EP3518625B1 - Driver circuit for supplying power to one or more leds - Google Patents

Description

The invention relates to a driver circuit for the power supply of one or more LEDs, in particular a driver circuit that includes regulation to a current value in order to achieve a desired dimming of the LEDs. An LED is understood to mean any semiconductor light source, which also includes organic semiconductor light sources, so-called OLEDs.

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

Thus, analog current control is preferred. A typical prior art circuit is shown in figure 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 to which a measuring resistor R _CS is connected to ground. The same current that flows through the LED load flows through the measuring resistor R _CS when it is switched on. The voltage drop across _RCS is in the integrated circuit measured and used to regulate a desired current level for the LED load in the integrated circuit.

However, a disadvantage of this type of analog current control is that the voltage drop across the measuring resistor can be very small with small current values. Due to currents that inevitably arise in the circuit, e.g. from switching transistors, the evaluation of the voltage drop across the current measuring resistor for the regulation of the current becomes useless with small current values. With such an analog current regulation, there is thus a minimum lower value up to which the current regulation can take place reliably. One way to get 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 higher currents leads to large losses and even thermal problems can occur.

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

C. Wesley Tipton ET AL: "Multi-Channel, Constant-Current Power Source for Aircraft Applications", 16 January 2018 discloses a driver circuit for a solid state light source according to the preamble of claim 1.

U.S. 2015/305103 A1 , WO 2013/028632 A1 , U.S. 2015/245441 A1 , EP 2 512 207 A1 and DE 100 13 215 A1 disclose further driver circuits for driving LEDs, which have a controllable resistor in the supply circuit of the LEDs.

The data sheet "HV9961 LED Driver with Average-Current Mode Constant-Current Control" discloses a driver circuit according to figure 1 .

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 to achieve a large dimming range and thereby avoids the disadvantages of pulse width modulation.

The object is achieved by a driver circuit for powering one or more LEDs according to claim 1.

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

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 switched on or off electronically. For example, the two resistors, which together form the current measuring resistor, are connected in parallel to ground in the circuit of the LEDs, with an electronic switch being provided in one of the two parallel branches, which can be opened or closed. As a result, the current measuring resistor is formed either by just one of the two resistors (when the switch is open) or by both resistors connected in parallel (when the switch is closed).

This allows two different values for the current measuring resistor to be implemented. More than just two resistors can also be provided in parallel. Furthermore, more than just one electronic switch can also be provided. Each of the resistors connected in parallel can also be connected in series 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 at least one resistor is switched on and off electronically by driving a gate of a field effect transistor, FET, whose source and drain are connected in series with the resistor. A FET has a very low resistance between the source and drain in the closed state, so 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 the 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 that is formed by a relay, such a circuit is simpler to construct and can also be implemented entirely as an integrated circuit.

In an example not according to the invention, 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 driver circuit switches over the current measuring resistor in order to correspondingly reduce or increase the voltage drop across the current measuring resistor.

According to the invention, however, the two current ranges overlap. This prevents frequent switching operations between the current measuring resistors if the current to be measured through the LEDs happens to fluctuate exactly in the range between the two current ranges. According to the invention, the measuring resistor is switched over to the lower value when the maximum of the lower current range is reached and, conversely, only switched back to the higher value when the minimum of the higher current range is reached. This type of hysteresis prevents frequent switching operations from occurring with small current fluctuations around the boundary between the current ranges, which could result in flickering of the LED.

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

According to an embodiment not according to the present invention, the lower current range can be 20 mA to 200 mA and the higher current range can be 200 mA to 1 A. However, it is provided according to the invention that the current ranges overlap, as described above, so that, for example, the lower current range has a maximum value of 250 mA, while the higher current range has a minimum value of 150 mA.

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

Figur 1

zeigt eine LED-Treiberschaltung gemäß dem Stand der Technik.

Figur 2

zeigt eine LED-Treiberschaltung gemäß einer Ausführungsform der vorliegenden Erfindung.

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

figure 1

Figure 1 shows a prior art LED driver circuit.

figure 2

12 shows an LED driver circuit according to an embodiment of the present invention.

An embodiment of a driver circuit for LEDs according to the present invention is in figure 2 shown. Similar to a prior art driver circuit disclosed in figure 1 is shown, an LED load 3, which can be formed by one or more LEDs (only one LED is shown in the figures) is driven via a supply voltage that is provided by an integrated circuit 2. The integrated circuit 2 provides a constant current supply through the LED load 3. A constant current can be set (not shown in the figure) in order to produce a desired dimming of the LED load. Furthermore, the LED load can be switched on and off by the integrated circuit. For this purpose, a field effect transistor 4 is connected to an output GATE of the integrated circuit 2, which connects the LED load 3 to ground when it is closed. A resistor R _CS is included in this circuit. Therefore, the same current flows through this resistor R _CS as through the LED load 3 flows. The voltage drop across the resistor R _CS is measured at an input CS of the integrated circuit 2 and is used to control the constant current through the LED. So far, the circuit according to the present invention is also the same as that in FIG figure 1 shown circuit according to the prior art.

