DE102018101796A1 - Driver circuit for powering one or more LEDs - Google Patents

Abstract

The invention relates to a driver circuit for the power supply of one or more LEDs, comprising a controllable constant current source for connecting one or more series-connected LEDs, and a current measuring resistor, which is connected in series with the one or more LEDs, wherein the driver circuit is arranged to determine, via a voltage drop of the current sense resistor, the current through the one or more LEDs to control the current to a predetermined value, the driver circuit being further configured to switch the current sense resistor to at least two different values of the two values for measuring the current through the one or more LEDs in a higher current range and to the lower of the two values for measuring the current through the one or more LEDs in a lower current range.

Description

The invention relates to a driver circuit for powering one or more LEDs, in particular a driver circuit, which comprises a control to a current value to achieve a desired dimming of the LEDs. An LED is any semiconductor light source to understand, including organic semiconductor light sources, so-called OLEDs to count.

The driver circuit of the type mentioned above are typically realized in two different variants. On the one hand, there is a pulse width modulation control in which LED current with pulse width modulated signal on and off. On the other hand, there is an analogous method in which the current can be regulated without interruption to a desired value. However, the pulse width modulation method is not considered to be preferred for general lighting, as it may lead to stroboscopic effects, in particular interference of cameras or interference of barcode scanners. In addition, health effects are feared by the stroboscopic effect.

Thus, the analog current control is preferred. A typical prior art circuit is in 1 shown. An LED load, which may include one or more LEDs, is controlled by an integrated circuit. The integrated circuit has an input CS at which a measuring resistor R _CS connected to earth. Through the measuring resistor R _CS when switched on, the same current flows through the LED load. The voltage drop over R _CS is measured in the integrated circuit and used to control a desired amperage for the LED load in the integrated circuit.

A disadvantage of this type of analog current control, however, is that at low current values, the voltage drop across the measuring resistor can be very small. By currents inevitably occurring in the circuit, e.g. resulting from switching transistors, therefore, at low current values, the evaluation of the voltage drop across the current measuring resistor for the control of the current is unusable. Thus, in such an analog current control results in a minimum lower value up to which the current control can be done reliably. One way to get to even smaller current values would be to increase the value of the current sense resistor. However, this has the consequence that the power loss at the resistor at high currents leads to large losses and even thermal problems can occur.

A regulation to low current values was therefore possible in the prior art only by the control according to the pulse width modulation method. Combined methods have also been used by applying, in a lower current range, the pulse width modulation method and a higher current range analog current control as previously described to provide a larger dimming range for LEDs. However, this again has the previously described disadvantage of the stroboscopic effect at low current values.

The object of the present invention is therefore to provide a driver circuit for the power supply of LEDs, which covers the widest possible current range for achieving a large dimming range and avoids the disadvantages of pulse width modulation.

The object is achieved by a driver circuit for the power supply of one or more LEDs according to claim 1.

A special feature of the driver circuit of the present invention is that the current measuring resistor is switched to at least two different values, the lower of the two values for measuring the current through the LEDs in a lower current range and the higher of the two values for measuring the current through the LEDs is applied in a higher current range. As a result, the current measuring resistor is adapted to the particular current range to be measured, so that the voltage drop across the current measuring resistor assumes an acceptable value, which can be easily processed by the driver circuit. This makes it possible to measure even small currents. Furthermore, the power converted at the current measuring resistor is limited because a lower measuring resistor is selected in the higher current range. This limits the power loss and the resulting thermal problems.

According to a preferred embodiment, the measuring resistor is formed by at least two resistors connected in parallel, of which at least one of the two resistors can be electronically connected or disconnected. For example, the two resistors, which together form the current measuring resistor, connected in parallel to each other in the circuit of the LEDs to earth, wherein in one of the two parallel branches, an electronic switch is provided which can be opened or closed. As a result, the current measuring resistor is formed either only by one of the two resistors (when the switch is open) or by both resistors connected in parallel (when the switch is closed).

As a result, two different values for the current measuring resistor can be realized. It can also be provided in parallel more than two resistors. Furthermore, more than just an electronic switch can be provided. It can also be connected in series with each of the resistors connected in parallel with an electronic switch, in which case at least one of the switches is closed during operation of the LED.

According to a preferred embodiment, the electronic connection and disconnection of the at least one resistor by driving a gate of a field effect transistor, FET whose source and drain is connected in series with the resistor. A FET has a very low resistance in the closed state between source and drain, so that the resistance of the field effect transistor itself is negligible. In particular, the internal resistance of the FET is largely thermally stable, so that measurement errors are minimized by a changing internal resistance of the FED.

