Short-pulse LED illumination system

Abstract

The invention relates to a short-pulse LED illumination system (100), comprising a circuit board (10) having a first side (10a) and an opposite second side (10b), and an LED (12), a switching element (14), at least one resistor (16), and at least one capacitor (18) fixed to the circuit board (10), the essence of which is that the LED (12), the switching element (14), the at least one resistor (16), and the at least one capacitor (18) are connected in series with each other and together define a loop-shaped circuit passing through the circuit board (10) that is openable and closable by the switching element (14), such that the LED (12) is arranged on the first side (10a) of the circuit board (10), and the switching element (14), the at least one resistor (16), and the at least one capacitor (18) are arranged on the second side (10b) of the circuit board (10).

Description

[0001] SHORT-PULSE LED ILLUMINATION SYSTEM

[0002] The invention relates to a short-pulse LED illumination system comprising a circuit board having a first side and an opposite second side, and an LED, a switching element, at least one resistor, and at least one capacitor fixed to the circuit board.

[0003] Since the beginning of photography, there has been a demand for generating short light pulses in order to freeze moving subjects. Scanning microscopes have also adopted the use of short light pulses during their development, since freezing a moving sample constitutes a very similar technical problem. In modern scanning microscopes, for scanning pathological samples in bright-field mode, a CMOS machine-vision sensor is generally used, which records an entire field of view at once. While the slide is in continuous motion, the light pulse generated by the light source determines the exposure time. A complete digitised pathological slide is produced from the resulting individual image fields after software-based stitching. For this reason, in digital scanners the illumination system is one of the most important sub-assemblies. The flash frequency determines the image acquisition frequency, while the illumination duration, and the chromatic and intensity homogeneity, have a major influence on image quality. The numerical aperture of the illumination unit determines the smallest optically resolvable stripe width, and inadequate illumination can generate significant stray light for the imaging.

[0004] At present, Xe arc lamps are used in scanning microscopes as short-pulse light sources. In an arc lamp, due to the high electric field strength, an arc discharge of the Xe gas emits an extremely large amount of light in a relatively short pulse of 2-3 ps, which is sufficient to freeze the sample. A disadvantage of this technology, however, is that the emission spectrum of Xe radiates significant power in the deep- UV range around 250 nm, which is not utilised during bright-field scanning, and therefore must in any case be filtered out immediately after the lamp. In addition, the formation of the arc is partly stochastic in nature, and therefore there may be a large variation in the power of successive pulses, which makes image-to-image intensity variation more frequent. The arc is typically 1 .5 mm long and 0.5 mm wide, and thus, in Fourier space, the resulting angular distribution is significantly anisotropic. To eliminate this, it is necessary to incorporate a strong diffuser element into the optical path, which entails light loss. A further disadvantage is that, during the use of arc lamps, a significant amount of heat is generated inside the device, since the total power consumption of the electronics is relatively high, and arc lamps generate a non-negligible amount of electromagnetic radiation, such that considerable care must be taken to filter the radiated electromagnetic interference.

[0005] LED-based illumination technology has undergone intensive development over recent decades, and high-power LED light sources are now commercially available which would have seemed inconceivable 10 years ago.

[0006] We have recognised that LEDs provide a number of preferred characteristics compared with Xe arc lamps, including, inter alia, substantially better efficiency and smaller size, and, due to their entirely solid-state construction, a longer expected lifetime, and they are also relatively less expensive. An LED light source is a sub-assembly of lower complexity than a xenon arc lamp, which is always preferred from a manufacturing-technology perspective. A further advantage is that it operates from extra-low voltage, in contrast to the order of 1000 V typically present in arc lamps. This simplifies the electrical safety measures required, the implementation of which would otherwise represent an additional cost, and the components used are also significantly less expensive in this voltage range. In addition to reducing the manufacturing cost, the solution also yields a perceptible improvement in product quality for users, since, owing to the improved homogeneity, it can provide improved image quality, and it also significantly reduces the audible noise of the device, since the main noise source at present is the individual discharges of the arc lamp.

