OPERATING CIRCUIT FOR AN LED LIGHT SOURCE

Description

TECHNICAL FIELD

[0001] The present invention relates to an operating circuit for an LED light source. BACKGROUND

[0002] To operate light-emitting diode (LED)-based lamps, hereinafter referred to as LED lamps, operating circuits are typically used which convert a country-specific mains voltage, for example an alternating voltage of 230V or 120V, into an operating signal suitable for operating the LEDs. This operating signal can be generated, for example, with a constant current level. Alternatively, the operating signal can also be generated with a constant voltage level. The operating signal can be generated in a two-stage manner: first, a DC voltage is generated using a boost converter, which is then converted into the operating signal using a buck converter. In this way, the operating signal can be generated efficiently and with a low noise level. In particular, power factor correction can be implemented using the boost converter.

[0003] However, during relatively rapid load changes, e.g. during dimming, the DC voltage generated by the boost converter can drop. This can be due in particular to the boost converter's relatively sluggish response. Such a drop in the DC voltage can, in turn, impair the operation of the buck converter to which it is fed. For example, this can lead to undesirably high current peaks in the operating signal, which can damage the LED device. Furthermore, it can lead to failure of the LED device. However, improving the response of the boost converter to avoid such problems is often hardly possible without impairing the efficiency of the boost converter and thus the entire operating circuit.

[0004] DE 10 2015 217 712 A1 describes a lighting circuit for operating a light source, e.g., an LED. The lighting circuit boosts a voltage from a battery and supplies it to the light source. An OPC circuit is provided to limit the current supplied to the light source.

[0005] DE 10 2016 214 014 A1 describes a lighting circuit for a vehicle light, in which an output voltage for supplying a light source is generated via a step-down converter. A protective circuit is provided to prevent damage to the light source in the event of load changes. The protective circuit has a differential circuit whose output signal depends on a downward slope of the output voltage. Depending on the output signal of the differential circuit, switching operations of the step-down converter can be influenced via a target current regulator.

[0006] DE 10 2012 017 397 A1 describes a power factor correction circuit for an LED lamp in which the power correction circuit can be switched between a slower and a faster control behavior.

[0007] There is therefore a need for technologies that enable reliable and efficient generation of an operating signal for an LED lamp via a buck converter. SUMMARY

[0008] According to the present invention, an operating circuit for an LED lamp according to claim 1 and a method according to claim 5 are provided. The dependent claims define further embodiments.

[0009] According to one embodiment, an operating circuit for an LED light source thus comprises an input stage for generating a DC voltage from a mains voltage supplied to the operating circuit, e.g., by means of a boost converter. The operating circuit further comprises a buck converter for converting the DC voltage into an operating signal for the LED light source. The operating circuit further comprises a controller. The controller is designed to monitor a voltage level of the DC voltage and a voltage level of the operating signal. Furthermore, the controller is designed to control a speed of an adaptation process for the operating signal depending on the monitored voltage levels. In this way, excessive load changes at the output of the input stage and the resulting drops in the DC voltage and instabilities in the buck converter can be avoided.

[0010] According to the invention, the controller is designed to monitor a difference between the voltage level of the DC voltage and the voltage level of the operating signal. In this way, it can be determined in particular whether there is a risk that the voltage level of the DC voltage will fall below the voltage level of the operating signal, which in turn could impair the operation of the buck converter. According to the invention, the controller is designed to reduce the speed of the adaptation process if the difference falls below a first threshold value. This threshold value can be used to ensure that the speed of the adaptation process is reduced before the voltage level of the DC voltage falls below the voltage level of the operating signal. Furthermore, the controller can be designed to increase the speed of the adaptation process if the difference exceeds a second threshold value.This makes it possible to return to the originally intended speed of the adjustment process when there is no longer any risk of the DC voltage level falling below the voltage level of the operating signal. The second threshold can be higher than the first threshold, thus achieving hysteresis behavior regarding changes in the speed of the adjustment process and avoiding excessively frequent changes in speeds near the threshold values.

[0011] The adjustment process occurs in conjunction with the dimming of the LED lamp. The controlled speed thus includes the dimming speed. Instabilities caused by rapid dimming processes in the generation of the operating signal for the LED lamp can thus be effectively avoided.

