WO2015166080A1

WO2015166080A1 - Capacitance-free led driver

Info

Publication number: WO2015166080A1
Application number: PCT/EP2015/059587
Authority: WO
Inventors: Stefan Dietrich; Sebastain STRACHE; Ralf Wunderlich; Stefan Heinen
Original assignee: Rwth Aachen
Priority date: 2014-05-02
Filing date: 2015-04-30
Publication date: 2015-11-05
Also published as: DE102014208305A1

Abstract

The invention relates to a capacitance-free LED driver. The LED driver has one or more outputs (A₁,... A_n) for driving a respective LED or multiple LEDs (LED_n,1... LED_n,m), a magnetic local energy store (L), a selection device (MUX, S₁, S₂,... S_n) for selecting the one or one of the multiple outputs (A₁,... A_n) in order to transmit energy from the local electric energy store (L) through the selected output (A₁,... A_n), and a switching unit (S) for selectively charging the magnetic local energy store (L), wherein the selection device does not select an output (A₁,... A_n) for transmitting energy when the switching unit (S) is providing a charging process for the magnetic local energy store (L).

Description

Capacitance-free LED driver

The invention relates to a capacitance-free LED driver.

Background of the invention

Conventional products of LED room lighting technology are based on a single-color application with defined color temperatures and luminous fluxes. Common color temperatures are defined in the range of over 5000K for cold white, 3500-5000K for neutral white, or below 3500K for warm white lighting.

The most general way to control light-emitting diodes is shown schematically in FIG.

In this case, starting from an arbitrary input voltage V on, the output voltage across the LED series circuit V out is regulated by a voltage converter, stored on a capacitor C and regulated by means of a freely selectable regulation device with voltage drop V ctrl.

The LEDs are powered by pulsed current in common applications. To provide and control any pulse current, a switch in series with the LED (s) is needed to close or break the circuit.

FIG. 1 shows a general solution for controlling an LED string, wherein the block named as a voltage converter is switched as an application-specific solution by means of AC-DC, linear regulators (LDO), switched DC-DC converters (DC-DC) Buck, switched DC-DC converters (Boost) DC-DC converter buck-boost, switched AC-DC converter (AC-DC) or a related converter can be realized. In this case, a series connection of light emitting diodes with the transistor S _{1 is} regulated in a pulse width modulated manner in this LED string by way of example. The LED string, formed by the series connection he l ... m LEDs, is therefore supplied with a predetermined supply voltage and a predetermined supply current. The light dimming is carried out by means of pulse width modulation (PWM) through the transistor Si in order to obtain the desired LED current l _LED .

While the forward voltage V _{F of} each light emitting diode results directly from its current-voltage characteristic (s), V is a variable quantity that has to be minimized for efficiency reasons. This minimization occurs on the one hand either by intelligent circuit design, or on the other hand by increasing the number m light-emitting diodes, so preferably V «V _F.

The illustrated generalization of the driving of light emitting diodes ensures at all times both the voltage across the LEDs V F and current through the LEDs. That at any time the voltage across the LEDs and current through the LEDs is defined.

Previous lighting systems have the advantage over LED systems that they are dimmable. To provide a serious alternative to these previous lighting systems, modern LED drivers are expected to provide increased functionality, full spectrum light and high dimming range.

For LED lighting with higher individuality and wide light spectrum, two different approaches are currently favored. The topology of FIG. 1 is thereby further developed into a multi-channel LED driver in the simplest case. A resulting schematic circuit of a multi-channel LED driver is shown by way of example in FIG.

A number of different colors, each represented by its individual LED string, are connected in parallel. Each LED strand l ... n has any number m of light-emitting diodes. The LED strings are connected to the simple output of the voltage converter together with a capacity and a regulation. The output voltage V out, which is stored on the capacitor C, is regulated according to the LED string with the highest voltage requirement, so that each additional LED strand with less supply voltage has sufficient voltage swing in order to be able to work meaningfully. Although this implementation is particularly simple, it has serious disadvantages.