[0007] We have also recognised, however, that there is currently no driver circuit by means of which a sufficiently high-intensity, yet ultra-short (~ 1 ps) light pulse required for scanning can be obtained from a high-power LED light source.

[0008] As part of our recognition, the short-pulse light source is used by the scanning microscope with a very low duty cycle, such that a plurality of solutions may be employed which would not be viable in other applications, but are particularly preferred here. In scanning microscopes, due to the scanner, the camera data transfer rate, and the mechanical limitations of sample movement, the image acquisition frequency is typically not greater than 150 Hz. The duty factor D = 1 ps / 6.67 ms = 1 .5e-4, and thus the resulting thermal loss is also approximately 1 / 10000 of that obtained if the typical current of 8 A were to flow through the LED and the limiting resistors. (Pdc = 8 A*24 V = 192 W) which, due to the duty factor, corresponds to an average power consumption of P = 28.8 mW. This is so low that no dedicated heat dissipation needs to be provided, and the generated heat can be dissipated appropriately merely by the design of the printed circuit board (PCB). As a result, owing to the low power resulting from the low duty cycle, there is no need for active feedback in the circuit, a high-efficiency switched-mode power supply, or other efficiency-increasing elements — which would be indispensable in most conventional applications — thus enabling the circuit to be minimised.

[0009] We have recognised that, in the driver circuit of the pulse-operated light source, parasitic inductance and capacitance terms may already significantly impede the achievement of 1 ps light pulses required in scanning microscopes, since they slow the rise of the current flowing through the LED and thereby ultimately increase the pulse duration. According to our recognition, by appropriately designing the driver circuit for the LED and minimising the size of the current loop, an LED illumination system can be provided in which the parasitic inductance can be substantially reduced (even by up to one half), and, at the same time, the stray magnetic field can also be significantly reduced. In this manner, it becomes possible to achieve an ultra-short pulse duration using a high-power LED.

[0010] The object of the invention is to provide an LED illumination system which is free from the disadvantages of the solutions according to the prior art. In particular, the object of the invention is to provide an LED illumination system by means of which a high-power (> 1000 Im), ultra-short (~ 1 ps) light pulse can be generated at a frequency commonly used in scanning microscopes (~ 150 Hz).

[0011] We have solved the problem, according to the invention, by providing an LED illumination system comprising a circuit board having a first side and an opposite second side, and an LED, a switching element, at least one resistor, and at least one capacitor providing the power supply of the LED, which are fixed to the circuit board.

[0012] The essence of the invention is that the LED, the switching element, the at least one resistor, and the at least one capacitor are connected in series with each other and together define a loop-shaped circuit passing through the circuit board that is openable and closable by the switching element, such that the LED is arranged on the first side of the circuit board, and the switching element, the at least one resistor, and the at least one capacitor are arranged on the second side of the circuit board. It is particularly preferred that the LED, the switching element, the at least one resistor, and the at least one capacitor are arranged on the first and second sides of the circuit board directly opposite each other, thereby being arranged so as to minimise the loop area.

[0013] Some preferred embodiments of the invention are defined in the dependent claims.

[0014] Further details of the invention are described by way of exemplary embodiments, with reference to the drawings. In the drawings,

[0015] Figure 1 is a schematic perspective view of an exemplary embodiment of an LED illumination system according to the invention,

[0016] Figure 2a is a schematic view of the embodiment of an LED illumination system according to the invention illustrated in Figure 1 , shown without the circuit board,

[0017] Figure 2b is a schematic view of a second exemplary embodiment of an LED illumination system according to the invention, shown without the circuit board, Figure 2c is a schematic view of a third exemplary embodiment of an LED illumination system according to the invention, shown without the circuit board.

[0018] Figure 1 shows a schematic view of an exemplary embodiment of a shortpulse LED illumination system 100 according to the invention. The illumination system 100 comprises a circuit board 10 having a first side 10a and an opposite second side 10b, and an LED 12, a switching element 14, at least one resistor 16, and at least one capacitor 18 fixed to the circuit board 10.

