EP4000353A1

EP4000353A1 - Light source driver circuit, optical measuring device comprising the light source driver circuit, device for checking value documents, and method for operating a light source load by means of the light source driver circuit

Info

Publication number: EP4000353A1
Application number: EP20746107.0A
Authority: EP
Inventors: Ulf EHRHARDT
Original assignee: Giesecke and Devrient Currency Technology GmbH
Current assignee: Giesecke and Devrient Currency Technology GmbH
Priority date: 2019-07-18
Filing date: 2020-07-13
Publication date: 2022-05-25
Also published as: DE102019005029A1; US20220264720A1; CN114175857A; WO2021008732A1

Abstract

The invention relates to a light source driver circuit which has a switch regulator with a voltage input for applying an input voltage, a voltage output for outputting an output voltage to be regulated for operating a light source load, and a regulating input for applying a regulating voltage for regulating the voltage level of the output voltage; a current source comprising a switch element and a voltage-controllable component which is arranged in series with the light source load, wherein a pulse signal is applied to a control connection of the switch element in order to connect a control connection of the voltage-controllable component to a voltage source in a first switch state of the switch element and to not connect the control connection of the voltage-controllable component to the voltage source in a second switch state of the switch element; and a regulating unit, a first input of which is connected to a first connection of the voltage-controllable component, a second input of which is connected to a second connection of the voltage-controllable component in order to tap a voltage drop across the voltage-controllable component in the first switch state, and an output of which is connected to the regulating input of the switch regulator in order to provide the regulating voltage, said regulating voltage being regulated on the basis of the voltage drop at the voltage-controllable component. The invention additionally relates to an optical measuring device comprising the light source driver circuit, to a device for checking value documents using the light source driver circuit, and to a method for operating a light source load using the light source driver circuit.

Description

Light source driver circuit, optical measuring device with the light source driver circuit, device for checking documents of value, and method for operating a

Light source load by means of the light source driving circuit

The invention relates to a light source driver circuit, an optical measuring device with the light source driver circuit, a device for checking documents of value with the light source driver circuit and a method for operating a light source load with the light source driver circuit.

The light source driving circuit or the optical measuring device is, for example, one

System component in a banknote processing machine for the recognition of machine-readable features. For example, machine-readable features are used in the case of securities, such as banknotes, passports or identity cards - hereinafter simply referred to as measurement objects - in order to be able to prove the authenticity of the measurement object. The measurement object is irradiated by means of fast-switched and bright flashes of light, and a characteristic response of the measurement object to these flashes of light is evaluated. With such a method, forgeries of the measurement objects can be reliably detected.

A device according to the invention for checking documents of value is intended to check a large number of measurement objects in the shortest possible time. Transport and processing speeds of several meters per second, in particular between 1 and 12 m / s, are desired in bank note processing machines. These processing speeds for testing an object to be measured, which is then exposed to several flashes of light, place high demands on the generation of the flashes of light.

The measurement objects to be tested are illuminated by means of at least one light source, in particular an LED. When operating light source loads, light source driver circuits with switching regulators are usually used. A general goal here is to operate a light source load with low current ripple and to lower a voltage that is required to control a light source load in order to reduce unnecessary power loss.

KR 2009 0060878 A and KR 10 102 88 60 B1 each propose LED driver circuits for lighting applications. In circuit variants, pulse width modulation (PWM) operations of the LED driver circuits are presented to ensure efficient

To enable brightness control with unchanged LED current and constant LED wavelength. None of the circuits is suitable for generating alternating pulse current values for the LED load. None of the circuits without a PWM operating mode can supply the pulse current value of the LED load of the steady state of the circuit immediately after switching on, ie application of the operating voltage. None of the circuits with PWM operating mode can supply the pulse current value of the LED load of the steady state of the circuit immediately after the PWM control pulses have been applied.

US 2009/0187925 A1 describes an LED driver circuit which supplies all LEDs connected in series with a constant current and ensures uniform illumination and optimal operating efficiency at low costs over a wide range of input / output voltage and temperature. Brief changes in the LED branch voltage, as would be provided for example in a pulsed current operation, would lead to a change in the LED current. This circuit is therefore not suitable for pulse operation.

In addition, data sheets for LED driver circuits are known, for example the LM3464 circuit from Texas Instruments or the ZXLD1362 circuit from Zetex Semiconductors. It is true that these solutions disclose minimizing a voltage drop in the drain-source voltage of a MOSFET in order to minimize power consumption.

However, none of the solutions shown are suitable for pulsed operation of the LED load, especially when varying pulse sequences, for example alternating pulse heights, are used to determine properties of a measurement object by emitting light through the LED driver circuit of an optical measuring device or a device for testing

To capture documents of value.

The invention is based on the object of realizing a highly efficient operation of a light source load, in particular an LED load, in which a variation in the voltage of a switching regulator is regulated. It should be possible to operate various light source loads. The number and type of light sources to be operated are not intended to be limiting. In addition, nominally different flow voltages of the light sources and fluctuations in the

Flow voltages can be compensated energy-efficiently during operation. For example, aging of an LED load or heating of an LED load or the use of fast, also cyclically varying, pulse trains - as is required, for example, in a device for checking documents of value - should not have any effect on the power consumption of the

Light source driver. A cyclically varying pulse sequence is a sequence of current pulses, which in particular can have different pulse lengths, pulse pauses and current strengths, which are repeated at fixed time intervals.

The object is achieved by the features described in the independent patent claims. Advantageous refinements of the invention are given in the dependent claims. According to the invention, a light source driver circuit is proposed. The light source driver circuit has a switching regulator with a voltage input for applying an input voltage and a voltage output for outputting a voltage to be regulated

Output voltage to operate a light source load and a control input to the

Applying a control voltage to regulate the voltage level of the output voltage.

At least one light source to be operated is provided as the light source load. An LED, also known as light emitting diode, or the interconnection of a plurality of LEDs that are connected in series or in parallel with one another is preferably provided as the light source load. A parallel connection of several is also conceivable

Series connections of LEDs (LED branches). In a further preferred embodiment, the light source load comprises at least one other, based on the same operating principle

Semiconductor light source, such as a laser diode, a resonant cavity light emitting diode, RC-LED for short, or an organic light-emitting diode, OLED for short. Other loads, such as incandescent lamps, motors or thermoelectric elements, can also be operated advantageously with the current driver according to the invention.

A switching regulator is a voltage regulator as the basis for an efficient voltage supply of a load, in this case the light source load, with the help of a periodically switched on electronic

Switching element and at least one energy store, for example a capacitive one

Energy storage and / or inductive energy storage. The switching regulator can be a

Have rectification.

The switching regulator regulates an input voltage applied (supplied) to the voltage input of the switching regulator, for example an AC input voltage or a

DC input voltage into a voltage output of the switching regulator that can be output

(available) output DC voltage, also referred to as output voltage. The DC output voltage preferably has a higher, lower or inverted voltage level compared to the input voltage.

For example, a DC voltage regulator, also referred to as a DC-DC regulator or DC power controller, is used as a switching regulator, which converts a DC input voltage supplied at the voltage input of the switching regulator into an output DC voltage that can be output at the voltage output of the switching regulator with a higher (buck-boost converter), lower (buck converter) or inverted voltage level.

A buck converter is preferably used as a switching regulator. A buck converter, also known as a step-down converter, regulates one at the voltage input of the Switching regulator supplied input voltage into an output voltage provided at the voltage output of the switching regulator with a - compared to the supplied input voltage - lower voltage level.

The switching regulator includes a control input for applying (supplying) a control voltage. This control voltage sets the voltage level of the output voltage to be output by the switching regulator. The voltage level of the output voltage is therefore dependent on the control voltage (= affinity). The voltage level of the control voltage is injectively, preferably injectively monotonically rising / falling and, in the special case, can be mapped bijectively onto the

Output voltage. A change in the control voltage is thus clearly converted into a change in the output voltage by means of the switching regulator. This dependence is preferably linear or logarithmic. A drop in the output voltage is only possible if the energy store of the switching regulator is discharged, i.e. a current is flowing from the switching regulator.

The one that can be tapped off at the voltage output of the switching regulator preferably changes

Output voltage linear with the control voltage provided at the control input. The slope of this linear function is called the proportionality factor. The proportionality factor is particularly preferably negative, so that the output voltage when the

Control voltage decreases. This enables a particularly simple control of the switching regulator.

The light source driver circuit according to the invention also has a current source with a switching element and a, arranged in series with the light source load,

voltage controllable component, wherein a pulse signal is applied to a control terminal of the switching element, wherein in a pulse phase of the pulse signal the switching element is switched to a first switching state in which a control terminal of the voltage controllable component is connected to a voltage source, and in a pulse pause of the pulse signal, the switching element is switched to a second switching state in which the control connection of the voltage-controllable component is not connected to the voltage source.

In a preferred embodiment, in the second switching state of the switching element, the

Control connection of the voltage controllable component connected to a reference potential, so that possibly existing charges flow away from the voltage controllable component.

