mehrere LED-Stränge mit Multiplexer;

Figur 3

ein erstes Ausführungsbeispiel einer erfindungsgemäßen Ansteuerschaltung mit Erfassung des Gesamtstroms der Slave-Stränge;

Figur 4a

den Verlauf des Fehlersignals beim Auftreten eines entscheidungserheblichen Fehlers;

Figur 4b

den zeitlichen Verlauf der Spannung U_Mess bei Ausfall einzelner LED-Stränge in dem Ausführungsbeispiel von Figur 3;

Figur 5

eine detailliertere Darstellung der Ausführungsform von Figur 3;

Figur 6

in schematischer Blockschaltbilddarstellung eine Ausführungsform der Fehlerdiagnose bei einer erfindungsgemäßen Ansteuerschaltung.

Further advantageous embodiments are defined in the subclaims. Exemplary embodiments of the invention are described in more detail below with reference to the attached drawings. They represent:

Figure 1

a circuit arrangement known from the prior art for controlling an LED strand with clocked current regulation;

Figure 2

a control circuit known from the prior art for a plurality of LED strings with multiplexer;

Figure 3

a first embodiment of a control circuit according to the invention with detection of the total current of the slave lines;

Figure 4a

the course of the error signal when a decision-relevant error occurs;

Figure 4b

the time course of the voltage U _Mess in the event of failure of individual LED strands in the embodiment of Figure 3;

Figure 5

a more detailed representation of the embodiment of Figure 3;

Figure 6

in a schematic block diagram representation of an embodiment of the fault diagnosis in an inventive control circuit.

The following are different for the same and equivalent elements Embodiments used the same reference numerals throughout.

In the control circuit shown in FIG. 3, a first LED strand 40 with two LEDs D1, D2 is designated as the master strand. Several second LED strands 42, 44 with LEDs D3, D4, D5, D6 are referred to as slave strands. Instead of the two LEDs shown per example, a plurality of LEDs can of course be arranged. This is essentially limited by whether the battery voltage U _Batt used for the supply is sufficient to apply the sum of the LED forward voltages.

The current flow through the master strand 40 is detected by means of a resistor R _shunt , the voltage U _shunt dropping across the resistor R _{shunt being} supplied to an LED control circuit 46. The latter supplies the control clock CLK for the switch S1 of the master line 40, as well as for the switches S2, S3 of the slave lines 42 and 44. The total current through the slave lines is determined via a resistor R _Mess , the value at the resistor R _Measuring falling voltage U _{Mess of} the diagnostic unit 50 is supplied. The latter also receives the battery voltage U _Batt and a signal 48 from the LED control unit 46. The diagnostic unit 50 in turn supplies a signal 58 to the LED control unit 46. The signals 48 and 58 are described in more detail below, as is the structure of the diagnostic unit 50. The output signal of the diagnostic unit 50 is sent to the base of a status transistor ST1 in the open collector Circuit laid. Information about the status of the LED array is provided at the collector of transistor ST1.

Figure 4b shows the temporal profile of the voltage drop across the resistance R _measured voltage U _measured. A user-defined voltage U _{OL is also entered} , which specifies a setpoint against which U _{Mess is} compared. The failure of a first slave line that occurs at time t1 has no influence, since the voltage U _{Mess is} greater than U _OL even after this failure. Only when a second slave line fails at time t2 does U _Mess fall below voltage U _OL , which leads to the generation of an information signal 47, see FIG. 4a. The status signal at the collector of the status transistor ST1 only goes from high to low when the second slave line fails at the time t2, that is to say at the time at which U _{Mess falls} below U _OL .

FIG. 5 shows the LED control unit 46 and the diagnostic unit 50 from FIG. 3 in more detail. A triangular generator 52 supplies a triangular signal U _D to the minus input of a comparator 54. A control unit 56 receives, on the one hand, the voltage U _Shunt dropping across the resistor R _Shunt and a reference voltage U _Ref , with which the mean value of the current I _LED through the LEDs of the Master strand 40 is set. Since in the present case the switch S1 is designed as a PNP transistor and the triangular voltage U _{D is} fed to the comparator 54 at its negative input, the controller 56 generates at its output in the event that the current I _LED through the master strand is too low, a smaller control voltage U _rule. In the event that the current I _{LED is} too high, the voltage U _{Regel is} increased by the controller 56. In the event of an error in the master strand 40, which prevents a current flow I _LED through the master strand 40, U _Regel would accordingly become very small, which would result in the switches S2, S3 of the slave strands 42, 44 from the comparator 54 would be controlled so that they would open fully. This would lead to the slave LED strings D3, D4, D5, D6 being connected to the full supply voltage U _Batt , which could result in the LEDs being destroyed. So that this does not occur, the voltage U _{Regel is} supplied as a signal 48 to the diagnostic unit 50, which is designed in such a way that a drop in U _Regel below a predeterminable threshold value is registered and switches off the triangle generator 52 via a connection 58.