According to a preferred embodiment, the FET can be controlled with a microcontroller. In this embodiment, the microcontroller only has to generate a voltage which is to be applied to the gate in order to open or close the FET between source and drain. Compared to an electronic switch, which is formed by a relay, such a circuit is easier to set up and can also be fully realized as an integrated circuit.

According to a preferred embodiment, the maximum of the lower current range corresponds to the minimum of the higher of the two current ranges. In this circuit, when the current to be measured reaches the limit of the two current ranges, the drive circuit switches the current sense resistor to decrease or increase the voltage drop across the current sense resistor, respectively.

However, according to an alternative embodiment, the two current ranges may also partially overlap. This prevents frequent switching operations between the current sense resistors if the current to be measured by the LEDs happens to oscillate exactly in the area between the two current ranges. For example, when the maximum of the lower current range is reached, the measuring resistor can be switched over to the lower value and, conversely, switched to the higher value only when the minimum of the higher current range has been reached. This type of hysteresis prevents frequent switching operations from occurring at low current fluctuations around the border between the current ranges, which could manifest as LED flickering.

According to a preferred embodiment, the current measuring resistor may assume at least two values, the lower value between 0.1 ohms and 0.5 ohms and the higher of the two values between 0.9 ohms and 1.7 ohms. With a supply current that is typically between 20 mA and 1 A, and with a current measuring resistor that is switched at about 200 mA, the power loss can be limited to a maximum of 0.5 W.

In a preferred embodiment, the lower current range may include 20 mA to 200 mA and the higher current range may comprise 200 mA to 1 A. It can also be provided that the current ranges also overlap, as previously described, so that e.g. the lower current range comprises a maximum value of 250 mA, while the higher current range comprises a minimum value of 150 mA.

According to a preferred embodiment, the current measuring resistor can also be more than just two values, e.g. three or four values, for three or four current ranges.

• 1 zeigt eine LED-Treiberschaltung gemäß dem Stand der Technik.
• 2 zeigt eine LED-Treiberschaltung gemäß einer Ausführungsform der vorliegenden Erfindung.

Further advantages and features of the present invention will become apparent from the following description of a preferred embodiment given in conjunction with the figures. The figures show the following:

• 1 shows a LED drive circuit according to the prior art.
• 2 shows an LED driving circuit according to an embodiment of the present invention.

An embodiment of a driving circuit for LEDs according to the present invention is shown in FIG 2 shown. Similar to a driver circuit of the prior art, which in 1 is shown, becomes an LED load 3 , which may be formed by one or more LEDs (only one LED shown in the figures) via a supply voltage provided by an integrated circuit 2 is provided, driven. The integrated circuit 2 provides a constant current supply through the LED load 3 , A constant current can be adjusted (not shown in the figure) to produce a desired dimming of the LED load. Furthermore, the LED load can be switched on and off by the integrated circuit. This is done at an output GATE of the integrated circuit 2 a field effect transistor 4 connected to the LED load 3 connects with earth when closed. In this circuit is a resistor R _CS intended. Through this resistance R _CS Therefore, the same current flows, which is also due to the LED load 3 flows. The voltage drop across the resistor R _CS is connected to an input CS of the integrated circuit 2 measured and used to control the constant current through the LED. As far as the circuit according to the present invention is also equal to the in 1 illustrated circuit according to the prior art.

In the circuit according to the invention according to 2 is also a second resistor R _CS 2 provided, which is parallel to the first resistor R _CS is connected and via a semiconductor switch, such as a field effect transistor 22 also connected to ground. When the field effect transistor 22 is turned on, ie there is a conductive connection between the drain and source, the resistance is R _CS 2 parallel to the resistor R _CS , so the current sense resistor for the integrated circuit 2 in total by the parallel connection of Rcs and R _CS 2 given is. The gate of the field effect transistor 22 is located at an output of a microcontroller 24 and is controlled by this. Furthermore, the microcontroller provides 24 a control voltage provided on an input LD of the integrated circuit 2 acts to adjust the supply current through the LED. The microcontroller 24 In this embodiment also serves to adjust the current for the desired dimming of the LED load. The microcontroller 24 generates a control voltage corresponding to the desired dimming, which is applied to the input LD of the integrated circuit 2 is present, the over the range switching from the microcontroller 24 selected current measuring resistor is taken into account.

In operation, the integrated circuit 2 the current set by the LED load. For dimming the LED, any current can be set between 20 mA and 1 A, for example.

