EP2294897A1

EP2294897A1 - Circuit arrangement and method for operating at least one led

Info

Publication number: EP2294897A1
Application number: EP08785941A
Authority: EP
Inventors: Ralf Hying; Peter Niedermeier; Oskar Schallmoser
Original assignee: Osram GmbH
Current assignee: Osram GmbH
Priority date: 2008-07-07
Filing date: 2008-07-07
Publication date: 2011-03-16
Also published as: WO2010003448A1; TW201008393A; KR20110040891A; US20110163693A1; CN102090143B; CN102090143A

Abstract

The present invention relates to a circuit arrangement for operating at least one LED, having an operational amplifier (12) with a non-inverting input and an inverting input as well as an output; a desired value predefining apparatus (10) which is coupled to the non-inverting input of the operational amplifier (12); a first connection and a second connection for the at least one LED, wherein the first connection is coupled to a connection for a DC supply voltage (V+); a transistor (T1) which is coupled in series with the first and second connections for the at least one LED and can be operated in an analogue manner, wherein the transistor (T1) has a control electrode, a reference electrode and a working electrode, wherein the control electrode of the transistor is coupled to the output of the operational amplifier (12), wherein the working electrode of the transistor (T1) is coupled to the connection for a DC supply voltage (V+); and a current measuring resistor (RShunt) which is coupled in series between the reference electrode of the transistor (T1) and a reference potential, wherein the voltage dropped across the current measuring resistor is coupled to the inverting input of the operational amplifier (12); wherein at least one load (RV1) is coupled in parallel with the first and second connections for the at least one LED. The invention also relates to a corresponding method for operating at least one LED.

Description

Confession

Circuit arrangement and method for operating at least one LED

Technical area

The present invention relates to a Schaltungsanord ^¬ tion for operating at least one LED with an operation amplifier with a plus and a minus input and an output, a setpoint specification device gekop ^¬ pelt with the positive input of the operational amplifier, a first and a second terminal for the at least one LED, wherein the first terminal is coupled to a connection for a DC supply voltage, a series with the first and second terminal for the at least one LED coupled analog betreibba ^¬ ren transistor, said transistor having a control electrode, a reference electrode and a working electrode, wherein the control electrode of the transistor is coupled to the output of the operational amplifier, wherein the working electrode of the transistor is coupled to the terminal for a DC supply voltage, and coupled in series between the reference electrode of the transistor and a reference potential th current measuring resistor, the voltage drop across the current sensing resistor voltage is clamping ^¬ ge ^¬ coupled to the negative input of the operational amplifier. It also relates to a corresponding method for operating at least one LED.

State of the art

A generic, known from the prior art circuit arrangement is shown in Fig. 1. there provides a setpoint specification device 10 at its output a target voltage U _so ii ready, which is coupled to the positive input of an operational amplifier 12. The operational amplifier 12 is supplied from a first source + V _CC , which provides a positive DC supply voltage, and from a second source V _S s, which provides a Ver ^¬ supply voltage of zero or a negative supply ^¬ DC voltage. Between the off ^¬ gear A of the operational amplifier 12 and its minus input of a feedback network is connected, which in the present case comprises the series connection of an ohmic resistor Rl and a capacitor Cl. The voltage dropping between the plus input and the minus input of the operational amplifier 12 is _denoted by U _Dlff . The output A of the operational amplifier 12 is connected to the control input ^¬ , in this case the gate terminal, a transistor Tl. Between a DC supply voltage V ₊ , which may correspond to the source + V _CC , and the working electrode, in this case the drain terminal, of the transistor Tl, an LED is connected across which a voltage U _LED drops. Between the reference electrode, in this case the source terminal, the transistor Tl and a Bezugspo ^¬ potential, a current measuring resistor R _S hunt is arranged, over which a voltage U _SHU nt drops. The voltage U is also _Shu nt gekop pelt ^¬ via a second resistor R2 to the negative input of operational amplifier 12th The operational amplifier 12 together with its feedback and the transistor Tl forms a linear regulator.

