EP1330143B1 - Operating device for light emitting diodes - Google Patents

Abstract

The arrangement has an electronic switch for the LED operating current and a pull-down device in parallel with the switch whose resistance is dimensioned so that the ratio of the operating current with the switch closed to the operating current with the switch open is described by a reduction factor of at least 10.

Description

Technical area

The invention relates to a circuit arrangement according to the preamble of claim 1. It is in particular an electronic operating device for operating light-emitting diodes which contains a device for setting the effective value of the operating current of the light-emitting diodes, this device having a pulsed mode of operation.

State of the art

For operation of light-emitting diodes, further abbreviated to LED, the following circuit arrangement is widespread: An energy source, for example a public supply network or a battery, supplies an electronic converter which provides a supply voltage. The LEDs are usually connected to the supply voltage via two connection cables. One or more LEDs can be operated on a supply voltage. Several LEDs are usually connected in series and form a so-called strand. Several strands can be connected in parallel, with cross-connections also possible between the strands. If several LEDs are connected to a supply voltage, they are usually combined to so-called. LED modules, which may also contain current limiting resistors. Together with a voltage regulation in the electronic converter, a desired operating current for connected LEDs is realized. The circuit arrangements in question are not only for the operation of LED suitable. It is also possible to use so-called Organic Light Emitting Devices (OLED).

The document EP 1 049 360 A2 (Jusuf) describes a programmable LED driver. With the aid of a multiplying digital-to-analog converter, the brightness of one LED with respect to other LEDs can be set very precisely. This is important when the color impression of the entire arrangement is to remain constant when dimming LED arrays with differently colored LEDs.

Often there is a desire to be able to set the RMS value of the operating current of the LEDs and without intervening in the electronic converter. This allows the dimming of the connected LEDs or the possibility of connecting different types or a different number of LEDs to one and the same supply voltage. The adjustment of the rms value of the operating current of the LEDs is realized by inserting an electronic switch into a connecting line. In which connection cable the electronic switch is inserted is in principle arbitrary. In the German patent application with the registration file 10136658.2 (Scilla) said adjustment of the RMS value of the operating current of the LEDs is described. The electronic switch can be designed inexpensively as a MOSFET. By opening the switch, the operating current of the LEDs is interrupted. By periodically opening and closing the electronic switch, a pulsed operation for the operation of the LEDs is realized. The ratio of the duration of the closed state of the switch to the duration of the open state determines a duty cycle. By a suitable duty cycle, a desired RMS value for the operating current of the LEDs can be set. The maximum value of the operating current of the LEDs, which occurs when the electronic switch is switched on, is specified by the supply voltage.

The electronic switch and an associated drive circuit can be combined together with the electronic converter to form a unit in a housing. But it is also possible to form the electronic switch and the drive circuit as a separate dimming unit in a separate housing. This dimming unit, when an adjustment for the rms value of the operating current of the LEDs is desired, can be switched between the electronic converter and the LEDs.

The pulse rate at which the electronic switch is opened and closed is usually above a frequency at which the human eye is single Light pulses can distinguish from each other. In practice, frequencies from 100 Hz up to several kilohertz are used. The connecting cable, in which no electronic switch is inserted, forms one pole of an output voltage. The other pole of the output voltage forms the pole of the electronic switch facing away from the supply voltage. The output voltage is fed to the LEDs or LED modules. It has a rectangular shape whose fundamental frequency is equal to the above pulse frequency. Especially with long connection lines, this can lead to electromagnetic incompatibility or radio interference. In order to prevent this and to comply with relevant regulations (eg CISPR 15), it often takes a lot of effort in the form of filters and shields.

Presentation of the invention

It is an object of the present invention to provide a circuit arrangement according to the preamble of claim 1, which accomplishes a pulsed operation of the LEDs, but compared to the prior art generates lower electromagnetic incompatibilities or radio interference.

