Driver circuit and method for operating a driver circuit

Abstract

The invention relates to a driver circuit for generating a current flow through a light source (5), in particular a laser diode (LD), and a method for operating the driver circuit, wherein a voltage source (3), the light source (5), and a series resistor are arranged in a circuit (2), wherein, in order to generate a series resistor for the light source (5) that can be controlled or regulated in discrete steps, the circuit (2) is branched into at least two parallel current branches (21, ..., 2N), and in each current branch (21, ..., 2N) at least one switchable parallel resistor (R1,...,RN) is provided.

Description

TECHNICAL AREA

[0001] The invention relates to the field of current control for controlling laser light sources. It is based on driver electronics and a method according to the preamble of the independent claims. STATE OF THE ART

[0002] The invention is based on WO 2013 / 046285 A1. This patent discloses a driver circuit for a light source (e.g., a light-emitting diode LED) which enables compensation of an ambient-dependent variable voltage drop across the LED. For this purpose, a second controlled shunt current driver path is provided in parallel to the first current driver path, the electrical resistance of which is regulated such that the heat dissipation in the shunt resistor counteracts the variable current through the LED and thereby stabilizes the light output of the LED.

[0003] DE 10 2016 212 928 A1 discloses a method for generating a laser pulse of an excitation laser in response to a trigger signal. The driver control signal is generated taking into account the time interval between the current and the previous trigger.

[0004] DE 103 93 192 T5 discloses a circuit for the common power supply of several light-emitting diodes (LEDs), wherein the LEDs are arranged in parallel current paths and each LED has its own current controller for regulating its light output.

[0005] DE 20 2010 017 580 U1 discloses a circuit arrangement for reducing the power dissipation of linear current drivers for LEDs. A control variable for the LED supply voltage is determined from the power dissipation of the driver stage and fed as a control signal to a control network for adjusting the LED supply voltage. The driver circuit is independent of the type of LED and is suitable for static or dynamic control. PRESENTATION OF THE INVENTION

[0006] The object of the present invention is to provide an improved driver circuit and a method for laser control which enable simplified control of the light power. This object is achieved according to the invention by the subject matter of the independent claims. Further features, in particular in dependent claims or combinations of dependent claims or combinations of embodiments, are optional and are therefore designated as in particular, preferred, etc., and are thus not essential to the invention and rather serve to achieve further advantages or effects.

[0007] In many laser processing applications of materials and tissues, it is necessary to generate laser beam pulses with specific temporal properties to optimize the efficiency of the processing or treatment and to minimize side effects: this concerns the pulse rise, pulse shape, and pulse decay. This is particularly relevant when using reproducible pulses (single pulses or pulse trains) below milliseconds for ophthalmic laser treatments. For example, pulse durations of a few microseconds are required for Selective Retina Laser Therapy (SRT). The pulses must exhibit a controlled, reproducible rise and shape for treatments at the process threshold. The present invention describes a suitable driver circuit and a method for controlling the output power of laser diodes or diode-pumped lasers.

[0008] In a first aspect, the invention comprises a driver circuit for generating a current flow through a light source, in particular a laser diode, wherein a voltage source, the light source, a series resistor, and a switch are arranged in a circuit, wherein, to generate a series resistor for the light source that can be adjusted in discrete steps, the circuit is branched into at least two parallel current branches, and each current branch contains at least one switchable parallel resistor. By adjusting the light source in discrete steps through the switching on or off of discrete parallel series resistors, analog or digital controllers for the series resistor can be avoided, and overall, a simple, fast, and fundamentally stable control of the driver circuit can be achieved. The light source is, in particular, a laser diode or a laser diode-pumped or LED-pumped laser resonator.

[0009] Various embodiments of the invention relate to, among other things: the use of a larger number N of current branches, e.g. N >= 2 or N >= 5 or N >= 10; the selection of identical or different parallel resistances; the control, i.e. switching on or off, of a switch per current branch by a microcontroller or programmable logic or digital electronics; and a voltage source with variable output voltage.

[0010] In a second aspect, the invention consists of a method for operating the driver circuit disclosed herein, wherein the series resistor is controlled in discrete steps by switching at least one of the switches on or off.

[0011] Various embodiments of this include, among others: a temporal sequence of switch configurations is implemented such that a predetermined current-time profile or luminous intensity-time profile is realized for the light source; a first fast temporal sequence of switch configurations is implemented such that short rise times of the luminous intensity, e.g. in the range of 0.1 - 100 microseconds, are achieved at the beginning of a current pulse; a second subsequent temporal sequence of switch configurations is implemented such that the luminous intensity of the light source is controlled as a function of a current measurement or luminous intensity measurement; and a third slow sequence of switch configurations is implemented such that fluctuations in the luminous intensity of the light source due to thermal effects are compensated during the course of a current pulse.

