Patent Document

CLAIM OF BENEFIT TO PRIOR APPLICATIONS 
     This application claims benefit of an earlier-filed United Kingdom Patent Application 1104785.9, filed Mar. 22, 2011, which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates to power conditioning units (inverters) for use with photovoltaic (PV) modules for delivering ac power either directly to the mains (grid) utility supply or for powering mains (grid) devices directly, independently from the mains utility supply. More particularly the invention relates to improved techniques for manufacturing such inverters, and to inverters manufactured by these techniques. 
     BACKGROUND TO THE INVENTION 
     We have previously described a range of improved techniques for increasing reliability and efficiency in photovoltaic inverters (see, for example, WO2007/080429 and others of our published patent applications). 
     We now particularly address problems which can arise with so-called microinverters. A microinverter is an inverter dedicated to one or a few PV panels, and may be defined as an inverter having a power rating suitable for connection to less than 10 or less than 5 panels (for example less than 1000 watts, often less than 600 watts) and/or as an inverter having a dc input voltage which is less than half a peak-to-peak voltage of the ac mains, more typically less than 100 volts dc or less than 60 volts dc. One of the advantages of a microinverter is that it can be physically located close to the PV panel or panels to which it is connected, thus reducing the voltage drop across the connecting cables (which can be significant). However, locating a microinverter adjacent to or on a PV panel brings other difficulties, in particular because such locations are subject to extreme temperature and environmental conditions including, for example, water, ice, humidity, and dry heat (depending upon the installation, up to or above 80° C.). 
     The very large temperature excursions, and in particular the extremes of high temperature which may be encountered, create particular difficulties. In addition a microinverter generates heat which increases the internal temperature of the electronic components above the local ambient conditions. Simple potting of the electronic components can in principle help to address some of these issues but in practice air bubbles and the like can give rise to local temperature hotspots (caused by the low thermal conductivity of air), which can lead to reliability problems and premature failure of the inverter. 
     There therefore exists a need for improved manufacturing techniques for solar photovoltaic inverters, in particular microinverters. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is therefore provided a solar photovoltaic inverter, the inverter comprising: a power conditioning circuit mounted on a circuit board, the power conditioning circuit having a dc power input to receive dc power from one or more photovoltaic panels and an ac power output to deliver ac power to an ac mains power supply; an electrically conductive shield enclosing said circuit board; and a plastic overmould over said conductive shield and said circuit board; wherein said electrically conductive shield has one or more holes to allow said plastic overmould to extend through said shield to cover said circuit board. 
     In embodiments the electrically conductive shield comprises a metal can having portions which fit opposite faces of the circuit board. In embodiments each portion has a flange which fits on, around or against a perimeter of the circuit board, so that the portions of the can clamp around the circuit board. The one or preferably more holes in the conductive shield (or can) enable the plastic, for example polyamide, overmould to be injection moulded over the combination of the circuit board and can. Thus the injection moulded plastic provides a sealed, solid plastic housing which encases the power conditioning circuit, the holes in the can enabling the plastic overmould to pass through the can to overmould the circuit board. The finished item is a robust, solid, sealed plastic unit, which is substantially free of air bubbles, and which provides a high degree of environmental protection. Furthermore the combined overmoulding and can arrangement facilitates the spreading and dissipation of heat from power components on the circuit board, helping to address the issues causing reliability problems. 
     In some preferred implementations one or more of the magnetic components that is coils, transformers and the like, is/are pre-coated with an elastic material such as silicone. This is because such magnetic components, in particular the core of such components (often comprising a ceramic material), have a different coefficient of thermal expansion to the plastic or polymer overmould. Thus by providing a relatively soft, compressible material between the coil or transformer and the overmould, the core is able to thermally expand without cracking. In some preferred embodiments the power conditioning circuit includes an RF transmitter and/or receiver, for example to permit monitoring and/or control of the solar inverter (as described in our UK patent application number 1017971.1 filed 25 Oct. 2010). In this case, advantageously, the electrical shield can also be employed as an antenna. The shield may float or may be coupled to a ground connection of the power conditioning circuit by a reactive component or circuit, in particular which has low impedance at low frequencies and a high impedance at higher frequencies. When the shield or can is functioning as an antenna preferably the hole or holes in the shield/can have a maximum dimension which is less than a free space wavelength of an operating frequency of the RF transmitter/receiver. More preferably the maximum dimension is less than half a wavelength, most preferably less than a quarter wavelength. In embodiments the power conditioning unit includes a transceiver operating in the 2.45 GHz ISM (industrial, scientific, medical) band, for example a ZigBee™ device, in which case preferably a hole has a maximum dimension of no more than 35 mm. 
