EP1278402B1 - Circuit for LED with temperature dependent current control - Google Patents

Abstract

The device has a regulator, demand value generator, temperature sensor, a subtracter with a demand current input that drives the regulator, a switch-off device that varies the current demand so the diode current is negligible if a switch-off signal is applied and a controller with a temperature connection, an temperature-dependent output to the subtracter and a temperature-dependent second output to the switch-off input of the switch-off device. The device has a regulator (1), demand value generator (5) and temperature measurement device (4), a subtracter (6) with the demand current value as input that drives the regulator, a switch-off device (7) that varies the current demand value so the diode current is negligible when a switch-off signal is applied and a controller (8) with connections for a temperature measurement parameter, a first output to the subtracter dependent on the temperature measurement value and a temperature-dependent second output to the switch-off input of the switch-off device.

Description

Technical area

The invention is based on a control gear for light-emitting diodes according to the preamble of claim 1. It concerns in particular the regulation of the Operating current of the LEDs as a function of the ambient temperature.

State of the art

The maximum permissible effective value of the operating current of LEDs, below short operating current, depends on the ambient temperature, in the following briefly called temperature. So that a light-emitting diode a desired life Achieved, the following conditions must be met by light-emitting diode manufacturers are specified more precisely. Up to a derating temperature the operating current remains constant. From the derating temperature begins a so-called derating; d. H. with increasing temperature the operating current decreases proportionally with increasing temperature. Above a shutdown temperature, the LED may not be operated, which is why then a shutdown must be done in the the operating current is negligibly small.

In the document EP 0 891 120 (Weis) it is proposed to use a temperature-dependent Resistor (PTC) in series with the light emitting diodes to switch. At a temperature increase increases the resistance of the PTC and the current through the LEDs decreases. Disadvantage of this solution is that there is no unique derating temperature below which the operating current is constant.

If a diode, in the present case a light-emitting diode, operated in the flow direction, so falls on her from a forward voltage. In the document DE 198 10 827 (Graf) is the Temperature dependence of the forward voltage exploited. A control circuit provides the current through the LEDs so that the forward voltage remains constant. Thus, the temperature of the LED is independent of the ambient temperature kept constant. Disadvantage of this solution is that under the derating temperature not a desired operating current is maintained.

Presentation of the invention

It is an object of the present invention to provide a control device for light-emitting diodes according to to provide the preamble of claim 1, which is a temperature-dependent Control of the operating current of the LEDs accomplished. Here is the operating current have a temperature dependence as in the section to the state The technique is described and recommended by LED manufacturers.

This object is achieved by a control gear having the features of the preamble of claim 1 by the features of the characterizing part of claim 1 solved. Particularly advantageous embodiments can be found in the dependent claims.

Operating devices for light-emitting diodes, which regulate the current through the light-emitting diodes, have In general, a control device, with a setpoint input to the one Current setting value is applied. Depending on the Stromeinstellwert the current through the LEDs are set. This can be done continuously by influencing one Operating voltage of an operating arrangement done. As for the operation of the Light emitting diodes of the effective value of the operating current is decisive, a regulation also done by means of a pulse width modulation.

Usually, an o. G. Operating device a setpoint generator, which is at a setpoint output provides a current setpoint. As the operating current of the temperature the control gear also has a temperature measuring device, which provides a temperature measurement that is linear from the ambient temperature depends.

According to the invention, the current setpoint is not directly the setpoint input of the Control device supplied. Rather, the operating device according to the invention a subtractor having first and second inputs and an output. The subtracter subtracts an electrical quantity at its second input of an electrical quantity at its first input and sets the result. ready for his exit. The current setpoint is inventively the first input supplied to the subtractor. The setpoint input of the control device is According to the invention connected to the output of the subtractor. Accordingly delivers the output of the subtractor the Stromeinstellwert.

The second input of the subtracter is connected to a first output of an inventive Control device connected. This will be a deduction value that the Provides control device at its first output, supplied to the subtractor. According to the invention is caused by the fact that the Stromeinstellwert equal to Current setpoint, reduced by a temperature-dependent pull-off variable. A task The control device now consists of the temperature measurement variable appropriate value of the deduction size is determined. For this purpose, the temperature measured fed to a temperature input of the control device. Up to an adjustable Derating start value, the temperature measured at the derating temperature assumes a value of the trigger size that is the current setpoint not reduced to determine the current setting value. Is the value of the Temperature measured above the Deratingstartwert, so the deduction size is proportional to the temperature measurement. According to the invention we thereby the Stromeinstellwert reduced proportional to the temperature measured. The transition from the constant Current setting value for the reduced current setting value at the derating temperature continuous according to the invention.