FIG. 6 shows an overview in a schematic representation in addition to the safety measures already mentioned, which can be implemented in the control circuit according to the invention. To separate the individual functions, the diagnostic unit 50 is divided into a block 50a, a block 50b and a block 50c. At block 50a, the drop across the resistance R _measured voltage U _measured is integrated in an integrator 60, that is, the average value, and the output signal of the integrator 60 supplied to a comparator 62nd The comparator 62 receives the voltage U _OL, obtained by a voltage divider comprising resistors R1 and R _OL from the voltage U _ref at its other input. The comparator 62 provides the signal 78 at its output.

Block 64 is used for undervoltage detection. As soon as the supply voltage U _{Batt of} the circuit comes close to the string voltage of the LEDs, that is to say the sum of all LED forward voltages, uncontrolled processes can occur during error diagnosis. For this purpose, the supply voltage U _{Batt is} compared in a comparator 66 against a reference _voltage U _Ref1 . The voltage U _Ref1 , that is to say the undervoltage _limit , can be determined by a voltage divider, which is preferably located completely outside the undervoltage detection unit 64. Alternatively, the voltage divider can be implemented in that a resistor is in the circuit and an adjustable resistor is external. The voltage U _Ref1 can then be _set by a user via the external resistor. However, it can also be provided, for example when using the control circuit in the automotive sector, where the overvoltage limit is specified (in the 14V / 12V electrical system this is 9V, in the 42V electrical system this is 30V), in order to save costs, to do without the manual adjustability and U _Ref to be determined with regard to the vehicle electrical system voltage. The undervoltage detection unit provides a signal 76 at its output.

The control voltage U _{Regel is} fed to the block 50b and compared in a comparator 68 against a reference _voltage U _Ref2 . If the voltage U _{Regel is lower} than the voltage U _Ref2 , the comparator 68 supplies a signal to a flip-flop 70, the output signal 72 of which can be used to _switch off the entire control _circuit to prevent the LEDs in the slave lines from being destroyed, or to switch off a master circuit. Trigger changeover in which a slave line is made the master line. Block 50b also receives signal 76 from undervoltage detection unit 64 in order to prevent erroneous generation of output signal 72 in the event that supply voltage U _Batt has dropped too far. This is because the reference _voltage U _{Ref2 is} often _obtained from the supply voltage U _Batt , and if an undervoltage occurs, a comparison with the voltage U _Regel could lead to incorrect results.

With the arrangement in block 74 there is a switch-on time delay for the drive circuit realized related to uncontrolled switching operations to prevent by switching on the control circuit. It creates a signal 80 at its output.

Signal 76, like the output signal 78 of block 50a and the output signal 80 of the switch-on delay circuit 74, is fed to block 50c, which drives the status transistor ST1. In block 50c it is ensured that a signal to the status transistor is only generated if the control circuit is not in a predetermined period after switching on, if there is no undervoltage and at the same time the voltage U _{Mess is} less than U _OL . The block 50c comprises a flip-flop 88, the signal 76 and the signal 80 "-linked" being applied to the reset input R of the flip-flop 88, while the signal 78 is applied to the set input S of the flip-flop 88. The use of the flip-flop 88 prevents a sporadic error signal in the event of contact problems. In the present case, an error signal that has been set remains as long as the control circuit is switched on. An enable input (not shown) can be provided to reset a set error signal.