This object is achieved by a circuit arrangement with the features of the preamble of claim 1 by the features of the characterizing part of claim 1. Particularly advantageous embodiments can be found in the dependent claims.

How strong the electromagnetic interference generated by a circuit in question, depends primarily on the amplitude of the alternating component of the above-mentioned output voltage. The output voltage is composed of a DC component and an AC component. The amplitude of the alternating component of the output voltage is hereinafter referred to briefly as the amplitude of the output voltage. The maximum value of the output voltage adjusts itself when the electronic switch is closed and is specified by the value of the supply voltage. By definition, the amplitude of the output voltage is half the difference between the maximum value and the minimum value of the output voltage. Given Maximum value thus determines the minimum value, the amplitude of the output voltage and thus the strength of the electromagnetic interference. The smaller the minimum value of the output voltage, the stronger the electromagnetic interference.

In the prior art, the minimum value of the output voltage is determined essentially by two quantities: a load impedance with the electronic switch switched off and an off-impedance of the electronic switch. The load impedance when the electronic switch is off is made up of the impedance of the leads and the impedance of the LEDs at very low operating current. The off-impedance of the electronic switch is the impedance of the electronic switch in the open state to understand. Lead impedances to the electronic switch can be considered in the off-impedance, but generally do not matter. Both variables, the load impedance with the electronic switch off and the off-impedance are subject to strong variations due, e.g. by specimen scattering, temperature, aging and selection of the LEDs so that the value of the minimum value is not accurately determined. In general, however, the amount of off-impedance exceeds the amount of load impedance by orders of magnitude. It can also be assumed that the real part of the relevant impedances essentially determines the properties of these impedances. As a result, the output voltage drops to the value 0 when the electronic switch is open. It thus follows that in the prior art, the amplitude of the output voltage is substantially equal to half the supply voltage.

According to the invention, a pull-down device is connected in parallel with the electronic switch. The value of the resistance of the pull-down device should be chosen so that in the parallel circuit of pull-down device and off-impedance the off-impedance can be neglected, so that variations of the off-impedance have no influence on the output voltage. By means of the pull-down device according to the invention, the minimum value of the output voltage can be set. The minimum value will differ significantly from 0 according to the invention and thus be substantially larger than in the prior art. This will be compared to the Prior art reduces the amplitude of the output voltage, resulting in a reduction of the electromagnetic interference according to the invention.

The value of the resistance of the pull-down device according to the invention may not be so small that it is in the order of magnitude of the resistance of the electronic switch in the closed state. In this case, the effect of the electronic switch and thus the effect of the pulsed operation would be limited. By opening the electronic switch, the operating current of the LEDs is lowered. By the ratio of the operating current with the electronic switch closed to the operating current with the electronic switch open, a lowering factor is described. A limitation of the pulsed operation according to the invention is not given if the value of the Absenkfaktors is at least 10. If the Absenkfaktor greater than 1000 so no effective inventive reduction of electromagnetic incompatibilities is given more.

The invention most cost-effective implementation of the pull-down device is a pull-down resistor. However, if the value of the Absenkfaktors remain constant, even if operating parameters such as load impedance or supply voltage change, the pulldown device according to the invention must be realized by an adjustable pull-down resistor. This can take the form of one or more semiconductors, eg FETs, which are operated as voltage-controlled resistors. Also, a non-continuously variable resistor, for. B. in the form of a switchable resistor cascade, is conceivable. The adjustable pull-down resistor is controlled by a controller. This detects a current lowering factor and adjusts the adjustable pull-down resistance so that a preset lowering factor is maintained. To detect the current descent factor, the regulator is supplied with the operating current of the LED or the current through the electronic switch and the adjustable pull-down resistor. Since the Absenkfaktor is defined by the operating current, the sole acquisition of the operating current is sufficient to determine the Absenkfaktor. Since the operating current is essentially the sum of the current through the electronic switch and the current through the adjustable pull-down resistor, it also satisfies these two currents to detect to determine the Absenkfaktor. As an approximation, with the electronic switch closed, the current through the electronic switch can be set equal to the operating current. In order to be able to make a statement about the lowering factor, the amplitude of the output voltage can also be evaluated. In the event that the supply voltage is constant, it is sufficient to evaluate the voltage across the electronic switch.