In a current range of 200 mA to 1 A, the microcontroller applies a voltage to the gate of the semiconductor switch passing through the FET 22 is given, so that a connection is switched through from drain to source. In this circuit state are the resistors R _CS and R _CS 2 connected to earth parallel to each other. The parallel connection of Res and R _CS 2 acts in this circuit state as a measuring resistor for the current control of the LED load. The integrated circuit 2 Measures the voltage drop across the parallel connection of the resistors R _CS and R _CS 2 to regulate the current through the LEDs to a constant value.

If the current for dimming the LEDs is to be regulated to a lower value, for example to a current in a range between 20 mA and 200 mA, is via the microcontroller 24 the field effect transistor 22 controlled to open the connection between drain and source. In this case, the supply current flows through the LED load 3 only through the resistance R _CS , so that in this circuit state of the measuring resistor for determining the current only through the resistor R _CS given is.

Thus, via the microcontroller 24 the current measuring resistor switched between two values according to a predetermined current range. In the lower current range, the measuring resistor is only through R _CS given, while for the higher current range of the measuring resistor by the parallel circuit of R _CS and R _CS 2 given is correspondingly lower. For example, the resistance R _CS and R _CS 2 are each 1.2 ohms, so that in the higher current range of 200 mA to 1 uA effectively only a current measuring resistor of 0.6 ohms is given. As a result, the power converted at the measuring resistor power is reduced at higher currents. However, since the current is in the higher measuring range, the voltage drop is still sufficiently high to allow a precise current measurement. In the lower current measuring range of the measuring resistance is increased accordingly, for example to 1.2 ohms, so that even at the lower to be measured supply currents still a sufficiently high voltage drop is given for the measuring resistor.

About the microcontroller 24 Furthermore, a control voltage is output to the integrated circuit 12 acts to match the current measurement to balance the selected measuring resistor by the corresponding factor.

The present invention is not limited to the illustrated circuit of two resistors R _CS and R _CS 2 limited. It is also possible to connect more than two resistors in parallel, and it is also possible to provide more than just two current ranges for measuring the current.

The remaining circuit for LED power supply corresponds to the embodiment in the prior art. As a result, the inventive range switching for the measuring resistor can also be realized simply with existing integrated circuits known from the prior art for an LED power supply.

LIST OF REFERENCE NUMBERS

2

integrated circuit

3

LED load

4

Semiconductor switches, e.g. Field Effect Transistor

22

Semiconductor switches, e.g. Field Effect Transistor

24

microcontrollers

R _CS , R _CS 2

Current sense resistor

Claims (10)

A driver circuit for powering one or more LEDs (3), comprising a controllable constant current source for connecting one or more serially connected LEDs (3), and a current sense resistor (R _CS , R _CS 2) connected in series with the one or more LEDs is connected, wherein the driver circuit is adapted to determine the current through the one or more LEDs (2) via a voltage drop of the current measurement resistor (R _CS, R _CS 2) in order to regulate the current to a predetermined value, characterized characterized in that the driver circuit is further adapted to switch the current sense resistor (R _CS , R _CS 2) to at least two different values, the lower of the two values being used to measure the current through the one or more LEDs (3) in FIG a higher current range and to the higher of the two values for a measurement of the current through the one or more LEDs (3) in a lower current range becomes. Driver circuit after Claim 1 Wherein the switching of the current measurement resistor (R _CS, R _CS 2) is controlled by a microcontroller (24) and the microcontroller further provides a control voltage which adjusts the generation of the constant current source corresponding to the selected measurement resistor (R _CS, R _CS 2). Driver circuit after Claim 1 or 2 , wherein the measuring resistor (R _CS , R _CS 2) by at least two resistors connected in parallel (R _CS , R _CS 2) is formed, of which at least one (R _CS 2) is electronically switched on or off. Driver circuit after Claim 3 , wherein the electronic connection and disconnection of the at least one resistor (R _CS 2) by driving a gate (G) of a field effect transistor, FET, (22), whose source (S) and drain (D) in series with the Resistor is switched. Driver circuit after Claim 4 with reference to Claim 2 wherein the gate (G) of the FET (22) is driven by the microcontroller (24). Driver circuit according to one of the preceding claims, wherein a maximum of the lower current range corresponds to a minimum of the higher of the two current ranges. Driver circuit according to one of Claims 1 to 5 , where the two current ranges partially overlap. Driver circuit according to one of the preceding claims, wherein the current measuring resistor can assume at least two values, wherein the lower value is between 0.1 ohms and 0.7 ohms and the higher values between 0.9 ohms and 1.7 ohms. Driver circuit according to one of the preceding claims, wherein the lower current range comprises 20 mA to 200 mA and the higher current range comprises 200 mA to 1 A. Driver circuit according to one of the preceding claims, wherein the current measuring resistor assumes more than two values, in particular three or four values, in order to switch over between more than two current ranges, in particular three or four current ranges.

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