This prevents operation of the LED above the shutdown temperature is, the operating device according to the invention has a shutdown device with a Shutdown. When the shutdown temperature is exceeded, the value exceeds the temperature measurement an adjustable shutdown. In this case, the Control device via a second output to the shutdown input a shutdown signal off, which causes the shutdown device to the Stromeinstellwert so change that the current through the light emitting diodes is negligible.

For the light-emitting diodes, the manufacturer gives a maximum operating current when reaching the shutdown temperature. The proportionality between the deduction size and the temperature measurement is inventively chosen so that when reach the shutdown temperature through the LEDs from the manufacturer for this temperature specified maximum operating current flows. This will achieve that for all Temperatures the maximum operating current is not exceeded.

Description of the drawings

Figur 1

die Temperaturabhängigkeit des maximal zulässigen Betriebsstroms einer Leuchtdiode

Figur 2

ein Blockdiagramm eines erfindungsgemäßen Betriebsgeräts

Figur 3

ein Ausführungsbeispiel für die erfindungsgemäße Erzeugung des Stromeinstellwerts

In the following, the invention will be explained in more detail with reference to embodiments. Show it:

FIG. 1

the temperature dependence of the maximum allowable operating current of a light emitting diode

FIG. 2

a block diagram of a control gear according to the invention

FIG. 3

an embodiment of the inventive generation of Stromeinstellwerts

The following are resistors by the letter R, transistors through the letter T, amplifiers through the letter A, diodes through the letter D respectively followed by a number.

FIG. 1 shows an example of the temperature dependence of the maximum permissible operating current of a light-emitting diode. These are manufacturer specifications for a type LA E675 light-emitting diode from Osram Opto Semiconductors. The operating current IF in mA is plotted in a graph above the ambient temperature TA in ° C. Up to a temperature TA of 70 ° C, the operating current IF is constant 70mA. Above 70 ° C begins the so-called derating. The derating temperature is therefore in the present example 70 ° C. In a temperature range between the derating temperature and the cut-off temperature, which in the example given is 100 ° C., the operating current IF decreases linearly with increasing temperature TA. Above the shutdown temperature of 100 ° C operation of the LED is excluded.

FIG. 2 shows a block diagram of an operating device according to the invention. A control device 1 supplies at its output 13 the operating current for the embrittenden LEDs 2. About a feedback line 3 is an actual value of the Operating current fed into an actual value input 12 of the control device 1.

The setpoint generator 5 provides a current setpoint at its setpoint output 51. This is inventively supplied to a first input 61 of an adder 6. An output 63 of adder 6 provides a current setpoint for a setpoint input 11 of the control device 1. At the second input 62 of the adder 6 is a Deduction value fed to a controller 8 provides at its output 82. According to the invention corresponds to the Stromeinstellwert that at the output 63 of Adder 6, the current setpoint minus the trigger value. In Fig. 2 the current setting value is calculated by an adder 6 whose second input 62 is inverted. This is indicated by a minus sign at the second input 62 indicated. In the same way, it is also possible to use an inverted subtraction value at the same time provide first output 82 of the controller 8. An inversion on the second input 62 is then no longer necessary.

It is also possible to invert the current setpoint and the current setting value, while the deduction value is not inverted.

Another way to calculate the Stromeinstellwerts is that the adder 6 is replaced by a subtractor. At the first input 61 is the Minuend, at the second input 62, the subtrahend is fed. The inversion a value is not necessary in this case.

The deduction value is in the control device as a function of the temperature certainly. For this purpose, a temperature measuring device 4 at its output 41 a Temperature measurement, which in the temperature input 81 of the control device. 8 is fed. Below the derating temperature, the controller provides a deduction value that does not affect the current setpoint, causing the Stromeinstellwert is equal to the current setpoint. Above the derating temperature rises the withdrawal value is linear with the temperature measured, so that the Stromeinstellwert decreases linearly with the temperature. The deduction is made by the controller 8 chosen so that the course of Stromeinstellwerts at the derating temperature is steady.