Generic circuit arrangements are ^{beispielswei ¬} se in LED projection applications, in particular in the so-called rear projection, used. In this case, the nominal value setting device applies signals to the operational amplifier, which can have very short switch-on pulses, up to 4 μs, and very short dark times, likewise up to 4 μs. How have appropriate analyzes erge ^¬ ben, the generic circuit arrangements worked unsatisfactory especially with very short turn-on or dark periods. This leads to projection results of inferior quality.

Presentation of the invention

The present invention is therefore based on the object be ^¬ relationship as further develop an aforementioned method in such an aforementioned circuitry that projection applications are made possible with a higher quality.

This object is achieved by a circuit arrangement having the features of patent claim 1 and a method having the features of patent claim 6.

The present invention is based on the finding that within the entire current range, which can flow through the LED and extends from 0 A to I _L EDmax, at a nominal current in the range of 0 A undesired, greatly deteriorating regulator properties of the linear regulator occur. The reason is that in practice it is never possible to control absolutely exactly to a current of 0 A, ie it always flows in, albeit with very little, positive or negative current. Since the circuitry used does not permit any negative currents, in this case the operational amplifier would have leave the linear controller mode. As a result, the control properties would deteriorate unacceptably. Thus, the controller has different dynamic behavior within the entire current range. A more detailed analysis of the processes at currents near 0 A can be found below, reference being made to FIG. 3.

Due to the fact that at least one further consumer is coupled parallel to the LED, the voltage across the LED, if suitably dimensioned, remains so small that the LED does not yet emit light, even though the analog-operable transistor is already in linear operation, since a positive Control voltage applied to it. The transistor can be turned on quickly, so that a time delay is avoided by the slew rate of the analog operable transistor. As a result, extremely short switch-on pulses and dark times can be achieved, which leads to a very high quality in projection ^¬ applications.

In the circuit arrangement according to the invention, therefore, regardless of manufacturer or color or manufacturing lot, an LED can be operated so that it does not light at a target current of 0 A, but the current regulator already engaged, i. in linear operation, is. For now given nominal current jumps, the controller can react with extremely small time constants.

Another advantage of one of the at least one LED connected in parallel consumer results from the fact that this is to discharge the capacity of the LED and its leads after switching off the current through the LED leads. As a result, an afterglow of the LED, which can be up to 1 microseconds in the prior art, avoided. In addition, negative current peaks due to Lei ^¬ tion inductances, which can be up to 1 V and therefore can lead to failure of the LED, reliably eliminated.

Preferably, the said at least one LED connected in parallel consumer represents at least one or more Ele ^¬ elements of the following selection: ohmic resistance, current sink constant-current diode.

Particularly preferably, the consumer comprises at least a first and a second partial consumer, wherein an electronic switch is arranged serially to the at least second partial consumer. This opens up the possibility to change the consumer depending on the light emitted by the LED color or to Berücksichti ^¬ supply of manufacturing tolerances, thereby to accommodate different threshold voltages. This can be set from which power the LED emits light. This can also take into account, for example, an aging of the LED or a temperature change of the LED.

Particularly preferably, the circuit arrangement wei ^¬ terhin comprises a microcontroller which is designed, the forward voltage of a to determine an LED between the first and the second terminal for the at least one LED coupled Minim ^¬ tens and to drive the / the electronic switches accordingly. This opens up the possibility of automatically becoming the most suitable consumer or consumers the switch at least one LED in parallel, ie the particular ^¬ dynamically during operation of the Minim ^¬ least one LED.

It is further preferred if a Rückkop ^¬ pelnetzwerk coupled between the output and the negative input of the operational amplifier. As a result, the Regelpara ^¬ meters of the linear controller and thus the circuit arrangement can be adjusted.

