An LLC DC-DC converter for use in a micro-inverter system

Abstract

　The invention relates to an LLC DC-DC converter for use in a micro-inverter system, which is capable of providing a double or quadruple gain of voltage. The LLC DC-DC converter comprises: a full-bridge circuit; a resonant transformer having a primary winding on one end thereof, and secondary and tertiary windings on the other end, wherein the start of said primary winding is connected to the output of said full-bridge circuit; a rectifier circuit connected to said other end of the resonant transformer; and two tapping switches connected to a DC-AC inverter in a way such that when the first tapping switch is turned on, the LLC DC-DC converter provides a double gain of voltage, and when the second tapping switch is turned on, the LLC DC-DC converter provides a quadruple gain of voltage.

Description

[0001] AN LLC DC-DC CONVERTER FOR USE IN A MICRO-INVERTER SYSTEM

[0002] FIELD OF THE INVENTION

[0003] This invention relates to an LLC (i.e. Inductance-Inductance-Capacitance) DC-DC (i.e. Direct-Current-to-Direct-Current) converter for use in a micro -inverter system, and more especially, an LLC DC-DC converter capable of providing a double or a quadruple gain of voltage.

[0004] BACKGROUND OF THE INVENTION

[0005] In a multi-input or multi-port solar micro-inverter system, one to several (usually 1, 2, 4 or 6) photovoltaic (PV) panels, each with the capability of handling an input voltage of 20Vdc - 60Vdc are connected to DC-DC converters which will step up the low input voltage to a high output voltage of 380V de - 420Vdc (400Vdc nominal) at the DC bus before feeding the voltage to an inverter which is subsequently connected to a single-phase grid. Fig. 1 depicts the building blocks (79) of a multiinput micro-inverter, e.g. a 2-in-l or 4-in-l micro-inverter, which connects 2 to 4 solar panels to the grid.

[0006] A conventional two-stage micro-inverter (refer to Fig. 2) includes a front-end comprising a high gain DC-DC converter and a back-end of a DC-AC inverter that has its output connected to the grid. For the DC-DC stage, an LLC topology is used due to its high efficiency and ability to provide intrinsic galvanic isolation.

[0007] However, the LLC converter has a major disadvantage, i.e. in order to achieve a high transformer efficiency, the input will be restricted to a narrow range of input voltage. For example, with a nominal input voltage of 40V de, in order to realize 400Vdc at the DC link, a gain of ten times is required from the LLC converter. To realize this high voltage gain, the conventional way is to increase the turns ratio of the transformer in the LLC converter. An LLC converter designed for a gain of ten times would work for any input voltage from 40Vdc upwards. Since the input voltage range of a typical solar panel ranges from 20V de to 60Vdc, there is an issue with the smaller voltages from 20Vdc to 40Vdc, where a gain of more than ten times is required. An LLC converter designed for a gain larger than ten times has a very high turns ratio, which means a larger size, higher cost, and high losses of the transformer.

[0008] To overcome this flaw of the conventional LLC converter, various secondary rectification topologies have been proposed. Some of the existing technologies are described below.

[0009] US 9130462 discloses a multi -transformer LLC (resonant) power converter having at least two transformers including a first transformer T i and a second transformer T2 in series includes a switch network configured for receiving input power including a first and second switched node. Resonant circuitry is coupled between the first and second switched node including a series combination of an inductor, a capacitor, a primary winding of Ti and a primary winding of T2. At least one switch is operable for providing a first mode that includes T2 in the resonant circuitry and a second mode that excludes T2 from the resonant circuitry. Secondary windings of T2 and Ti are connected electrically in parallel for driving an output capacitor through respective rectifiers which provide conversion from AC to DC.

[0010] US 20200083818 Al discloses an interleaved LLC half-bridge series resonant converter having an integrated transformer includes a power supply, a magnetic core, a first converter, a second converter and an output load circuit. The magnetic core has first and second outer columns and a center column. The first converter includes a first switch circuit, a first resonant tank, a first transformer, and a first rectifier circuit. The first transformer is coupled to the first resonant tank and includes a first primary winding wound on the first outer column and a first secondary winding wound on the second outer column. The second converter includes a second switch circuit, a second resonant tank, a second transformer and a second rectifier circuit. The second transformer is coupled to the second resonant tank and includes a second primary winding wound on the first outer column and a second secondary winding wound on the second outer column.

[0011] The existing technologies use additional devices, i.e. capacitors and diodes, in the doubler or quadrupler circuits to achieve a double, quadruple or higher gain without considering the effect of the additional devices. The devices not only increase the cost and complexity, but also introduce additional power losses which affect the overall system efficiency.