[0012] Further embodiments, advantages and applications of the invention will become apparent from the dependent claims, the combinations of claims and from the following description and figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] They show schematically <tb>Fig. 1 <sep>a simple self-stabilizing driver circuit according to the state of the art; <tb>Fig. 2 <sep>a first embodiment of a driver circuit according to the invention; <tb>Fig. 3 <sep>a second embodiment of a driver circuit with microcontroller according to the invention; <tb>Fig. 4, 5 <sep>third embodiment of a driver circuit with microcontroller and feedback according to the invention; <tb>Fig. 6 <sep>a current-power characteristic of a light-emitting diode (LED) or laser diode (LD); <tb>Fig. 7 <sep>a largely ideal temporal behavior of current and light output of an LED or LD; and <tb>Fig. 8 <sep>a real temporal behavior of current and light power of a laser diode.

[0014] In the figures, identical or functionally equivalent parts are provided with the same reference numerals. WAYS TO IMPLEMENT THE INVENTION

[0015] Fig. 1 shows the principle, known per se, of a driver circuit 1 for generating light emission 50 by means of a circuit 2 with a voltage source 3, a light source 5, in particular a laser diode LD, a series resistor R, and a switch S. The circuit is self-stabilizing and requires no active components or controllers. The current can be easily set by the voltage of the voltage source 3 and the resistance value R. By opening and closing the circuit 2 with the switch S (e.g., MOSFET), the current, and thus the light, can be pulsed. A disadvantage of this is that the setpoint current can only be varied by changing the voltage of the voltage source 3 or by changing the resistance value R, which is technically difficult to implement. In addition, a high voltage of the voltage source 3 can potentially result in high power dissipation in the resistor R.

[0016] Conventional driver circuits are based, for example, on analog electronics: In principle, the current can be regulated, for example, using operational amplifiers and corresponding power semiconductors. A disadvantage here is that controllers can be inherently unstable, and setting the controller parameters and selecting the components is not trivial, especially in the case of variable control setpoints. Digital electronics or digital switching controllers are also common: Switching regulator current controllers generally offer the possibility of good energy efficiency. A disadvantage here, too, is that complex control algorithms are required. Furthermore, very fast microcontrollers or, for example, field-programmable gate arrays (FPGAs) are necessary.

[0017] The invention relates to a driver circuit 1 and a method for operating the driver circuit 1 according to the independent claims. Exemplary embodiments are given below.

[0018] Fig. 2 shows an embodiment of a driver circuit 1 with a circuit 2 which is branched into N parallel current branches 21, ..., 2N, each of which has a resistor R1,...,RN, also called a parallel resistor, and a switch S1,...,SN, in particular an on / off switch S1,...,SN, in series.

[0019] The principle presented here allows the development of simple electronic hardware and software for generating light pulses in laser diodes and laser systems. The principle is based on the setup described in Fig. 1, "voltage source with series resistor". However, the series resistor is newly implemented as a combination of several individually switchable resistors R1,...,RN.

[0020] The total resistance R can be calculated depending on the switch positions. In particular, R<-1>= (R1an<-1>+ ... + Rnan<-1>) , where the index “an” denotes the resistance R1an, ..., Rnan in each of the switched-on current branches and the running index n <= N.

[0021] Figures 3-5 show that the switches S1 to SN can be controlled by digital electronics or a microcontroller 4. The times at which the switches S1,...,SN change their state, i.e., a new switch combination is set, are variable. According to the invention, this can happen very slowly or very quickly.

[0022] According to the embodiment shown in Fig. 3, the microcontroller 4, or alternatively a digital electronics unit, is connected to each switch S1,...,SN via a control path (A1,...,AN). According to the embodiment shown in Fig. 4, a light sensor 6, 7, in particular comprising a beam splitter 6 and photodetector 7, is additionally provided for the light source 5 for measuring the luminous flux of the light source 5, which is connected to the microcontroller 4 or alternatively to the digital electronics unit via a feedback path 70 for luminous flux control. According to the embodiment shown in Fig. 5, a current sensor 8 for measuring the current through the light source 5 is provided in the circuit 2 and is connected to the microcontroller 4 or alternatively to the digital electronics unit via a feedback path 80 for current control. The embodiments shown in Figs. 4 and 5 can also be implemented in combination with each other, i.e.,that feedback is used for both current and light output.