     In some implementations of the technique, dc input and/or ac output cables are overmoulded by injection moulding together with the shield and circuit board. This can help reduce the risk of water ingress through the cables. However an alternative approach, in embodiments preferable, is to replace in particular the dc cable or cables by a modular connector system in which a first interface part is mounted on the circuit board and then overmoulded to leave what is, in effect, a standard interface to the microinverter. Then any of a set of second mateable interface parts may be mated with this first interface part to connect to the photovoltaic panel or panels. This is advantageous because there is at present no universal standard for dc connection to a PV panel or module, and thus were the dc cable or cables to be overmoulded multiple different inverter versions would be needed to interface with multiple different types of PV panel. Instead the aforementioned approach enables a common manufacturing procedure followed by a customisation of the device to interface to a particular panel or panels. 
     Thus the inverter may be provided with a set of second interface parts each configured to provide a connection to a different type of photovoltaic panel. Additionally or alternatively the set of second interface parts may be configured to make connection with two or more PV panels of the same type, for example to provide the advantages described in our GB1009430.8 and U.S. Ser. No. 12/947,116 patent applications (incorporated by reference). Patent application US12/947,116 is now published as U.S. Patent Publication No. 2011/0298305. Furthermore, because the first interface part is sealed by the overmould, there are no particularly stringent environmental requirements on the modular connector system since this is sealed at/behind the first interface to the circuit board. 
     In some preferred embodiments the plastic overmould is also configured to define one or more mounting points for the inverter, for example to define a mounting plate and/or one or more tags, ears or the like optionally bearing mounting holes. 
     in preferred embodiments the inverter is a microinverter, and is preferably configured for mounting behind or adjacent to the PV panel. 
     In a related aspect the invention provides a method of manufacturing a solar photovoltaic inverter, the method comprising: providing a power conditioning circuit mounted on a circuit board, the power conditioning circuit having a dc power input to receive dc power from one or more photovoltaic panels and an ac power output to deliver ac power to an ac mains power supply; providing an electrically conductive shield enclosing said circuit board; and injection moulding a plastic overmould over said conductive shield and said circuit board; wherein said electrically conductive shield has one or more holes to allow said plastic overmould to extend through said shield to cover said circuit board. 
     Preferred embodiments of the method further comprise pre-coating a coil or transformer of the inverter with an elastic material prior to the injection moulding, to make provision for thermal expansion of the core of the coil or transformer. 
     Some preferred embodiments use the shield (or can) as an antenna for an RF communications circuit coupled to a microcontroller of the power conditioning circuit. In this case preferably the one or more holes are arranged so that they have a maximum dimension less than a wavelength, half wavelength or quarter wavelength of a frequency of operation of the RF circuit. 
     As previously mentioned, In some preferred embodiments the dc power input of the circuit board is provided by a modular connector system for connecting to any of a set of photovoltaic panels (with either the same, or different, types of dc connections). 
     In embodiments the modular connector system comprises a first interface part and a set of second interface parts to interface with the set of photovoltaic panels, the first part being mateable with any of the second parts. The method may then further comprise overmoulding the first interface part, selecting one or more second interface parts to connect to a PV panel, and then mating the selected second interface part or parts with the first interface part (or parts) subsequent to the overmoulding. 
     In preferred embodiments the electrically conducted shield comprises a metal can having two parts within which the circuit board is sandwiched to, in combination with the overmoulding, spread and dissipate heat from one or more power components on the circuit board. Such power components may include, for example, one or more power semiconductor switching devices. 
     In embodiments the overmoulding is used to mount the microinverter on or adjacent to a PV panel, and the injection moulded overmoulding is configured to provide a mounting plate or the like. 
