WO2001069979A1

WO2001069979A1 - Control of light-emitting diodes (leds)

Info

Publication number: WO2001069979A1
Application number: PCT/EP2001/002639
Authority: WO
Inventors: Werner Ludorf; Peter Hein
Original assignee: Tridonicatco Gmbh & Co. Kg
Priority date: 2000-03-17
Filing date: 2001-03-08
Publication date: 2001-09-20
Also published as: EP1269798B1; AU2001248339A1; DE50102479D1; EP1269798A1; ATE268538T1; DE10013208A1

Abstract

The invention relates to constant current sources (KSQ) having operation amplifiers (OP) for operating LED arrays. Residual currents caused by the offset voltages of the operation amplifier (OP) can occur in the off-phases Toff of the controlling PWM signals and can flow through the LED arrays, whereby said signals which are varied for dimming the LED arrays. This is a disadvantage, especially in the lower dimming range. This is why the potential is increased on the feedback side of the transistor (Q1) of the constant current source (QSK) by means of the potentially occurring offset voltage of the operation amplifier (OP). Moreover, a light-emitting diode can be used on the feedback side on control switches having constant current sources which are provided with transistors (Q1). Said diode at least partially replaces the current detector resistor (R3).

Description

Control of light emitting diodes (LEDs)

The present invention relates to a control circuit for at least one light-emitting diode.

The invention relates generally to the field of light-emitting diodes (LEDs) and - more precisely - to the field of using such LEDs for lighting purposes. Although the use of LEDs for display applications has long been known, the use of these light-emitting diodes for lighting purposes has only recently developed. The reason for this is, among other things, 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 only recently achieved a satisfactory degree of efficiency.

The use of LEDs for lighting purposes, in particular in a matrix arrangement, in order to form a type of spotlight, is known, for example, from US Pat. No. 6,016,038.

FIG. 12 shows a control circuit as it is present in products which are sold by the Color Kinetics company and which essentially correspond to the US patent mentioned.

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

As can be seen in FIG. 12 and is also known in particular from US Pat. No. 6,016,038, a pulse width modulated (PWM) signal is applied to the base connection of the transistor Q1 for dimming the light-emitting diode LED. The advantage of the PWM signal is that the light-emitting diode LED, which implements changes in the current flow practically instantaneously, is either fully or not driven at all. The efficiency is in these states significantly larger compared to intermediate values in which a diode current flows between zero and maximum current flow through the LED. For dimming, which is specified by an external control signal on a PWM control circuit, the pulse duty factor of the PWM signal on the NPN transistor Q1 (at a constant frequency) is changed according to the cited US patent. By increasing the dead times of the PWM signal, the luminosity of the LED that is perceptible to the human eye is dimmed.

In general, there is the problem of high heat development when using LEDs for lighting purposes, since the LEDs and the associated control circuits (constant current source, etc.) have to be packed very closely into the matrix arrangements in order to achieve a sufficient luminosity. A problem is also the heat development at the current sensor measuring resistor R2, which is connected in the negative feedback of the constant current source on the emitter side.

It is therefore an object of the present invention to improve a control circuit for light-emitting diodes with constant current sources on the feedback side of the transistor of the constant current source.

This object is achieved by the features of independent claims 1 and 6, which each propose efficiency-increasing measures on the feedback side of the transistor.

According to the first aspect of the invention, a control circuit is therefore provided for at least one light-emitting diode, which has a constant current source. The constant current source has an operational amplifier, the output of which is connected to the control connection of an (external) transistor. The light-emitting diode is connected in series with a first output-side connection of the transistor. A second output-side connection of the transistor is fed back to the inverting input of the operational amplifier. According to the invention, a component is provided on the feedback side of the transistor that raises the potential of the feedback side of the transistor by at least the value of an offset voltage of the operational amplifier if no or only an infinitesimal current flows through the light-emitting diode.

Of course, this definition also includes a component that always increases the potential on the feedback side of the transistor by a predetermined value greater than or equal to the offset voltage of the operational amplifier. Such a component is typically a voltage source. Alternatively, the component on the feedback side can be a further diode and in particular a light-emitting diode, which converts the voltage drop and the current flow into a further light output. The light-emitting diode on the feedback side is advantageously a red light-emitting diode, since the forward voltage of red light-emitting diodes is lower than that of green or blue light-emitting diodes.