If the shutdown temperature is reached, the control device 8 is at its second Output 83 a shutdown signal to the input 71 of the shutdown device 7 off. Thereupon the shutdown device 7 influences the setpoint generator via its output 72 in the way that at the setpoint output 51, a desired value to the adder. 6 is issued, which has an operating current to follow, the negligible is.

Thus, an inventive operating device is described in Fig. 2, which is a temperature-dependent Operating current supplies, as shown in principle in Fig. 1.

In Fig. 3 is the circuit diagram of an embodiment of this invention given how a Stromeinstellwert is generated from the current setpoint.

An adder is formed of an operational amplifier A1 and resistors R1, R2 and R3. Via R2, a first input of the adder with the inverting Input connected by A1. Over R1 becomes a second input of the adder connected to the inverting input of A1. R3 connects the output of A1 with its inverting input. The output of A1 forms the output of the Adder's VST. There is the sum of the two input signals, ie the current setting value, available in inverted form. Becomes a non-inverted form required, the adder is followed by an inverter. The non-inverting input from A1 is connected to a potential called Virtual Mass VM becomes. The virtual ground VM forms the reference potential for the inputs of the adder. The virtual mass VM is derived by means of a voltage divider from the resistors R4 and R5, which is between a reference voltage VR and a ground potential M is connected.

At the first input of the adder a current setpoint VS is fed. There is also connected a shutdown device. In the present example, the Turn-off device only from an NPN bipolar transistor T1. This can be realize the shutdown device cost. It is also possible for one other electronic switch to use. The collector of T1 is with the Current setpoint connected while the emitter is at ground potential M. The base from T1 forms a turn-off input, which is connected to the output of a comparator A2 is connected. A2 is realized by an operational amplifier. Once A2 Shutdown signal outputs to T1, the current setpoint is set to ground potential, whereby At the output of the adder VST, a value is output which has no operating current allows more.

A temperature measuring device is formed from the series connection of a resistor R6 and a reference diode D1 and an operational amplifier A3. The Series connection of R6 and D1 is between the reference voltage VR and the Ground potential M, wherein the cathode of D1 to ground potential M lies. The connection point of D1 and R6 is with the non-inverting input connected by A3. The inverting input of A3 is with the output connected by A3. Thus, A3 forms a voltage follower and sets at his Output the forward voltage of D1 available, which is a temperature measurement forms. The forward voltage of semiconductor diodes is inversely proportional to the temperature. Ie. the temperature measured variable is in inverted form. That's why In the present embodiment, an adder and no subtractor is used. For D1, basically any semiconductor diode can be used. As it is, however it is important that the same temperature is relevant for D1 as for the ones to be operated Light emitting diodes, is advantageously used for D 1, a diode, the type of corresponding operating diodes.

The output of A3, to which the temperature variable is applied, becomes the inverting one Input from A2 supplied. The non-inverting input is connected to a center tap of a voltage divider formed by resistors R7 and R8, which is connected between the reference voltage VR and the ground potential M. The ratio of the resistors R7 and R8 is chosen so that at the shutdown temperature the comparator A2 outputs a turn-off signal and T1 in a conductive state added.

The output of A3, to which the temperature measurement is applied, feeds one the ground potential M lying voltage divider consisting of resistors R9 and R10. The center tap of this voltage divider is with the non-inverting Input of an operational amplifier A4 and with a first input terminal of a Switch S1 connected. The switch S1 can be both mechanically and be executed electronically. The second input pole of the switch S1 and the inverting input from A4 are connected to the virtual ground VM. Of the Output terminal of the switch S1 is connected to the second input of the adder. The output of A4 controls the switch S1. The ratio of resistances R9 and R10 are tuned to the potential of the virtual mass VM, that the changeover switch is switched at the derating temperature. Below At the derating temperature, the switch sets the virtual ground to the second one Input of the adder. Thus, the Stromeinstellwert VST at the output of the adder regardless of the temperature. According to the invention is at the output of the switch S 1 immediately after exceeding the derating temperature a potential which corresponds to the virtual mass VM. The course of the voltage at the output of the switch S 1 at the derating temperature is therefore steady. Above the derating temperature is via the switch S 1, a reverse proportional to the temperature Trigger value switched to the second input of the adder.