     In embodiments the injection moulding process comprises injecting the overmould plastic or polymer into an injection moulding tool in which the circuit board and conductive shield (and optionally connecting cables) are located, under pressure to expel air. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures, in which: 
         FIG. 1  shows an outline block diagram of an example power conditioning unit; 
         FIGS. 2   a  and  2   b  show details of a power conditioning unit of the type shown in  FIG. 1 ; 
         FIGS. 3   a  and  3   b  show details of a further example of solar photovoltaic inverter; 
         FIGS. 4   a  and  4   b  show an exploded views of solar photovoltaic inverters according to embodiments of the invention; 
         FIG. 5  shows details of an antenna connection for the solar inverters of  FIG. 4 ; and 
         FIGS. 6   a  and  6   b  show, respectively, an example of an overmoulded solar photovoltaic inverter according to an embodiment of the invention, and elements of the inverter of  FIG. 6   a  prior to overmoulding. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Power Conditioning Units 
     By way of background, we first describe an example photovoltaic power conditioning unit. Thus  FIG. 1  shows photovoltaic power conditioning unit of the type we described in WO2007/080429. The power converter  1  is made of three major elements: a power converter stage A,  3 , a reservoir (dc link) capacitor C dc    4 , and a power converter stage B,  5 . The apparatus has an input connected to a direct current (dc) power source  2 , such as a solar or photovoltaic panel array (which may comprise one or more dc sources connected in series and/or in parallel). The apparatus also has an output to the grid main electricity supply  6  so that the energy extracted from the dc source is transferred into the supply. 
     The power converter stage A may be, for example, a step-down converter, a step-up converter, or it may both amplify and attenuate the input voltage. In addition, it generally provides electrical isolation by means of a transformer or a coupled inductor. In general the electrical conditioning of the input voltage should be such that the voltage across the dc link capacitor C dc  is always higher than the grid voltage. In general this block contains one or more transistors, inductors, and capacitors. The transistor(s) may be driven by a pulse width modulation (PWM) generator. The PWM signal(s) have variable duty cycle, that is, the ON time is variable with respect to the period of the signal. This variation of the duty cycle effectively controls the amount of power transferred across the power converter stage A. 
     The power converter stage B injects current into the electricity supply and the topology of this stage generally utilises some means to control the current flowing from the capacitor C dc  into the mains. The circuit topology may be either a voltage source inverter or a current source inverter. 
       FIG. 2  shows details of an example of a power conditioning unit of the type shown in  FIG. 1 ; like elements are indicated by like reference numerals. In  FIG. 2   a  Q 1 -Q 4 , D 1 -D 4  and the transformer form a dc-to-dc conversion stage, here a voltage amplifier. In alternative arrangements only two transistors may be used; and/or a centre-tapped transformer with two back-to-back diodes may be used as the bridge circuit. In the dc-to-ac converter stage, Q 9 , D 5 , D 6  and Lout perform current shaping. In alternative arrangements layout may be located in a connection between the bridge circuit and the dc link capacitor. Transistors Q 5 -Q 8  constitutes an “unfolding” stage. Thus these transistors Q 5 -Q 8  form a full-bridge that switches at line frequency using an analogue circuit synchronised with the grid voltage. Transistors Q 5  and Q 8  are on during the positive half cycle of the grid voltage and Q 6  and Q 7  are on during the negative half cycle of the grid voltage. 
     Control (block) A of  FIG. 1  may be connected to the control connections (e.g. gates or bases) of transistors in power converter stage A to control the transfer of power from the dc energy source. The input of this stage is connected to the dc energy source and the output of this stage is connected to the dc link capacitor. This capacitor stores energy from the dc energy source for delivery to the mains supply. Control (block) A may be configured to draw such that the unit draws substantially constant power from the dc energy source regardless of the dc link voltage V dc  on C dc . 
     Control (block) B may be connected to the control connections of transistors in the power converter stage B to control the transfer of power to the mains supply. The input of this stage is connected to the dc link capacitor and the output of this stage is connected to the mains supply. Control B may be configured to inject a substantially sinusoidal current into the mains supply regardless of the dc link voltage V dc  on C dc . 
     The capacitor C dc  acts as an energy buffer from the input to the output. Energy is supplied into the capacitor via the power stage A at the same time that energy is extracted from the capacitor via the power stage B. The system provides a control method that balances the average energy transfer and allows a voltage fluctuation, resulting from the injection of ac power into the mains, superimposed onto the average dc voltage of the capacitor C dc . The frequency of the oscillation can be either 100 Hz or 120 Hz depending on the line voltage frequency (50 Hz or 60 Hz respectively). 