By means of the pull-down device according to the invention, the above-described variations of the amplitude of the output voltage, which are caused by the off-impedance of the electronic switch, are eliminated. What remains are variations caused by the load impedance. These can be eliminated according to the invention by a pull-up device which is connected to the output voltage and is thus connected in parallel to the LEDs. The pulldown dimensioning rules outlined above also apply when the pullup facility is present.

According to the invention, the pull-up device is realized inexpensively by a pull-up resistor. The value of the pull-up resistor must be between the values given for the amount of load impedance with the electronic switch open and closed. When the electronic switch is closed, the operating current through the LEDs is high and thus the load impedance is low. Advantageously, the value of the pull-up resistor is chosen to be greater than this low load impedance, so that the electronic converter provides substantially energy to the LEDs and not to the pull-up resistor. When the electronic switch is open, the operating current through the LEDs is low and, due to the non-linear characteristics of the LEDs, the load impedance is high. The value of the pull-up resistor must be lower than this high load impedance, so that according to the invention the variations of the high load impedance do not affect the amplitude of the output voltage.

Both pull-up and pull-down resistors do not necessarily have to be implemented by a single resistor. Of course, the realization by parallel or series connection of multiple resistors is possible. Parallel to the resistors z. B. be switched to reduce the slope of the output voltage and reactances.

Brief description of the drawings

Figur 1

ein Blockschaltbild einer erfindungsgemäßen Schaltungsanordnung zum Betrieb von LEDs,

Figur 2

den zeitlichen Verlauf der Ausgangsspannung einer Schaltungsanordnung zum Betrieb von LEDs nach dem Stand der Technik,

Figur 3

den zeitlichen Verlauf der Ausgangsspannung einer erfindungsgemäßen Schaltungsanordnung zum Betrieb von LEDs,

Figur 4

ein Blockschaltbild einer erfindungsgemäßen Schaltungsanordnung zum Betrieb von LEDs mit einstellbarem geregeltem Pulldown-Widerstand.

In the following the invention will be explained in more detail with reference to an embodiment with reference to drawings. Show it:

FIG. 1

a block diagram of a circuit arrangement according to the invention for the operation of LEDs,

FIG. 2

the timing of the output voltage of a circuit arrangement for operating LEDs according to the prior art,

FIG. 3

the time profile of the output voltage of a circuit arrangement according to the invention for the operation of LEDs,

FIG. 4

a block diagram of a circuit arrangement according to the invention for the operation of LEDs with adjustable regulated pull-down resistor.

In the following, resistors are denoted by the letter R, switches by the letter S, voltages by the letter V, and joints by the letter J each followed by a number.

Preferred embodiment of the invention

FIG. 1 shows a block diagram of a circuit arrangement according to the invention for operating LEDs. An electronic converter 1 supplies at the junctions J1, J2 a supply voltage V1 to a dimming module 2. The dimming module 2 supplies at the junctions J3, J4 an output voltage V2 to the LEDs 3. A piece of a first connecting line from the electronic converter 1 to the LEDs 3rd is formed by a connection from the junction J1 to the junction J2 in the dimming module 2. A piece of a second connecting cable from the electronic converter 1 to the LEDs 3 leads from the junction J2 2 via an electronic switch S1 in the dimming module to the junction J4. A drive circuit 4 controls the electronic switch S1 so that the LEDs 3 are operated in a pulsed manner. According to the invention, a pull-down resistor R2 is connected in parallel to the electronic switch S1. The function and the sizing rules for R2 can be found in the section for illustrating the invention. This applies equally to a pull-up resistor according to the invention, which is connected in the dimming module 2 between J3 and J4.