[0012] According to a further embodiment, a method for operating an LED light source is provided. The method can be implemented using the operating circuit presented above. The method comprises generating a DC voltage from a mains voltage, e.g., using the above-mentioned input stage of the operating circuit. Furthermore, the method comprises converting the DC voltage into an operating signal for the LED light source, e.g., using the above-mentioned step-down converter of the operating circuit. Furthermore, the method comprises monitoring a voltage level of the DC voltage and a voltage level of the operating signal, and controlling a speed of an adaptation process for the operating signal depending on the monitored voltage levels.

[0013] According to the invention, the method monitors a difference between the voltage level of the DC voltage and the voltage level of the operating signal. The speed of the adjustment process is reduced if the difference falls below a first threshold, and / or the speed of the adjustment process is increased if the difference exceeds a second threshold. The second threshold is preferably higher than the first threshold.

[0014] The features set out above and features described in more detail below may be used not only in the corresponding explicitly set out combinations, but also in further combinations or in isolation. SHORT DESCRIPTION OF THE CHARACTERS

[0015] The present invention is explained in more detail below using preferred embodiments with reference to the figures. Figure 1schematically illustrates an operating circuit according to an embodiment. Figure 2A shows example signal curves during a dimming process. Figure 2B shows exemplary signal waveforms during a dimming process with monitoring of voltage levels and speed control according to an embodiment. Figure 3 shows a flowchart illustrating a method for operating an LED light source according to an embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

[0016] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of embodiments, which are explained in more detail in connection with the drawings.

[0017] The embodiments explained below refer to operating circuits for LED lamps.

[0018] Figure 1 shows an operating circuit 100 that supplies an LED light source 200. The LED light source 200 can, for example, comprise one or more LEDs, which can be connected in series in a string, for example. However, more complex circuit arrangements with one or more light-emitting diodes are also possible.

[0019] The operating circuit 100 has inputs 111, 112 for receiving a mains voltage Vn. The mains voltage Vn is an alternating voltage, e.g., with a country-typical effective value of 230V or 120V. The operating circuit 100 also has an input stage 120. The input stage generates a direct voltage Vdc from the mains voltage Vn. In the example shown, the input stage 120 includes a boost converter that generates the direct voltage Vdc provided by the input stage 120 with a voltage level that is higher than the effective value of the mains voltage Vn. For example, a target value of the voltage level of the direct voltage Vdc can be approximately 400V. The boost converter 125 can be implemented, for example, as a boost converter. Furthermore, the input stage 120 can, for example, have a rectifier, e.g., a bridge rectifier.

[0020] The operating circuit 100 further includes a step-down converter 140, which generates an operating signal S for the LED lamp 200 from the DC voltage Vdc. The operating signal S is supplied to the LED lamp 200 via outputs 113, 114. The step-down converter 140 can be implemented, for example, as a buck converter. By combining the step-up converter 125 of the input stage 120 with the step-down converter 140, power factor correction can be implemented in the operating circuit 100, and noise components in the generated operating signal S can be reduced.

[0021] Furthermore, in the example shown, the operating circuit 100 supports a dimming function for the LED lamp. The dimming function enables adjustment of the brightness of the light generated by the LED lamp 200. This adjustment can be made via control inputs supplied to the controller 150, which are Figure 1by a dimming control signal DIM. The dimming function can be based, for example, on current control of the operating signal S, pulse width modulation of the operating signal S, or pulse frequency modulation of the operating signal S.

[0022] To implement the dimming function, the controller generates control signals which are fed to the down converter 140 (in Figure 1illustrated by dashed arrows). These control signals cause the buck converter 140 to adapt the operating signal S accordingly, wherein this adaptation process takes place essentially uniformly over a certain period of time. For example, a dimming process can include that the voltage level of the operating signal S is increased uniformly over the period of time from a first value to a second value. Likewise, a dimming process can include that the voltage level of the operating signal S is reduced uniformly over the period of time from a first value to a second value. Furthermore, a dimming process can include that a current IL generated by the operating signal S through the LED lamp is increased uniformly over the period of time from a first value to a second value.Furthermore, a dimming process can include that a current IL generated by the operating signal S through the LED lamp is reduced uniformly from a first value to a second value over the period of time.

[0023] The dimming process can be configured via parameters stored in the controller 150. In particular, a speed of the dimming process can be controlled. In the present case, it is assumed that the dimming process can take place either at a higher first speed or at a lower second speed, and that it is also possible to switch between these speeds during the dimming process. However, it is understood that more than two dimming speeds could also be provided. Corresponding parameters can be stored in the controller 150 for each of the speeds. These parameters can, in particular, include control parameters for controlling the voltage level of the operating signal S and / or the current IL generated by the operating signal S. The parameters can be configurable via a configuration interface of the operating circuit 100.