Said component can be connected in series to form an ohmic resistor. The ohm see resistor provides a certain linear portion of the feedback side. According to a further aspect of the present invention, a drive circuit for a light-emitting diode is provided, which has a constant current source with a transistor. The light emitting diode is connected in series to a first output-side connection of the transistor. A second connection on the output side of the transistor is fed back to the control connection (base or gate) of the transistor. A further light-emitting diode is provided on the feedback side of the transistor.

This further light-emitting diode can be connected in series to form an ohmic resistor. The further light-emitting diode is advantageously a red light-emitting diode, the advantages of which have already been explained above. The transistor can be a bipolar transistor. In particular, pulse width modulated voltage signals can be applied to the control terminal of the transistor. The at least one light-emitting diode is then dimmed by changing the pulse duty factor and / or the frequency of the pulse width modulated (PWM) voltage signals at the control connection of the transistor.

An advantageous development of the invention relates to measures by means of which the dimensions of the control circuit can be kept as compact as possible. In order to achieve this, the control circuit consists at least partially of a multi-layer circuit in which passive components - for example resistors, conductor tracks and the like - are integrated. This integration is possible in particular if the light-emitting diodes are operated at high frequencies, since then correspondingly lower capacitance or inductance values can be used in the circuit. In the present case, a frequency range from 200 kHz to 1 MHz has proven to be particularly suitable. An increased radiation of electromagnetic high-frequency fields, which is initially caused by the frequency increase, can be avoided by suitable shielding measures which, owing to the reduced dimensions of the circuit, can be carried out simply. The integration of components can take place, for example, using a multilayer printed circuit board technology. The multilayer circuit is preferably implemented by an LTCC (Low Temperature Cofired Ceramic) structure, which consists of a plurality of low-sintered ceramic layers or foils arranged one above the other, between which there are conductor tracks. Compared to conventional printed circuit board technology, this miniaturization of the circuit can be achieved again with this LTCC technology, which has been newly developed in recent years and is known, for example, from EP 0 581 206 A2. In addition to the conductor tracks, inductors and capacitors in particular can also be integrated into the multilayer circuit. Furthermore, the ceramic material has the advantage that it conducts heat relatively well, which means that larger outputs can be achieved with the same construction volume, since heat loss is better radiated. The heat dissipation is preferably increased again by embedding the ceramic structure in a metallic housing. In this way, an effective shielding of the high-frequency fields emitted by the control circuit into the environment can also be achieved.

At the frequencies mentioned above, a large part of the components of the control circuit can be integrated into the multi-layer circuit. However, the remaining passive components and semiconductor assemblies are still to be attached to the surface or outside the ceramic structure. In order to achieve the smallest possible space requirement for this, the semiconductor modules are preferably mounted on the ceramic substrate using the known flip-chip (FC) technology. In this case, a plastic layer is introduced between the semiconductor, which is mounted without a housing, and the contacts on the surface of the carrier substrate, which on the one hand is electrically conductive perpendicular to the contacting plane and is insulating in the contacting plane, and on the other hand, which results from a different thermal expansion of the semiconductor assembly and the ceramic substrate Absorbs voltages and thus prevents destruction of the semiconductor module.

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

1 shows a schematic view of a drive circuit for an LED

Array with light emitting diodes (LEDs) of different colors (R, G, B), 2 shows a drive circuit according to a first aspect of the invention, in which an improvement has been made on the feedback side of the constant current source, which brings advantages in dimming,

3 shows a modification of the embodiment of FIG. 2,

4 shows a drive circuit according to yet another aspect of the invention,

5a-10 show the manufacturing steps of a multilayer ceramic (LTCC)

Structure,

11 shows an enlarged section of an LTCC structure, and

Fig. 12 shows a control circuit for a light emitting diode according to the prior art

Technology.

Referring first to Figure 1, a general view of a drive circuit for LED arrays, i.e. more specifically, of arrays with LEDs of different colors (R, G, B) are explained. An AC / DC converter provides an essentially regulated output voltage V +. In the first exemplary embodiment of FIG. 1, a constant current source KSQ_R, KSQ_G, KSQ B is provided for each of the different colors of LEDs (R, G, B), which are controlled by a PWM (pulse width modulation) control circuit PWM_R, PWM_G or PWM_B. It is also possible to provide a constant current source for LED arrays of different colors. The PWM control circuits PWM_R, PWM G, PWM_B are supplied with an external control signal, for example from a bus, which specifies dimming positions for the various LED arrays LED_R, LED_G and LED_B.