The use of the term “current source” instead of the term “current sink”, which can also be used, for this component of the light source driver circuit is arbitrary. It should be noted that the choice of the respective term is only determined by a current direction on

Output of the current source / current sink is defined. So with a power source a

Output current delivered, while with an inverse definition of the current direction the same Component would be called a current sink. Since the current source used here is operated in series with the light source load, the choice of the term “current source” or “current sink” only depends on an actual position of the light source load in relation to the current source / current sink. Since the actual position is not restrictive according to the invention, the term current source can be used synonymously with the term current sink. In this application, the term current source is used for this component of the light source driver circuit.

A current source is an active two-terminal network in the light source driver circuit, which supplies an electrical current at its connection point to the light source load. The current strength of the supplied current depends only slightly or, ideally, not at all on the electrical voltage at its connection point, so that the electrical current is almost independent of the connected light source load (the connected consumer). For example, if the voltage changes by 1 V, the current only changes by 0.1%. The power source is connected in series with the light source load so that the current supplied by the power source is the current through the light source load.

The power source comprises a switching element, for example an electronic switch or an electromechanical switch or a mechanical switch. An electronic switch, for example a semiconductor switch, is preferably used. The switching element is switched from a first switching state (for example closed) to a second switching state (for example open) by means of a pulse signal at its control connection. In a pulse phase of the pulse signal, the switching element is switched to a first switching state. In a pulse pause in the pulse signal, the switching element is switched to a second switching state. In the first switching state of the switching element of the current source, the current source is switched to active and in the second switching state of the pulse signal, the current source is switched to inactive. The pulse signal, preferably a binary switching signal, is connected to a control connection of the

Switching element (for example a gate connection of a switching transistor) applied. The pulse signal is, for example, an output signal from the control terminal of the

Switching element connected microcontroller.

A switching element of the switching regulator is different from the switching element of the current source and is operated independently of the switching element of the current source by means of a pulse signal which is generated in the switching regulator itself

In addition to the switching element, the current source also includes a voltage-controllable component, preferably a field effect transistor, or FET for short. The switching element of the current source is connected with a first connection to a control connection of the voltage-controllable component. In the first switching state of the switching element, the control connection of the voltage controllable Component connected to a voltage source, the current source supplies in this first

Switching state (closed) an electric current. The voltage-controllable component thus provides an output current of the current source in the first switching state. The

The voltage level of the voltage source sets the current level of the output current of the current source. In the second switching state of the switching element, the control connection of the voltage-controllable component is not connected to the voltage source; in this second switching state (open) the current source does not deliver any output current.

The voltage-controllable component is connected with a first connection to a connection of the light source load. The output current of the power source thus flows through the light source load. This means that the output current, which is set by the voltage level at the control connection of the voltage-controllable component in the pulse phase of the pulse signal and is provided by the voltage-controllable component, also flows through the light source load in the first switching state of the switching element, whereby the light source load Emits light. This also means that in the second switching state of the switching element no output current is provided by the current source, and thus no current flows through the light source load, so that the light source load does not emit any light in the second switching state.

In addition, the light source driver circuit has a control unit, the first input of which is connected to a first connection of the voltage-controllable component of the power source, and whose second input is connected to a second connection of the voltage-controllable component of the power source in order to prevent a voltage drop across the voltage-controllable component in the the first switching state (closed). The output of the control unit is connected to the control input of the switching regulator in order to apply (provide) the control voltage to the switching regulator.

The control voltage is controlled by the control unit as a function of the voltage drop in the first switching state (closed) on the voltage-controllable component. This regulation of the control voltage by means of the control unit takes place in such a way that the voltage drop across the voltage-controllable component is minimal.

The control unit according to the invention regulates the output voltage output of the switching regulator to a value which is equal to the sum of the voltage drop across the light source load plus the desired minimum voltage drop across the

voltage controllable component is.

If the current source has a current measuring resistor (shunt) in series with the light source load and the voltage controllable component, the output voltage output of the switching regulator is regulated to a value that is equal to the sum of the voltage drop across the Light source load plus the targeted minimum voltage drop across the

voltage controllable component plus the voltage drop across the current measuring resistor.

By regulating the voltage drop across the voltage-controllable component to a minimum, the energy dissipated in the voltage-controllable component is reduced to a minimum, and thus the energy consumption of the light source driver circuit is reduced.

In addition, a variation in the output voltage of the switching regulator is compensated for. For a given light source driver circuit, for example, this compensation enables both the number of light sources and their interconnection to one another (in series or in parallel) to be varied. Nominally different light source leakage voltages and

Fluctuations in the light source leakage voltages during the operation of the light source driver circuit due to aging or temperature fluctuations inside or outside the circuit (heating / cooling) are also compensated for in an energy-efficient manner.

The pulse signal at the control connection of the switching element of the power source is also referred to as a pulse train. The pulse signal is a periodically repeating change in the

Voltage levels at the control connection of the switching element of the current source, with a current flowing through the voltage-controllable component as a result of a pulse phase (first switching state) and no current flowing through the voltage-controllable component in a pulse pause (second switching state). As a result of this cyclical change in the switching state, the control connection of the voltage-controllable component is connected or not connected to the voltage source in accordance with the pulse signal, as a result of which the current source is switched on and off periodically. When the pulse signal is applied, the current source supplies a pulsed current for the light source load. This pulse signal leads to cyclic pulse currents and, as a result, to periodic ones

(cyclical) switching on and off of the light source load. The current through the light source load is preferably 0 A in the pulse pause.

The pulse signal consists of a sequence of at least two individual pulses. Each single pulse comprises a single pulse phase (eg voltage at "HIGH" level) and a single pulse pause (eg voltage at "LOW" level). A single pulse phase and a single pulse pause result in a single pulse period. The individual pulse periods of the at least two individual pulses are preferably of the same length, that is to say the individual pulses have a fixed frequency. This frequency is preferably between 100 Hz and 50 kHz (corresponding to a single pulse period between 20 ps and 10 ms). When the light source driver circuit is used in a device for checking bank notes, these frequencies enable a spatially resolved test of moving bank notes at typical processing speeds between 1 and 12 m / s. The pulse signal can be a so-called burst signal. The burst signal consists of at least one burst consisting of a limited number of individual pulses. The sum of all

Individual pulse periods of a burst result in a burst phase. A burst preferably consists of 5 to 50 individual pulses. When the light source driver circuit is used in a device for checking bank notes, this enables a spatially resolved examination of a moving bank note, the light source only being switched on when the bank note is present in the measurement area.

The burst signal can be a periodically recurring signal. The period between two successive bursts is the burst pause. A burst phase and a burst pause result in a burst period. The burst period is preferably between 10 ms and 1 s When the light source driver circuit is used in a device for testing

Banknotes thus result in one burst per banknote at typical processing speeds between 1 and 12 m / s.

The pulse signal for switching the switching element can be a pulse-width-modulated signal so that a pulse duty factor of the pulse signal is variable. The pulse duty factor indicates the ratio of the pulse phase to the pulse period duration for the periodic sequence of pulses.

In a preferred embodiment, the control unit has a storage capacitor for increasing and decreasing a voltage level of the control voltage. In order to increase the control voltage, a charge is introduced into the storage capacitor. In order to reduce the control voltage, a charge is taken from the storage capacitor. The

Storage capacitor is therefore a dynamic charge storage device. The resulting average voltage across the storage capacitor is applied as a control voltage to the control input of the switching regulator. This enables fluctuations in the

Voltage drop across the light source load and variations in the output voltage of the switching regulator. The voltage drop across the voltage-controllable component in the first switching state (closed) causes the voltage-controllable component to be in its optimal state

Operating point operated with a minimal voltage drop in the first switching state (closed).

The storage capacitor is part of a linear, time-invariant system in the control unit. The storage capacitor is preferably a part of two resistor-capacitor elements, or RC elements for short, in order to create two integrating, continuous-time, linear, time-invariant transmission elements in the control unit that are easy to implement.

The time constant of the RC element for charging the storage capacitor must assume such a value that the highest rate of change of the storage capacitor voltage is smaller, preferably 2 times smaller than the quotient of the lowest rate of change of the switching regulator output voltage and the switching regulator proportionality factor. The rate of change of the storage capacitor voltage can also be even smaller, but would then unnecessarily lengthen the adjustment phase. The rate of change of the switching regulator output voltage is the quotient of the smallest light source load current in the pulse phase and the capacity of the energy store at the output of the switching regulator. The dimensioning condition for the time constant ensures that at the end of the adjustment phase there is no undershoot of the switching regulator output voltage, which would lead to an excessively low voltage drop above the

voltage controllable component can lead, which in consequence to an undesired

Reduction of the light source current can result.