[0024] In the Figure 1In the example shown, the controller 150 further comprises a voltage monitoring function 155. The voltage monitoring function 155 monitors the voltage level of the DC voltage Vdc and the voltage level of the operating signal S. The voltage level of the DC voltage Vdc is referred to below as V1, and the voltage level of the operating signal S is referred to below as V2. In particular, the voltage monitoring function 155 forms a difference between the voltage level V1 of the DC voltage Vdc and the voltage level V2 of the operating signal S, ie, determines a voltage difference according to Δ V = V 1 − V 2 .

[0025] The voltage difference ΔV is then subjected to a threshold comparison. The speed of the dimming process is controlled based on the threshold comparison. In particular, the speed of the dimming process is reduced if the threshold comparison shows that the voltage difference ΔV falls below a first threshold T1. The speed of the dimming process is increased again if the threshold comparison shows that the voltage difference ΔV exceeds a second threshold T2. To avoid frequent speed changes due to a ping-pong effect, it is advantageous to select the second threshold T2 higher than the first threshold T1.In order to reliably prevent the voltage level V1 of the DC voltage V DC from falling below the voltage level V2 of the operating signal S, it is further advantageous to select the first threshold value T1 and the second threshold value T2 as finite values ​​other than zero.

[0026] The increase and decrease in the speed of the dimming process can be realized by initially initiating the dimming process at the aforementioned higher first speed, and the controller 150 switching to the aforementioned lower second speed when the voltage difference ΔV falls below the first threshold T1. If the voltage difference ΔV then increases again and exceeds the second threshold T2, the controller 150 switches back to the higher first speed. The speed of the dimming process can thus be dynamically adjusted during the dimming process if the threshold comparison indicates that there is a risk that the voltage level V1 of the DC voltage Vdc will fall below the voltage level of the operating signal S.By reducing the speed of the dimming process, a slower load change is achieved at the output of the input stage 120, so that a further drop in the DC voltage Vdc generated by the input stage 120 is prevented.

[0027] Figures 2A and 2B show exemplary signal curves to illustrate the control of the speed of the dimming process described above. In particular, Figure 2A Signal curves for an example in which a dimming process from 1% power of the LED lamp (corresponding to IL = 5mA) to 100% power of the LED lamp (corresponding to IL = 500mA) was carried out in a time period of Δt = 200ms without controlling the speed of the dimming process. Figure 2B shows a comparison of signal curves for a corresponding dimming process with the control of the dimming speed. Specifically, Figures 2A and 2Bthe voltage level V1 of the DC voltage Vdc, the voltage level V2 of the operating signal S, the voltage difference ΔV and the current IL as a function of time t. Furthermore, Figure 2B the position of the first threshold T1 and the second threshold T2. In the example shown, T1=50V and T2=60V were assumed. In addition, Figure 2B the course of a control signal CTRL for selecting between the first speed and the second speed. A low level of the control signal CTRL corresponds to the use of the higher first speed, and a high level of the control signal CTRL corresponds to the use of the lower second speed. The first speed corresponds to a change in the current IL of approximately 10 mA / ms. The second speed corresponds to a change in the current IL of approximately 5 mA / ms, i.e., approximately half the first speed. Figures 2A and 2BThe start and end of the dimming process are marked by vertical dashed lines.

[0028] As in Figure 2A As can be seen, initiating the dimming process causes a load change, which causes oscillations in the voltage levels V1 and V2. As a result, the voltage level V1 of the DC voltage Vdc falls below the voltage level V2 of the operating signal S at the position of the vertical arrow. This results in instability in the operation of the buck converter 140, which leads to a high peak in the current IL.

[0029] In the example of Figure 2B Switches to the lower second speed occur at time positions indicated by vertical arrows. These switches are temporary and only occur until the voltage difference rises above the second threshold T2 again. However, by temporarily reducing the speed of the dimming process, it can be avoided that, as with Figure 2A the voltage level V1 of the DC voltage Vdc falls below the voltage level V2 of the operating signal S. Accordingly, no undesirable peaks of the current IL are observed. Due to the temporary change to the lower second speed, in the example of Figure 2B The dimming process is extended by approximately 10% to Δt=220 ms. Nevertheless, the dimming process is significantly shorter than if it were performed exclusively using the lower second speed.

