DE102010002081A1 - LED voltage measurement - Google Patents

Abstract

The invention relates to an operating circuit for at least one light-emitting diode path (L) with at least one light-emitting diode (LED), comprising a switching regulator circuit (SRS), to which a direct voltage is supplied and which provides a constant current for the at least one light-emitting diode path (L), parallel to that Light-emitting diode path (L) the primary side (P1) of a current mirror (S) is connected and a measuring voltage (Umess) proportional to the voltage drop across the light-emitting diode path (L) is tapped on the secondary side (P2) at a resistor (R2).

Description

The present invention relates to a voltage measuring circuit for one or more light-emitting diodes (LEDs) and to operating circuits for LEDs with such a circuit for measuring the voltage across an LED path which has one or more LEDs. The LEDs can u. a. be organic or inorganic LEDs.

It is basically known to use an operating circuit with switching regulators, in particular buck converters, buck converters, boost converters, flyback converters, etc. for driving LEDs (these switching regulators can all also be used in the context of the present invention). In this case, a control unit controls a clocked semiconductor power switch, by means of which an inductance is magnetized in its on state, wherein the inductance in the off state of the switch then eg. Via the LED discharges or demagnetizes. Depending on the circuit, however, the current does not have to flow through the light-emitting diode path during the switch-on and switch-off phase (see, for example, step-up converter).

For the operation of LEDs, the use of an operating circuit with control is advantageous. This requires a returned measured variable, for example the voltage drop across the LEDs or current flowing through the LEDs. It is advantageous to know the voltage across the LED track. It may also be advantageous to use the LED voltage for fault monitoring (eg short-circuit detection, overload detection, etc.).

DE 10 2006 034 371 A1 shows an operating circuit for measuring the voltage across the LED track, as in the adjacent 5 is shown. By means of a switch S1, an inductance L1 is selectively charged and discharged, which forms a freewheeling path with a diode D1 and the LED path L. Parallel to the LED path L, a filter / smoothing capacitor C1 is connected.

In this circuit, on the one hand, a so-called. Bus voltage V _in , ie, a DC supply voltage, and on the other hand, the LED cathode voltage V _kath measured. A control unit (in 5 not shown) determines from the difference between the two values the voltage U _LED across the LED path L, which has one or more LEDs connected in series. The measurement of the bus voltage V _in takes place in this prior art via a Meßabgriff M1 at the midpoint of a two resistors R10, R11 having voltage divider ST1, which is connected in parallel to a supply voltage source VQ. The measurement of the LED cathode voltage V _kath takes place at a measuring tap M2 at the midpoint of a voltage divider ST2 having two resistors R20, R21, which is connected in series with the LED path L on the cathode side.

With this form of voltage measuring circuit, a power loss occurs at each voltage measuring tap M1, M2, that is to say at both voltage dividers ST1, ST2. Such a power loss can, for example, up to several 100 mW per LED channel, d. H. per LED track, with LED lighting means having multiple of these channels.

In addition, the problem arises that the current flowing continuously through the voltage divider for the LED cathode voltage measurement, especially for very efficient LEDs, can lead to unwanted glare of the LEDs, even if the LEDs should actually be switched off. This glow could only be stopped by switching off the supply voltage or by short-circuiting the LEDs. For this purpose, an additional anti-malfunction circuit would be necessary, the additional effort and possibly power loss would result.

Furthermore, when removing an LED from the circuit, a large output voltage is produced: the cathode-side potential is pulled to zero because of the fact that the filter capacitor C1 is charged via the voltage divider ST2, while an arbitrarily high supply voltage (for example 400 volts) is applied to the anode side ) may be present. In a so-called "hot-swapping", d. H. When replacing an LED during operation, there is a risk that the LED can be destroyed by high pulse currents, which are caused by the discharge of the filter capacitor.

The invention is therefore based on the object to provide a voltage measuring circuit, in which at least one of the above-mentioned problems is rudimentary reduced.

This object is achieved according to the invention by the features of the independent claims. The dependent claims further form the central idea of the invention in a particularly advantageous manner.

Thus, in a first aspect, the invention deals with a voltage measuring circuit for a light-emitting diode path with at least one light-emitting diode. A DC voltage is supplied to the light-emitting diode path. Parallel to the light-emitting diode path, the primary side of a current mirror is connected. On the secondary side of the current mirror is now according to the invention to a measuring resistor a tapped to the voltage drop across the LED path proportional measurement voltage.