[0019] In the context of the present invention, the circuit board 10 is understood to be a printed wiring board known per se (in short: PCB), the function of which is to combine the components of the system 100, such as the LED 12, the switching element 14, the at least one resistor 16, and the at least one capacitor 18, into a unitary circuit, while, by means of the formed wiring, providing the electrical connection between the components and between connection points, as is known to the person skilled in the art. The circuit board 10 according to the invention is configured such that both its first and second sides 10a, 10b are provided with conductor tracks (not shown in the drawings), that is, the circuit board 10 is suitable, on both the first side 10a and the second side 10b, for establishing electrical connections between the components of the system 100. The circuit board 10 comprises a plurality of PCB vias 1 1 passing through the circuit board 10 and electrically connecting the conductor tracks of the first and second sides 10a, 10b. The PCB vias 1 1 enable the flow of electrical signals and currents between the first and second sides 10a, 10b of the circuit board 10. They function as channels, providing a seamless connection for the components and enabling efficient transmission of electrical signals within the complex network of the circuit board 10. The PCB vias 1 1 may be formed, for example, as microscopic bridges enabling communication and integration of different circuit elements within the layers of the circuit board 10, as will be apparent to the person skilled in the art.

[0020] The LED 12 is configured as a high-power LED 12 having a maximum luminous flux of more than 1000 Im. In an exemplary embodiment, the LED 12 is an Osram KW CULPM1.TG LED; however, depending on the circumstances, a different type of high-power LED suitable for pulsed operation may of course also be used. It is noted that the light pulse generated by the LED 12 can be coupled into the scanning microscope by means of a suitable holder arrangement and coupling optics, as is known to the person skilled in the art. However, since these do not form part of the present invention, they are not shown in the drawings, for the sake of clarity.

[0021] The illumination system 100 according to the invention comprises at least one resistor 16 connected in series with the LED 12, by means of which the magnitude of the current flowing through the LED 12 can be regulated. The at least one resistor 16 may be configured, for example, as a high-power thick-film resistor known per se. In a particularly preferred embodiment, the system 100 comprises a plurality of resistors 16 connected in parallel with each other, which together are connected in series with the LED 12. An advantage of this embodiment is that, in pulsed operation, the individual resistors 16 are subjected to a lower load and thus to reduced stress, and, moreover, the dissipation of the generated Joule heat is also facilitated by the larger combined surface area of the plurality of resistors 16.

[0022] The illumination system 100 further comprises the at least one capacitor 18 connected in series with the LED 12 and the at least one resistor 16, the function of which is to store and provide the energy required for operating the LED 12. During operation of the system 100, the at least one capacitor 18 is discharged through the LED 12. Since the energy of the light pulse produced by the LED 12 is proportional to the voltage of the capacitor 18, in a particularly preferred embodiment the at least one capacitor 18 is not fully discharged during the flashing of the LED 12, but only about 1 % of the stored energy is utilised, in order to ensure that, during the light pulse, the voltage of the capacitor 18, and thus the luminous output of the LED 12, decreases as little as possible. This can be achieved by appropriately dimensioning the at least one capacitor 18 and the LED 12. In an exemplary preferred embodiment, the at least one capacitor 18 and the LED 12 are dimensioned such that a maximum energy storable by the at least one capacitor 18 is at least fifty times an energy required to flash the LED 12 with a pulse having a full width at half maximum of 1 ps at a maximum power of the LED 12. The at least one capacitor 18 may be configured, for example, as a multilayer ceramic capacitor of the MLCC type known per se, or as another high-capacitance capacitor. In a preferred embodiment, the system 100 comprises, coupled to the at least one capacitor 18, a charging circuit 19 for regulating the voltage of the at least one capacitor 18 (not shown in the drawings), by means of which the voltage of the capacitor 18, and thus the energy of the light pulse produced by the LED 12, can be set. Like any physical device, the capacitor 18 is made of materials having a finite resistance, and, to take this into account, it is customary to introduce the concept of so-called equivalent series resistance, ESR. The ESR, which at high frequency is the alternating-current impedance of the capacitor, is a temperature- and frequency-dependent value, and comprises the resistance of the dielectric, and the direct-current resistance of the connections between the terminals, the dielectric, and the electrodes, as is known to the person skilled in the art. The ESR characteristic of the capacitor 18 determines the total l2R loss, which is particularly important in switched-mode and powerelectronics applications. Capacitors having a higher ESR value supply the external circuit with greater difficulty, because they charge and discharge more slowly. In a particularly preferred embodiment, the system 100 therefore comprises a plurality of capacitors 18 connected in parallel with each other, which together are connected in series with the LED 12 and the at least one resistor 16. An advantage of this embodiment is that, by paralleling the capacitors 18, the resultant ESR can be reduced.

