Patent Publication Number: US-2021195801-A1

Title: Distribution point unit (dpu) with improved thermal management and electrical isolation

Description:
FIELD OF THE INVENTION 
     The present invention relates to a Distribution Point Unit (DPU) with improved thermal management and electrical isolation. The DPU is used in data communication networks that employ both fibre optic and copper lines. 
     BACKGROUND TO THE INVENTION 
     FTTdp (Fiber to the Distribution Point) brings the fibre optic connection point of a fibre-optic telecommunications network closer to the premises (home) of an end user (customer) in comparison to Fiber to the Node (FTTN) broadband architectures. FTTdp requires access to Customer Premises Equipment (CPE) located at the premises of a customer and which includes, as a minimum, a modem and POTS (Plain Old Telephony System) wiring. In this broadband network architecture the last few meters (e.g. up to 200 metres) of the broadband connection supplied through a passive optical network (PON) are provided by the existing twisted pair cabling or copper wire (copper lead-ins) to the premises that is currently used for legacy technologies such as POTS and xDSL. 
     In a gigabit passive optical network (GPON), networking protocols can provide 2.4 Gbps download speed for up to 20 kilometres and the NG-PON2 networking protocol can provide 40 Gbps download speed for up to 60 kilometres. To match the increased speed capabilities that come with bringing the fiber nodes closer to the home, the access technology through the copper wire is improving to provide higher data transfer speeds. Such access technology on the copper wire can include G.fast (1 Gbps download speed for up to 100 metres) and its successor XG.FAST (3.5 Gbps download speed for up to 100 metres). The signal frequency of G.fast starts at 106 MHz (it can be doubled to 212 MHz) and XG.fast uses between 350 MHz and 500 MHz. This copper wire access technology provides more bandwidth for transferring data over copper at higher speeds. To address the problem of signal attenuation at higher frequencies, G.fast and XG.FAST both use vectoring to generate an anti-phase signal to cancel the majority of interference. 
     A distribution point unit (DPU) is typically a small piece of telecommunications equipment that is used in broadband network architecture such as FTTdp. In use, the DPU is placed in an existing underground communications pit or secured outdoors to a wall or power pole, depending on suitability and other factors. In one example, the DPU includes one fibre-optic cable port or tail for its upstream connection to the optical network, an optical-electrical signal converter that converts incoming optical signal and electrical signals into electrical and optical signals, depending on traffic direction to/from the premises, and directs network traffic to four or more downstream copper wire or coaxial cable ports or fly wires of the DPU. The downstream connections connect to a device for connecting the premises to the telecommunications network, such as a Network Connection Device (NCD), for example, the modem at the customer (user) premises. A Reverse Power Unit (RPU) located at the premises provides a power feed to the DPU via the copper wire. 
     The placement of the DPU in the external environment requires it to be environmentally hardened to withstand harsh and varied weather and environmental conditions for many years. The DPU should be operable in very challenging circumstances such as immersion in water or mud, and long exposure to direct sunlight in summer. The DPU should operate in extreme temperatures since temperatures can range from −23° C. to 51° C. in certain countries. 
     Other criteria for DPUs include that they are required to be tamper-resistant, capable of sending a notification if they have been tampered with, be inexpensive to manufacture and install, and the connections at the DPU should be accessible outside the DPU housing to enable a technician of a network provider to connect them with copper lead-ins and the fibre-optic cable leading to the DPU without opening the DPU housing. 
     It is an important function of a DPU to manage the temperature of its electronics. The DPU needs to dissipate towards the environment heat generated from electronic components and optical components housed within the DPU during operation to avoid these components over-heating beyond a maximum operating temperature range. Over-heating may cause the DPU to be unstable, unreliable and malfunction and therefore require a technician to make an on-site visit to repair or replace the DPU. Thus, metallic housing components are often used to ensure adequate heat transfer away from electronic components. However, it is equally essential to ensure electrical isolation of relevant components housed within the DPU. Finding a suitable lay out that addresses these two requirements is not a straight forward exercise. 