FIG. 2 shows a control circuit for at least one LED 1, which uses an operational amplifier OP.

The current flow through the LED1 corresponds to the current flow through the emitter resistor R3.

The output of the operational amplifier OP is connected to the control terminal of the external transistor Ql. In the present exemplary embodiment, the output of the operational amplifier is connected to the base of an NPN bipolar transistor Q1. The feedback causes the voltage difference V _r -V _IN at the emitter resistor R3 on, so that the current through the resistor R3 and thus through the LED LED1 (the base current is negligible)

I = (V _C -V _E1N) / R3)

is.

The potential of the negative feedback side of transistor Ql is raised by a constant current source V _c . The potential increase is at least as large as an average offset voltage, as typically occurs in operational amplifiers, and is therefore in the range of at least about 2 mV.

The circuit of FIG. 2 has the following advantages when dimming the light-emitting diode LED by means of pulse width modulated signals (PWM signals):

In the off time of the PWM signals would be if the constant current source V _c is not provided on the feedback side of the transistor, could (in which V _a = 0), an offset voltage to the control terminal abut (base of transistor Ql). The transistor Q1 would thus be conductive, even if only very slightly, and a low current would also flow through the light-emitting diode LED1 in these switch-off phases. Thus, the light emitting diode LED1 would inadvertently not be switched off in the switch-off phases. As a result, it is not possible to achieve indefinitely small dimming levels through the offset voltage, since the offset voltage always ensures a small current flow through the light-emitting diode LED1 without the constant current source V _c being provided.

By means of the constant current source V _c on the negative feedback side (emitter side) of the transistor Ql, the potential on the negative feedback (emitter) side of the transistor Ql is raised to a higher potential than the potential of the control terminal. In the example of FIG. 2, the potential of the emitter of the transistor Q1 is therefore at a higher potential than the base of the transistor Q1, so that any current flow is reliably prevented (the “diode” base-emitter blocks). As a result, any current flow is prevented in the switch-off phases of the PWM signals, so that the light-emitting diode LED1 is safely switched off. Of course, the voltage V must be _a significantly above the potential of the constant voltage source V _c during the ON phases of the PWM signals.

Figure 3 shows a modification of the embodiment of Figure 2, in which instead of the constant voltage _source V _c an electronic component with non-linear current Voltage characteristic, ie more precisely, a diode LED2 is used. In general, a component is used on the feedback side of the transistor, which at zero current or infinitesimally small current through the light-emitting diode LED1 already raises the potential on the feedback side. As shown in FIG. 3, this component can be a diode and in particular a light-emitting diode LED2, which serves to increase the light generation of the LED arrangement. This further light-emitting diode LED2 on the feedback side of the transistor can serve as a complete or, as shown in FIG. 3, partial replacement of the current detector resistor R3 in the constant current source. The second LED2 on the feedback side of the transistor Q1 represents a current detector light-emitting diode, so to speak.

This component, which causes a voltage rise on the feedback side at zero current or very low current, reliably suppresses the current flow through the LED1 in the switch-off phases.

In addition to the voltage source and the diode, any component that has a very large resistance value in the area of low currents can be used.

According to the aspect of the invention, as shown in FIGS. 2 and 3, residual currents are prevented by the LED 1 which, in the case of constant current sources KSQ with operational amplifier OP, in the switch- _off phases T _{off of} the controlling PWM signals, which varies for dimming the LED arrays will occur due to offset voltages of the operational amplifier OP. In the lower dimming ranges in particular, the residual LED currents hinder the implementation of low dimming levels. To reduce these currents, the emitter or source potential of the bipolar or MOSFET transistor is raised according to the invention by means of a voltage source to such an extent that the offset voltages of the operational amplifier are compensated or overcompensated during the switch- _off phase T _off .

Figure 4 shows another aspect of the present invention. The state of the art shown in FIG. 5 is again assumed, and in so far as the components are the same, reference is made to the detailed description of FIG. 5. In FIG. 4, in the case of a control circuit with a constant current source KSQ with pure current feedback (without operational amplifier OP), the current detector resistor on the feedback side is partially replaced by a light-emitting diode LED2, which converts part of the voltage occurring on the feedback side into light. The light yield is thus improved while reducing the heat development. Even if only a partial replacement of the ohmic resistor R2 is advantageous with regard to the linearity of the feedback loop, the current detector resistor R2 in the feedback loop can alternatively also be completely replaced by the LED2.