[0023] The system 100 according to the invention further comprises the switching element 14 connected in series with the LED 12, the at least one resistor 16 and the at least one capacitor 18. In the context of the present invention, the switching element 14 is understood to be an electronic component suitable for closing, for a short period of time (on the order of ps), the circuit formed by the LED 12, the at least one resistor 16 and the at least one capacitor 18. In a preferred embodiment, the switching element 14 comprises a MOSFET 14’, that is, an insulated-gate fieldeffect transistor. It is noted that, in this embodiment, the switching element 14 preferably further comprises other elements commonly used with switched-mode MOSFETs 14’, for example a gate driver circuit 20 known per se (not shown in the drawings), which controls the MOSFET 14’ based on an external signal, for example a signal from a scanning microscope.

[0024] Figures 2a to 2c illustrate exemplary embodiments of the LED illumination system 100 according to the invention, shown without the circuit board 10. It is noted that Figures 2a to 2c merely illustrate the order, within the circuit, of the LED 12, the at least one resistor 16, the at least one capacitor 18 and the switching element 14 connected in series with each other, and, in the figures, the direction of current is anticlockwise. In the embodiments illustrated in Figures 2a and 2b, for example, in the circuit the switching element 14 is arranged after the LED 12 and before the at least one capacitor 18 with respect to the direction of current flow in the circuit. The switching element 14 may be connected in series directly after the LED 12 (see Figure 2a), or indirectly, with the at least one resistor 16 interposed (Figure 2b). That is, in Figure 2a the at least one resistor 16 is arranged after the at least one capacitor 18 and before the LED 12 with respect to the direction of current flow in the circuit, whereas, in the embodiment shown in Figure 2b, the at least one resistor 16 is arranged after the LED 12 and before the switching element 14 with respect to the direction of current flow in the circuit. The embodiments illustrated in Figures 2a and 2b are particularly preferred, since, in this case, an N-channel MOSFET 14’ can be used in the switching element 14, which can be operated with good efficiency and thus with a low series resistance. In the embodiment shown in Figure 2c, the switching element 14 is arranged after the at least one capacitor 18 and before the LED 12 with respect to the direction of current flow in the circuit. In this case, a P- channel MOSFET 14’ is to be used in the switching element 14, or an N-channel MOSFET in a so-called “high-side” configuration, as is known to the person skilled in the art.

