The invention relates to a light source for integration in a headlamp comprising a printed circuit board, on which at least one light-emitting diode (LED) and a converter unit for supplying power to the LED are arranged, and to a method for producing such a light source.
Such light sources are finding increasing numbers of applications in particular in the motor vehicle sector in headlamps for main beam or indicator light functions and are typically combined with optical elements such as reflectors, projectors and fiberoptic conductors.
A conventional LED headlamp used in practice has, for example, one or more LED modules and, in order to supply a constant current to the LEDs, one or more offset electronic ballasts. The connection of the LED modules to the ballasts is performed by means of a plug-type and cable connection. Since pulse-width-modulated signals are used for actuating the LEDs, the lines required for this emit radiation in a range which can result in interference in the case of electrical or electronic devices in the surrounding environment. This problem is generally combated with the aid of output filters in the control device, but this results in limitations in the case of low duty factors since said duty factors are downwardly limited by the filter elements. In addition, the use of boost and buck converters which is required in offset control devices results in poor efficiency of the control device and requires a correspondingly large heat sink. An LED headlamp therefore places more stringent requirements on the internal cable harness and in addition requires a greater amount of installation space than conventional halogen headlamps, for example.
The integration of a converter unit and at least one LED on a common printed circuit board has the advantage that no dedicated control device which is offset from an LED module needs to be used, and this therefore simplifies the application, i.e. both the design of the headlamp and the fitting of the light source, and reduces or avoids outgoing electromagnetic radiation. For example, light sources are known which consist of completely closed-off, standardized modules and provide a defined thermal, electrical and optical interface on the outside. In this case, a disadvantageous restriction of the design freedom as regards optical and thermal properties is accepted in order to achieve a maximum degree of replaceability and standardization.
In the known light sources, four or more LEDs are connected in series and are supplied by a common voltage converter, for example a SEPIC (single-ended primary inductance converter), which steps up or steps down the available vehicle electrical distribution system voltage correspondingly; in this case, the LEDs are embodied as chip-on-board modules. Both measures reduce the production costs of the standardized light source, but in particular the voltage converter for step-up conversion requires relatively complex or voluminous cooling owing to the poor energy efficiency.
In contrast, the object of the invention consists in providing a more compact light source which additionally permits variable placement of LEDs.
This object is achieved in the case of a light source of the type mentioned at the outset by virtue of the fact that the converter unit and the at least one LED are in the form of surface-mounted components (surface-mounted devices, SMD), wherein the converter unit has one or more step-down converters. In particular, all of the converter circuits of the converter unit are step-down converters. Such a light source is characterized by particularly low heat losses in the voltage conversion since purely step-down converters have a higher electrical efficiency than any other type of switching controllers. Therefore, more simple and more compact cooling can be used. In addition, the light source can be matched flexibly to different requirements owing to the LEDs provided as SMD component parts and converter units during production without any changes to the printed circuit board. For example, population with LEDs can be reduced in comparison with full population of the printed circuit board and/or the populated positions can be selected appropriately. Overall, a substantially more flexible and more compact and therefore even more easily integratable and universally useable light source therefore results, which has a low overall weight and whose installation is associated with minimum wiring complexity. By virtue of the partitioning of the system functions, in addition as far as possible reuse of the components in different applications and therefore scale effects are achieved.
The LED configuration should advantageously be selected such that each LED has a supply voltage which is equal to or less than a supply voltage of the converter unit. In the case of a typical vehicle electrical distribution system voltage of from 6 to 18 V, this can mean that at most two LEDs are connected in series. A further advantage of such a circuit consists in that failure of a single LED can be diagnosed easily and reliably with the aid of a measurement of the LED string voltage.
Therefore, preferably only step-down converters, but not step-up converters, are therefore provided as the actual converter.
If a greater number of LEDs is required, it is favorable if the converter unit has a plurality of step-down converters and is in the form of an integration set (system-in-package, SiP). By virtue of integration in an SiP, the space requirement can be kept relatively low in comparison with separate voltage converters and nevertheless a plurality of highly efficient converters can be designed for supplying power to the LEDs.
In order to avoid the need for an inefficient step-up converter when using more than two LEDs, it is furthermore advantageous if a parallel circuit comprising a plurality of LEDs is arranged on the printed circuit board and is connected to the converter unit. Owing to the parallel circuit, an addition of the LED forward voltages is prevented and it is also possible for only (at least) one step-down converter to be provided for supplying power to a plurality of LEDs from a vehicle electrical distribution system voltage.
In order to cool the LEDs and the converter unit, it is further advantageous if the printed circuit board is a thermal substrate, preferably a metal-core printed circuit board, in particular with an aluminum or copper core. By virtue of the special printed circuit board, heat is spread directly at the LEDs and, as a result, the LED junction temperature is kept low.
If a cooling device, preferably a cooling plate or a die-cast body, is connected, in particular welded, to the printed circuit board on a side opposite the LED, improved dissipation of the power losses generated of the LEDs and the converter unit can be achieved. Owing to the relatively high efficiency of the voltage converter (step-down converter) used, the cooling device can generally be more compact than in the case of known light sources.