In a preferred embodiment, the following values are specified:

lowest rate of change of the switching regulator output voltage dUSRAl / dt = 165 V / s

Switching regulator proportionality factor KS = 5.5

Rate of change of the storage capacitor voltage dUSKl / dt = dUSRAl / dt / Ks / 2 = 15 V / s Charging voltage of the RC element UL = 5 V

Time constant of the RC element x (charging) = UL / dUSKl / dt = 0.3 s

A discharge of charges from the storage capacitor in pulse pauses is necessary so that the control unit can regulate the switching regulator output voltage to its maximum value again after switching off the light source pulses, which corresponds to the initial state. The

The time constant of the RC element for discharging the storage capacitor should assume a value such that the switching regulator output voltage only increases by a small value in the pulse pauses. The switching regulator output voltage preferably increases by less than 0.1 V in a pulse pause. The increase in the switching regulator output voltage leads to an increased voltage drop across the voltage-controllable component, which results in a higher power loss in the voltage-controllable component. It is not necessary to select a time constant for discharging which guarantees an equal increase in the switching regulator output voltage under all operating conditions (time periods for pulse phase and pulse pause), especially with very long pulse pauses. With longer pulse pauses and unchanged pulse phase, the pulse duty factor (= pulse phase / pulse period duration) is reduced. With unchanged

Light source current in the pulse phase and a reduced pulse duty factor, the average power loss in the voltage-controllable component is reduced. It is therefore only necessary to determine the value for the longest pulse pause at which the power loss has increased

The voltage drop across the voltage controllable component is not greater than that

Power loss with the same light source current value in the pulse phase and the shortest pulse pause (= largest pulse duty factor). From this value for the longest pulse pause and the value to be defined for increasing the switching regulator output voltage in the pulse pause, determines the time constant of the discharge. As with the charging process, the

Discharge process the switching regulator proportionality factor must be included in the calculation.

In a preferred embodiment, the following values are specified:

Increase of the switching regulator output voltage in the pulse pause dUSRA2 = 0.1 V

With a minimum voltage drop across the voltage controllable component of 1.5 V (without increase), the power loss in the voltage controllable component increases by the same factor as the voltage increase: (0.1 V + 1.5 V) / 1.5 V = 1.07

Switching regulator proportionality factor KS = 5.5

Reduction of the storage capacitor voltage dUSK2 = dUSRA2 / KS = 0.0182 V

Longest pulse pause TPause = 0.125 s

Rate of change of the storage capacitor voltage dUSK2 / dt = dUSK2 / TPause = 0.146 V / s maximum storage capacitor voltage USKmax = 2.6 V

Time constant of the RC element x (discharge) = USKmax / dUSK2 / dt = 18 s

If, in another embodiment, the time constant of charging is to be increased (to reduce the rate of change of the storage capacitor voltage), then the

Time constant of the discharge can be increased by the same factor in order to avoid a further increase in the power loss in the voltage-controllable component.

In a preferred embodiment, the control unit has a comparison unit which provides a comparison voltage at its output as a function of the voltage drop across the voltage-controllable component. The comparison voltage is preferably a binary voltage, which is a particularly simple implementation of the following

Control voltage setting unit allows. The comparison unit can be designed as a comparator. The output of the comparison unit is with an input of a

Control voltage setting unit connected to the control unit. Such a modular design enables a more flexible design of the control unit. The

The control voltage setting unit controls the

Control voltage. The output of the control voltage setting unit is connected to the control input of the switching regulator in order to provide the control voltage.

In a preferred embodiment, the control voltage is through the

Control voltage setting unit increased when the (binary) comparison voltage has a first state and the control voltage is reduced by the control voltage setting unit when the comparison voltage has a second state different from the first state. The level of the comparison voltage is in steady-state operation

Consider the light source driver circuit, i.e. when a control phase (= start-up phase) of the switching regulator and the control unit has ended. In a preferred embodiment, the comparison unit comprises a comparator, the first input of which is connected to the first connection of the voltage-controllable component, and a direct voltage source. The first connection of the DC voltage source is connected to the second input of the comparator, and the second connection of the

The DC voltage source is connected as the second input of the control unit to the second connection of the voltage-controllable component. The DC voltage source supplies a

Reference voltage level to the second input of the comparator, which is connected to the

Voltage level of the first input of the comparator is compared. Depending on the comparison result, a comparison voltage is generated at the output of the comparator

provided. In this way, a light source driving circuit is provided, whose

Comparison unit the voltage drop of the voltage controllable component with a

DC reference voltage compares to generate the comparison voltage. This structure is particularly space-saving and has a low power consumption.

In a preferred embodiment, a diode is in the connection between the first connection of the voltage-controllable component and the first input of the comparator

introduced whose anode is connected to the first input of the comparator, and whose cathode is the first input of the control unit with the first terminal of the

voltage controllable component is connected. The diode has a blocking function in order to prevent a current flow through the light source load in the pulse pause.

In a preferred embodiment, a first connection of a storage capacitor of the comparison unit is connected to the anode of the diode, and a second connection of the

Storage capacitor of the comparison unit is connected to the second input of the control unit. The storage capacitor of the comparison unit is different from the storage capacitor of the control voltage setting unit described above. With an alternating pulse sequence, the storage capacitor ensures that the control voltage is optimized for the highest current intensity that occurs during the pulse phase. At the highest current intensity, the lowest voltage drop occurs across the voltage-controllable component.

In a preferred embodiment, a voltage source with a high internal resistance is arranged at the connection between the anode of the diode and the first input of the comparator. The voltage level of this voltage source is greater than the voltage level of the DC voltage source at the second input of the comparator. This means that the comparison voltage is reliably set to "HIGH" level when the circuit is started (see below). In the starting state, this causes a minimum control voltage and thus a maximum output voltage of the switching regulator, so that the intended current flows through the light source load when it is switched on for the first time. Even if the light source load changes while the Light source driver enables the voltage source to increase the output voltage of the switching regulator.

Thus, when the current source is inactive (pulse pause of the pulse signal or second switching state), the storage capacitor of the comparison unit is charged to the voltage level of the voltage source at the first input of the comparator. Due to the higher voltage level at the first input, a first state of the comparison voltage is provided at the output of the comparator. If the current source is then switched on by means of the pulse phase of the pulse signal and a voltage drops across the voltage-controllable component that is smaller than the difference between the voltage level at the first input of the comparator and the voltage level of the forward voltage of the diode in the forward direction, the storage capacitor breaks Discharge the voltage controllable component via the diode to a voltage value which corresponds to the sum of the voltage across the voltage controllable component and the voltage level of the forward voltage of the diode in the forward direction. If this voltage level at the first input of the comparator, achieved by the discharge process, is greater than the voltage level at the second input of the comparator, the first state of the comparison voltage is initially provided at the output. This leads to the control voltage being changed, which leads to a change in the output voltage of the switching regulator. This change leads to a changed voltage drop across the voltage-controllable component and, as a result, to a switchover of the voltage level of the comparison voltage at the comparator output.

In another preferred embodiment, the control unit is as

Computer program product implemented installed in a microcontroller. The voltage drop on the voltage-controllable component is digitized by means of AD conversion and made available to the microcontroller. This generates a corresponding control voltage in accordance with the processes described here. This control voltage is converted into an analog voltage signal by means of DA conversion and then fed to the switching regulator at the control input. This enables flexible reprogramming of the control parameters.

In a further aspect of the invention, an optical measuring device is provided. This optical measuring device has at least one light source for illuminating a measurement object. This light source, in particular an LED, is operated by means of a light source driver circuit of the type described above. A pulse signal is used for the power source in order to generate cyclic pulse currents with the power source, which then also flow through the light source load and cyclically switch the light source load on and off. This cyclical switching on and off of the light source load is used to illuminate the above-mentioned measurement object. The optical measuring device is used in particular for the recognition of machine-readable security features on documents of value. The optical measuring device can be part of a device for checking documents of value. In a further aspect of the invention, a device for checking documents of value with a machine-readable security feature is provided with a measuring area for receiving documents of value as test objects and an optical measuring device according to the preceding type for illuminating the security feature. The device according to the invention tests a large number of measurement objects in the shortest possible time. In particular, transport speeds through the measuring range of several meters per second are provided. This stands for the testing of a measurement object, which is then tested with several

Flashes of light are applied, only a very short period of time, for example 0.02 s. This requires short switch-on and switch-off times for the light source load, which are effected by means of the pulse signal. In a preferred embodiment, the device for checking documents of value also has a detector, the detector detecting a response from the security feature in response to the lighting and converting it into an electronic one

Converts output signal. Compared with visual detection, this enables a more precise check of the security feature and thus improved security against forgery. In a preferred embodiment, the device for checking documents of value also has a processor, the processor evaluating a property of the security feature (e.g. authenticity, document class) as a function of the output signal of the detector and outputting the result of the evaluation. This enables the device for testing to be integrated in an industrial environment, e.g. in a bank note processing machine, as well as a more precise analysis of the security feature and thus improved protection against forgery. A microprocessor (NI, 24) of the light source driver according to the invention is also preferably the processor of the device for checking documents of value.