The current mirror has at least two transistors. Of these, preferably one each connected to the primary and one on the secondary side of the current mirror. To improve the behavior of the current mirror but also circuits with a larger number of transistors come into question.

The transistors may be bipolar transistors and / or field-effect transistors.

The at least two transistors can also have identical electrical properties. They can be designed as an integrated component.

The measuring resistor on the secondary side of the current mirror is connected in a preferred embodiment in series with the secondary-side transistor.

Another resistor on the primary side of the current mirror is preferably connected in series with the primary-side transistor. This serves to dimension the current to be mirrored.

In a further aspect, the invention deals with an operating circuit for at least a light-emitting diode path with at least one light-emitting diode. This operating circuit has a switching regulator circuit. The switching regulator circuit is supplied with a DC voltage. In addition, the switching regulator circuit provides a constant current for the at least one light emitting diode path. Parallel to the light-emitting diode path, the primary side of a current mirror is connected. On the secondary side, a voltage proportional to the voltage drop across the light-emitting diode path is tapped off at a resistor.

The operating circuit may have a voltage measuring circuit according to the invention, as has been described above.

The current flowing through the light-emitting diode path is preferably set by means of a coil and the timing of a switch controlled by a control and / or regulating circuit.

The measuring voltage can be compared as a feedback signal with a setpoint to clock depending on a possible difference as a control variable the switch. It serves as a monitor, for example as a detection of errors or for a shutdown. All evaluation functions, d. H. Control and fault monitoring and possibly shutdown) can be integrated in the example. As IC running control / regulating device.

When the switch is energized, the coil, which demagnetizes when the switch is off, for example, over the at least one LED strip. The control and / or regulating circuit can determine the time duration between a switch-off and a subsequent switch-on of the switch as a function of the voltage across the at least one light-emitting diode and a time-constant characteristic variable of the coil.

The control and / or regulating circuit can also detect the coil characteristics over the rising slope of the coil current and by the inclusion of the coil voltage.

In addition, in a preferred embodiment, the control and / or regulating circuit does not detect the current through the at least one light-emitting diode.

The control and / or regulating circuit may be an integrated circuit.

The control and / or regulating circuit can control the switch in the form of PWM-modulated signals.

The switching regulator may be, for example, a boost converter, a buck converter, a flyback converter, etc.

Parallel to the light-emitting diode path, a capacitor is preferably connected, in particular for smoothing the DC voltage.

Further features, advantages and features of the present invention will now be explained with reference to the figures of the accompanying drawings and the detailed description of exemplary embodiments.

1 shows a first embodiment of the LED voltage measuring circuit according to the invention,

2 shows a second embodiment of the LED voltage measuring circuit according to the invention,

3 shows a third embodiment of the LED voltage measuring circuit according to the invention,

4 shows a fourth embodiment of the LED voltage measuring circuit according to the invention, and

5 shows an LED voltage measuring circuit, as known from the prior art DE 10 2006 034 371 A1 is known and explained in the introduction.

1 to 4 show embodiments of the LED voltage measuring circuit according to the invention 1 , Here is the LED voltage measurement circuit 1 executed such that no measuring resistor (R20, R21) in series with the LED line L is required.

As in 1 shown schematically, according to the invention, the LED voltage U _LED reproducing, measuring signal U _MESS coupled by means of a current mirror S.

2 to 4 show examples of possible embodiments of the current mirror S, wherein, however, any further embodiments of current mirrors, for example, can also be found with an even higher number of transistors, application.

In 2 the current mirror is formed with three transistors T1, T2, T3, of which two T2, T3 are arranged on the secondary side P2 of the current mirror, wherein the transistors T1, T2, T3 are formed as bipolar transistors.

In 3 the current mirror S is constructed with a transistor T1 on the primary side P1 and a transistor T2 on the secondary side P2, wherein the transistors T1, T2 are formed as bipolar transistors.

In the example of 4 Finally, the current mirror S is constructed with a transistor T1 on the primary side P1 and a transistor T2 on the secondary side P2, wherein the transistors T1, T2 are formed as MOSFETs. Of course, a current mirror S can also be constructed with three MOSFETs.