In the circuit according to the invention figure 2 A second resistor R _CS 2 is additionally provided, which is connected in parallel to the first resistor R _CS and is also connected to ground via a semiconductor switch, for example a field effect transistor 22 . If the field effect transistor 22 is switched on, ie there is a conductive connection between drain and source, the resistor R _CS 2 is in parallel with the resistor R _CS , so that the current measuring resistor for the integrated circuit 2 is given overall by the parallel connection of R _CS and R _CS 2 is. The gate of the field effect transistor 22 is connected to an output of a microcontroller 24 and is controlled by it. Furthermore, the microcontroller 24 provides a control voltage which acts on an input LD of the integrated circuit 2 in order to set the supply current through the LED. In this embodiment, the microcontroller 24 is also used to set the current for the desired dimming of the LED load. The microcontroller 24 generates a control voltage corresponding to the desired dimming, which is present at the input LD of the integrated circuit 2, the current measuring resistor selected by the microcontroller 24 via the range switchover being taken into account.

In operation, the integrated circuit 2 sets the current through the LED load. Any current can be set between 20 mA and 1 A, for example, to dim the LED.

In a current range of 200 mA to 1 A, the microcontroller applies a voltage to the gate of the semiconductor switch, which is given by the FET 22, so that a connection from drain to source is switched through. In this circuit state, the resistors R _CS and R _CS 2 are connected in parallel to ground. In this circuit state, the parallel connection of R _CS and R _CS 2 acts as a measuring resistor for the current regulation of the LED load. The integrated circuit 2 measures the voltage drop across the parallel combination of resistors _RCS and _RCS 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, eg to a current in a range between 20 mA and 200 mA, the field effect transistor 22 is controlled via the microcontroller 24 in order to open the connection between drain and source. In this case, the supply current flows through the LED load 3 only through the resistor R _CS , so that in this circuit state the measuring resistor for determining the current is given only through the resistor R _CS .

The current measuring resistor is thus switched over between two values via the microcontroller 24 in accordance with a predetermined current range. In the lower current range, the measuring resistance is given only by _RCS , while for the higher current range the measuring resistance is given by the parallel connection of _RCS and _RCS 2 is correspondingly lower. For example, the resistance R _CS and R _CS 2 can each be 1.2 ohms, so that in the higher current range from 200 mA to 1 μA there is effectively only a current measuring resistor of 0.6 ohms. As a result, the power converted at the measuring resistor is reduced for larger currents. But since the current is higher measuring range, the voltage drop is still sufficiently high to enable precise current measurement. In the lower current measuring range, the measuring resistor is correspondingly increased, for example to 1.2 ohms, so that even with the lower supply currents to be measured, there is still a sufficiently high voltage drop for the measuring resistor.

Furthermore, a control voltage is output via the microcontroller 24, which acts on the integrated circuit 12 in order to compensate the current measurement corresponding to the selected measuring resistor by the appropriate factor.

The present invention is not limited to the illustrated circuit of two resistors R _CS and R _CS 2 . More than two resistors can also be connected in parallel and more than just two current ranges can also be provided for measuring the current.

The rest of the LED power supply circuitry is the same as the prior art embodiment. As a result, the range switching according to the invention for the measuring resistor can also be easily implemented using existing integrated circuits known from the prior art for an LED power supply.

REFERENCE LIST

2

integrated circuit

3

LED load

4

Semiconductor switch, e.g. field effect transistor

22

Semiconductor switch, e.g. field effect transistor

24

microcontroller

RCS, RCS2

current sensing resistor

Claims (6)

1. A driver circuit for powering one or more LEDs (3), comprising a controllable constant current source for connecting one or more series-connected LEDs (3), and

   a switchable current sense resistor (RCS, RCS2) connectable in series with the one or more LEDs, wherein the driver circuit is configured to measure the current through the one or more LEDs (3) via a voltage drop of the current sense resistor (RCS, RCS2) in order to regulate the current to a predetermined value, wherein the switching of the current sense resistor (RCS, RCS2) is controlled by a microcontroller (24) of the driver circuit and the microcontroller further provides a control voltage which adjusts the current generation at the constant current source in accordance with the selected sense resistor (RCS, RCS2),

   wherein the driver circuit is further configured to switch the current sense resistor (RCS, RCS2) to at least two different values, wherein switching to the lower of the two values for measuring the current through the one or more LEDs (3) in a higher current range and to the higher of the two values for measuring the current through the one or more LEDs (3) in a lower current range,

   characterised in that the two current ranges partially overlap, and in that the driver circuit is configured to switch the current sense resistor (RCS, RCS2) to the lower value upon reaching the maximum of the lower current range and vice versa to switch back to the higher value only upon reaching the minimum of the higher current range.
2. The driver circuit according to claim 1, wherein the sense resistor (RCS, RCS2) is formed by at least two resistors (RCS, RCS2) connected in parallel, of which at least one (RCS2) is electronically connectable or disconnectable.
3. The driver circuit according to claim 2, wherein the electronic connection or disconnection of the at least one resistor (RCS2) is effected by driving a gate (G) of a field effect transistor, FET, (22), the source (S) and drain (D) of which is connected in series with the resistor.
4. The driver circuit according to claim 3, wherein the gate (G) of the FET (22) is driven by the microcontroller (24).
5. The driver circuit according to any one of the preceding claims, wherein the current sense resistor can take at least two values, wherein the lower value is between 0.1 Ohm and 0.7 Ohm and the higher value is between 0.9 Ohm and 1.7 Ohm.
6. The driver circuit according to any one of the preceding claims, wherein the current sense resistor takes more than two values, in particular three or four values, to switch between more than two current ranges, in particular three or four current ranges.

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