[0030] Figure 3 illustrates an exemplary method according to which an LED lamp can be operated using the above-described operating circuit 100. The LED lamp can, for example, correspond to the above-described LED lamp 200.

[0031] In step S10, a DC voltage is generated from a mains voltage. This may involve generating the DC voltage at a voltage level that is higher than an effective value of the mains voltage. The DC voltage may, in particular, be generated from the mains voltage using a boost converter. As described in connection with the operating circuit 100, the boost converter may be part of an input stage of a two-stage operating circuit for the LED light source.

[0032] In step S20, the DC voltage is converted into an operating signal for the LED lamp. This can be done using a buck converter. As described in connection with the operating circuit 100, the buck converter can be part of a two-stage operating circuit for the LED lamp and can be arranged downstream of an input stage of the operating circuit.

[0033] In step S30, a voltage level of the DC voltage generated in step S10 and a voltage level of the operating signal generated in step S20 are monitored. As explained in connection with the operating circuit 100, this can be done by controlling an operating circuit for the LED light source. Specifically, a difference between the voltage level of the DC voltage and the voltage level of the operating signal is monitored.

[0034] In step S40, during a dimming process, a speed of an adjustment process of the operating signal is controlled depending on the voltage level monitored in step S30. This can in particular involve a selection between two or more speeds. The speed of the dimming is thus controlled depending on the monitored voltage level, e.g., as described above, by selecting the higher first speed or lower second speed. In particular, the speed of the adjustment process is reduced if the difference between the voltage levels falls below a first threshold. Furthermore, the speed of the adjustment process can be increased if the difference between the voltage levels exceeds a second threshold. To provide hysteresis, the first voltage level is preferably lower than the second voltage level.

[0035] It is understood that a wide variety of modifications are possible in the examples presented above. For example, an operating circuit based on the concepts described above could be implemented with various types of input stages or step-down converters, without being limited to the above-mentioned examples of an input stage with a rectifier and a step-up converter and a downstream stage with a step-down converter implemented as a buck converter. Furthermore, it is understood that the two-stage controllability of the speed of the adjustment process used in the examples presented is merely an example, and that multi-stage or quasi-continuous speed control could also be used. Furthermore, it is understood that the operating circuit can be at least partially integrated into the LED light source to be supplied.

Claims

1. Operating circuit (100) for an LED lamp (200), comprising: - an input stage (120) for generating a DC voltage (Vdc) from a mains voltage (Vn) supplied to the operating circuit (100); - a step-down converter (140) for converting the DC voltage (Vdc) into an operating signal (S) for the LED lamp (200); and - a controller (150) which is designed to: - monitor a difference between the voltage level (V1) of the DC voltage (Vdc) and the voltage level (V2) of the operating signal (S), and control a speed of an adjustment process of the operating signal (S) depending on the monitored difference, wherein the adjustment process is a dimming process and the controller (150) is designed to reduce the speed of the dimming process if the difference falls below a first threshold value (T1), wherein during the adjustment process, control signals are generated which are supplied to the step-down converter (140) and the control signals cause the step-down converter (140) to adjust the operating signal (S) accordingly.

2. Operating circuit (100) according to claim 1, wherein the controller (150) is designed to increase the speed of the dimming process if the difference exceeds a second threshold value (T2).

3. Operating circuit (100) according to claim 2, wherein the second threshold value (T2) is higher than the first threshold value (T1).

4. Operating circuit (100) according to any of the preceding claims, wherein the input stage has a step-up converter (155).

5. Method for operating an LED lamp (200), wherein the method comprises: - generating a DC voltage (Vdc) from a mains voltage (Vn); - converting the DC voltage (Vdc) into an operating signal (S) for the LED lamp (200); and - monitoring a difference between the voltage level (V1) of the DC voltage (Vdc) and the voltage level (V2) of the operating signal (S); and - controlling a speed of an adjustment process of the operating signal (S) depending on the monitored difference, wherein the speed of the dimming process is reduced if the difference falls below a first threshold value (T1), and wherein during the adjustment process, control signals are generated which are supplied to the step-down converter (140) and the control signals cause the step-down converter (140) to adjust the operating signal (S) accordingly.

6. Method according to claim 5, comprising: - increasing the speed of the dimming process when the difference exceeds a second threshold value (T2).

7. Method according to claim 6, wherein the second threshold value (T2) is higher than the first threshold value (T1).