Abstract:
A light system for growing plants indoors comprising a light source mounted on a single, two sided FR4 based PCB board.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention provides a LED light utilized in the field of indoor horticulture lighting, the light comprising at least three LEDs each emitting light of a different frequency. 
     2. Description of the Prior Art 
     High intensity discharge (“HID”) lights have been used in indoor horticulture for many years. Specifically, there are two types of HID lights used for indoor growing: metal halide (MH) and high pressure sodium (HIPS) based systems. MH lights have a blue tint and are typically used during vegetative growth. HPS lights emit a yellowish/red tint used for the flowering portion of the grow process. 
     The field of light emitting diodes (LED) is a rapidly advancing technology that has the promise to significantly reduce power consumption for general lighting as well as for indoor horticulture. Over the last five years, ever brighter and more efficient LED emitters in the 3-5 W range have been developed which is a significant improvement over the LEDs of 10 years ago that did not exceed 50 mW. When used for indoor growing, LED based lights have the advantage of being higher efficiency than other lights. In addition, LEDs can be focused on the photo-synthetically active regions of the light spectrum, namely blue and red (400-500 nm and 600-700 nm respectively) without wasting energy on the green (500-600 nm) region which is not very useful to plants during the vegetative phase. LEDs emit light in a uni-directional fashion, eliminating the need for reflectors, further improving efficiency. Finally, LEDs have a lifetime of over 50,000 hours compared to less than 10,000 for HID systems, which reduces their overall cost of operation. 
     Key elements in designing an LED light fixture are:
         1. Thermal management: LEDs are most efficient and last longer at lower temperatures.   2. Driver Circuit: LEDs are typically connected in series and driver circuits are typically switching mode power supplies with constant current control.   3. Primary optics (and secondary optics if necessary) to focus the light energy where needed.       

     High brightness LEDs (LEDs with power greater than 1 W) are typically mounted on metal core printed circuit boards (MCPCBs). However, MCPCBs are relatively new technology and typically cost three times or more compared to FR4 PCBs (fiberglass based PCBs). In addition, hole plating can not be accomplished with MCPCBs. In this regard, it is difficult to integrate the power supply/LED driver circuit with high brightness (HB) LED on the same MCPCB. Most HB LED light fixtures mount LEDs on an MCPCB then use a separate FR4 based PCB for the power supply/driver circuit resulting in higher assembly costs. 
     What is desired is to provide an improved LED circuit board for use in indoor plant growing operations, the LED circuit board integrating all the system components on a single PCB and wherein heat generated by the system components is significantly reduced. 
     SUMMARY OF THE INVENTION 
     The present invention provides a LED based optic fixture for growing plants wherein a single, two sided. FR4 based PCB is used which comprises the following:
         1. Two switching mode power supplies (SMPS) rated at 75 W each;   2. Two constant current LED driver circuits;   3. High Brightness (HB) LEDs (two strings). Thermal vias are used to thermally connect the HB LEDs on one side of the PCB to the heat sink on the other side. In addition, the through hole and surface mounted technology (SMT) components are combined on the PCB to lower assembly cost.       

     The present invention significantly lowers the cost of manufacturing an LED light fixture by using:
         1. A low cost FR4 based PCB;   2. A fully automated SMT based assembly; and   3. A single SMT placement and reflow solder run for the LED, SMPS and driver circuit.       