[0012] SUMMARY OF THE INVENTION

[0013] The above-mentioned drawbacks and difficulty are overcome by an LLC DC-DC converter for use in a micro-inverter system, capable of providing a double or a quadruple gain of voltage, comprising: a full -bridge circuit; a resonant transformer including: a primary winding on one end thereof, the start of said primary winding being connected to the output of said full-bridge circuit; secondary and tertiary windings on the other end thereof, wherein said secondary and tertiary windings are integrated into a plurality of secondary printed circuit boards, each of said secondary printed circuit board being wound by said secondary and tertiary windings in a way such that said secondary winding encloses said tertiary winding, and the plurality of secondary printed circuit boards being stacked; a rectifier circuit connected to said other end of said resonant transformer in a way such that: the start of said secondary winding is connected to a first node diverging to a first set of two diodes, and the end of said secondary winding is connected to a second node diverging to a first set of two capacitors, wherein the input ends of said first sets of diodes and capacitors converges at a third node, and the output ends of said first sets of diodes and capacitors converges at a fourth node; and the start of said tertiary winding is connected to a fifth node diverging to a second set of two diodes, and the end of said tertiary winding is connected to a sixth node diverging to a second set of two capacitors, wherein the input ends of said second sets of diodes and capacitors converges at a seventh node, and the output ends of said second sets of diodes and capacitors converges at said third node; a first tapping switch, input of which is connected to said third node; and a second tapping switch, input of which is connected to the output of said first tapping switch at an eighth node, and output of which is connected to said fourth node; wherein said seventh and eighth nodes are connected to a DC-AC inverter in a way such that: when said first tapping switch is switched on, the LLC DC-DC converter provides a double gain of voltage; and when said second tapping switch is switched on, the LLC DC-DC converter provides a quadruple gain of voltage. Advantageously, the LLC DC-DC converter provides a significant reduction in the power diode drop when it operates for a double gain of voltage, and is capable of operating within a wider range of input voltage for both double and quadruple gains of voltage.

[0014] Typically, a photovoltaic (PV) panel is connected to the input of said full bridge circuit to provide an input voltage to the LLC DC-DC converter, said LLC DC-DC converter being configured in a way such that: when the input voltage is within a first range, said first tapping switch is switched off and said second tapping switch is switched on; and when the input voltage exceeds the maximum of the first range, but is within a second range, said first tapping switch is switched on and said second tapping switch is switched off.

[0015] Typically, said tertiary winding has an annular ring width of at least three times smaller than that said secondary winding has.

[0016] Typically, the LLC DC-DC converter also comprises: a resonant capacitor connected between the output of said full-bridge circuit and the start of said primary winding; or a first resonant capacitor connected between the output of the start of said secondary winding and said first node, and a second resonant capacitor connected between the output of the start of said tertiary winding and said fifth node.

[0017] Advantageously, the use of said first and second resonant capacitors reduces current stress through each of the resonant capacitors, and facilitates said secondary and tertiary windings to be tuned resonantly to compensate for any leakage mismatch between said secondary and tertiary windings.

[0018] Typically, said primary winding is wound onto a plurality of primary printed circuit boards in two ways, in a first way said primary winding is wound onto a primary printed circuit board in an fl-like contour such that a first type of primary printed circuit board is formed, and in a second way said primary winding is wound onto a primary printed circuit board in an a-like contour such that a second type of primary printed circuit board is formed, the first and second types of primary printed circuit boards being stacked in an alternate manner, each primary printed circuit board being connected to an adjacent primary printed circuit board by a plurality of half vias configured on the sides thereof, wherein the plurality of half vias is configured on the opposing sides in an interleaved manner.

[0019] Typically, said secondary and tertiary windings are wound onto a plurality of secondary printed circuit boards which comprises two types of secondary printed circuit boards, a first type of which has input and output ports configured in a reverse manner to those of a second type, wherein: said first and second types of secondary printed circuit boards stack in an alternate manner; said secondary winding of said first types of secondary printed circuit board is connected to that of said second types of secondary printed circuit board by a plurality of half vias configured on a side thereof; said tertiary winding of said first types of secondary printed circuit board is connected to that of said second types of secondary printed circuit board by a plurality of half vias configured on the opposing side thereof; and adjacent secondary printed circuit boards are connected by a plurality of inter-board solders configured on two opposing sides thereof, adjacent inter-board solders being spaced transversely in an alternate manner.

[0020] Advantageously, introducing said tertiary winding to said other side of said resonant transformer does not increase the cost for copper as said secondary and tertiary windings are integrated in a secondary printed circuit board.

[0021] Typically, said first and second tapping switches are MOSFETs.

[0022] Typically, the first range is 20-38Vdc, and the second range is 38-60Vdc.