[0023] The term microcontroller 4 is also intended to include programmable logic. In particular, the microcontroller 4 includes software that is implemented in the driver circuit 1 and is part of the driver circuit, or that is programmed to execute the method.

[0024] In further embodiments, some or all parallel resistors R1,...,RN have the same resistance value; or some or all parallel resistors R1,...,RN have different resistance values, in particular doubled resistance values ​​according to the formula RN=2<(N-1)>R1, where N = number of current branches or number of parallel resistors R1, ..., RN.

[0025] In exemplary embodiments, the resistance values ​​can be doubled compared to the nearest resistor, for example R1 = 1Ω, R2 = 2Ω, R3 = 4Ω, R4 = 8Ω, R5 = 16Ω, ... , RN = 2 <n>Ω, or more generally written: R2 = 2 * R1, R3 = 4 * R1, R4 = 8 * R1, R5 = 16 * R1, ..., RN = 2<(N-1)>* R1.

[0026] In other embodiments not shown, the voltage source 3 may include means for changing its output voltage; and / or the light source 5 may be a laser diode LD or a laser diode-pumped or light-emitting diode (LED)-pumped laser system, in particular a diode-pumped solid-state laser DPSSL or a coherent optically pumped semiconductor laser OPSL.

[0027] The method for operating the driver circuit 1 consists in a circuit 2 comprising a voltage source 3, a light source 5, a series resistor and a switch, wherein the circuit 2 is branched into at least two parallel current branches 21, ..., 2N, in each current branch 21, ..., 2N a parallel resistor R1,...,RN and a switch S1,...,SN for switching on or off the parallel resistor R1,...,RN are provided, wherein the series resistor is controlled or regulated in discrete stages by switching on or off at least one of the switches S1,...,SN.

[0028] Preferably, N current branches 21, ..., 2N are present, where N = a natural number greater than or equal to 2. Furthermore, 2 <n>Switch configurations by setting the N switches S1,...,SN to on or off are present and in particular switchable, wherein the series resistor is controlled or regulated stepwise by a temporal sequence of switch configurations.

[0029] Fig. 6 shows a typical characteristic curve of a laser diode (LD) or light-emitting diode (LED) for the emitted luminous power Pout [in watts] as a function of the current I [in amperes], shown here for a junction temperature Tj of 25 °C. Light emission only occurs after the current I has exceeded a certain threshold Ith1.

[0030] Fig. 7 shows a largely ideal behavior of the time characteristic of current and luminous power of a laser diode (LD) or light-emitting diode (LED). In this context, the emitted luminous power of laser diodes or LEDs and laser systems (DPSSL, OPSL, etc.) in the time domain considered here is often such that the output luminous intensity Pout follows the electric current I through the laser diode very rapidly.

[0031] Fig. 8 shows an example in which the laser diode or laser system does not exhibit a directly proportional relationship between the output power and the current in the time domain under consideration. The oscillation and decay occur only after a certain delay. Overshoot can also occur under certain circumstances.

[0032] Real laser diodes and real voltage sources therefore have non-ideal properties. The parasitic series inductance of the laser diode and the supply line has a major effect. In addition, the ohmic and inductive internal resistance, a limited capacitance, and non-ideal load-regulating properties of the voltage source result in a brief voltage change when the switches are turned on.

[0033] Such non-idealities of laser diodes or laser systems can be reduced or largely compensated by the driver circuit 1 according to the invention and the method according to the invention and also by the disclosed embodiments.

[0034] In exemplary embodiments, a temporal sequence of switch configurations is implemented such that a predetermined current-time profile or light output-time profile Pout(t) is realized for the light source 5, in particular to at least partially compensate for non-ideal behavior of the light source 5 and / or the voltage source 3 and / or supply lines when the light source 5 is switched on. This also provides the possibility of laser pulse shaping or laser pulse adaptation ("pulse design" or "shape engineering"), i.e., the possibility of designing the laser pulse shape (Pout(t)) in such a way as is optimal for the application, in particular ophthalmological treatment.

[0035] In particular, a first temporal sequence of switch configurations can be implemented such that, to generate short rise times of the light output, e.g., in the range of 0.1 to 100 microseconds, a very small series resistance is set at the beginning of a current pulse, and this series resistance is increased stepwise by changing the switch configuration, in particular after each 1 microsecond. The first temporal sequence is therefore preferably implemented on a very short timescale, in particular microseconds or sub-microseconds such as e.g., 1 ns to 1000 ns or preferably 100 ns to 1000 ns.