The forward voltage V _{F of} the m LEDs correlates with the color spectrum of the emitted light, which is bound directly to the band gap of the material used. Blue LEDs emit light at a wavelength of about 400 nm to 450 nm and typically have a forward voltage in the region of 3.5 V. In comparison, the forward voltage of red LEDs emitting light with a wavelength of about 650 nm is about 2.6 V. The control of the output voltage of the multicolor multichannel LED driver shown is therefore typically based on the LED strand with the smallest wavelength of light, if the number <m ₁ = ... = m _{n of} all LEDs in the different LED strands is identical ,

With the same number of serially connected light emitting diodes, the voltage drop across the control transistor in each leg is the difference between high bandgap LEDs and low bandgap LEDs. Even with a small number of m series-connected LEDs, the difference in the forward voltage for blue and red LEDs, for example, is already essential, so that at high LED current can lead to thermal destruction of the switch. Overall, it should be noted that the requirements for energy efficiency with this circuit arrangement are difficult, if any, to achieve.

Therefore, a different LED driver solution is necessary if the number of serial LEDs is high and thus the voltage difference V _{F is} high, strongly different LED string configurations, ie different numbers of LEDs per LED string, or comparatively high LED current are used.

FIG. 3 shows a further schematic circuit of a multi-channel LED driver, which can be regarded as a development of the schematic circuit of FIG. 2, which avoids some of the disadvantages discussed.

In the circuit shown in Figure 3, each LED string is powered by an individual output of the voltage converter. The capacity Ci ... "stores the regulated operating voltage V _{out> 1 n of} the LED string, which results from the voltages V _LED and V _ctr i for the _forward voltage, respectively the control voltage. The switch Si. n controls the pulse current I LED and the respective block Ctrl represents the regulation. Individual and different LED strand applications, different LED currents and supply voltages can thereby be provided. Due to the complete isolation of the individual LED outputs, the control of the colors is independent of each other. Each color can therefore be controlled in an amplitude modulated and / or pulse width modulated manner as anticipated by the color setting of the system. Furthermore, in this circuit, the potential energy efficiency compared to the previously discussed circuit of Figure 2 is greatly increased, since in this case, the control _voltage V _ctri each LED strand can be minimized.

However, the circuit of Figure 3 is also disadvantageous, since now the solution of the problems shown in Figure 2 with a greatly increased system complexity and the resulting form factor is paid.

Since the voltage of each LED string is stored in an appropriate capacity, the number of external components used increases linearly with the number of voltage transformer outputs. In addition, the number of switching transistors increases linearly and the control of the converter is considerably complicated, since each output is controlled individually. In addition to the greatly expanded use of components, which is driving costs, the space requirement also increases.

It should not go unmentioned another problem with multi-channel LED drivers. Here, the minimum color occurring time is problematic, since it can cause flicker effects here, as soon as the pulses of the individual colors fall within the perceptible range.

Based on this situation, it is an object of the invention to provide a capacitance-free LED driver, which makes it possible to provide high energy efficiency with low circuit complexity.

The object is achieved by a device according to claim 1. Further advantageous embodiments are in particular subject of the dependent claims. The invention will be explained in more detail with reference to the figures. In these shows: 1 is a schematic circuit of a single-channel LED driver,

2 is a schematic circuit of an honor channel LED driver,

3 shows a further schematized circuit of a multi-channel LED driver,

4 shows a schematized circuit of a first embodiment of a single-channel LED driver according to the invention,

5 shows a schematized circuit of a second embodiment of a single-channel LED driver according to the invention,

6 shows a schematized circuit of a first embodiment of a multi-channel LED driver according to the invention, 10

7 shows a schematized circuit of a second embodiment of a multi-channel LED driver according to the invention,

8 shows a schematized circuit of a third embodiment of a multi-channel LED driver according to the invention,

Fig. 9 is a schematic circuit of a fourth embodiment of a multi-channel LED driver according to the invention, and

10 is a conceptual representation of a color sequence.

FIG. 4 shows a schematic circuit of a capacitance-free LED driver according to a first embodiment of the invention.

The capacitance-free LED driver has an output A ₁ for driving an LED or a plurality of LEDs LED _nl ... LED _{nm of} a LED string. The capacitance-free driver further comprises a magnetic local energy store L and a selector S ₁ for selectively selecting from the one output Α to drive energy from the local magnetic energy store L through the selected output Ai.

