EP1185147B1 - Voltage supply for Leds in lighting applications - Google Patents

Abstract

The voltage supply has a constant current source with a transistor with control and output connections. A LED or LED arrangement is connected in series with an output connection. A control loop detects the voltage drop between the transistor output connections and/or between the output connection connected to the LED and the control connection and sets the supply voltage so the voltage drop corresponds to a value above the saturation voltage. The voltage supply has a constant current source (2) to which a supply voltage is applied and that contains a transistor (3) with a control connection and two output connections, whereby the light emitting diode (5) or LED arrangement is connected in series with a transistor output connection. A control loop (6) detects the voltage drop between the transistor output connections and/or between the output connection connected to the LED and the control connection and sets the supply voltage so that the voltage drop corresponds to a value above the transistor saturation voltage. Independent claims are also included for the following: an arrangement of light emitting diodes and a method of supplying voltage to a constant current source for at least one light emitting diode.

Description

The present invention relates to a control circuit for at least one Light-emitting diode or for a light-emitting diode arrangement, on a light-emitting diode arrangement and a method for supplying a constant current source with a Supply voltage.

The invention relates generally to the field of light emitting diodes (LEDs) and more specifically - in the field of the use of such LEDs Illumination purposes. Although the use of LEDs for display applications It has long been known to use these light emitting diodes Only recently developed for lighting purposes. One of the reasons for this is that the Yield (light output per watt) has only recently reached such values that the efficiency of LED lighting devices is satisfactory. In particular the blue LEDs necessary for the generation of white light have recently achieved a satisfactory level of efficiency.

The use of LEDs for lighting purposes, especially in a Matrix arrangement in order to form a type of radiator is, for example, from the U.S. Patent 6,016,038 is known.

FIG. 4 shows a control circuit as it is present in products manufactured by the company Color Kinetics are sold and essentially the US patent mentioned correspond.

The LED LED is controlled with a constant current source KSQ. The Constant current source has a bipolar transistor, with the light emitting diode LED the collector of an NPN transistor is connected. The emitter of transistor Q1 Constant current source KSQ is grounded by means of an ohmic resistor R2 connected and via the PWM circuit for regulating the current to the control connection of transistor Q1 fed back. The NPN transistor provides a switchable Current drain (also known as current sink or in English "current sink"). The diode current is detected by means of the ohmic resistor R2 and by means of Change in base voltage regulated to a setpoint.

The transistor Q1 can only keep the output current constant as long as it is not oversaturated, ie as long as the voltage U _CE between its collector and its emitter is greater than the saturation _voltage U _CESAT . The saturation voltage is usually between about 0.7 and 1.2 volts. According to the prior art, the supply voltage U _CC is therefore applied, which is clearly above the sum of the flux voltage at the light-emitting diode LED and the saturation _voltage U _CESAT . A safety margin is thus created in order to be able to reliably compensate for production-related or thermal tolerances or fluctuations in the flow voltage of the diode. A sufficiently large safety margin is thus added to the usual saturation _voltage U _CESAT .

A control circuit of the type described above is also from, for example GB 2 333 593 known. Furthermore, US Pat. No. 5,929,617 describes a so-called LDO (low dropout) regulator for delivering a regulated voltage to a load, the Falling voltage is monitored via the transistor of the circuit arrangement. If this voltage falls below a certain minimum value, the current supplied to the load is reduced to cause the transistor to transition to prevent non-adjustable range.

If light-emitting diodes are used for lighting purposes and are accordingly packed in a thick matrix-like manner, the heat development or heat dissipation is a very big problem. If now a supply voltage for the reasons mentioned above U CC = (Saturation voltage of the transistor + safety margin + Voltage drop at the LED) is selected, the voltage selected as the safety margin generally drops as a power loss at the collector base section of the transistor Q1, which means further heat generation.

In view of the above problem, it is an object of the present invention that Heat development in control circuits for light-emitting diodes for lighting purposes to reduce. Of course, the principle that the Transistor of the constant current source must not saturate.

This object is solved by the features of the independent claims.

According to the invention, a control circuit is therefore provided for at least one light-emitting diode or light-emitting diode arrangement. The drive circuit has a constant current source, to which a supply voltage is applied and which has a transistor with a control connection and two output-side connections, the light-emitting diode or light-emitting diode arrangement being connected in series with one of the output-side connections of the transistor. According to the invention, a control loop is provided which, in the event that a bipolar transistor is used, detects the collector-emitter voltage and / or the collector base voltage of the transistor. If, however, an FET transistor is used, the voltage drop between the two output-side connections of the transistor is detected. The supply voltage of the constant current source is then set in such a way that the voltage drop U _CE between the two output-side connections of the transistor corresponds to a desired value which is above the saturation _voltage U _{CESAT of} the transistor.

The control loop can be a function for signal processing of the detected Have voltage drop.