In the exemplary embodiments of FIG. 3 or FIG. 4, it is advisable to use a red light-emitting diode as light-emitting diode LED2 in the feedback loop, since the forward voltage, i.e. the necessary voltage drop across the diode, which is minimally necessary for the light to be emitted by the light-emitting diode, is lower for red light-emitting diodes than for green, blue or white.

The structural design of the control circuits according to the invention will now be discussed below, which is particularly suitable when using control frequencies in the range from 200 kHz to 1 MHz. The LTCC multilayer circuit already mentioned is ideal for integrating the passive components. The production of such a ceramic multilayer structure will now be explained with reference to FIGS. 5a-10.

The basic building block of an LTCC structure is an approx. 100-130 μm thick, low-sintering ceramic film - for example made of aluminum oxide, which is mixed with glass particles and other filler material - as shown in plan view in FIG. 5a. The first processing step consists in punching via holes 11 in the ceramic film 10. 5b shows the correspondingly processed ceramic film 10 in section I-I of FIG. 5a. Before the firing process, the diameter of the via holes 11 is approximately 250 μm. In the next work step shown in FIGS. 6a and 6b, the plated-through holes 11 are then filled with a conductive material, usually with a conductor paste, which contains a relatively high solids content.

Corresponding to the desired circuit structure, 10 conductor tracks 12 are then printed on the top of the ceramic film (FIG. 7). This is usually done using a screen printing process. Silver, silver / palladium, gold or copper pastes are used for the plated-through holes and for the conductor tracks. In order to avoid deflections, the material composition of the conductor pastes is selected such that they shrink to the same extent as the ceramic layers 10 themselves during subsequent sintering.

The processing steps just described are first carried out separately for each ceramic film 10. The individual layers of punched and printed ceramic films 10 are then stacked and aligned as shown in FIG. 8. They are then stacked in a press mold and laminated with the application of heat and pressure, so that a coherent ceramic structure is formed. This is finally sintered into a high-strength ceramic structure, a homogeneous ceramic substrate 13 with a connected interconnect network 14 being integrated therein, as shown in FIG. 9.

In the final processing step shown in FIG. 10, the components that cannot be integrated into the ceramic circuit, for example various semiconductor assemblies 15, are then attached and contacted on the upper side of the ceramic substrate 13. In this case, conductor tracks 16 can also be subsequently applied to the upper side. Finally, the entire complex is provided with connections and surrounded by a metallic housing, which on the one hand increases heat dissipation and on the other hand shields the high-frequency electromagnetic fields that arise during operation.

FIG. 11 again shows an area of the LTCC structure in section on an enlarged scale. The dividing lines between the individual original ceramic layers are also shown, even if — as described above — a homogeneous ceramic structure 13 is formed after the lamination and sintering. As can be seen on the right-hand side of the figure, the vertically running conductor tracks 11, which are formed by the via holes punched out in the first method step, can also extend over several levels. The main advantage of using an LTCC structure is that not only conductor tracks 11 or 12 but also other passive components can be integrated in the multilayer circuit. In the course of the processing step shown in FIG. 7, in addition to the conductor paste for the general conductor tracks 12, other materials with a specific conductivity can also be applied to the top of an individual ceramic layer 10, so that a resistor 21 can be fully integrated into the ceramic structure 13 in this way can. In addition to the usual through-hole 11, larger vertical holes can also be punched into a ceramic layer 10, for example. These can then be filled with a material 20 with a specific dielectric constant, so that capacities integrated in the ceramic substrate 13 can be realized by the layer arrangement of conductor track - dielectric 20 - conductor track shown in FIG. 11.

Furthermore, an inductance within the ceramic structure 13 could be realized by a spiral-like conductor track printed on an individual ceramic layer 10. However, it would also be conceivable to distribute the different windings of the inductance over several conductor track levels. Such structures are also called planar inductors. To increase the inductance, an opening or recess could also be provided in the ceramic substrate 13, which is filled with a suitable core material, for example ferrite.

Only inductance and capacitance values up to a certain level can be achieved for the components integrated in the multi-layer circuit. However, these values are sufficient for correct operation of the drive circuit according to the invention at frequencies in the range between 200 kHz and 1 MHz.