The capacitance-free LED driver further comprises a switching unit S for selectively charging the magnetic local energy store L, wherein in times in which the switching unit S provides a charge of the magnetic local energy store L, the selector does not select the output Ai for driving. That is, the present invention solves the listed complexity problems of the conventional multi-channel LED driver with the same number of passive components of the single-channel LED driver and maximum system efficiency by a minimum number of switches. A circuit similar thereto, which differs only in the type of the magnetic memory is shown in Figure 5. While in FIG. 4 the magnetic memory L is embodied as a coil, in FIG. 5 the magnetic memory is, for example, a secondary winding of a transformer so that, for example, a suitable adaptation of the primary voltage to the conditions of the LED strands can be provided (k: l). As a result, the adaptability of the circuit is significantly improved, whereby efficiency gains can be achieved.

That is, the circuit consists of a parallel connection of a (AC) voltage source, a magnetic memory L and l ... m serially connected LEDs. The parallel circuit is separated and controlled by two (ideal) switches S and Si. The two switches work in phase opposition. In other words, with the switch S closed and the switch S 1 open, the magnetic memory L is charged by the positive input voltage V in at the node V _sw and the current I _T rises.

If the switch S is opened and Si is closed, the LEDs are polarized in the forward direction and a compensation current equivalent to k * l _T flows from the reference potential in Figure 5 or in Figure 4 via the LEDs to V _sw . This current flow across the LEDs results in emitted light.

Due to the defined relationship between current and voltage in the LEDs defines the number l ... m and the band gap of the LEDs in this switch position, the maximum voltage at point V _sw - in Figure 5 and V _sw in Figure 4. With the defined voltage at point V _sw - in Figure 5 and V _sw in Figure 4, the voltage drop across the magnetic memory L so that the slope of the discharge is known. As long as energy is present in the magnetic memory L, V _sw remains negative in FIG. 5 or V _sw in FIG. 4 and l _T sinks until the system otherwise shuts itself off. That is, the charged magnetic memory L is discharged by the freewheeling current by closing the switch S ₁ by the LEDs, so that when discharging light is emitted through the LEDs.

In the steady state, continuous operation is maintained by alternately adding energy to the magnetic memory L and power consumption by radiative recombination in the LEDs through the switches S and S 1. The location and type of switch within the strand are arbitrary. Since the forward voltage of the LEDs is set by the LEDs with a defined current, voltage regulation for this topology is not necessary. Any capacities for providing the output voltage can be saved.

Furthermore, the invention also allows the provision of multi-channel LED drivers.

By connecting several, different LED strands in parallel, the topology develops into a multi-color multi-channel LED driver, as shown in FIGS. 6 and 7. While in FIG. 6 the magnetic memory L is designed as a coil, in FIG. 7 the magnetic memory is for example a secondary winding of a transformer, so that e.g. a suitable adaptation of the primary voltage to the conditions of the LED strings can be provided (k: l). As a result, the adaptability of the circuit is significantly improved, whereby efficiency gains can be achieved.

Here, as before, an alternating operation of the discharge switch S _a ... S _n with S provided, so that for each discharge phase only one LED strand emits light and discharges the magnetic memory L. Alternatively, this plurality of parallel discharge switches Si... S "can also be understood as a multiplexer MUX. This in turn leads to controlled current in the respective selected LED strand and thus to controllable output spectrum.

Thus, an example of stable steady state condition is satisfied, and a suitable switching frequency averages the LED current and the discrete light spectra of the LEDs to a continuous light spectrum for the human eye, without being perceived as flicker.

The topology is ideal for using very different LED configurations by automatically adjusting the individual voltage drop. The magnetic memory L is discharged depending on the number of LEDs in a string of LEDs at different speeds, as is due to the magnetic memory L, an individual voltage drop through the individual LED strand results. During the switching phase of a particular LED string, the other LED strings are turned off, so that a clearly defined current flows at all times, reducing the control demand to a single current control loop for the discharge current. With this invention, the number of external components used is reduced to only one magnetic memory L, which dramatically reduces the cost of the circuit and lowers the form factor. In addition, the complexity of the circuit is reduced and only one current control loop is necessary because there is no voltage regulation. Automatic adjustment of the drooping current via the LEDs eliminates the need for an output voltage and keeps the losses to a minimum.

By using a transformer as a magnetic memory L even a direct connection to the AC mains is possible and yet the circuit is on the secondary side galvanically isolated from the primary circuit, so there are no security risks for the end user.

The topology shown above may optionally occur with or without a rectifier as shown in FIG. The type and realization of the rectification and rectifier is irrelevant to the function of the capacitance-free, transformer-based multi-channel LED driver. The drive circuit can be co-supplied, for example, as shown in FIG. 9, by means of a suitable voltage supply via the secondary side. The drive circuit can be supplied with power via one or more auxiliary windings on the secondary side, for example, and can therefore also be implemented in the low-voltage range. The type and realization of the voltage supply of the drive circuit and the type and realization of the drive circuit is arbitrary.