     SUMMARY OF THE INVENTION 
     The inventive concept arises from a recognition that effective thermal management and electrical isolation of electrical components from the metallic housing components of the DPU enables the DPU to have long term reliability. A particular challenge arises in the context of hermetically sealed DPUs that prevent the ingress of dirt and moisture when the DPU is installed in a harsh environment. The inventive concept recognises that passive thermal management, relying solely on the thermo-dynamics of conduction, convection and radiation to complete the heat transfer process, is important in such cases. 
     In a first aspect, the present invention provides a Distribution Point Unit (DPU), comprising an at least partially metallic housing having an exterior surface and interior surface. The DPU further comprises an electronic board including electronic components housed within the housing. The DPU further comprises at least one heat sink housed in displaceable manner within the housing and configured to conduct heat generated by at least some of the electronic components at the electronic board into the housing. The DPU further has at least one resilient thermal pad placed between one side of the electronic board and the heat sink in at least partial, physical contact making engagement, the thermal pad being heat conductive and electrically insulating to electrically isolate the first heat sink from the electronic board components. Finally, the DPU also includes at least one mechanical fastener engaging with the heat sink and operable from an exterior of the housing, the arrangement being such that in response to tightening of the mechanical fastener (i) the heat sink is pressed against the interior surface of the housing to maintain contact (and preferably a force fit) between the heat sink and the interior surface and (ii) the engagement between the thermal pad, the heat sink and components of the electronic board is maintained. 
     The present invention, in another aspect, provides a Distribution Point Unit (DPU) having improved thermal management and electrical isolation. The DPU comprises an at least partially metallic housing having an exterior surface and interior surface. The DPU also comprises an electronic board including electronic components housed within the housing. The DPU also comprises a first resilient thermal pad placed on a first side of the electronic board. The DPU also comprises a first heat sink configured to rest against the first thermal pad. The DPU also comprises a first mechanical fastener that is operable from an exterior of the housing, and which engages with the first heat sink. The configuration is such as to draw the first heat sink against the interior surface of the housing in response to tightening of the first mechanical fastener, whereby intimate physical (or positive) contact between the first heat sink and the interior surface of the housing is achieved. The first thermal pad is heat conductive and electrically insulative to electrically isolate the first heat sink. The first thermal pad is devised such that during tightening of the first mechanical fastener, its resilient nature and shape allows it to maintain surface-contact with the electronic board as well as the first heat sink, thereby providing a heat dissipation path between electronic components of the electronic board and the interior surface of the housing. 
     Preferably, the heat dissipation path between the electronic components, first thermal pad, first heat sink and the at least partially metallic housing is substantially without an air gap. 
     The DPU may further comprise a second resilient thermal pad placed on a second side opposite the first side of the electronic board. The DPU may further comprise a second heat sink configured to rest against the second thermal pad. The DPU may further comprise a second mechanical fastener that is operable from the exterior of the housing, and which engages with the second heat sink. The configuration is such as to draw the second heat sink against the interior surface of the housing in response to tightening of the second mechanical fastener, whereby intimate physical (or positive) contact between the second heat sink and the interior surface of the housing is achieved. The second thermal pad is electrically insulative to electrically isolate the second heat sink. The second thermal pad is devised such that during tightening of the second mechanical fastener, its resilient nature and shape allows it to maintain surface-contact with the electronic board as well as the second heat sink, thereby providing a heat dissipation path between electronic components of the electronic board and the interior surface of the housing. Preferably, the heat dissipation path is substantially without an air gap. 
     The above described arrangement allows mounting of the electronic board in electrically isolated manner within the at least partially metallic housing, and then drawing the first (and second) heat sink(s) into an essentially forced (and positive), abutting contact with the inside surface of the housing, whilst the first (and second) thermal pad(s) maintain physical contact with all or at least the most relevant, heat-generating components of the DPU attached to the electronic board. It will be understood that the physical contact need not extend to all heat-generating components, as long as the majority thereof remain in heat-conducting contact with the thermal pad(s). 
     The DPU may further comprise a first manifold and preferably a second manifold configured to support the electronic board and prevent physical contact between the electronic board and the at least partially metallic housing, and for positioning the heat sinks relative to the electronic board. The manifold(s) are advantageously comprised of a selected electrically insulating material, and may also comprise materials conducive to heat transfer from the electronic board to the partially metallic housing. Physical isolation helps with surge protection and enables the DPU to be installed without an external grounding point. Installing an earth point or grounding point may frequently cost more than the entire installation cost of the DPU. Therefore there is a cost advantage in providing a DPU that does not require an external earth. 