     The final assembly is faster and done at a lower cost since additional assembly and wiring for the LED driver circuits is not required and the overall design enables the device to be easily sealed from humidity. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawing therein: 
         FIG. 1  is a top view of the LED grow light circuit board in accordance with the teachings of the present invention; 
         FIG. 2  is a side view along line  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a bottom view of the circuit board shown in  FIG. 1 ; 
         FIG. 4  is a side view along line  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a detail of a portion of the circuit board shown in  FIG. 1 ; 
         FIG. 6  is a view along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is an enlarged area view at arrow  7  of  FIG. 3 , the other squares being identical thereto; 
         FIG. 8  is a simplified assembly view of the circuit board components shown in  FIG. 1 ; 
         FIG. 9  shows the circuit board of  FIG. 8  assembled; and 
         FIG. 10  is a view along line  10 - 10  of  FIG. 9 . 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1 and 2 , a top view of the LED grow light fixture  10  of the present invention is illustrated. In accordance with the teachings of the invention, a single FR 4  based two sided printed circuit board (PCB)  12  has a top layer  14  and bottom layer  16 . A plurality of constant current LED emitter components  18  are mounted on layer  14  using conventional PCB component mounting processes, such as the surface mount technology (SMT) which allows SMT components to be more densely populated on a PCB. The LEDs generate radiation of five different wavelengths corresponding to the colors blue, deep blue, red, deep red and white. A pair of switching mode supplies  20  and  22  (preferably 75 W each) are formed on layer  14  of PCB  12  as illustrated. Since the total power of fixture  10  is 130 W, in order to maintain the proper dimensions of the, the design preferably is split such that each power supply drives half of the LEDs on the fixture. The LEDs are connected in series forming two distinct circuits driven by two separate power supplies. As shown in  FIG. 2 , all the through hole components  17  (connector),  19  (capacitor),  21  (capacitor),  23  (choke),  27  and  29  (inductors),  31  (capacitor),  33  and  35  (inductors),  37  (capacitor),  39  (inductor) and  41  (capacitor) are mounted on the bottom layer  16 . The through hole components have leads that extend into plated holes (vias) and are soldered on top surface layer  14 ) (illustrated by reference numeral  24 ) are mounted to bottom layer  16 . The through holes, or thermal vias, are also used to manage the heat generated by the LEDs and power supplies; the details of using the vias to manage the heat are described in application notes issued, for example by OSRAM Opto Semi conductors GmbH, Regensburg, Germany and Phillips Electronics, N.V., the Netherlands, suppliers of LED emitters. The thermal vias  42  thermally connect the high brightness LEDs mounted on top layer  14  to heat sink  60 , secured to bottom layer  16 . The vias  42  are plated holes formed on the LED pads  40  to provide heat conduction (all the holes shown in the figures are vias). 
     PCB  12  is mated to heat sink  60  using mounting screws  50  ( FIG. 8 ) extending through a plurality of screw holes  51 . A high thermal conductivity dielectric  52  ( FIGS. 8 and 9 ) is inserted between PCB  12  and heat sink  60 . A dielectric material which has been successfully utilized is the Bergquist SilPad 400. 
     PCB  12  has all the surface mount components on the same side as the LED emitters and the through-hole components, such as large inductors  29 ,  33 ,  35 , and  39  are mounted on bottom layer  16  away from the LED emitters. 
       FIGS. 5 and 6  shows LED emitter  18  secured to heat sink pad layer  40  with vias  42  and  FIG. 6 , in addition, shows bottom layer heat sink  44  and LED conducting pads  43 . It should be noted that the high brightness LEDs used on fixture  10  have three terminals that are soldered to the top layer  14  of PCB  12 . The anode/cathode terminal  43  are electrically conducting whereas the slug (not shown in the figures) is used for thermal management purposes. Heat sink layers  40  and  44  are copper layers that are used as part of the thermal management (i.e. dissipation of heat) for fixture  10  and not for its electrical conducting characteristics. Heat sink  60  is preferably made of aluminum and is a separate component that combines with layers  40  and  44  to dissipate the heat generated during the time when fixture  10  is powered on. Referring to  FIG. 6 , the thermal path is as follows: heat from LED  18  travels through slug within the LED to top layer  40  through vias  42  to the bottom layer  44 , through dielectric  52  to heat sink  60 . 
       FIG. 7  is a bottom view of a LED emitter  18  showing a plurality of vias extending through heat sink layer  44 . 
       FIG. 8  is an assembly view showing the various components that comprise fixture  10  and  FIG. 9  shows those components assembled. Finally,  FIG. 10  illustrates how mounting screws  50  join PCB  12  to heat sink  60  (extruded aluminum pieces  58  are attached to heat sink  60  and function as the casing for fixture  10 ). 
     It should be noted that prior art LED fixture designs employ separate SMPS and driver circuit PCBs. The driver circuits control the current furnished to the LEDs so that LED current is maintained substantially constant throughout their operation. Other designs manufacture LED drivers which encompass the SMPS and LED constant current driver circuit that are used in many LED light fixtures. 
     The present invention thus combines switching mode power supply (SMPS) technology on the same PCB that encompasses high brightness LEDs along with their thermal management components and driver circuitry provides a LED grow light that is unique and less costly than currently available systems. 
     While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.