Particularly advantageous can be made possible with the invention, a flicker-free operation.

In this case, the individual LED strands are controlled in series via the selection device. As a result, a time-averaged light is generated via the combination of amplitude and / or pulse width modulation.

In the following, a color sequence is referred to as a time sequence of an averaged light spectrum of serially driven, discrete LED light spectra. For example, the different LED strands have different LED types. To avoid flicker, the minimum frequency of a color sequence is now chosen to be above the perceivable range. This range can be normative or empirically determined. As a rule, however, a range of 150 Hz or above should meet this requirement.

A common problem with switched drivers is that they tend to produce noise. To avoid this, the frequency of the total current can now be selected such that the frequency of the total current is above the threshold of hearing. Typically, with frequencies of the total current above 20 kHz, this problem can be avoided.

In order to achieve a suitable color depth and color rendering of the light, the switching frequency of the individual light packets of the discrete spectra is selected to be high in order to produce a mixture. This will be explained below with reference to FIG. In this case, the number of light packets can be spectrally dependent and can be varied by the user. That An adjustment device for color control by the user is provided. Alternatively, however, there may also be a fixed specification, e.g. LED lamps, which generate light of a certain temperature by means of various LED strings and via the control explained below.

In the following it will be assumed with reference to FIG. 10 that the LED driver has 4 LED strings, wherein a first LED string has white LEDs W, a second LED string has red LEDs R, a third LED string has green LEDs G and a fourth LED string has blue LEDs B. The selection of the LEDs is only to be understood as illustrative and not limiting.

Hereinafter, the individual LED strands are also referred to as color, i. also "white" light of an LED is treated as a color.

That The LED driver now has at least four outputs and each of these outputs is assigned a different LED string.

In order to meet the above requirements, the LED strands are now controlled so that each individual color is individually imperceptible and the overall color thus remains flicker-free. This can be done for example by a digital control of the LED driver with respect to the selection device. In this case, for example, the selection device can be understood as a parallel connection of individual switches Si, S ₂ ,... S _n . The switch positions can be mapped logically as 1 for a closed switch and logically as 0 for an open switch.

Preferably, the driver is then designed so that the period, ie a color sequence, is subdividable into more than 2 ^kl , preferably 2 ¹⁰ or more individual steps, k indicating the number of color spaces used. The higher the number of individual steps, the better the color is adjustable or the dimming range is improved.

Since, as described above with reference to FIGS. 6 to 9, only one LED strand is always active, an LED driver according to the invention can be used here in a particularly simple manner.

The frequency of the total current can also be controlled digitally by the switching unit S. This can be realized in a particularly simple manner by the drive circuit shown in FIG.

The presented invention allows each color sequence to be recreated. It is only necessary to ensure that the conditions for the flicker-free of each individual color with respect to the previous color sequence is maintained.

In order to avoid the computational effort required for this, it can therefore be provided that the color sequences for each or all possible (adjustable) color temperatures are fixed. For example, FIG. 10 indicates that light of a specific temperature can be generated by 900 light packets of the white LEDs, 73 red light packets, 29 green light packets, and 22 blue light packets. Ie a color sequence is distributed in the example on 1024 packages. That is, the color space is divided into 10 bits. In principle, only 2 ^{(4 11} = 8 switch settings would be necessary to allow color control of exemplary 4 colors, but the color experience would then be very limited.

In order to fulfill the requirement for the Flickerfreiheit, the individual colors are distributed over the color sequence. In this example, for example, the color with the lowest frequency, ie blue, is selected. Since this color only occurs a few times, is now around the Flickerfreiheit too ensure this color is always switched only in a single package. These packages are now distributed as evenly as possible over the course of time.

This can e.g. be done by dividing the 1024 switch positions into 22 sections of almost equal length, in which the individual colors are switched in a specific order. For example, in Figure 11, the order is chosen to initially select 40 or 41 white packets, i. the corresponding switch of the LED string is switched on and off 40 or 41 times in succession, followed by exactly one blue packet, then 3 or 4 red packets are selected and then 1 or 2 green packets are selected. As far as a variable number is indicated above, this results from the fact that the respective total numbers for packages of a color are not divisible by 22.