The LED voltage measuring circuit according to the invention 1 takes place in an operating circuit 2 Application designed to operate a light-emitting diode link L. However, it should be understood that the voltage measuring circuit according to the invention does not affect the operating circuit 2 is limited, but rather can be used in any circuit in which a voltage measurement is to be made. In this respect, the invention is not limited to the field of LEDs, but can be used in a circuit with any load.

The operating circuit 2 An input DC voltage Vin or a rectified AC voltage is supplied.

A switching regulator circuit SRS has a series connection between a semiconductor power switch S1 (for example, a MOSFET) and a freewheeling diode D1. In the switched-on state of the switch S1, the series circuit magnetizes an inductance L1 by means of the current flowing through the switch S1. In the switched-off state of the switch S1, the energy stored in the coil L1 discharges in the form of a falling current i through the light-emitting diode path LED (the coil L1 is demagnetized). This results in a current profile in which rising cycles and declining cycles alternate with the periodicity of the switch control. Decisive for the light output is the temporally averaged current. The triangular current through the light-emitting diode path can be smoothed by a filter capacitor C1.

It is a control and / or regulating circuit SR is provided, which sets the clocking of the switch S1, for example in the form of PWM-modulated signals as a manipulated variable of the control of the LED power. As the feedback signal to which is regulated (and which, for example, is compared with a target value), at least the current flowing through the LED path L is measured. This LED current can be measured at any point in the LED current path.

However, in addition to the LED current, the voltage across the LED path is of interest for the correct operation of the LEDs. The voltage across the LED track is measured by means of the current mirror circuit described.

The light-emitting diode path has one or more parallel, but preferably in series LEDs and / or OLEDs. These may be, for example, monochromatic LEDs, color-converted white LEDs and / or RGB-LED modules. In the case of the latter, it is particularly advantageous if each luminous color is arranged in a separate LED segment ("LED channel") and each LED segment is regulated via its own feedback signal, such as the current flowing in the LED segment.

Parallel to the light-emitting diode path, the primary side P1 of a current mirror S is connected. The current mirror can have a transistor T1, T2 and a series-connected resistor R1, R2 on the primary side as well as on the secondary side. The transistors in the embodiment of FIG 4 to field effect transistors (FETs), in particular MOSFETs. However, the current mirror can also be any other embodiment known from the prior art.

The actual measurement of the LED voltage takes place through the secondary side P2 of the current mirror S. Due to the current mirror function, the identical current flows on the secondary side as on the primary side of the current mirror S and thus through a secondary-side measuring resistor R2. The voltage Umess at this resistor R2 thus reflects the LED voltage Uled again. The resistor R1 on the primary side P1 serves to dimension the measuring voltage Umess picked up at the resistor R2. The measured voltage Umess is thus proportional to the voltage Uled, wherein the factor between the two voltages can be adjusted by the size of the resistors R1 and R2.

An advantage is now that the losses that occur in the prior art (see above) due to the measurement circuit for the bus voltage in the idle state, now no longer exist because the bus voltage no longer needs to be measured to the voltage across the LED Track to determine.

In standby mode (i.e., applied supply voltage, but no light emission from the LEDs), the charge, i. H. the voltage across the capacitor C1 parallel to the LED discharged via the resistor R1 of the primary side P1 of the current mirror S. Thus, the voltage across the LED goes to zero. Even in standby mode, therefore, no current will flow through the resistor R1 or R2 of the current mirror. Thus, the power loss in standby mode goes to zero stationary.

Since the light-emitting diode path L is no longer connected to ground via a measuring circuit on the cathode side, the problem of unwanted glaring of the LEDs is also reduced.

Another advantage is that when the LED is removed from the circuit, the voltage across the capacitor C1 is discharged through the resistor R1 on the primary side P1 of the current mirror. When the LED is then reinserted, the voltage of the charged capacitor C1 does not drop off directly, but rather a voltage across the LED will be applied again when it is switched on again. In the prior art described above, the problem is that when re-inserting the cathode-side potential is pulled to zero due to the measuring channel, while the anode side, the supply voltage of, for example, 400 volts applied, thus resolved.

If a so-called 'hot-swapping' is done, which corresponds to the removal and re-insertion of an LED during operation, the risk that the LED is destroyed is significantly smaller than in the prior art. The 'hot-swapping' is electrically equivalent to an interrupt error, i. H. that the LED track is interrupted and then supply voltage again.