According to the invention, a light-emitting diode arrangement is also provided, which has at least two groups of light-emitting diodes of different colors, wherein again each group has at least one light emitting diode. According to the invention is for each group of light emitting diodes has a control circuit as described above intended.

According to a further aspect of the invention, a method for regulating the supply voltage of a constant current source for at least one light-emitting diode is provided. The constant current source has a transistor with a control connection and two output-side connections and the light-emitting diode is connected in series with one of the output-side connections of the transistor. In the event that a bipolar transistor is used, the method has the step of detecting the collector-emitter voltage and / or the collector base voltage of the transistor. If, however, an FET transistor is used, the voltage drop between the two output-side connections of the transistor is detected. In a further step, the supply voltage of the constant current source is set in such a way that the voltage drop between the two output-side connections of the transistor corresponds to a desired value which is above the saturation voltage U _{CESAT of} the transistor.

The detected voltage drop can be processed as a signal in the feedback loop.

Fig. 1

zeigt eine schematische Ansicht einer Ansteuerschaltung für LED's (genauer gesagt LED-Arrays), bei der die Konstant-Versorgungsspannung der Konstantstromquelle geregelt wird.

Fig. 2

zeigt eine detaillierte Ansicht der Schaltung von Figur 1,

Fig. 3

zeigt die Anwendung jeweils einer Konstantstromquelle, Versorgungsspannung und Regelschleife für je eine Gruppe an farbigen Leuchtdioden in einem LED-Array, und

Fig. 4

zeigt eine bekannte Ansteuerschaltung für Leuchtdioden für Beleuchtungszwecke.

Further features, advantages and properties of the present invention will become apparent from the following description and exemplary embodiments and with reference to the figures of the accompanying drawings.

Fig. 1

shows a schematic view of a control circuit for LEDs (more precisely LED arrays), in which the constant supply voltage of the constant current source is regulated.

Fig. 2

shows a detailed view of the circuit of Figure 1,

Fig. 3

shows the application of a constant current source, supply voltage and control loop for each a group of colored light-emitting diodes in an LED array, and

Fig. 4

shows a known control circuit for light emitting diodes for lighting purposes.

It should be noted in advance that in the exemplary embodiments, the transistor Constant current source is an npn bipolar transistor connected as a current sink. Of course, this can be done using a pnp transistor as a switchable current source be replaced. In addition, both cases are also in FET technology, e.g. with IGBTs, realizable.

FIG. 1 shows a schematic view of a control circuit for an LED array 5, that several groups 10, 11, 12 each consisting of several red, green or has blue LEDs. In the embodiment of Fig. 1 that entire LED array 5 supplied with constant current sources 2. For everyone Constant current sources 2 is a common feedback or control loop 6 provided that - as will be explained later - as the actual value Voltage drop at the output terminals of the transistor is detected and closed a power section 1 with regulated (adjustable) DC output voltage. The power unit 1 supplies the constant current sources 2 with a regulated DC supply voltage.

To different lighting effects, for example by changing the Illuminance (dimming) of different groups 10, 11, 12 of the LEDs of the To achieve LED arrays 5, a sequence controller (controller) 9 is provided, which Target current by changing the voltage signals at the control connections of the Can set transistors of constant current sources 2. Thus, brightness and Color mixing changes are generated.

The structure of the constant current source 2 is shown in detail in FIG. essential Component of constant current source 2 is a transistor, here an npn bipolar transistor 3, the collector side with the light-emitting diode or a light-emitting diode arrangement 5 is connected and its emitter is connected to a measuring resistor (ohmic resistor) 4 connected is. Since the current through the measuring resistor 4 essentially the current through the LED 5 (the base current can be neglected), is the Diode current and can therefore be used to regulate the diode current by changing the Voltage at the control terminal (base) of transistor 3 can be used.

As already said at the beginning, the voltage difference between the collector and the emitter of the transistor 3 U _{CE must be} greater than the saturation _voltage U _{CESAT of} the transistor 3 if this is to be able to _withstand the current through the light-emitting diode even with a slightly fluctuating supply voltage U _CC to keep constant. However, as already stated, there is the problem that the voltage drop of the light-emitting diode 5 (forward voltage) can fluctuate due to production or also due to thermal changes. If the supply voltage U _CC in the prior art would correspond exactly to the sum of the forward voltage of the light-emitting diodes 5 and the saturation _voltage U _{CESAT of} the transistor 3, there is the problem that the collector-emitter voltage U _CE to the transistor 3 is below the saturation _voltage U _{CESAT of} the transistor 3 can drop if the forward voltage 5 of the LED is unexpectedly large due to its tolerance, for example, or rises over time. In this case of saturation, the transistor Q1 could no longer keep the diode current constant.