Further advantageous embodiments will become apparent from the dependent claims.

The presented in connection with a circuit arrangement according to the invention preferred embodiments and their advantages apply accordingly, as far as ^¬ applicable, for the inventive method.

In a preferred embodiment of the method according to the invention, the step of coupling takes place in such a way that a positive current constantly flows through the at least one LED during operation of the circuit arrangement.

Short description of the drawing (s)

In the following we will now describe an embodiment of a circuit arrangement according to the invention with reference to the accompanying drawings. Show it :

Figure 1 is a schematic representation of a known from the prior art circuit arrangement for operating at least one LED. 2 shows a schematic representation of an embodiment of a circuit arrangement according to the invention; and

3 shows the time profile of various sizes of the circuit arrangements of FIGS. 1 and 2.

Preferred embodiment of the invention

The reference numbers introduced with reference to FIG. 1 apply mutatis mutandis to the same or similar components of the embodiment of the present invention shown in FIG. They will not be reintroduced.

In Fig. 2, the operational amplifier 12 is shown in more detail. In particular, between the positive input of the operational amplifier 12 and the positive input of an ideal operational amplifier 14 contained in the operational amplifier 12, a voltage source U _{0F is plotted} , which reproduces the so-called offset voltage. Depending on the _lot or the aging condition or other parameters, the offset voltage U _{0F can be} positive or negative. It is not critical if the differential voltage U _D iff between the positive and the negative input of the operational amplifier 12 is positive, that is, the offset voltage U _{0F is} positive. Then, when namely the current I _LED is close to zero, however, is the most current sensing resistor R _S hunt ^¬ from falling voltage U _Shu nt almost zero, positive. At the output A of the operational amplifier 12, a small, positive voltage is applied to the control electrode of the transistor Tl. As a result, the transistor Tl remains conductive and can quickly increase the power when needed. However, the LED is undesirable.

On the other hand, it is even more critical if the offset voltage U ₀ F is negative. For clarity, reference is made to the time profiles of the voltage U _so U, U _G s and U _shU nt in Fig. 3. Above the time course of the setpoint voltage Usoii is shown, which may be rectangular, for example, where curve a) the time course for the case shows that U ₀ F is greater than zero, and curve b) the time course for the case that U ₀ F is less than zero. The processes for U _0F greater than zero have already been mentioned. For the case where U _0F is less than zero, at the output A of the operational amplifier 12 to a ne gative voltage ^¬. The operational amplifier 12 'will' settle so that the voltage U _shunt the current measurement resistor R _shunt becomes negative. This is not possible because the Tran ^¬ sistor Tl may not more than in the non-conducting state are added. This causes no ge ^¬ loop current longer present. the voltage at the output a of the operational amplifier decreases to V _ss, where ^¬ may be at V ₅ s is zero or less than zero. V _S S is in a preferred embodiment, -15 V, and is shown in Figure 3 is not reproduced to scale for the sake of clarity.

Now when the operational amplifier to be brought out of this situation again in an area with a positive current I _LED 12, is the operational amplifier 12 is first "so at the ready", ie in such gesättig ^¬ th state that it is dynamic very slowly. This arises from the curve shown in FIG. 3 at the bottom: Curve a) corresponds to curve a) in FIG mean curve, while curve b) corresponds to the curve b) in the middle diagram. As shown, the voltage U _shu nt increases only with a significant time delay .DELTA.t when U _{0F is} negative and the operational amplifier 12 was operated in a phase with a current I _LED close to 0 A. This time delay results in short-duration switch-on pulses and dark periods being not reproduced at all or incorrectly, in particular far too short. This is especially evident when one considers that Δt can be up to 10 μs and more.