In a further aspect of the invention, a method for operating a light source load by means of a light source driver circuit according to the type described above is provided. The pulse signal is first switched on to the control connection of the

Switching element to the control connection of a voltage controllable component with a

To connect voltage source. In addition, the voltage drop across the voltage-controllable component is picked up by means of the control unit. In addition, the control voltage is provided by means of the control unit, the control voltage being controlled as a function of the voltage drop across the voltage-controllable component. In addition, the control voltage is received in the switching regulator and the output to be controlled is output

Output voltage for operating the light source load using the control voltage to control a voltage level of the output voltage, the output voltage preferably decreasing linearly with the control voltage.

The invention and further embodiments and advantages of the invention are explained in more detail below with reference to figures, the figures merely being exemplary embodiments of the Describe the invention. The same components in the figures are provided with the same reference symbols. The figures are not to be regarded as true to scale; individual elements of the figures can be shown exaggeratedly large or exaggeratedly simplified.

Fig. 1 shows a first embodiment of a principle of a light source driver circuit according to the invention;

Fig. 2 shows a second embodiment of a principle of a light source driver circuit according to the invention;

Fig. 3 shows a first embodiment of a circuit for a

Fichtquelle driver circuit according to the invention based on the principle of FIG. 1;

Fig. 4 shows a second embodiment of a circuit for a

Fichtquelle driver circuit according to the invention based on the principle of FIG. 2;

5 shows a first exemplary embodiment of a flow diagram of a

Method according to the invention for operating a Fichtquellen-Fast;

FIG. 6 shows a first voltage / current-time curve of selected signals in the light source driver circuit according to FIG. 3;

FIG. 7 shows a selected part of the voltage / current-time curve shown in FIG. 6;

FIG. 8 shows a selected portion of the voltage / current-time curve shown in FIG. 7;

FIG. 9 shows a second voltage-time profile of signals in the light source driver circuit according to FIG. 3;

FIG. 10 shows a first partial area of the voltage-time curve shown in FIG. 9;

FIG. 11 shows a second partial area of the voltage-time curve shown in FIG. 9;

FIG. 12 shows a portion of the voltage-time curve shown in FIG. 9;

FIG. 13 shows a portion of the voltage-time curve shown in FIG. 12; and FIG. 14 shows a partial area of the voltage-time curve shown in FIG.

Fig. 1 shows a first embodiment of a principle of an inventive

Light source driver circuit. A switching regulator N8 has a voltage input N8_l for applying an input voltage U6. The switching regulator N8 has a voltage output N8_2 for outputting an output voltage U5 to be regulated. The switching regulator N8 has a control input N8_3 for applying a control voltage U4 to control the voltage level of the output voltage U5.

The voltage output N8_2 is connected to a connection of the light source load 3. The light source load 3 is shown here by way of example as an LED V3. According to the invention, the operation of a plurality of LEDs is also provided as the light source load 3, which LEDs are interconnected in series or in parallel. It is also conceivable to connect several series circuits of LEDs (LED branches) in parallel, or to use other semiconductor light sources based on the same operating principle. The anode of the light source load 3 is connected to the voltage output N8_2.

A power source 1 is provided in the light source driving circuit. The term “current source” is used throughout the description of the figures regardless of a current direction at the output of current source 1 (connection Vl_l and Vl_2 of a voltage-controllable component VI). The term “power source” can be interchanged with the term “current sink”.

The current source 1 has a switching element N3 and a voltage-controllable component VI, shown here by way of example as a field effect transistor, FET. A first connection Vl_l of the FET is connected to the cathode of the light source load 3 as a current output of the current source 1. A second connection Vl_2 of the FET is connected to a first connection of the current measuring resistor RI (shunt). A second connection of the current measuring resistor RI is with a

Reference potential connected.

A control connection Vl_3 of the FET is connected to a first connection N3_l of the switching element N3. A second connection N3_2 of the switching element N3 is connected to a first connection of a voltage source N2. A second connection of the voltage source N2 is connected to the reference potential. A pulse signal U7 is applied to a control connection N3_3 of the switching element N3. This pulse signal U7, as a switching signal for the switching element N3, has a pulse phase by means of which the switching element N3 is switched to a first switching state (closed) and has a pulse pause by means of which the switching element N3 is switched to a second switching state (open). The switching element N3 is, for example, an electronic switching element, for example a transistor. In Fig. 1, the switching element N3 is in the second Switching state (open) shown in which the control terminal Vl_3 of the FET is not connected to the first terminal of the voltage source N2. In the first switching state (not shown) of the switching element N3, the control terminal Vl_3 of the FET is connected to the first terminal of the voltage source.

The first connection Vl_l of the FET VI is connected to a first input 2_1 of a control unit 2. The second connection V 1_2 of the FET V 1 is connected to a second input 2_2 of the control unit 2. As a result, a voltage drop across the FET V 1 can be tapped by the control unit 2. An output 2_3 of the control unit 2 is connected to the control input N8_3 of the switching regulator N8 in order to provide the control voltage U4, the control voltage U4 being controlled as a function of a voltage drop at the FET V 1.

The switching regulator N8 is, for example, a standard DC-DC buck converter, the function of which does not need to be explained in more detail. For example, the switching regulator N8 can be implemented with a combination of an integrated circuit TPS541540 from Texas Instruments and a resistor at the feedback input. The use of other integrated circuits is not excluded.

In Fig. 1, a comparison unit 21 and a control voltage setting unit 22 are the

Control unit 2 indicated, which are described in more detail in FIG.

The principle of the light source driving circuit shown in Fig. 1 will be explained below.

The switchable current source 1 can be switched on and off by means of the pulse signal U7. The output current level of the current source 1 is set via the voltage source N2. Any changes in the output current level must be synchronized with the pulse signal in such a way that a cyclically varying sequence of current pulses results from the light source load.

The light source load V3 is derived from the highly efficient switching regulator N8 with its

Output voltage U5 supplied. Its input voltage U6 is a supply voltage of, for example, 24 volts DC. Other voltage levels or types of voltage for the input voltage U6 are not excluded, so one could also be

AC voltage can be applied to switching regulator N8, which is then rectified.

So that the power source 1 is operated with a high degree of efficiency, the

The voltage drop between the first connection Vl_l and the second connection Vl_2 of the voltage controllable component VI during the first switching state (closed) should be as small as possible. If the voltage controllable component VI is an FET, then the voltage drop between the connections Vl_l and Vl_2 is referred to as the drain-source voltage UDS, and the voltage drop between the inputs V 1_3 and V 1_2 is referred to as the gate-source voltage UGS. An FET has a threshold voltage Vth of 1.8 V, for example, which is characterized in that a usable drain current flows for UGS> Vth, in particular a current through the light source load. The drain-source voltage preferably fulfills the condition UDS> UGS-Vth, so that the drain current, in particular the current through the light source load, is as independent as possible of UDS.

The switching regulator N8 is connected to the control input N8_3 with the output 2_3 of the control unit 2. The output voltage U5 of the switching regulator N8 preferably decreases linearly with an increase in the control voltage U4. Another dependency between U4 and U5 can also exist. The values of U4 and U5 can be a bijective mapping. The voltage level of the output voltage U5 can thus be determined using the voltage level of the

Control voltage U4 can be set.

The level of the control voltage U4 is controlled by the control unit 2.

In the first switching state of the switching element N3 - current source 1 switched on - is on

The voltage drop across the FET VI is tapped and the control voltage U4 is regulated accordingly.

In the second switching state of the switching element N3 - current source 1 switched off - the control voltage U4 is not regulated.

During the operation of the light source driver circuit (second and third operating phase, see below), a pulse signal U7 is applied to the switching element N3 in order to switch the current source 1 on and off periodically (cyclically). It follows from this that the light source load 3 is switched on or off in accordance with the pulse signal U7. In this way, for example, flashes of light are generated, which are emitted onto a measurement object, for example a bank note, in order to obtain a

to receive and evaluate characteristic response to it. This enables, for example, an authenticity check of machine-readable features on a measurement object by a bank note checking system.

An increased power loss of the FET during a second operating phase, the “adjustment phase” (see explanations relating to FIGS. 6 to 14), of the light source driver circuit is shown in FIG

Component selection and the thermal design of the circuit board must be taken into account.

Fig. 2 shows a second embodiment of a principle of an inventive

Light source driver circuit. The principle of the light source driver circuit of FIG. 2 corresponds to the principle of the light source driver circuit of FIG. 1, so that reference can be made in full to the description of FIG. 1. Only the differences between FIGS. 1 and 2 are explained below. In contrast to FIG. 1, in FIG. 2 the control unit 2 is not provided with a comparison unit 21 and a

Control voltage setting unit 22, but alternatively with an AD converter 23, a microcontroller 24 and a DA converter 25. This difference will be explained in detail in FIG. The detection of the takes place in the microcontroller 24

Voltage difference on the FET based on a converted from analog to digital

Voltage value across the first connection Vl_l and the second connection Vl_2, and the setting of a digital control voltage corresponding to the digital voltage drop. In the subsequent DA converter, the set digital control voltage is converted into an analog control voltage, which is then fed to the switching regulator N8 as control voltage U4.