     Two control blocks control the system: control block A controls the power stage A, and control block B power stage B. An example implementation of control blocks A and B is shown in  FIG. 2   b . In this example these blocks operate independently but share a common microcontroller for simplicity. 
     In broad terms, control block A senses the dc input voltage (and/or current) and provides a PWM waveform to control the transistors of power stage A to control the power transferred across this power stage. Control block B senses the output current (and voltage) and controls the transistors of power stage B to control the power transferred to the mains. Many different control strategies are possible. For example details of one preferred strategy reference may be made to our earlier filed WO2007/080429 (which senses the (ripple) voltage on the dc link)—but the embodiments of the invention we describe later do not rely on use of any particular control strategy. 
     In a photovoltaic power conditioning unit the microcontroller of  FIG. 2   b  will generally implement an algorithm for some form of maximum power point tracking. In embodiments of the invention we describe later this or a similar microcontroller may be further configured to control whether one or both of the dc-to-dc power converter stages are operational, and to implement “soft” switching off of one of these stages when required. The microcontroller and/or associated hardware may also be configured to interleave the power transistor switching, preferable to reduce ripple as previously mentioned. 
     Now to  FIG. 3   a , this shows a further example of a power conditioning unit  600 . In the architecture of  FIG. 3  a photovoltaic module  602  provides a dc power source for dc-to-dc power conversion stage  604 , in this example each comprising an LLC resonant converter. Thus power conversion stage  604  comprises a dc-to-ac (switching) converter stage  606  to convert dc from module  602  to ac for a transformer  608 . The secondary side of transformer  608  is coupled to a rectifying circuit  610 , which in turn provides a dc output to a series-coupled output inductor  612 . Output inductor  612  is coupled to a dc link  614  of the power conditioning unit, to which is also coupled a dc link capacitor  616 . A dc-to-ac converter  618  has a dc input from a dc link and provides an ac output  620 , for example to an ac grid mains supply. 
     A microcontroller  622  provides switching control signals to dc-to-ac converter  606 , to rectifying circuit  610  (for synchronous rectifiers), and to dc-to-ac converter  618  in the output ‘unfolding’ stage. As illustrated microcontroller  622  also senses the output voltage/current to the grid, the input voltage/current from the PV module  602 , and, in embodiments, the dc link voltage. (The skilled person will be aware of may ways in which such sensing may be performed). In some embodiments the microcontroller  622  implements a control strategy as previously described. As illustrated, Microcontroller is coupled to an RF transceiver  624  such as a ZigBee™ transceiver, which is provided with an antenna  626  for monitoring and control of the power conditioning unit  600 . 
     Referring now to  FIG. 3   b , this shows details of a portion of an example implementation of the arrangement of  FIG. 3   a . This example arrangement employs a modification of the circuit of  FIG. 2   a  and like elements to those of  FIG. 2   a  are indicated by like reference numerals; likewise like elements to those of  FIG. 3   a  are indicated by like reference numerals. In the arrangement of  FIG. 3   b  an LLC converter is employed (by contrast with  FIG. 2   a ), using a pair of resonant capacitors C 1 , C 3 . 
     The circuits of  FIGS. 1 to 3  are particularly useful for microinverters, for example having a maximum rate of power of less than 1000 Watts and or connected to a small number of PV modules, for example just one or two such modules. In such systems the panel voltages can be as low as 20 volts and hence the conversion currents can be in excess of 30 amps RMS. 
     Manufacturing Techniques 
     We will now describe techniques which enable a solar microinverter to be encapsulated to provide a combination of thermal management, dielectric resistance, environmental robustness and good electromagnetic emissions performance. 
     Referring now to  FIGS. 4   a  and  4   b , these show an exploded 3-D view of a solar photovoltaic inverter  400  according to an embodiment of the invention. The solar inverter comprises a power conditioning circuit, for example of the type shown in  FIGS. 3   a  and  3   b , mounted on a circuit board  402 , having, in the illustrated example, two dc power inputs  404  and an ac power output  406 , each comprising a cable connection to the circuit board  402 . The circuit board is provided with a conductive shield comprising first and second portions  408   a, b  of a can which substantially encloses the circuit board  402 , fitting around the perimeter of the circuit board. The can may be formed, for example, from 0.8 mm-1 mm aluminium, and provides EMC (electromagnetic compatibility) shielding, as well as a thermal conductor for heat spreading/dissipation. 