Claims (16)

Control circuit for an LED array, which comprises a first LED strand (40) and at least a second LED strand (42; 44), a switch (S1, S2.) In series with each LED strand (40, 42, 44) , S3) and each LED strand (40, 42, 44) has a supply connection via which it can be connected to a supply voltage (U _Batt ), each switch (S1, S2, S3) being controllable in such a way that a Current flow in the associated LED string is made possible with
a control circuit (46) which is designed to control the switch (S1) of the first LED strand (40) in such a way that a constant mean value of the current (I _LED ) flowing through the first LED strand (40) is achieved, the Control circuit (46) for controlling at least one switch (S2, S3) of a second LED string (42, 44) is designed,
characterized in that the drive circuit further comprises: a total current detection device (R _measurement ) with which an actual variable (U _measurement ) can be determined, which corresponds to the sum of the currents through at least two, in particular through all second LED strands (42, 44), and a comparison unit (50, 50a) in which the actual variable (U _measurement ) can be compared with a predefinable target variable (U _OL ). Control circuit according to claim 1,
characterized in that the target variable (U _OL ) is adjustable by a user. Control circuit according to claim 1 or 2,
characterized in that the comparison unit (50, 50a) is designed to output an information signal (78) when the actual variable (U _measurement ) falls below the target variable (U _OL ). Control circuit according to one of the preceding claims,
characterized in that it comprises a monitoring unit (50, 50b) with which the current flow through the first LED strand (40) can be monitored. Control circuit according to claim 4,
characterized in that the monitoring unit (50, 50b) is designed in such a way that the control circuit (46) is switched off when a current flow in the first LED strand (40) is detected outside a predeterminable tolerance range. Control circuit according to claim 4,
characterized in that the monitoring unit (50, 50b) is designed such that when a current flow is detected in the first LED strand (40) outside a predeterminable tolerance range, the first LED strand (40) is switched off and a second LED strand (42 , 44) is made the first LED strand. Control circuit according to one of the preceding claims,
characterized in that it further comprises an undervoltage _{detection device} (64) which is designed to _{emit an undervoltage warning signal} (76) when the supply voltage (U _Batt ) drops below a predeterminable value (U _Ref1 ). Control circuit according to claim 7,
characterized in that the predeterminable value (U _Ref1 ) is equal to or greater than the sum of the forward voltages of all LEDs of an LED string (40, 42, 44). Control circuit according to one of claims 7 or 8,
characterized in that the predeterminable value (U _Ref1 ) can be set manually or can be predefined. Control circuit according to one of Claims 3 to 9,
characterized in that it further comprises an output unit (50, 50c, ST1) to which the information signal (78) and / or the undervoltage warning signal (76) can be transmitted. Control circuit according to claim 10,
characterized in that the output unit (50, 50c, ST1) comprises at least one transistor (ST1) which is in the open collector circuit and whose base for transmitting the information signal (78) with the comparison unit (50a) and / or Transmission of the undervoltage warning signal (76) is connected to the undervoltage detection device (64). Control circuit according to one of the preceding claims,
characterized in that it further comprises a switch-on time delay device (74) which is designed to deactivate the output unit (50, 50c, ST1) for a predetermined time after switching on the drive circuit. Control circuit according to one of Claims 10 to 12,
characterized in that the output unit (50, 50c, ST1) comprises a flip-flop (88), the base of the transistor (ST1) having the output of the flip-flop (88) and the set input (S) of the flip-flop (88) the undervoltage detection device (64) for transmitting the undervoltage warning signal (76) and / or with the comparison unit (50a) for transmitting the information signal (78). Control circuit according to one of claims 12 or 13,
characterized in that the switch-on time delay device (74) is designed to apply a switch-on delay signal (80) to the reset input (R) of the flip-flop (88) of the output unit (50, 50c, ST1) for the duration of the switch-on time delay. Control circuit according to one of the preceding claims,
characterized in that the actual variable (U _measurement ) corresponds to an average over time of the sum of the currents through at least two, in particular through all second LED strands (42, 44). Method for operating an LED array which comprises a first LED strand (40) and at least a second LED strand (42, 44), a switch (S1, S1, 40) being connected in series with each LED strand (40, 42, 44) S2, S3) and each LED string (40, 42, 44) has a supply connection via which it can be connected to a supply voltage (U _Batt ), comprising the following steps: a) triggering the switch (S1) of the first LED string (40) with a trigger signal (CLK) so that a constant mean value of the current flowing through the first LED string (40) (I _LED ) is achieved, and triggering at least one second LED string (42, 44) with the same control signal (CLK), b) measuring an actual variable (U _measurement ) which corresponds to the sum of the currents through at least two, in particular through all second LED strands (42, 44), c) Comparing the actual variable (U _measurement ) with a predefinable target variable (U _OL ).

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