     The mechanical fastener may comprise an integral deformable radial sealing member arranged to seal an interface between the fastener and a port or opening through which the fastener extends into the inside of the at least partially metallic housing. 
     The mechanical fastener may be a sealing screw with an integral or separate o-ring. 
     The thermal pad(s) can advantageously be made from a silicone elastomer loaded with thermal conductive filler to provide both thermal conductivity as well as the required resilient compressibility degree to ensure physical contact is maintained between most if not all the heat-generating components housed on the electronic board and the heat sinks as these are displaced into forced abutment with the inner surface of the at least partially metallic housing. 
     The thermal pad(s) may be made from SR-1000C thermal conductive silicone rubber. 
     Noting that electronic boards (such as PCBAs) typically carry most components on one side and the opposite side is used to provide the electrically conductive tracks between components, and assuming centric positioning of the board proper within the at least partially metallic housing with respect to its mayor external walls, it is advantageous to provide the second thermal pad with a thickness that is greater than that of the first pad. This allows the second thermal pad to expand (i.e. undergo the same degree of decompression) by the same degree as the first thermal pad in the process of moving the first and second heat sinks into pressed-on engagement against the inner surfaces of the at least partially metallic housing. 
     Advantageously, the second heat sink may be larger geometrically or by mass than the first heat sink. This measure caters for differential heat generation at the component carrying (first) side of the electronic board and the (second) side of the board where the electrically conductive tracks prevail. 
     Preferably, the first heat sink can comprise a threaded hole to receive the first mechanical screw fastener, and the hole is preferably located about the centre of the first heat sink to achieve even tightening of the first heat sink against the inner surface of the housing upon tightening via the first mechanical fastener. Advantageously, a similar arrangement will be present at the second heat sink. 
     Advantageously, the first and the second heat sinks will exhibit a substantially planar engagement surface which can be brought into planar abutment against the inner surface of the at least partially metallic housing, thereby to provide an increased heat transfer area. 
     The heat sink bodies will preferably be made from a same metallic material as those parts of the housing that are metallic, thereby minimising contact corrosion and achieving even heat transfer across the interface of the abutting components. 
     In order to improve heat transfer to an exterior of the at least partially metallic housing, the outer surface can be provided with heat dissipation ribs or similar structures. 
     Other advantages and preferred features according to the invention will become apparent to those of ordinary skill upon reading the following description of preferred, non-limiting embodiments of the invention described with reference to the accompanying figures in which like reference numbers denote like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a DPU incorporating various thermal and electric insulation management aspects in accordance with the present invention; 
         FIG. 2  is a partially exploded perspective view showing the main components of the DPU of  FIG. 1 , including an inner DPU-subassembly comprising a PCBA with an opto-electronic converter, an electrically isolating PCBA support structure and heat sink elements, an outer metallic casing and a housing end-cap comprising the optical and electrical connections for coupling the DPU between an optical cable network and POTS wire infrastructure (not shown); 
         FIG. 3  is an exploded perspective view of the inner DPU-subassembly illustrated in  FIG. 2 , illustrating its main components; and 
         FIG. 4  is a side elevation of the DPU of  FIG. 1  in assembled state, partially in section. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A distribution point unit (DPU) is illustrated in  FIG. 1  and shown generally at reference numeral  10 . The DPU  10  comprises a casing  31  including an end cap  20 . The casing  31  is also referred to as an enclosure or housing. The end cap  20  is where optical cable  40  and electrical cables  41  are securely attached and sealed. The end cap  20  comprises at least one port  30  for entry of at least one cable  40  carrying optical signals and four additional ports  30  for cables  41  carrying electrical signals. The DPU  10  also comprises an electronic board (e.g. a PCBA  110 , see  FIG. 3 ) comprising electronic components for processing the optical and electrical signals carried via cables  40 ,  41  and transcribing these information carrier formats, as is known in the art, as well as power circuitry required for running electronic components. 