However, not all components of the control circuit can be integrated into the multi-layer circuit 13 in the LTCC technology. For example, the operational amplifier is formed by a semiconductor module 15, which cannot be integrated into the ceramic layer 13. Such semiconductor assemblies 15 are then preferably contacted on the upper side of the multilayer circuit 13 by means of flip-chip technology. In this case, an anisotropically electrically conductive plastic is introduced between the top of the ceramic substrate 13 and the unhoused semiconductor circuit 15, which is electrically conductive perpendicular to the flip-chip contacting plane and has an insulating effect in the contacting plane. As electrically conductive particles, the plastic contains, for example, irregularly shaped metal pieces or else smaller balls or fibers, which bring about contact between the surface contacts 18 of the ceramic substrate 13 and the connection pads 17 of the semiconductor assembly 15. Furthermore, this plastic 18 also absorbs voltages which can result from a different thermal expansion of the ceramic material and the semiconductor module 15. This flip-chip technology enables a very high contact density, so that it also contributes to a volume reduction of the entire circuit. The light-emitting diodes themselves can also be applied to the top of the multilayer circuit 15 using this technology. Of course, resistors 22 or inductances of the circuit can also be arranged on the surface as discrete parts.

By means of the present invention, improvements are made on the feedback side of the transistor of the constant current source in the case of control circuits for LEDs with constant current sources, a control circuit having extremely compact dimensions being able to be realized by the developments described last.

Claims

Expectations

1. Control circuit for at least one light-emitting diode (LED1), having a constant current source (KSQ) connected, which has an operational amplifier (OP), the output of which is connected to the control terminal of a transistor (Ql), the light-emitting diode (LED1) being connected to a first output-side connection of the transistor (Ql) is connected in series and a second output-side connection of the transistor (Ql) is fed back to the inverting input of the operational amplifier (OP), characterized in that on the feedback side of the transistor (Ql) a component (Vc, LED2) is provided that the potential of the feedback side of the transistor (Ql) increases at least by the value of an offset voltage of the operational amplifier (OP) when no or only an infinitesimal current flows through the light-emitting diode (LED1).

2. Control circuit according to claim 1, characterized in that the component is a voltage source (Vc).

3. Control circuit according to claim 1, characterized in that the component is a further diode (LED2).

4. Control circuit according to claim 3, characterized in that the diode is a light emitting diode (LED2).

5. Control circuit according to one of the preceding claims, characterized in that the component is connected in series with an ohmic resistor (R3).

6. control circuit for a light-emitting diode, comprising a constant current source with a transistor (Ql), the light-emitting diode (LED1) being connected in series to a first output-side connection of the transistor (Ql) and a second output-side connection of the transistor (Ql) being fed back to the control connection of the transistor (Ql) , characterized in that a further light-emitting diode (LED2) is provided on the feedback side of the transistor (Ql).

7. Control circuit according to claim 6, characterized in that the further light-emitting diode (LED2) is connected in series to an ohmic resistor (R2).

8. Control circuit according to claim 6, characterized in that the further light-emitting diode is a red light-emitting diode (LED2).

9. Drive circuit according to one of the preceding claims, characterized in that the transistor is a bipolar transistor (Ql).

10. Control circuit according to one of the preceding claims, characterized in that pulse width modulated (PWM) voltage signals are applied to the control connection of the transistor (Ql).

11. Control circuit according to claim 10, characterized in that the voltage signals have frequencies in the range between 200 kHz and 1 MHz.

12. Control circuit according to one of the preceding claims, characterized in that that it has at least one multilayer circuit (13), in which passive components (20, 21) of the control circuit are integrated.

13. Control circuit according to claim 12, characterized in that a multilayer circuit consists of a plurality of stacked circuit boards, on the upper and / or lower sides of which conductor tracks made of a conductive material are applied and which have through-holes for connecting different line levels, which also have a conductive Material are filled.

14. Control circuit according to claim 12, characterized in that it is in the multilayer circuit to an LTCC structure (13), which consists of several superimposed low-sintered ceramic layers (10), between which there are conductor tracks (12), the Have ceramic layers (10) through holes (11) for connecting conductor tracks (12) of different levels.

15. Control circuit according to one of claims 12 to 14, characterized in that the multilayer circuit (13) is surrounded by a metallic housing.

16. Control circuit according to one of claims 12 to 15, characterized in that semiconductor modules (15) on the surface of the multilayer circuit (13) are attached by means of flip-chip technology.