[0036] Alternatively or additionally, a second temporal sequence of switch configuration can be implemented such that, following the first temporal sequence, the luminous flux of the light source 5 is regulated as time progresses, depending on a current measurement in circuit 2 or a luminous flux measurement of the power source 5, by stepwise changing the series resistor. The second temporal sequence is preferably implemented on a slower time scale than the first temporal sequence, e.g., in the range of 1–1000 microseconds, or preferably 5–500 microseconds, or particularly preferably 10–200 microseconds.

[0037] Alternatively or additionally, a third temporal sequence of switch configurations can be implemented such that, during a current pulse, fluctuations in the light output of the light source 5 due to thermal effects, particularly in the voltage source 3 and / or in the parallel resistors R1,...,RN, are compensated by discretely or stepwise changing the series resistor. The third temporal sequence is preferably implemented on an even slower time scale, e.g., in the range of seconds or minutes.

[0038] In further embodiments, the voltage source 3 is controlled or regulated such that the voltage is increased at the beginning of a current pulse and then gradually reduced, in particular to realize an approximately rectangular curve shape of the luminous power Pout(t) of the light source 5 as a function of time t and / or to keep the power loss in the resistors low.

[0039] In further embodiments of the driver circuit and the method, the start-up of the light source, in particular of a laser or laser diode-pumped laser system, can be improved by continuously or during a lead time before the desired laser light emission reducing the pump current for the laser 5, in particular for the laser diode 5 or pump laser diode 5, to a value just below the threshold current Ith1 by switching the switches S1,...,SN. The lead time can be selected, for example, in the range of 1 microsecond to 100 microseconds. REFERENCE MARK LIST

[0040] 1 Driver circuit, laser driver circuit, LED driver circuit 2 Circuit 21,...2N Parallel current paths, parallel current branches 3 Voltage source 4 Microcontroller, digital electronics 5 Light source, laser, laser system, laser diode, laser diode-pumped laser system, diode-pumped solid-state laser (DPSSL), coherent optical pumped semiconductor laser (OPSL) 50 Emitted light 6, 7 Light sensor 6 Beam splitter 7 Photodetector, light power measurement 70 Feedback path for light power control 8 Current sensor, current measurement 80 Feedback path for current control 9 Current [Amperes] through the laser diode 10 Light power [Watts] of the laser diode LD Laser diode R Electrical resistance R1,...,RN Electrical resistances 1, 2, ..., N in parallel current paths S Switch, MOSFET S1, ..., SN Switch in parallel current paths, MOSFETs N Number of parallel current paths A1,...,AN Control paths 1, 2, ..., N from the microcontroller to the 1st, 2nd, ..., N.ten switch I Current [Ampere] through the light source I(LD) Current flow through the laser diode Ith1 Current threshold of the laser diode Pout Light power [Watt] of the light source Tj Temperature of the laser diode, junction or barrier temperature t Time [seconds].< / n> < / n> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb> < / sep> < / tb>

Claims

1. Driver circuit (1) for generating a current flow through a light source (5), in particular a laser diode (LD) or light-emitting diode (LED), wherein a voltage source (3), the light source (5), a series resistor and a switch are arranged in a circuit (2), characterized in that, in order to generate a series resistor for the light source (5) that can be controlled or regulated in discrete steps, the circuit (2) is branched into at least two parallel current branches (21, ..., 2N) and in each current branch (21, ..., 2N) at least one switchable parallel resistor (R1,...,RN) is provided.

2. Driver circuit (1) according to claim 1, characterized in that in each current branch (21, ..., 2N) at least one switch (S1,...,SN) is provided for switching on or off the parallel resistor (R1,...,RN).

3. Driver circuit (1) according to one of the preceding claims, characterized in that a) parallel resistors (R1,...,RN) with the same resistance value are present; and / or b) parallel resistors (R1,...,RN) with different resistance values ​​are present, in particular in a series with each resistance value doubled (RN=2<(N-1)>R1).

4. Driver circuit (1) according to one of claims 2-3, characterized in that a) in each current branch (21, ..., 2N) the switch (S1,...,SN) is arranged in series with the parallel resistor (R1,...,RN); and / or b) the switches (S1,...,SN) are MOSFET switches.

5. Driver circuit (1) according to one of claims 2-4, characterized in that a digital electronics or a microcontroller (4) is provided for controlling the switches (S1,...,SN), in particular that the digital electronics or the microcontroller (4) is connected to each switch (S1,...,SN) via a control path (A1,...,AN).