     As best seen in  FIGS. 2 and 3 , the PCBA  110  is housed a support structure  70 ,  80  which, as a sub-assembly  300 , is inserted and secured within the casing  31  to protect the entire sub-assembly  300  from damage and being contaminated by water and dust. The end cap  20  is devised to hermetically seal-off the open end of casing  31  and be secured to casing  31  in replaceable manner. 
     The DPU housing  31  is made from aluminium. Electrical isolation between the electronics carried on the PCBA  110  and the housing  31  is required to avoid electrically shocking someone who handles the DPU  10  and to avoid transmitting lightning or external shocks along the cables  40 ,  41  to the electronics and other equipment, or users. The casing  31  does not have an external earthing or grounding point. Consequently, the support structure  70 ,  80 , described in more detail below, and which forms part of sub-assembly  300 , is devised to support the PCBA  110  in electrically isolated manner within casing  31 . Furthermore, heat generated by electric components on the PCBA  110  has to be transferred from within the casing  31  to the DPU&#39;s surroundings. In accordance with one inventive concept of the present invention, the DPU  10  is provided with heat sink bodies  50 ,  60  to manage the temperature of its electronics. The DPU  10  has a top heat sink  50  located at the upper side of the DPU  10  and a base heat sink  60  located at the lower side of the DPU  10 . These heat sinks  50 ,  60  cooperate with casing  31  to conduct heat towards heat-radiating structures (such as fins or ribs) formed integrally on exterior surfaces of the casing  31 . 
     Heat Sinks 
     In the preferred embodiment, the heat sinks  50 ,  60  are die cast bodies made from the same aluminium as casing  31 . 
     Using heat sink body  60  as an example, the board-facing surface  62  of heat sink body  60  (but similarly that of heat sink body  50 ) is structured and shaped to reflect the lay-out and shape of critical heat generating components carried on the surface of PCBA  110  which faces the respective heat sink  50 ,  60 . These DPU components will typically include the optical interface (BOSA), main processors, DSL line drivers and other minor components. The heat sink bodies  50 ,  60  are also structured and shaped to accommodate the placement of thermal pads  90 ,  100  which are described in detail below, between their board-facing surfaces  52 ,  62  and the PCBA  110 . 
     As is best seen in  FIG. 4 , both heat sink bodies  50 ,  60  have a substantially smooth and planar casing-facing (outer) surface  51 ,  61  which mirrors the structure of the inward facing inner surface  32  of casing  31 , thereby enabling substantially air-gap-free planar abutment of these surfaces against each other, for reasons explained below. 
     It will be further noted that the foot-print size of bottom heat sink  60  is larger than that of upper heat sing body  50 . The lower side heat sink  60  contacts a large area of the PCBA  110  because there are mainly low-profile components on the underside of the PCBA  110  that press into the thicker thermal base pad  100 . 
     As noted, the heat sink bodies  50 ,  60  have a predetermined shape and structure taking into account the geometry and topography of the main PCBA  110  and daughter PCBA  120  to ensure a close fit which brings the heat sink bodies  50 ,  60  in very close proximity to the main PCBA  110  and daughter PCBA  120 . Electrical isolation and the shape and location of daughter PCBA  120  are also considered when determining the shape for the heat sink bodies  50 ,  60 . The daughter PCBA  120  covers part of the top side of the PCBA  110  and also some components are relatively high or have a complex shape. These factors also determine an optimal shape for the heat sinks  50 ,  60 . Thermal imaging may be initially performed to identify the heat generating areas to optimally determine the topography of the board facing side  52 ,  62  of the heat sink bodies  50 ,  60 . 
     The profiling of the heat sink bodies  50 ,  60  allows for good direct thermal contact (via the thermal pads  90 ,  100 ) to certain PCBA component bodies having a variable height. The contact surface area is maximised. The grade of aluminium used for the heat sink bodies  50 ,  60  ensures optimal thermal conductivity. Preferably, casting grade A413 aluminium is used. 
     These thermal management measures allow heat generated by power-consuming and generating components of the PCBA  110  to transfer via the thermal pads  90 ,  100  and heat sink bodies  50 ,  60  to the casing  31  and then out to the ambient environment. Within the DPU  10 , these components provide direct thermal conduction paths to facilitate thermal management. The DPU  10  should also have electrical isolation between the active components and the conductive aluminium heat sink bodies  50 ,  60 , main body  31  and end cap  20 . 