Fig. 3 shows a first embodiment of a circuit of an inventive

Light source driver circuit based on the principle of FIG. 1. The description of FIG. 1 also applies to FIG. 3, so that reference can be made in full to the description of FIG. 1. Therefore, only the differences between FIGS. 1 and 3 are explained below.

In contrast to FIG. 1, the current source 1 of FIG. 3 is a precision current source which additionally contains a digital-to-analog converter N2 and an operational amplifier N4.

As in FIG. 1, the current source 1 also includes the switching element N3 and the

voltage-controllable component VI, shown here as a field effect transistor, FET. The first connection Vl_l of the FET is the current output of the current source 1 with the cathode of the

Light source load 3 connected. The light source load is shown here as a series connection of LEDs V3 to Vn. Since in FIG. 3 the load current flows from the cathode of the light source load to the connection V1_1 of the current source 1, the term “current sink” is more appropriate from a purely circuit theory point of view.

In an embodiment variant not shown in FIG. 3, a first connection Vl_l of the voltage controllable component VI is connected to the output N8_2 of the switching regulator N8 and a second connection V 1_2 of the voltage controllable component V 1 is connected to the anode of the light source load 3, for example the anode of the first LED Vn of all the series-connected LEDs V3 to Vn, and the cathode of the LED V3 is connected to the first connection of the current measuring resistor RI. In a further embodiment variant not shown in FIG. 3, a first connection Vl_l of the voltage controllable component VI is connected to the output N8_2 of the switching regulator N8, a second connection Vl_2 of the voltage controllable component VI is connected to a first connection of the current measuring resistor RI, and a The second connection of the current measuring resistor RI is connected to the anode of the light source load 3, for example the anode of the first LED Vn of all LEDs V3 to Vn connected in series. The cathode of the LED V3 is connected to the reference potential.

The design of these design variants is easily possible for a person skilled in the art of light source drivers. Since the output current of the current source 1 flows from the connection V1_2 to the light source load in these design variants, it is pure

From a circuit theory point of view, the term "power source" is more appropriate.

Since the topology of the “current source” 1 is the same in all design variants and only the direction of the output current changes towards the light source load 3, the term “current source” is used here in general.

The second connection Vl_2 of the FET VI is connected to the first connection of the current measuring resistor RI (shunt). The second connection of the current measuring resistor RI is with the

Reference potential connected.

The control connection V 1_3 of the FET V 1 is connected to an output of an operational amplifier N4. The positive input of the operational amplifier N4 is connected to the first connection N3_l of the switching element N3. The negative input of the operational amplifier N4 is connected to the first connection of the current measuring resistor RI.

The second connection N3_2 of the switching element N3 is connected to an output of the voltage source N2, provided here as a DA converter. One input of the DA converter is connected to a microcontroller N 1. Alternatively (not shown), the second connection N3_2 of the switching element N3 is connected to an analog output of the microcontroller NI, the DA converter then being an integral part of the microcontroller NI.

The pulse signal U7 is in turn applied to the control connection N3_3 of the switching element N3. The pulse signal U7 is generated by the microcontroller NI. This pulse signal U7 has a pulse phase by means of which the switching element N3 is switched to a first switching state (closed) and a pulse pause by means of which the switching element N3 is switched to a second

Switching state (open) is switched. The switching element N3 is, for example, an electronic switching element, for example a transistor. In Fig. 3, the switching element N3 is shown in the second switching state (open), in which the first input (positive input) of the Operational amplifier N4 is not connected to the output of the DA converter N2. In a preferred embodiment (not shown), in the second switching state of the switching element N3, the first input of the operational amplifier N4 is connected to the reference potential, so that any charges that may be present flow away from the operational amplifier. As a result, the reference potential is also present at the output of the operational amplifier, so that it is ensured that no current flows through the light source load.

In the (not shown) first switching state (closed) of the switching element N3, the first input (positive input) of the operational amplifier N4 is connected to the output of the DA converter N2.

As in lig. 1, in lig. 3 the first connection V1_l of the LET is connected to the first input 2_1 of the control unit 2. The second connection V 1_2 of the FET is connected to the second input 2_2 of the control unit 2. As a result, a voltage drop UDS across the FET can be tapped by the control unit 2. The output 2_3 of the

Control unit 2 is connected to the control input N8_3 of the switching regulator N8 in order to provide the control voltage U4, the control voltage U4 depending on the

Voltage drop across the FET V 1 is regulated.

The control unit 2 of FIG. 3 comprises a comparison unit 21, a

Control voltage setting unit 22 and a NAND gate Dl. Instead of a NAND gate Dl, another digital gate could also be used to time-couple the pulse signal U7 and the output signal of the comparison unit 21.

The comparison unit 21 comprises a comparator N5, the first input (positive input) of which is connected to an anode of a diode V2. The cathode of the diode V2 is connected to the first terminal Vl_l of the FET and provides the first input 2_1 of the

Control unit 2. The anode of the diode V2 is also connected to a first connection of a resistor R2. A second connection of the resistor R2 is with a

Voltage source U2 connected. The anode of the diode V2 is also connected to a first connection of a storage capacitor CI.

A second connection of the storage capacitor CI is connected to the second connection Vl_2 of the FET and represents the second input 2_2 of the control unit 2. The second connection of the storage capacitor CI is connected to a second connection

DC voltage source U 1 connected. A first connection of the DC voltage source U 1 is connected to a second input (negative input) of the comparator N5.

The output of the comparator N5 is connected to a first input Dl_l of the NAND gate Dl. A second input Dl_2 of the NAND gate Dl is connected to the output of the Microcontroller NI connected, which provides the pulse signal U7. An output Dl_3 of the NAND gate Dl is connected to a control input of a switching element N6

Control voltage setting unit 22 connected. The switching element N6 is, for example, an FET analog switch.

A first input connection of the switching element N6 is connected to a first connection of a resistor R3 of the control voltage setting unit 22. A second connection of the resistor R3 is connected to a voltage source U3 of the control voltage setting unit 22. A second input connection of the switching element N6 is connected to a first connection of a resistor R4 of the control voltage setting unit 22. A second connection of the resistor R4 is connected to the reference potential.

An output terminal of the switching element N6 is connected to a first terminal

Storage capacitor C2 of the control voltage setting unit 22 connected. A second

The connection of the storage capacitor C2 is connected to the reference potential.

By applying a corresponding signal, the control connection of the switching element N6 has the effect that either the first connection of the resistor R3 is connected to the first connection of the storage capacitor C2, or that the first connection of the resistor R4 is connected to the first connection of the storage capacitor C2.

The resistor R3 and the storage capacitor C2 form a first RC element. The resistor R4 and the storage capacitor C2 form a second RC element. The time constants of both RC elements are chosen so that under all three operating conditions (start condition,

Adjustment phase, phase of the regulated state) the function of the light source driver circuit is ensured. In a dimensioning suggestion - that is not

Restricting the subject matter of the invention is - the resistor R4 is much larger than the resistor R3, in particular R4 is at least 10 times larger than R3, for example the ratio R4 / R3 is equal to 60. This ensures that the control voltage U4 during one pulse period is only changes slightly.

The first connection of the storage capacitor C2 of the control voltage setting unit 22 is connected to an input of an amplifier stage N7. An output of the amplifier stage N7 provides the control voltage U4 and thus represents the output 2_3 of the control unit 2. In a preferred embodiment, the amplifier stage N7 has a gain of +1. This results in a particularly simple circuit design with few electronic components.

The principle of the light source driving circuit shown in Fig. 3 will now be explained. The switchable current source 1 can be switched on and off by means of the pulse signal U7. The output current level of the current source is set via the voltage source N2, here an analog output value of the DA converter or the microcontroller NI. Any changes in the output current level must be synchronized with the pulse signal in such a way that a cyclically varying sequence of current pulses results from the light source load.

The light source load 3 becomes the highly efficient switching regulator N8 with its

Output voltage U5 supplied. Its input voltage U6 is a supply voltage of, for example, 24V direct voltage.

So that the power source 1 is operated with a high degree of efficiency, the

The voltage drop between the first connection Vl_l and the second connection Vl_2 of the voltage controllable component during the first switching state (closed) should be as low as possible. For example, the voltage drop is approximately 1.5 volts. In a regulated state of the light source driver circuit of FIG. 3, the voltage between the first connection Vl_l and the second connection Vl_2 of the voltage controllable component is set by the comparison unit 21, in particular by the voltage level of the DC voltage source U1.

The switching regulator N8 is connected to the control input N8_3 with the output 2_3 of the control unit

2 and thus also to the output of the amplifier stage N7 of the control voltage setting unit 22. The output voltage U5 of the switching regulator N8 decreases linearly, for example, when the control voltage U4 increases. Thus, the tension level of the

Output voltage U5 can be set using the voltage level of the control voltage U4. The level of the control voltage U4 is controlled by the control unit 2.