It is advantageous for optimum heat distribution if the cooling device consists of the same material as a metal core of the printed circuit board. In addition, any mechanical stresses between the printed circuit board and the cooling device can thus be reduced and the connection of the two parts, for example by means of welding, is simplified.
It has proven to be particularly advantageous if the converter unit has an interface for the transmission of diagnostic and/or control data, which is preferably in the form of a single-wire interface, in particular in the form of a local interconnect network (LIN) interface. A central monitoring and/or control device can be connected to the light source via such an interface so that any malfunctions can be communicated to other systems, for example, and control of the light source is made possible without manipulation of the supply voltage.
A further improvement in comparison with the prior art can be achieved if, together with the converter unit and the at least one LED, a binning coding element, in particular a binning coding resistor, is arranged on the printed circuit board. “Binning” is understood to mean in this context a class division of the LEDs used depending on the luminous efficacy and tone thereof. When the binning of the LEDs used is known, a uniform luminous color can be achieved in the case of differently classified LEDs by corresponding matching of the supply current. In the case of an arrangement on the printed circuit board, the binning coding element does not need to be read externally via an additional line, for example a power supply interface, as has been conventional until now in the prior art, but can set the current assigned to a binning, i.e. a class division of the LEDs, directly, i.e. locally on the printed circuit board. The integration of the binning coding element is made possible by virtue of the fact that the binning of the placed LEDs is known during manufacture and therefore the binning coding element assigned to the respective binning can be selected immediately and likewise placed. The binning coding element may be, for example, a binning coding resistor or a logic module programmed with the respective binning setting. Advantageously, the solution specified here is much more reliable, for example less sensitive to moisture or dirt on the printed circuit board, avoids compatibility problems when purchasing LEDs, and is at the same time less expensive than gauging of a binning coding resistor.
It has furthermore proved to be favorable if a shunt for measuring an LED current is connected in series with at least one LED, preferably between the LED and ground. Owing to the integration of LEDs and converter unit on only one printed circuit board, a short-circuit strength of the LED driver, i.e. of the converter unit, with respect to a short circuit to ground or with respect to the supply voltage is not required. A shunt for current measurement can therefore be arranged upstream (“highside”) and downstream (“lowside”) of an LED string in relation to the voltage. In this case, the downstream arrangement, namely between the LEDs and ground, has the advantage of a much lower level of complexity in terms of circuitry for the current measurement, with the result that costs for the light source can be reduced.
The method of the type mentioned at the outset achieves the abovementioned object by virtue of the fact that, in the case of the production of light sources of the abovementioned type, in the case of an otherwise identical design of the light source, the number and position of the LEDs connected to the printed circuit board are configured. Using the in each case present configuration, therefore, a range of differently populated printed circuit boards can be produced, as a result of which the flexibility of the present light source in terms of the optical and thermal properties is considerably increased in comparison with a completely standardized light source.
In this context, it has proven to be particularly favorable if at least one LED is connected to the printed circuit board by reflow soldering. Reflow soldering enables a simple, quick and reliable connection and can at the same time be modified easily corresponding to a configured placement scheme by virtue of any superfluous soldering points being omitted or not being provided with the solder.
The invention will be explained in further detail below with reference to particularly preferred exemplary embodiments, to which the invention is not intended to be restricted, however, and with reference to the drawings, in which, specifically:
a shows a schematic block circuit diagram of a compact light source having a single step-down converter;
b shows a variant of the light source shown in
a shows a schematic block circuit diagram as shown in
b shows a variant of the light source shown in
In order to cool the component parts 2, 5, in particular the LEDs 2, which are subject to losses, the printed circuit board 3 is in the form of a metal-core printed circuit board (IMS), for example with an aluminum or copper core 11 (cf.
As is shown in
In addition to the connections 14 to a vehicle electrical distribution system, the converter unit 5 has an LIN interface 15 (cf.
The variant of the light source 1 shown in
However, it is also possible for heat sinks consisting of other metals or of ceramic to be used. Very efficient heat dissipation from the SMD component parts 2, 5 to the surrounding environment, for example, is achieved by such a cooling device 18. If necessary, in addition forced convection can be achieved with the aid of a fan.
During the regulation of the power MOSFET 8, the current controller 24 also takes into consideration the resistance value of a binning coding resistor 29, which does not belong to the converter unit 5, but is likewise arranged on the printed circuit board 3 and is connected to the current controller 24. The binning coding resistor 29 to be placed is selected depending on the binning of the LEDs 2 during manufacture of the light source 1, with the result that the present light source 1 can be produced with a universally useable converter unit 5 largely irrespective of the binning of the respectively placed LEDs 2.
In the circuit shown in
In the case of the light source 1 illustrated in
In a manner comparable with
Number | Date | Country | Kind |
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10 2013 202 282.4 | Feb 2013 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2014/052058 | 2/3/2014 | WO | 00 |