As can be seen in Figure 2, in the present invention, the collector-emitter voltage and / or the collector base voltage as the actual value of the control loop 6 detected. It should be noted that the collector-emitter voltage differs from the collector base voltage only by the relatively constant amount (e.g. 0.6 volts) of the base-emitter voltage, since the base-emitter path is diode-like Has properties.

In the feedback loop 6, the detected voltage signal is processed as an actual variable and fed to a comparator 8, to which the setpoint for the collector-emitter voltage and / or collector base voltage is also fed at an input 7. Depending on the detected actual value / setpoint deviation, the comparator 8 then sends a control signal to the power unit 1, which has an AC / DC converter with a regulated output. Thus, depending on the detected difference, the supply voltage U _{CC is} changed in such a way that the detected actual value corresponds to the target value.

The setpoint U _CE is selected somewhat above the saturation _voltage U _CESAT (generally between about 0.7 and 1.2 volts) transistor. Correspondingly, the target value for the collector base voltage U _{CB is} the value of the collector-emitter voltage U _CE minus the voltage that drops across the base-emitter path of the transistor 3.

Due to the regulation of the supply voltage U _CC depending on the detected voltage drop across the transistor 3, the voltage drop across the transistor 3 can be selected just above the saturation voltage U _{CESAT of} the transistor 3, so that the power loss and the heat development associated with the transistor 3 is reduced can be.

Figure 3 shows a modification of the principle of Figure 1 in that for each Group 10, 11, 12 of the red, blue and green light-emitting diodes of the LED array 5 one Constant current source 2 is provided, each of which has a feedback loop 6 for itself and has a power unit 1 for the supply voltage. The other Groups of light-emitting diodes of the LED array 5 are provided by their own constant current sources 2 'supplied, which have their own control loop and their own power section. Of course, there is still a common sequence control 9 for everyone Constant current sources provided by appropriate lighting control of the various groups 10, 11, 12 of the light-emitting diodes of the LED array 5 various lighting effects, such as color transitions and the like can achieve. The Sequence control 9 can be controlled, for example, by a bus.

Claims (7)

1. Control circuit for at least one light emitting diode or light emitting diode arrangement,
   having a constant current source (2), to which a supply voltage (1) is applied and which has a bipolar transistor (3) with a control terminal and two output terminals, the light emitting diode (5) or light emitting diode arrangement being connected in series with one of the output terminals of the transistor (3),
   characterised by
   a feedback or control loop (6) having means for

   detection of the collector-emitter voltage (U_CE) and/or of the collector-base voltage (U_CB) of the transistor (3), and

   setting the supply voltage (1) of the constant current source (2) such that the voltage drop between the two output terminals of the transistor (3) corresponds to a desired value which lies above the saturation voltage of the transistor (3).
2. Control circuit for at least one light emitting diode or light emitting diode arrangement, having a constant current source (2), to which a supply voltage (1) is applied and which has an FET transistor (3) with a control terminal and two output terminals, the light emitting diode (5) or light emitting diode arrangement being connected in series with one of the output terminals of the transistor (3) ,
   characterised by
   a feedback or control loop (6) having means for

   detection of the voltage drop between the two output terminals of the transistor (3), and

   setting the supply voltage (1) of the constant current source (2) such that the voltage drop between the two output terminals of the transistor (3) corresponds to a desired value which lies above the saturation voltage of the transistor (3).
3. Control circuit according to any preceding claim,
   characterised in that,
   the feedback or control loop (6) is constituted for signal processing of the detected voltage drop (U_CE, U_CB).
4. Light emitting diode arrangement,
   having at least two groups of light emitting diodes of different colour, each group having at least one light emitting diode,
   characterised in that,
   for each group of light emitting diodes there is provided a control circuit in accordance with any preceding claim.
5. Method of voltage supply of a constant current source for at least one light emitting diode, the constant current source (2) having a bipolar transistor (3) with a control terminal and two output terminals, and the light emitting diode (5) being connected in series with one of the output terminals of the transistor (3),
   having the following steps:

   detection of the collector-emitter voltage (U_CE) and/or of the collector-base voltage (U_CB) of the transistor (3), and

   setting the supply voltage (1) of the constant current source (2) such that the voltage drop between the two output terminals of the transistor (3) corresponds to a desired value which lies above the saturation voltage of the transistor (3).
6. Method of voltage supply of a constant current source for at least one light emitting diode, the constant current source (2) having an FET transistor (3) with a control terminal and two output terminals, and the light emitting diode (5) being connected in series with one of the output terminals of the transistor (3) ,
   having the following steps:

   detection of the voltage drop between the two output terminals of the transistor (3), and

   setting the supply voltage (1) of the constant current source (2) such that the voltage drop between the two output terminals of the transistor (3) corresponds to a desired value which lies above the saturation voltage of the transistor (3).
7. Method according to claim 5 or 6,
   characterised in that,
   the detected voltage drop (U_CE, U_CB) is signal processed in a feedback or control loop (6).

Images