The pulse signal U7 is, for example, binary and has a logical “LOW” level to switch the switching element N3 to a second switching state, and a logical “HIGH” level to switch the switching element N3 to a first switching state. The specific voltage levels of the two levels are not relevant to the invention, and the assignment of the levels to the switching states of the switching element N3 is not relevant to the invention.

FIG. 4 shows a second exemplary embodiment of a light source driver circuit according to the invention based on the principle of FIG. 2. Only the differences from FIG.

3 to avoid unnecessary repetition. Instead of the analog

Circuit elements according to FIG. 3, in FIG. 4 the control loop is at least partially digital, the voltage drop being converted into a digital value by means of an AD converter 23, which can be evaluated by a microcontroller 24. The Microcontroller 24 then forms the comparison unit 21 and shown in FIGS. 1 and 3, respectively

Control voltage setting unit 22 to regulate a digital control voltage. The digital control voltage generated in this way is converted into an analog control voltage U4 by means of a DA converter 25 and made available to the switching regulator N8 at control input 2_3. The control unit 2 can be designed entirely in the form of a computer program product. The microcontroller N 1 is preferably at the same time the microcontroller 24 for regulating the control voltage U4. In an embodiment variant, the DA converter 25 and / or the AD converter 23 is part of the microcontroller 24. This enables a reduced number of components and a lower energy consumption.

All of the light source driver circuits of FIGS. 1 to 4 of the present invention have three temporal phases of operation. The first operating phase is called the “start condition”, the second operating phase is called the “adjustment phase”, the third operating phase is called the “phase of the regulated state”; for further details, reference is made to FIGS. 6 to 14.

The principle of the light source driver circuit of the invention is explained in more detail below with reference to the specific embodiment according to FIGS. 6 to 14.

In the “start condition” phase, the respective input and supply voltages are applied to the corresponding components of the light source driver circuit. In the case of a light source driver circuit according to FIG. 3, the input voltage U6 is applied to the switching regulator N8, the voltage U1 to the second input of the comparator N5, the voltage U2 to the resistor R2 and the voltage U3 to the resistor R3 in this first phase . The operating voltages required to supply the operational amplifier N4, the comparator N5, the amplifier N7, the microcontroller N 1 and the DAC N2 are also applied. In this first phase, the control voltage U4 in each light source driver circuit has an unregulated value, for example a voltage value of 0V, and the pulse signal U7 has a constant “LOW” level, resulting in a permanent second

Switching state of the switching element N3 is switched, and so the current source 1 is permanently deactivated in this first phase. With reference to FIG. 3, the storage capacitor C 1 of the comparison unit 21 is charged to the voltage level of the voltage source U2. The voltage level of the voltage source U2 is greater than the voltage level of the

DC voltage source Ul at the negative input of the comparator N5. The (permanent) logical “LOW” level of the pulse signal U7 at the second input Dl_2 of the NAND gate Dl also keeps the output Dl_3 of the NAND gate Dl at a logical “HIGH” level. The logical "HIGH" level of the output Dl_3 of the NAND gate Dl is applied to the control connection of the switching element N6, here an electronic switch, and switches it

Switching element N6 in a state not shown in FIG. 3, in which the first connection of the resistor R4 with the first connection of the storage capacitor C2 of the

Control voltage setting unit 22 is connected. This makes the storage capacitor C2 discharged or is held in the discharged state, and the control voltage U4 decreases or remains at a minimum value. If a minimum value of the control voltage U4 is zero volts, for example, then the output voltage U5 is regulated to its maximum value of typically 19 V DC by the switching regulator N8.

When the pulse signal U7 changes its logic state from logic “LOW” to logic “HIGH” for the first time, a second operating phase begins, the “adjustment phase” of the light source driver circuit of FIGS. 1 to 4. The first

Switching state of switching element N3 - current source 1 switched on - switched. The

The voltage drop UDS at the FET is then greater than a difference from the FET according to FIG

Voltage level of the direct voltage Ul and a forward voltage Uf_V2 of the diode V2 in the direction of flow. The comparator N5 of FIG. 3 thus initially continues to have a logic “HIGH” level at its output. The “HIGH” level of the comparator N5 and the “HIGH” level of the pulse signal U7 switch the NAND gate Dl at the output to a logical “LOW”. This switching of the output of the NAND gate Dl is to the

Control connection of the switching element N6 is provided, whereupon the switching element N6 switches over (into the switching state shown in FIG. 3). The first connection of the resistor R3 is thus connected to the first connection of the capacitor C2, as a result of which the storage capacitor C2 is charged via the resistor R3. As the voltage at the storage capacitor C2 increases, so does the control voltage U4.

If at the same time there is a sufficient average current of, for example, at least 60 mA through the light source load, the negative proportionality constant between the control voltage U4 and the output voltage U5 in the light source driver circuit according to FIGS. 1 to 4 causes the output voltage U5 to drop by means of the switching regulator N8 . The pulse sequence of the voltage U7 and the input signal in the DAC N2 are to be selected so that there is a sufficient average current through the light source load. As the output voltage U5 falls, the drain-source voltage UDS of the FET decreases (UDS is the voltage drop between the first connection Vl_l and the second connection Vl_2 of the FET VI). With reference to FIG. 3, the following applies: If the voltage drop UDS is smaller than the difference from the

Voltage level of the direct voltage Ul and a forward voltage Uf_V2 of the diode V2 in the flow direction, the output of the comparator N5 changes (flips) the comparison voltage from a first state (logical "HIGH" level) to a second state (logical "LOW" level) ). This toggle switches the NAND gate Dl at the output Dl_3 to logic “HIGH”, whereupon the switching element N6 switches over and connects the first connection of the resistor R4 to the first connection of the storage capacitor C2. The storage capacitor C2 is thus partially discharged again and the control voltage U4 is reduced. Both in the "regulation phase" and in the "phase of the regulated state" of the

In the light source driver circuit of FIGS. 1 to 4, a pulse signal U7 is applied to the switching element N3 in order to switch the current source 1 on and off periodically (cyclically). It follows from this that the light source load 3 is switched on or off in accordance with the pulse signal. In this way, for example, flashes of light are generated that are emitted onto a measurement object in order to receive and evaluate a characteristic response. This enables, for example, an authenticity check of machine-readable features on measurement objects.

In a pulse period of each pulse (= pulse phase and pulse pause), the

Storage capacitor C2 of the control voltage setting unit 22 is slightly charged and also slightly discharged. A mean voltage across the storage capacitor C2 sets a stable voltage value for the control voltage U4 and thus the output voltage U5 of the switching regulator N8. This stable voltage value of the output voltage U5 operates the current source 1 at the optimal operating point of a current-voltage characteristic of the FET. The voltage value of the DC voltage source U 1 defines the drain-source voltage UDS of the FET in the pulse phase of the “phase of the regulated state”.

The second operating phase “adjustment phase” ends when the mean value of the control voltage U4 no longer increases monotonically over a longer period of, for example, 10 pulse periods, in particular more than 5 ms, but U4 only alternates between two values within this period. At the end of the “adjustment phase”, the “phase of the regulated state” begins - the actual operating phase of the light source driver circuit.

By means of the control unit 2 of the light source driver circuit of FIGS. 1 to 4, a stable output voltage value of the output voltage U5 is obtained, which reflects the nominally different light source flow voltages and fluctuations in the light source flow voltages during operation of the light source driver circuit due to aging or temperature fluctuations inside or outside the circuit (heating / Cooling) as well as voltage fluctuations of the switching regulator N8. An increased power loss on the voltage controllable component VI during the "adjustment phase" of the light source driver circuit must be taken into account in the component selection and in the thermal design of the circuit board.

5 shows a first exemplary embodiment of a flow diagram of a method 100 according to the invention for operating a light source load by means of a light source driver circuit according to the type described above.

In a first step 101, a pulse signal is applied to a switching element in order to activate a

Control connection of a voltage controllable component with a voltage source in one To connect pulse phase and not to connect in a pulse pause. In subsequent step 102, a voltage drop across the voltage-controllable component is tapped by means of a control unit. In step 103, a comparison unit in the control unit is used to compare whether the voltage drop UDS is greater than the difference between the voltage level of the direct voltage Ul and a forward voltage Uf_V2 of the diode V2 in the flow direction.

If the answer is yes in step 103, a comparison voltage is switched to a first state (step 104). In a step 105, a control voltage is then set by means of the

Control unit 2 regulated (increased here), the control voltage being regulated as a function of the voltage drop on the voltage-controllable component. In a subsequent step 106, the control voltage is received in the switching regulator and an output voltage of the

Switching regulator is reduced and output to operate the light source load, the output voltage preferably decreasing linearly when the control voltage increases. The reduction in the output voltage leads to a reduction in the voltage drop UDS.

In the case of no in step 103, a comparison voltage is switched to a second state (step 107). Subsequently, in a step 108, a control voltage U4 is controlled by means of the control unit 2 (here reduced), the control voltage being controlled as a function of the voltage drop on the voltage-controllable component. In a subsequent step 109, the control voltage is received in the switching regulator and an output voltage of the

Switching regulator increased and output to operate the light source load, the output voltage preferably increasing linearly with a decrease in the control voltage. Increasing the output voltage leads to an increase in the voltage drop UDS.