Since the two transistors T1 and T2 of the current mirror should have identical electrical properties, the two transistors are preferably integrated in one component.

Functionally, the measurement of a DC current in a measuring branch P2, which is not connected in series with the light-emitting diode path, can thus be effected by the current mirror measurement. There is also no difference measurement (bus voltage minus cathode voltage) instead, but a direct measurement of the voltage across the LED track. Thus, a measurement of the light-emitting diode path voltage is performed by means of a measuring resistor, which is not connected in series with the light-emitting diode path.

LIST OF REFERENCE NUMBERS

1

Voltage measuring circuit

2

operating circuit

V _in

DC input voltage

SRS

Switching regulator circuit

S1

switch

D1

diode

L1

inductance

S

current mirror

P1

Current mirror primary side

P2

Current mirror secondary side

R1

Resistor of the current mirror primary side

T1

Transistor of the current mirror primary side

R2

Current mirror secondary side resistance

T2

Current mirror secondary side transistor

T3

Third transistor of the current mirror

C1

capacitor

L

LEDs route

LED

led

U _LED

Voltage across the LED strip

U _MEASURE

measuring voltage

U _KATH

Voltage potential at the cathode side of the LED track L

SR

Control and / or regulating circuit

ST1

first voltage divider

ST2

second voltage divider

R10, R11

Resistors of the first voltage divider

R20, R21

Resistors of the second voltage divider

VQ

Supply voltage source

M1, M2

Measuring taps at the center points of the voltage dividers ST1, ST2

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102006034371 A1 [0004, 0033]

Claims (11)

Voltage measuring circuit for a light-emitting diode path (L) with at least one light emitting diode (LED), which is supplied with a DC voltage, characterized in that parallel to the light emitting diode path (L), the primary side (P1) of a current mirror (S) is connected and on the secondary side (P2 ) of the current mirror (S) on a measuring resistor (R2) a voltage drop (U _LED ) over the light-emitting diode path (L) proportional measuring voltage (U _MESS ) is tapped. Voltage measuring circuit according to claim 1, characterized in that the current mirror (S) at least two transistors (T1, T2), preferably three transistors (T1, T2, T3), of which at least one on the primary side (P1) and at least one on the Secondary side (P2) of the current mirror (S) is connected. However, current mirrors with a larger number of transistors are also possible. Voltage measuring circuit according to one of the preceding claims, characterized in that the at least two transistors (T1, T2) are bipolar transistors or field-effect transistors. Spnnungsmessschalung according to any one of the preceding claims, characterized in that at least two transistors (T1, T2) have identical electrical properties and are preferably formed as an integrated component. Voltage measuring circuit according to one of the preceding claims, characterized in that the measuring resistor (R2) on the secondary side (P2) of the current mirror is connected in series with the secondary-side transistor. Voltage measuring circuit according to one of the preceding claims, characterized in that a resistor (R1) on the primary side (P1) of the current mirror (S) in series with the primary-side transistor (T1) is connected, which serves a dimensioning of the measuring voltage (Umess). Operating circuit for at least one light-emitting diode path (L) with at least one light-emitting diode (LED), comprising a switching regulator circuit (SRS) to which a DC voltage (V _IN ) is supplied and which provides a constant average current for the at least one light-emitting diode path (L), characterized in that parallel to the light-emitting diode path (L), the primary side (P1) of a current mirror (S) is connected and on the secondary side (P2) to a resistor (R2) to a voltage drop across the light emitting diode path (L) proportional measuring voltage (Umess) is tapped. Operating circuit according to claim 7, characterized in that the operating circuit comprises a voltage measuring circuit according to claims 1 to 6. Operating circuit according to Claim 7 or 8, characterized in that the current flowing through the light-emitting diode path (L) is adjusted by means of a coil (L1) and a switch (S1) clocked by a control and / or regulating circuit (SR). Operating circuit according to one of claims 7 to 9, characterized in that the measuring voltage (Umess) is used as feedback signal for monitoring, for detecting an error, for switching off or for controlling the timing of the switch (S1). LED module, comprising at least one LED, which is supplied by an operating circuit according to one of claims 7 to 9.

Images