The following elements can be assigned to the control device: R4, R5, R7, R8, R9, R10, A2, A4, S1 and the reference voltage VR. Following size can be defined: The output of A3 forms the temperature and is measured a temperature input of the control device, which is formed by the inverting input from A2 and from a terminal of R9. A first exit the control device forms the output pole of the switch S1. There will the deduction size provided. The output of A2 forms a second output the control device, which provides the shutdown signal.

Claims (6)

1. Operating device for operating light-emitting diodes having the following features:

   regulating device (1) for regulating the operating current (IF) of the light-emitting diodes (2) having a desired value input (11), to which a current set value (VST) can be fed,

   desired value transmitter (5), which outputs a current desired value (VS) at a desired value output (51),

   temperature measuring device (4) which provides a temperature measured variable which is linearly dependent on an ambient temperature (TA),

   characterized in that the operating device comprises the following features:

   subtractor having a first (61) and a second input (62) and an output (63) which subtracts an electrical variable at its second input (62) from an electrical variable at its first input (61) and makes the result available at its output (63), the current desired value (VS) being fed to the first input (61), and the output (63) being connected to the desired value input (11) of the regulating device (1),

   disconnection device (7) having a disconnection input (71) which alters the current set value (VST) when a disconnection signal is applied to the disconnection input (71) such that the current through the light-emitting diodes is negligible,

   control device (8) which has the following connections:

   temperature input (81), to which the temperature measured variable is fed,

   first output (82) which makes available a subtraction variable whose value is fed to the second input (62) of the subtractor, the value of the subtraction variable being set such that, for the case in which the value of the temperature measured variable is below a derating start value which can be set, the current desired value is present at the output of the subtractor and, for the case in which the value of the temperature measured variable is above the derating start value, the value of the subtraction variable is proportional to the value of the temperature measured variable, the value of the variable at the output of the subtractor having a continuous profile in the case of the derating start value,

   second output (83) which is connected to the disconnection input (71) of the disconnection device (7) and outputs a disconnection signal for the case in which the value of the temperature measured variable is above a disconnection value which can be set.
2. Operating device according to Claim 1, characterized in that the temperature measured variable is derived from a forward voltage across a light-emitting diode.
3. Operating device according to Claim 1, characterized in that the disconnection device contains an electronic switch (T1) which closes when a disconnection signal is applied to the disconnection input (71) and thus connects the desired value output to an earth potential (M).
4. Operating device according to Claim 1, characterized by the following features:

   the subtractor has a connection for a virtual earth (VM) which forms a reference potential for the inputs (61, 62) of the subtractor,

   the connection for the virtual earth (VM) of the subtractor is connected to a reference voltage (VR) which can be set,

   the control device (8) has a changeover switch (S1) which may also be in the form of an electronic switch by means of which the first output (82) of the control device (8) can be switched over between two potentials,

   the position of the changeover switch (S1) is dependent on a comparison between the level of the potential of the virtual earth (VM) and a fraction, which can be set, of the temperature measured variable,

   for the case in which the fraction, which can be set, of the temperature measured variable is smaller than the level of the potential of the virtual earth (VM), the changeover switch (S1) is brought into a position in which the first output (82) of the control device (8) is connected to the virtual earth (VM),

   for the case in which the fraction, which can be set, of the temperature measured variable is greater than the level of the potential of the virtual earth (VM), the changeover switch is brought into a position in which the first output of the control device is connected to the fraction, which can be set, of the temperature measured variable,

   the fraction, which can be set, of the temperature measured variable is set such that it corresponds to the potential of the virtual earth if the temperature measured variable assumes the derating start value.
5. Operating device according to Claim 1, characterized in that a proportionality factor, which determines the value of the subtraction variable as a function of the value of the temperature measured variable, is selected such that, when the disconnection value of the temperature measured variable is reached, the subtractor outputs a current set value (VST) which induces an operating current in the light-emitting diodes (2) which corresponds to a maximum permissible operating current given a maximum permissible operating temperature for the light-emitting diodes (2).
6. Operating device according to one of Claims 1 to 5, characterized in that the subtractor is replaced by - an adder (6) and the temperature measured variable is fed in inverted form to this adder (6).

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