[0025] In the LED illumination system 100 according to the invention, the LED 12, the switching element 14, the at least one resistor 16, and the at least one capacitor 18 are connected in series with each other and together define a loop-shaped circuit passing through the circuit board 10 that is openable and closable by the switching element 14, such that the LED 12 is arranged on the first side 10a of the circuit board 10, and the switching element 14, the at least one resistor 16, and the at least one capacitor 18 are arranged on the second side 10b of the circuit board 10. In the embodiment shown in Figure 1 , for example, the LED 12 is arranged on the first side 10a of the circuit board 10, and the switching element 14, the at least one resistor 16 and the at least one capacitor 18 are arranged on the second side 10b of the circuit board 10. The direction of current in the loop is indicated by an arrow. It is noted that, depending on the circumstances, embodiments are of course also conceivable in which, in addition to the LED 12, one or two of the switching element 14, the at least one resistor 16 and the at least one capacitor 18 are arranged on the first side 10a, next to the LED 12. By virtue of the fact that the components of the system 100 are fixed to the first and second sides 10a, 10b of the circuit board 10, the individual components can be arranged very close to each other, since the thickness of the circuit board 10 is negligible even compared with the dimensions of the components. In this manner, the loop area, and thus the parasitic inductance and capacitance, can be substantially reduced. In order to reduce the parasitic inductance and capacitance further, in a particularly preferred embodiment the LED 12, the switching element 14, the at least one resistor 16, and the at least one capacitor 18 are arranged on the first and second sides 10a, 10b of the circuit board 10 and are configured to minimise the loop area. The loop area can, for example, be minimised in that two and two of the components of the illumination system 100 are arranged on the first and second sides 10a, 10b such that the components located on the same side 10a, 10b are arranged closely adjacent to each other, while the components located on different sides 10a, 10b are separated by the circuit board 10 but are arranged as close as possible to each other, “one below the other” (not shown in the drawings).

[0026] It is clear that other alternative solutions to those embodiments presented herein may also be conceived by the person skilled in the art, which nevertheless fall within the scope defined by the claims.

Claims

Claims1. A short-pulse LED illumination system (100), comprising a circuit board (10) having a first side (10a) and an opposite second side (10b), and an LED (12), a switching element (14), at least one resistor (16), and at least one capacitor (18) fixed to the circuit board (10), characterised in that the LED (12), the switching element (14), the at least one resistor (16), and the at least one capacitor (18) are connected in series with each other and together define a loop-shaped circuit passing through the circuit board (10) that is openable and closable by the switching element (14), such that the LED (12) is arranged on the first side (10a) of the circuit board (10), and the switching element (14), the at least one resistor (16), and the at least one capacitor (18) are arranged on the second side (10b) of the circuit board (10).

2. The LED illumination system (100) according to claim 1 , characterised in that the LED (12), the switching element (14), the at least one resistor (16), and the at least one capacitor (18) are arranged on the first and second sides (10a, 10b) of the circuit board (10) and are configured to minimise the loop area.

3. The LED illumination system (100) according to claim 1 or 2, characterised in that, in the circuit, the switching element (14) is arranged after the LED (12) and before the at least one capacitor (18) with respect to a direction of current flow in the circuit.

4. The LED illumination system (100) according to claim 3, characterised in that the at least one resistor (16) is arranged after the at least one capacitor (18) and before the LED (12) with respect to the direction of current flow in the circuit.

5. The LED illumination system (100) according to claim 3, characterised in that the at least one resistor (16) is arranged after the LED (12) and before the switching element (14) with respect to the direction of current flow in the circuit.

6. The LED illumination system (100) according to any one of claims 3 to 5, characterised in that the switching element (14) comprises an N-channel MOSFET(14’).

7. The LED illumination system (100) according to any one of claims 1 to 6, characterised in that the at least one resistor (16) is configured as a high-power thick-film resistor.

8. The LED illumination system (100) according to any one of claims 1 to 7, characterised in that the LED illumination system (100) comprises a plurality of resistors (16) connected in parallel with each other.

9. The LED illumination system (100) according to any one of claims 1 to 8, characterised in that the at least one capacitor (18) is configured as a multilayer ceramic capacitor.

10. The LED illumination system (100) according to any one of claims 1 to9, characterised in that the LED illumination system (100) comprises a plurality of capacitors (18) connected in parallel with each other.1 1 . The LED illumination system (100) according to any one of claims 1 to10, characterised in that the at least one capacitor (18) and the LED (12) are dimensioned such that a maximum energy storable by the at least one capacitor (18) is at least fifty times an energy required to flash the LED (12) with a pulse having a full width at half maximum of 1 ps at a maximum power of the LED (12).

12. The LED illumination system (100) according to any one of claims 1 to1 1 , characterised in that the switching element (14) comprises a gate driver circuit

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