Raising the voltage U _to n such that it always being larger than SSER ^¬ U _0F, U _0F whether is positive or negative, would cause U _G s is always greater than zero and so-with a flow of current I _LED by the LED takes place even if this is not desired. To prevent this, it is now, see Fig. 2, provided according to the invention, the LED at least one consumer R _V i parallel to scarf ^¬ th. Further consumers are provided, of which one in Fig. 2, namely the consumer R _V , is shown playfully at ^¬ . A switch, in this case the switch S1, which is controlled by a microcontroller 16, is preferably arranged in series with these consumers. The microcontroller 16 is designed to determine the flux voltage of the LED and to control the switches Si in such a way that, in sum, the appropriate consumer total resistance of the LED is connected in parallel. Suitable here means that the threshold voltage of the LED, ie the voltage from which the LED emits light, by the use of the parallel circuit of a or several additional consumers just below.

Claims

claims

1. Circuit arrangement for operating at least one LED with

- An operational amplifier (12) having a plus and a minus input and an output; - A setpoint specification device (10) coupled to the positive input of the operational amplifier (12) gekop ^¬ pelt;

- A first and a second terminal for the at least one LED, wherein the first terminal is coupled to a terminal for a DC supply voltage (V ₊ );

- a series with the first and second terminal for the at least one LED coupled analog betreib ^¬ cash transistor (Tl), wherein the transistor (Tl) having a control electrode, a reference electrode and a working electrode, wherein said control electric ^¬ de of the transistor to the output of Operations ^¬ amplifier (12) is coupled, wherein the working ^¬ electrode of the transistor (Tl) is coupled to the terminal for a DC supply voltage (V ₊ ); and

- A serially coupled between the reference electrode of the transistor (Tl) and a reference ^¬ th current measuring resistor (Rshunt), wherein the voltage drop across the current measuring resistor to the Mi ^¬ nus input of the operational amplifier (12) gekop ^¬ pelt; characterized, in that at least one consumer (Rvi) is coupled in parallel to the first and second terminals for the at least one LED.

2. A circuit arrangement according to claim 1, characterized in that the consumer (Rvi), at least one or more elements of the following selection ^¬ re comprising: ohmic resistance; Current sink; Constant current diode.

3. Circuit arrangement according to one of claims 1 or 2, characterized in that the consumer comprises at least a first (R _V i) and a second partial load (R _V2 ), wherein se ^¬ riell to at least the second partial consumer (R _V2 ) an electronic switch (Sl) is arranged.

4. A circuit arrangement according to claim 3, characterized in that the circuit arrangement further comprises a microcontroller (16) which is designed, the forward voltage of a coupled between the first and the second terminal for the at least one LED min ^¬ least one LED to determine and the / the electronic switch (Sl) to control accordingly.

5. Circuit arrangement according to one of the preceding claims, characterized a feedback network (R1, C1, T1, Rshunt2 R2) is coupled between the output and the minus input of the operational amplifier.

6. A method for operating at least one LED on a circuit arrangement with an operational amplifier (12) having a plus and a minus input and an output; a set point setting device (10) coupled to the plus input of the operational amplifier (12); a first and a second check circuit for the at least one LED, wherein the first terminal is coupled to a connection for a versor ^¬ supply DC voltage; a transistor (T1) coupled in series with the first and second terminals for the at least one LED, the transistor (T1) having a control electrode, a reference electrode and a working electrode, the control electrode of the transistor (T1) being connected to the output of the operational amplifier (T1). 12), wherein the working electrode of the transistor (Tl) is coupled to the terminal for a DC supply voltage (V ₊ ); and a serially coupled between the reference ^¬ electrode of the transistor (Tl) and a reference potential coupled current measuring resistor (Rshunt), the falling of the current sensing resistor voltage at the minus input of the operational amplifier (12) is coupled Ü over; characterized by the following step:

Coupling at least one consumer (Rvi) in parallel with the first and second terminals for the at least one LED.

7. The method according to claim 6, characterized in that the step of Koppeins takes place such that thereby during operation of the circuit constantly a positive current flows through the at least one LED.