The method of the flowchart of FIG. 5 can be used as an operating method (operating method) in each of the light source driving circuits illustrated in FIGS. 1 to 4.

In the analog embodiments according to FIG. 1 and FIG. 3, all of them run

Process steps in parallel.

FIG. 6 shows a first voltage / current-time curve of selected signals of the light source driver circuit shown in FIGS. 1 to 4, in particular FIG. 3. 6 shows a voltage curve of the output voltage U5 to be regulated of the switching regulator N8 in a period of 0 seconds to 4 seconds, a voltage curve at the first terminal Vl_l of the FET VI in a period of 0 seconds to 4 seconds

Voltage curve at the second connection V 1_2 of the FET VI in a period of 0 seconds to 4 seconds, a voltage curve of the control voltage U4 in a period of 0 seconds to 4 seconds, a voltage curve of the pulse signal U7 in a period of 0 seconds to 4 seconds, a Voltage curve at the output Dl_3 of the digital gate Dl in a period of 0 seconds to 4 seconds and a current profile of the current through the current resistance RI in a period of 0 seconds to 4 seconds.

The voltage-time curve of FIG. 6 is divided into the three operating phases. The period from 0 seconds to 0.4 seconds shows the start conditions, as mentioned above with: U5 at 19 volts, U4 at 0 volts, U7 at a permanent "LOW" level. The current source 1 is thus deactivated in this operating phase, which is represented by the current value of 0A of the current I_R1 through the current measuring resistor RI. The current I_R1 corresponds to the current through the light source load 3 and is therefore also referred to below as the light source current. According to FIG. 3, the “LOW” level of the pulse signal U7 leads to a “HIGH” level at the output Dl_3 of the NAND gate Dl.

During this period of the “start condition”, the control voltage U4 in the amount of 0 V due to the dependency between the voltages U4 and U5 (e.g. negative linear) generates an output voltage U5 of the switching regulator N8 to be controlled at a maximum level of 19 volts

Switching regulator N8 output. A voltage drop between the first connection V1_l and the second connection V1_2 of the voltage controllable component VI (FET) is thus a maximum and in this first phase, for example, 18 volts. (The voltage curves were obtained through a simulation. Due to the limitations of the simulation program, the value for the voltage drop is 1 V lower. In the implemented circuit, the

Voltage drop also 19 V.) Since the maximum drain-source voltage UDS is present in the start condition, a sufficient light source current can be provided at the first pulse (start of the adjustment phase) regardless of the selected light source load.

The second operating phase, the "adjustment phase", begins with the first switchover of the

Pulse signal U7 from logical "LOW" to logical "HIGH" at 0.4 seconds. The current source 1 is activated in pulse phases of the pulse signal U7, which means, for example, a

Light source current I_R1 is set to 1 ampere. Other current values are also possible. The pulse signal U7 can, for example, be a burst signal and have a predefined number of individual pulses, also referred to as a burst. An exemplary pulse signal is shown in FIG. 8. The invention is not restricted to burst pulse signals according to FIG. 8.

The second operating phase, the “adjustment phase”, ends at 2.2 seconds, which can be seen from the end of an increase in the control voltage U4, and what is shown in FIG. 6 in particular by a constant mean voltage UDS (difference between the voltages at the connections Vl_l and Vl_2 of the FET) is shown. In addition, the output Dl_3 of the NAND gate Dl switches in the “phase of the regulated state” with a lower frequency. FIG. 7 shows a selected sub-area (= here also area selection) of the voltage / current-time curve shown in FIG. 6 between 3.26 seconds and 3.44 seconds, see also the marking “area selection” in FIG. 6 7 shows the voltage-time curves or the current-time curve in the third operating phase “phase of the regulated state”. The voltage scale for U4 is enlarged by a factor of 100 and is included in the

displayed scale range shifted. In this illustration, a slight increase and decrease in the control voltage U4 due to the charging and discharging of the storage capacitor C2 according to FIG. 6 - caused by the switching of the output Dl_3 of the NAND gate Dl - is clearly shown. Thus, in a period of between 3.3 seconds and 3.41 seconds - that is, a period of approximately 5 bursts - the control voltage U4 is continuously and slightly reduced, since the output Dl_3 of the NAND gate Dl is constantly on logic " HIGH ”level is held. The first connection of the resistor R4 is therefore connected to the first connection of the capacitor C2. The resulting time constant of the second RC element (formed from R4 and C2) is significantly greater than the burst phase of the pulse signal U7. The repeated presence of a “LOW” level at the output Dl_3 of the NAND gate Dl in the time periods from 3.28 seconds to 3.3 seconds and from 3.41 seconds to 3.43 seconds leads to switching element N6, see above that the first terminal of the resistor R3 is connected to the capacitor C2. This leads to a charging of the storage capacitor C2 and thus to an increase in the control voltage U4. As a result, U4 fluctuates around a regulated value with a small amplitude.

FIG. 8 shows a selected partial area of the voltage / current-time curve shown in FIG. 7 between 3.4 seconds and 3.42 seconds, see also the marking “area selection” in FIG. 7. FIG 7 shows the voltage-time curves or the current-time curve in the third operating phase “phase of the regulated state”

Voltage scale for U4 enlarged by a factor of 100 and shifted to the scale range shown. In this illustration, an exemplary pulse signal U7 is shown, whose

Burst period is 21 milliseconds. The burst phase of this burst period has 30 individual pulses, each with a single pulse phase of 100 microseconds and one

Have single pulse periods of 500 microseconds. The resulting individual pulse pause is therefore 400 microseconds. This sequence of 30 individual pulses is known as a “burst”, which is repeated at intervals of a burst pause.

8 shows the relationship between the pulse signal U7, the resulting voltage drop UDS (difference between the voltage of the first connection Vl_l and the voltage of the second connection Vl_2) and the current I_R1 through the current measuring resistor RI. It can be seen that the last six individual pulses of the burst shown (period between 3.412 and 3.415 seconds) cause the output Dl_3 of the NAND gate Dl to flip and thus lead to an increase in the control voltage U4. FIG. 9 shows a second voltage-time curve of selected signals of the light source driver circuit shown in FIGS. 1 to 4, in particular in FIG. 3. 9 shows a voltage profile of the output voltage U5 to be regulated of the switching regulator N8 in a period of 0 seconds to 4 seconds, a voltage profile at the first terminal Vl_l of the FET in a period of 0 seconds to 4 seconds

Voltage curve at the second connection V 1_2 of the FET in a period of 0 seconds to 4 seconds and a voltage curve of the control voltage U4 in a period of 0 seconds to 4 seconds.

The voltage-time curve in FIG. 9 is also divided into the three operating phases. The period from 0 seconds to 0.4 seconds shows the start conditions, as already mentioned above with: U5 at 19 volts, U4 at 0 volts, U7 at a permanent “LOW” level (not shown). Power source 1 is thus deactivated during this period.

During this period, the “start condition” is set by the control voltage U4 in the amount of 0 V due to the dependency between the voltages U4 and U5 (e.g. vice versa

proportional) an output voltage U5 to be regulated from switching regulator N8 at a maximum level of 19 volts is output by switching regulator N8. A voltage drop between the first connection V1_l and the second connection V1_2 of the voltage-controllable component VI (FET) is thus a maximum and in this first phase it is also 19 volts. (The curve of the voltage U5 has been shifted upwards by 0.4 V for better visibility.)

The second operating phase “adjustment phase” begins with the first switchover of the pulse signal U7 from logical “LOW” to logical “HIGH”. The current source 1 is thus activated in the pulse phases. The pulse signal U7 can, for example, have a burst signal with a predefined number of individual pulses, also referred to as a burst. Such a pulse signal is shown in FIGS. 10-14.

Under such operating conditions, the second operating phase ends at 2.3 seconds, which can be seen from the end of a rise in the control voltage U4, and what is shown in FIG. 9 in particular by a constant mean voltage UDS (difference between the voltages at the

Connections Vl_l and Vl_2) is shown.

FIG. 10 shows a first partial area of the voltage-time curve shown in FIG. 9. The location of the sub-area is indicated in the upper illustration of FIG. The sub-area shown in FIG. 10 is selected at the beginning of the second operating phase “adjustment phase”. One recognizes a large voltage drop UDS (difference between the voltages at the connections Vl_l and Vl_2), which corresponds to a high power loss during the pulse phase. The Voltage value V 1_2 already has a constant amplitude even with the first pulses, so that the desired light source current is also already present with the first pulses.

FIG. 11 shows a second part of the voltage-time curve shown in FIG. 9. The location of the second sub-area is indicated in the upper illustration of FIG. 11. The sub-area shown in FIG. 10 is at the beginning of the third operating phase “phase of the regulated

State "selected.