     Manifolds 
     As noted above, the DPU  10  also needs to provide for electrical isolation between the active (power generating and consuming) components and the conductive aluminium heat sink bodies  50 ,  60 , main casing  31  and end cap  20  which is also made of aluminium. Two plastic manifolds provide this functionality, a top manifold  70  and a base manifold  80  which are shaped such as to assemble into a casing that surrounds the PCBA  110 . The manifolds  70 ,  80  operate synergistically with the heat sink bodies  50 ,  60  to manage the temperature and provide electrical isolation. The manifolds  70 ,  80  assist in positioning the heat sink bodies  50 ,  60  relative to the PCBAs  110 ,  120 , by including shape conforming openings or through holes  71 ,  81  in which the heat sink bodies  50 ,  60  are received and constrained for movement to and away from the casing  31 . 
     During assembly, the manifold halves  70 ,  80  are joined together, with the PCBA  110  and thermal pads  90 ,  100  housed within the cavity defined by the two manifolds  70 ,  80 , and secured to each other using mechanical fasteners, for example, 6 screws or plastic pins  301 . The plastic pins  301  are inserted into sockets integrally formed in the manifolds  70 ,  70  and are secured during assembly via an interference fit. The heat sink bodies  50 ,  60  are then inserted into the receptacles  71 ,  81  to form a single unit (completed sub-assembly  300 ). Cooperating guide and placement/support structures are present on the outside of the manifolds  70 ,  80  and at the inner surfaces of casing  31 , thereby allowing sub-assembly  300  to easily slide into and be properly located within the DPU casing  31 . The open side of the DPU  10  is then covered by the end cap  20  to provide a sealed DPU  10 . 
     The heat sink bodies  50 ,  60  and PCBA  110  are held in place during assembly by the plastic manifolds  70 ,  80 . The plastic manifolds  70 ,  80  also provide critical electrical isolation of the components of the PCBA  110  from the casing  31  by avoiding direct physical contact between the PCBA  110  and the casing  31  when the DPU  10  has been installed. 
     As can be seen in  FIG. 2 , a separate manifold  130  is provided to cover and protect the daughter PCBA  120 . 
     Thermal Pads 
     The thermal pads  90 ,  100  are placed on the upper and lower surface of the PCBA  110  during assembly of the sub-assembly  300 . In use, the heat sink bodies  50 ,  60  rest against these pads  90 ,  100  to draw heat away from the electronics. These pads  90 ,  100  conduct heat and are also electrically insulative to electrically isolate the heat sink bodies  50 ,  60 . The thermal pads  90 ,  10  are elastomeric, heat-conductive and electrically isolating. The thermal pads  90 ,  100  are sandwiched between the two aluminium heat sink bodies  50 ,  60  and the PCBA  110 , respectively, and are held in place during assembly by the plastic manifolds  70 ,  80 . 
     The two electrically isolating and thermally conductive pads  90 ,  100  fill the space and air gaps between the electrical components and the profiled, board-facing surfaces  52 ,  62  of heat sink bodies  50 ,  60 , preferably with a 0.2 mm interference. This interference ensures the thermal pads  90 ,  100  remain securely in position and retain good thermal contact to the surface of both the heat sink bodies  50 ,  60  and the PCBA  110 . 
     The material for the pads  90 ,  100  has a high electrical insulation rating. Preferably, the material is a silicone elastomer loaded with thermal conductive filler. More preferably, the material is a thermal conductive silicone rubber. Even more preferably, the material is SR-1000C thermal conductive silicone rubber. A minimum 1 mm thickness for the thermal pads  90 ,  100  when installed exceeds the isolation requirements for the DPU  100 . In one embodiment, the top pad  90  is 1.2 mm thick (to accommodate the 0.2 mm interference fit referred to above) and the base pad  100  is 2 mm thick. If adjacent components are located close to but outside the footprint of the heat sink bodies  50 ,  60  on the PCBA  110 , it is preferred for a portion of the thermal pads  90 ,  100  to drape or cover these components too, and extend beyond the perimeter edge of the heat sink bodies  50 ,  60  and provide improved electrical isolation. 