FIG. 2 shows the profile of the output voltage V2 over the time t of a circuit arrangement for operating LEDs according to the prior art (without R1 and R2). The course is essentially rectangular. The maximum value of V2 is always assumed when the electronic switch S1 is closed, the minimum value of V2 is always assumed when S1 is open. The maximum value of V2 essentially corresponds to the supply voltage V1 that the electronic converter 1 supplies. The minimum value of V2 is approximately 0. This results in the amplitude of V2 according to the prior art, essentially the value $$\frac{V}{}$$$$\frac{1}{2}$$

FIG. 3 shows the profile of the output voltage V2 over the time t of a circuit arrangement according to the invention for operating LEDs (with R1 and R2). As in Figure 2, the course is substantially rectangular. The maximum value of V2 is always assumed when the electronic switch S1 is closed, the minimum value of V2 is always assumed when S1 is open. The maximum value of V2 essentially corresponds to the supply voltage V1 that the electronic converter 1 supplies. The minimum value is V3, whereby if the above-mentioned remarks on the dimensioning of R1 and R2 are observed, the following essentially holds: $$\mathrm{<figure-callout\; id="3"\; label="V"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">V</figure-callout>}3=V1\frac{R1}{\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">R</figure-callout>}1+\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">R</figure-callout>}2}$$

This results in the amplitude of V2 is essentially an inventive value of $$\frac{\mathrm{<figure-callout\; id="1"\; label="V"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">V</figure-callout>}1}{2}\frac{R2}{\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">R</figure-callout>}1+\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">R</figure-callout>}2}$$

In particular, in the case that R1 is substantially greater than R2, according to the invention results in a strong reduction in the amplitude of V2 over the prior art.

FIG. 4 shows a block diagram of a circuit arrangement according to the invention for operating LEDs with adjustable regulated pull-down resistance. Compared with FIG. 1, in FIG. 4 the pull-down resistor R 2 is designed as an adjustable resistor, which is controlled by a control device 5. The control device 5 adjusts the pull-down resistor R2 so that a predetermined preset value stored in the control device is maintained. By a measuring device 6, the control device 5 is supplied with a measured value for the operating current of the LEDs. The amplitude of the alternating component of the operating current of the LEDs is a measure of the current Absenkwert. The current Absenkwert can thus be determined in the control device 5.

Claims (8)

1. Circuit arrangement for operating light emitting diodes (3) that have an operating current which flows substantially through an electronic switch (S1), characterized in that a pull-down device (R2) is connected in parallel with the electronic switch (S1), the resistance of said pull-down device having a value designed so that the ratio of the operating current with electronic switch (S1) closed to the operating current with electronic switch (S1) open is specified by a pull-down factor that assumes at least the value 10.
2. Circuit arrangement for operating light emitting diodes (3) according to Claim 1, characterized in that a pull-up device (R1) is connected in parallel with the light emitting diodes (3).
3. Circuit arrangement for operating light emitting diodes (3) according to Claim 1, characterized in that the pull-down device (R2) is implemented by a pull-down resistor (R2).
4. Circuit arrangement for operating light emitting diodes (3) according to Claim 1, characterized in that the pull-down device (R2) is implemented by an adjustable pull-down resistance (R2), where a control device (5) adjusts the value of the pull-down resistance so that a preset pull-down factor is maintained.
5. Circuit arrangement for operating light emitting diodes (3) according to Claim 4, characterized in that the adjustable pull-down resistance (R2) is implemented by at least one semiconductor element.
6. Circuit arrangement for operating light emitting diodes (3) according to Claim 4, characterized in that the adjustable pull-down resistance (R2) is implemented by a plurality of switchable resistors.
7. Circuit arrangement for operating light emitting diodes (3) according to Claim 2, characterized in that the pull-up device (R1) is implemented by a pull-up resistor (R1).
8. Circuit arrangement for operating light emitting diodes (3) according to Claim 1, characterized in that the electronic switch (S1) is implemented by a MOSFET.

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