     Each of can portions  408   a, b  is provided with a set of holes  410  (not visible in can portion  408   a ) and these enable the entire assembly to be overmoulded in an injection moulding process so that the encapsulation becomes the mechanical housing of the device. By providing holes  410  the encapsulating material is able to expel air from the assembly. This means that there is no condensation, no issues associated with thermal expansion of the air, and the injection moulding process ensures that there are no hot spots from residual air bubbles when the inverter is in use. 
     The injection moulding process is performed in the usual way, by providing a suitable injection moulding tool within which the assembly to overmould is located, the overmoulding, for example of polyamide then being applied under pressure. The mould or tool may be shaped to enable the escape of air through air vents, for example in the parting line of the mould. 
     The result is a plastic overmould  412 . In  FIG. 4 , for ease of representation, this is not shown as extending through can portion  408   a  but nonetheless in practice the overmould coats the circuit board  402 . Similarly for ease of representation the lower part of overmould  412  is not shown in  FIG. 4   a . In the illustrated example overmould  412  includes strain relief features  412   a  for cables  404 ,  406 . The overmould process is able to provide a high degree of environmental sealing/protection, for example up to IP67 or IP68. A high degree of hermetic sealing is also useful where an inverter may need to have a long shelf life, to ensure that there is minimal moisture ingress. The circuit board  402  may include, for example, a transformer  402   a , and to prevent cracking of overmoulded core this is preferably pre-coated in silicone to allow for thermal expansion. 
       FIG. 4   b  shows another example of a solar photovoltaic inverter  450 , very similar to that of  FIG. 4   a , according to an embodiment of the invention. Like elements to those of  FIG. 4   a  are indicated by like reference numerals. In the arrangement of  FIG. 4   a  the shielding and overmould are asymmetric with respect to the printed circuit board assembly  402 . Again not all of holes  410  are shown, and again the full extent of the plastic overmould is omitted, for clarity. 
     In  FIG. 4   b  the base portion of overmould  412  comprises a base plate with locking features to match an interface base  414 , for mounting the inverter on a photovoltaic panel. Optionally the interface base  414  may be incorporated into the overmould  412 . 
     The PCB assembly  402  of  FIG. 4   b  also includes a modular connector system  416 , comprising a connector plate which is overmoulded to form a seal behind the plate. This facilitates a manufacturing process in which standard form inverters are overmoulded and then afterwards cable connectors added for the photovoltaic panels by mating a suitable cable connector to the standard interface  416  of the modular connector system. 
     In embodiments one or both can portions  408   a, b  may be employed as the antenna  626  of the RF transceiver  624  of the  FIG. 3   a . Referring to  FIG. 5 , the antenna/shield may either be allowed to float or it may be grounded via an RF choke  502  making connection to a ground line  500  of the inverter. 
     Where one or both of can portions  408   a, b  is used as an antenna it is preferable that hole portions  410  have maximum dimension which is no greater than the wavelength at the frequency of operation of RF transceiver  624 , preferably no greater than a quarter wavelength so that the holes are effectively ‘invisible’ to the RF signal. In embodiments the RF transceiver  624  is a ZigBee™ (transceiver) operating at approximately 2.4 GHz, in which case the quarter wavelength dimension is 31.25 mm (although in practice this will be modified a little by the effect of the dielectric overmoulding of the can/antenna). 
     Referring now to  FIG. 6   a , this shows a further embodiment of an overmoulded solar photovoltaic inverter  700 , showing a view from above and two side elevations. The inverter  700  has a plastic overmould  702 , which forms the body of the inverter, into which is moulded a mounting plate  704 . In alternative embodiments the mounting may be formed from the overmould itself. The inverter has a pair of cables  706   a,b  for positive and negative dc connections to a photovoltaic panel, for example of standard MC4 type, and an ac mains output cable  706  bearing a suitable connector at the end. 
       FIG. 6   b  illustrates components of the inverter  700  prior to overmoulding, showing top and side views of the inverter  700 , cross-sectional views of top and bottom electrically conductive shield (Faraday cage) components  750 ,  760 , and the mounting plate  704 . As can be, seen the Faraday cage incorporates a plurality of holes to enable the overmoulding to be performed after coating the circuit board with silicone or the like. 
     No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Technology Category: 4