FIG. 12 shows an exemplary partial area from the third operating phase “phase of the regulated state” of the voltage-time curve shown in FIG. 9. Instead of

The voltage curve of the output voltage U5 to be regulated of the switching regulator N8 shows a voltage curve at the output Dl_3 of the digital gate Dl. U4 was measured for this figure via an AC coupling, so that only the deviation from the mean value is shown. Similar to FIG. 7, the change in voltage U4 due to the temporary charging and discharging of storage capacitor C2 is shown here. Reference is made to the statements relating to FIG. 8,

FIG. 13 shows a partial area of the voltage-time curve shown in FIG. The location of the sub-area is indicated in the upper illustration of FIG. Similar to FIG. 8, an exemplary pulse signal, represented by Vl_l and Vl_2, is shown in a time-stretched manner

Presentation presented. U4 was measured for this figure via an AC coupling, so that only the deviation from the mean value is shown. The last 8 logical “LOW” level pulses shown at the output Dl_3 of the NAND gate Dl (of FIG. 3), with a length of approx

Microseconds arise during a phase of the control process in which the voltage across CI is almost identical to the DC voltage source Ul. It can be seen that these last eight logical “LOW” level pulses of the output Dl_3 of the NAND gate Dl do not lead to any relevant increase in the control voltage U4. The number of "LOW" level pulses of output Dl_3 can vary due to the analog control principle.

FIG. 14 shows a partial area of the voltage-time curve shown in FIG. The location of the sub-area is indicated in the upper illustration of FIG. 14. It is shown that the

Voltage difference UDS at the FET is 1.44 volts (difference between Vl_l and Vl_2). This corresponds to the difference between the voltage level of 2 volts of the direct voltage source Ul and the forward voltage in the forward direction of the diode D2. This low voltage difference UDS causes a minimal power loss of the FET.

Within the scope of the invention, all elements described and / or drawn and / or claimed can be combined with one another as desired. List of reference symbols

1 switchable power source

N3 switching element

N3_l First connection of the switching element

N3_2 Second connection of the switching element

N3_3 Control connection of the switching element

VI Voltage controllable component, FET

V 1_1 First connection of the FET

V 1_2 Second connection of the FET

Vl_3 control connection of the FET

UDS voltage drop across the FET, drain-source voltage RI shunt resistor, current measuring resistor

N4 operational amplifier

N2 digital-to-analog converter, voltage source

U7 pulse signal

2 control unit

2_1 First input of the control unit

2_2 Second input of the control unit

2_3 Output of the control unit

21 Comparison unit

V2 diode

CI storage capacitor

N5 comparator

U 1 DC voltage source

R2 resistor

U2 voltage source

22 Control voltage setting unit

R3 resistance

R4 resistor

C2 storage capacitor

N6 switching element

N7 amplifier

23 AD converter

24 microcontrollers

25 DA converters

Dl digital gate, NAND

Dl_l First input on the NAND

Dl_2 Second input on the NAND

Dl_3 output on the NAND

3, V3 to Vn light source load

N8 switching regulator

U6 input voltage

U4 control voltage

U5 output voltage to be regulated

N8_l voltage input

N8_2 voltage output

N8_3 control input

101 to 109 process steps

Claims

1.Light source driver circuit comprising:

- A switching regulator (N8) with a voltage input (N8_l) for applying a

Input voltage (U6), a voltage output (N8_2) for outputting an output voltage (U5) to be regulated for operating a light source load (3) and a control input (N8_3) for applying a control voltage (U4) to regulate the voltage level of the

Output voltage (U5);

- A current source (1) with a switching element (N3) and a voltage-controllable component (VI) arranged in series with the light source load (3), a pulse signal (U7) being sent to a control connection (N3_3) of the switching element (N3) is applied, wherein in a pulse phase of the pulse signal (U7) the switching element (N3) is switched to a first switching state in which a control terminal (Vl_3) of the voltage-controllable component (VI) is connected to a voltage source (N2), and in a Pulse pause of the pulse signal (U7) the switching element (N3) is switched to a second switching state in which the control connection (Vl_3) of the

voltage controllable component (VI) is not connected to the voltage source (N2); and

- A control unit (2) whose first input (2_1) is connected to a first connection (Vl_l) of the voltage-controllable component (VI), and whose second input (2_2) is connected to a second connection (Vl_2) of the voltage-controllable component (VI) is connected, and the output (2_3) is connected to the control input (N8_3) of the switching regulator (N8) to provide the control voltage (U4) to the switching regulator (N8), the control voltage (U4) depending on a voltage drop (UDS) between the connections (Vl_l) and (Vl_2) of the voltage controllable component (VI) is regulated.

2. Light source driver circuit according to claim 1, wherein the control unit (2) has a storage capacitor (C2) to the voltage level of the control voltage (U4) in

To increase or decrease the dependence of the voltage drop (UDS) on the voltage-controllable component (VI), whereby the control voltage (U4) is regulated.

3. Light source driver circuit according to one of the preceding claims, wherein the

Control unit (2) has a comparison unit (21) which has a

Provides comparison voltage, the voltage level of which depends on the voltage drop (UDS) across the voltage-controllable component (VI), the output of the comparison unit (21) being connected to an input of a control voltage setting unit (22) of the control unit (2) which determines the voltage level of the control voltage ( U4) controls depending on the comparison voltage, wherein the output of the control voltage setting unit (22) is connected to the control input (N8_3) of the switching regulator (N8) in order to provide the control voltage (U4).

4. Light source driver circuit according to claim 3, wherein the control voltage (U4) is increased by the control voltage setting unit (22) when the comparison voltage has a first state, and wherein the control voltage (U4) is reduced by the control voltage setting unit (22) when the Comparison voltage has a second state different from the first state.

5. Light source driver circuit according to one of claims 3 or 4, wherein the

Comparison unit (21) comprises a comparator (N5) and a direct voltage source (Ul), the first input of the comparator (N5) being connected to the first connection (Vl_l) of the voltage-controllable component (VI), and the second input of the comparator ( N5) is connected to a first connection of the direct voltage source (Ul), and a second connection of the direct voltage source (Ul) is connected as a second input (2_2) of the control unit (2) to the second connection (Vl_2) of the voltage-controllable component (VI).

6. Light source driver circuit according to claim 5, wherein in the connection between the first terminal (Vl_l) of the voltage controllable component (VI) and the first input of the comparator (N5) a diode (V2) is introduced, the anode of which with the first input of the Comparator (N5) is connected, and its cathode as the first input (2_1) of the

Control unit (2) is connected to the first connection (Vl_l) of the voltage-controllable component (VI).

7. The light source driving circuit according to claim 6, wherein a first terminal is a

Storage capacitor (CI) of the comparison unit (21) is connected to the anode of the diode (V2), and a second connection of the storage capacitor (CI) of the comparison unit (21) is connected to the second input (2_2) of the control unit (2).

8. Light source driver circuit according to one of the preceding claims 2 to 7, wherein the storage capacitor (C2) of the control unit (2) is part of a first RC element, the time constant of the first RC element assumes a value such that the highest

Rate of change of the storage capacitor voltage is smaller than the quotient of the lowest rate of change of the switching regulator output voltage and the switching regulator proportionality factor.

9. Light source driver circuit according to claim 8, wherein the storage capacitor (C2) of the control unit (2) is also part of a second RC element, the time constant of the second RC element assumes such a value that the switching regulator output voltage only increases by a small value in the pulse pauses.

10. Light source driver circuit according to claim 9, wherein the first RC element or the second RC element is selected as a function of the comparison voltage by means of a switching element (N6) of the control voltage setting unit (22).

11. Light source driver circuit according to one of the preceding claims, wherein the control unit (2) installed as a computer program product executable in one

Microcontroller (NI) is introduced.

12. An optical measuring device comprising at least one light source (V3) as a light source load (3) for illuminating a measurement object and a light source driver circuit according to one of the preceding claims for operating the light source (V3).

13. Arrangement with a light source driver circuit according to one of claims 1 to 11 and a light source load (3).

14. Device for checking documents of value, in particular bank notes, with a

Machine-readable security element with an optical measuring device according to Claim 12 for illuminating the security element.

15. The method (100) for operating a light source load (3) by means of a light source driver circuit according to one of claims 1 to 11, wherein the method (100) comprises the following steps:

- Switching on (101) the pulse signal (U7) to the control connection (N3_3) of the switching element (N3) in order to connect the control connection (Vl_3) of the voltage controllable component (VI) to the

To connect voltage source (N2);

- Tapping (102) of the voltage drop (UDS) between the connections (Vl_l) and (Vl_2) of the voltage-controllable component (VI) by means of the control unit (2);

- Control (105, 108) of the control voltage (U4) by means of the control unit (2), the

Control voltage (U4) depending on the voltage drop (UDS) over the

voltage controllable component (VI) is regulated;

- Receiving the control voltage (U4) in the switching regulator (N8) and outputting the output voltage (U5) to be regulated for operating the light source load (3) using the control voltage (U4) for regulating (106, 109) the voltage level of the output voltage (U5 ), whereby the control voltage (U4) is preferably inversely proportional to the output voltage (U5).