In addition, it is possible to control certain color gradients for effect lighting and / or to provide certain randomities in successive color sequences.

This means that the intelligent serial control of different LED strings generates a time-averaged light via the combination of amplitude and pulse width modulation. This ensures that the frequency of a color sequence does not fall within the perceptible range. Insofar as alternating voltage sources are shown in the figures to explain some embodiments of the invention, the design of the switching device or of the selection device is thereby in each case encompassed such that only in a corresponding half-wave the magnetic memory is charged. In this case, the local magnetic memory L can also be charged in at least part of the corresponding first (positive) half-wave. A discharge can subsequently take place in the same (positive) half-wave and / or in the subsequent (negative) half-wave. On the other hand, the invention also includes a rectified AC voltage, as well as the case of DC sources. That is, in the same way, the energy can be obtained from a switching regulator. Furthermore, it can also be provided that the LED driver has an output which is equipped with a diode and / or a resistor (not shown in the figures). Hereby, the local energy store L controlled energy can be removed without causing one or more light emitting diodes to light up. This can be advantageous, for example, if the local energy store otherwise continues to increase in charge despite normal discharge via the LED outputs and charge in the previous way. Such a condition can be extremely short, for example Discharge times occur in which less energy can be removed from the local energy storage L than is recharged in the shortest time controllable.

Although in the figures to illustrate some embodiments of the invention AC voltage sources are shown, of course, a DC voltage source can be used, since due to the switching unit S, a current change can be caused.

Claims

claims

Capacitance-free LED driver, wherein the LED driver has one or more outputs (Ai, ... A _n ) for driving one LED or a plurality of LEDs (LED _n ... LED _nm ), comprising a magnetic local energy store (L) .

selecting means (MUX, Si, S ₂ , ... S _n ) for selectively selecting the one or more outputs (Ai, ... A _n ) to extract energy from the local magnetic

Energy storage (L) through the selected output (A _{1 (} ... A _n ) to drive,

a switching unit (S) for selectively charging the magnetic local energy storage

(L),

wherein in times _when the switching unit (S) provides charging of the magnetic local energy store (L), the _selector does not select an output (A _l7 ... A _n ) for driving.

LED driver according to claim 1, characterized in that the magnetic local energy store (L) is part of a transformer, wherein the selective charging is magnetic.

LED driver according to claim 1 or 2, characterized in that the LED driver is powered by means of a switching regulator with energy.

LED driver according to one of the preceding claims, characterized in that the switching frequency of the switching unit (S) is higher than the perception threshold for flicker, in particular higher than 160 Hz, in particular higher than 20 kHz.

LED driver according to one of the preceding claims, characterized in that a plurality of outputs (Ai, ... A _n ) are provided, wherein at least one output one or more LEDs of a first color space and a further output one LED or multiple LEDs of a second Color space are assigned, wherein the first color space and the second color space are different.

LED driver according to claim 5, characterized in that the LED driver has an adjusting device for color control.

7. LED driver according to claim 5 or 6, characterized in that the minimum average repetition rate of the selector (MUX, Si, S ₂ , ... S _n ) for selectively selecting a particular output higher than the perception threshold for flicker, in particular higher than 160 Hz, is.

8. LED driver according to one of the preceding claims 5 to 7, characterized in that the LED driver digitally drives the selection device (MUX, Si, S ₂ , ... S _n ).

9. LED driver according to one of the preceding claims 5 to 8, characterized in that the LED driver digitally controls the switching unit (S).

10. LED driver according to one of the preceding claims 5 to 9, characterized in that the LED driver digitally drives the selection device (MUX, S _{1 (} S ₂ , ... S _n ), wherein the selection device depends on the setting for Color control is controlled periodically.

11. LED driver according to claim 10, characterized in that the period is subdividable into more than 2 ^kl , preferably 2 ¹⁰ or more individual steps, where k indicates the number of color spaces used.

12. LED driver according to one of the preceding claims, characterized in that the LED driver is designed for direct operation with AC voltage, wherein the local magnetic energy store (L) is charged in at least part of a first half-wave of the AC voltage during the Energy storage is discharged in a second half cycle of the AC voltage.

13. LED driver according to one of the preceding claims, characterized in that one of the plurality of outputs (Ai, ... A _n ) is connected to a diode and / or a load resistor to controlled energy from the local energy storage (L) without causing light-emitting diodes to light up.