6. Driver circuit (1) according to claim 5, characterized in that a) a current sensor (8) for measuring the current through the light source (5) is provided in the circuit (2) and is connected to the digital electronics or the microcontroller (4) via a feedback path (80) for current control; and / or b) a light sensor (6, 7) for measuring the luminous power (Pout) of the light source (5) is provided for the light source (5) and is connected to the digital electronics or the microcontroller (4) via a feedback path (70) for luminous power control.

7. Driver circuit (1) according to one of the preceding claims, characterized in that a) the voltage source (3) comprises means for changing its output voltage; and / or b) the light source (5) is a laser or a laser diode (LD) or a laser diode-pumped or LED-pumped laser system, in particular a diode-pumped solid-state laser (DPSSL) or a coherent optically pumped semiconductor laser (OPSL); and / or c) the driver circuit (1) comprises means for carrying out the method according to one of claims 8-10.

8. Method for operating a driver circuit (1) according to one of the preceding claims, wherein a voltage source (3), a light source (5), a series resistor and a switch are arranged in a circuit (2), wherein the circuit (2) is branched into at least two parallel current branches (21, ..., 2N), in each current branch (21, ..., 2N) a parallel resistor (R1,..., RN) and a switch (S1,...,SN) for switching on or off the parallel resistor (R1,...,RN) are provided, characterized in that the series resistor is controlled or regulated in discrete stages by switching on or off at least one of the switches (S1,...,SN).

9. Method according to claim 8, characterized in that N current branches (21, ..., 2N) are present, wherein N = natural number greater than or equal to 2, such that 2 <n>Switch configurations are available by setting the N switches (S1,...,SN) on or off and are in particular switchable, and that the series resistor is controlled or regulated stepwise by a temporal sequence of switch configurations.

10. Method according to claim 9, characterized in that a temporal sequence of switch configurations is implemented such that a predetermined current-time profile or luminous intensity-time profile (Pout(t)) is realized for the light source (5).

11. Method according to one of claims 9 to 10, characterized in that the temporal sequence of switch configurations is implemented such that (i) a non-ideal behavior of the light source (5) and / or the voltage source (3) and / or of supply lines when switching on the light source (5) is at least partially compensated; and / or (ii) that, for improved start-up of the laser system (5), the current for pumping the laser system (5) is continuously controlled or regulated to a value just below the threshold current (Ith1) or during a lead time before a desired laser light emission (50), in particular selected in the range of 1 microsecond - 100 microseconds.

12. Method according to one of claims 10-11, characterized in that a) a first temporal sequence of switch configuration is implemented such that, to generate short rise times of the light output, e.g. in the range of 0.1 - 100 microseconds, a very small series resistor is set at the beginning of a current pulse and this series resistor is increased stepwise by changing the switch configuration, in particular after each time interval in the range of 1 microsecond or in the range of 1 ns - 1000 ns; and / or b) a second temporal sequence of switch configuration, in particular on a time scale in the range of 1 - 1000 microseconds, is implemented such that, as time progresses, in particular after the first temporal sequence, the light output (Pout) of the light source (5) is regulated as a function of a current measurement in the circuit (2) or a light output measurement of the current source (5) by stepwise changing the series resistor;and / or c) a third temporal sequence of switch configurations, in particular on a time scale in the range of seconds or minutes, is implemented such that, during a current pulse, fluctuations in the luminous power (Pout) of the light source (5) due to thermal effects, in particular in the voltage source (3) and / or in the parallel resistors (R1, ..., RN), are compensated by stepwise changing the series resistor.; 13. Method according to one of claims 8-12, characterized in that the voltage source (3) is controlled or regulated such that the voltage is increased at the beginning of a current pulse and is then gradually reduced, in particular to realize an approximately rectangular curve shape of the luminous power (Pout(t)) of the light source (5) as a function of time (t) and / or to keep the power loss in the resistors low.

14. Method according to one of claims 9 to 13, characterized in that the light source is a laser system (5), e.g. a laser diode (5) or a diode-pumped laser (5), and the temporal sequence of switch configurations is carried out such that the laser pulse shape and laser pulse frequency is adapted or optimized for ophthalmic applications, in particular for selective retina laser therapy.

15. Method according to claim 14, characterized in that the laser pulses are generated with a duration in the range of 0.5 - 50 microseconds, preferably 1 - 10 microseconds; and / or the laser pulses are generated with a rapid reproducible rise in the range of 0.1 - 5 microseconds, preferably 0.5 - 2 microseconds.< / n>