     Mechanical Fastener for Positive Connection Between Casing and Heat Sinks 
     Since thermal conductivity increases with pressure between metal surfaces, the heat sink bodies  50 ,  60  are secured positively to the main body  31  through respective mechanical pressure joints, whereby an optional thermal paste/thermal grease film can be present at the interface. To this end, tamper proof sealing screws  200 ,  210  are used (e.g. M6 thread). These are fastened to the heat sink bodies  50 ,  60  in their threaded holes  53 ,  63  and extend through the upper and lower walls, respectively, of casing  31  and have their screw heads located in respective holes in the upper and lower walls, such that these can be tightened from the exterior of the casing  31 , a top screw  200  being screwed into a threaded hole  53  located about the centre of the top heat sink body  50  and a base screw  210  being screwed into a threaded hole  63  located about the centre of the base heat sink body  60 . 
     To ensure the DPU  10  remains hermetically sealed during prolonged use in a harsh environment, the M6 sealing screws  200 ,  210  each are provided with a integral deformable radial sealing member (e.g. an o-ring) providing a radial and axial seal at the screw holes  201  when they are tightened. The screws  200 ,  210  may be tightened with a torque driver. As the sealing screws  200 ,  210  are fastened to their maximum limit, they draw or pull the heat sink bodies  50 ,  60  outwards towards and into pressured abutment against the inner surface of the housing  31  and maximises the contact surface area. This positive (or pressure connection) enhances thermal conduction between the heat sinks  50 ,  60  and the inner surface of the casing  31  which then radiates into the environment from the outer, profiled surface of casing  31 . 
     Although a sealing screw has been described, wherein the sealing arrangement is at the screw head via the o-ring, other or additional sealing mechanisms can be used, e.g. a seal or packing surrounding the inner surface of the through-holes  201  of the housing  31  for screws  200 ,  210 . 
     In contrast to prior DPUs, the heat sink bodies  50 ,  60  are positively connected to the main body  31  by the M6 tamper proof sealing screws  200 ,  210  rather than relying solely on the elasticity of the thermal pads  90 ,  100  to make and maintain the critical thermal connection. 
     That is, the features of the DPU  10  contributing to thermal management include the combined contributions provided by the (profiled, die-cast aluminium) heat sink bodies  50 ,  60 , the electrically isolating thermal pads  90 ,  100 , and the positive securement of the heat sink bodies  50 ,  60  in surface-abutting relationship against the inner surface of the outer casing  31  through the mechanical fasteners  200 ,  210 . 
     The completed sub-assembly  300  comprising the PCBA  110 , manifolds  70 ,  80 , thermal pads  90 ,  100  and heat sink bodies  50 ,  60  is assembled into the casing  31  along its internal draft using a small amount of the thermal grease applied on the contact surface of the heat sinks  50 ,  60 . For example, FUMIO FUC-03 thermal grease 0.78 W/m/k is used. The thermal paste/thermal grease assists with heat conduction. 
     The contact between the heat sink bodies  50 ,  60  and main body  31  is enhanced because the contact provided in the preferred embodiment does not solely rely on the resilience of the thermal pad  90 ,  100 . There is a risk that the thermal pads  90 ,  100  may relax over time. Since the DPU  10  should have long term reliability, eliminating this risk as a cause for potential malfunction of the DPU  10  is advantageous. 
     Unless specified to the contrary, any and all components herein described are understood to be capable of being manufactured and, as such, may be manufactured together or separately. 
     Moreover, in interpreting the disclosure, all terms should be interpreted in the broadest reasonable manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. 
     Although the technology herein has been described with reference to particular examples, it is to be understood that these examples are merely illustrative of the principles and applications of the technology. In some instances, the terminology and symbols may imply specific details that are not required to practice the technology. For example, although the terms “first” and “second” may be used, unless otherwise specified, they are not intended to indicate any order but may be utilised to distinguish between distinct elements. 
     It is therefore to be understood that numerous modifications may be made to the illustrative examples and that other arrangements may be devised without departing from the spirit and scope of the technology.