[0023] BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

[0025] Fig. 1 is a schematic diagram depicting a parallel connection of several DC-DC converters to a DC bus coupled to a single DC-AC inverter;

[0026] Fig. 2 shows a typical topology of a two-stage micro-inverter;

[0027] Fig. 3 is a schematic diagram depicting, according to one form of the invention, a single input micro-inverter with 2 interleaved LLC converters; Fig. 4 is a schematic diagram depicting, according to one form of the invention, a single LLC converter formed by a full bridge structure followed by a transformer with two secondary side windings which are connected to a voltage doubler or quadrupler circuit;

[0028] Fig. 5 shows an operation with the quadrupler (4x) circuit enabled, and the doubler (2x) circuit disabled, which occurs when Vin is 38V and below;

[0029] Fig. 6 shows an operation with the quadrupler (4x) circuit disabled, and the doubler (2x) circuit enabled, which occurs when Vin is 38V and above;

[0030] Fig. 7 shows the switches S5 and S6 which perform a tapping function to select either the doubler (2x) circuit when S5 is activated or the quadrupler (4x) circuit when S6 is activated;

[0031] Fig. 8 shows a comparison between a conventional quadrupler circuit and the invention during a nominal operation (doubler mode);

[0032] Fig. 9 shows a comparison between a conventional quadrupler circuit and the invention during an extended operation (quadrupler mode);

[0033] Fig. 10 shows simulated waveforms when the LLC converter according to one form of the invention operates between an input voltage (Vin) between 20Vdc and 60Vdc, with a threshold voltage at 38VdC;

[0034] Fig. 11 shows a zoom-in view of current through diodes DI, D2, D3 and D4 at Vin < 38Vdc(e.g. Vin = 35 Vdc);

[0035] Fig. 12 shows a zoom-in view of current through diodes DI, D2, D3 and D4 at Vin > 38Vdc(e.g. Vin = 40Vdc);

[0036] Fig. 13 shows the primary winding according to one form of the invention;

[0037] Fig. 14 shows a primary winding structure according to one form of the invention;

[0038] Fig. 15 shows the secondary and tertiary windings according to one form of the invention;

[0039] Fig. 16 shows an integrated structure of the secondary and tertiary windings according to one form of the invention; and Fig. 17 shows the inter-board soldering connection according to one form of the invention.

[0040] DESCRIPTION OF THE PREFERRED EMBODIMENT

[0041] The invention provides an LLC DC-DC converter which is connected to a DC-AC (i.e. Direct-Current-to-Altemating-Current) inverter for a typical 2 stage solar micro-inverter design. The DC-DC converter could be interfaced to a photovoltaic (PV) panel in an interleaved manner as the first stage of a 2-stage solar microinverter design. As shown in Fig. 3, the DC-DC converter 80 comprises a set of interleaved LLC resonant converters, each of which has an LLC resonant tank which includes an inductor, a capacitor and the utilization of leakage inductance from a transformer. More specifically, the LLC resonant converter comprises a full bridge circuit interfaced to a PV panel to receive power therefrom, a high frequency transformer operating near its resonant frequency, and a voltage doubler (2x) or quadrupler (4x) circuit. The function of this DC-DC converter is to provide a gain for a wide range of PV voltages ranging from 20Vdc to 60Vdc so that its output can be maintained at 380Vdc to 420Vdc before an inversion to the grid voltage of 240Vac- As shown in Fig. 3, the DC-AC block 81 subsequently feeds to the grid.

[0042] The invention provides a DC-DC converter which facilitates the micro-inverter to function with a wide range of input voltage while keeping the cost low and maintaining the efficiency. A zoom-in block diagram of the DC-DC converter is shown in Fig. 4.

[0043] A single LLC circuit as shown in Fig. 4, comprises a full bridge 2 (S1-S4), a resonant capacitor 61 (Cr), a resonant transformer 62 (Tri), four diodes D1-D4, four capacitors C1-C4 and two MOSFETs functioning as AC switches 67 (68 (S5) and 69 (S6)). For the case of an interleaved LLC converter design, as shown in Fig. 3, both the top and bottom LLC circuits share the switches 68 (S5) and 69 (S6).

[0044] The secondary side of the transformer 62 comprises secondary and tertiary windings 64, 65 which are connected to a plurality of nodes 71, 72, 75, 76 for diodes and capacitors. The LLC circuit operates with a double or quadruple gain depending on the input voltage provided by the PV panel, which ranges from 20Vdc to 60Vdc typically.

[0045] The two operation modes of the secondary side of the LLC circuit transformer are shown in Fig. 5 and 6. Fig. 5 shows that the converter operates with the quadrupler circuit enabled, and the doubler circuit disabled, when Vin is 38V and below, whereas Fig. 6 shows that the converter operates with the quadrupler circuit disabled, and the doubler circuit enabled, when Vin is 38V and above.

[0046] When the solar irradiance is sufficient to drive the solar micro-inverter to start up from 20Vdc up to a threshold of 38Vdc, the secondary side of the LLC circuit operates as shown in Fig. 5, where the switch S5 is turned OFF, and S6 is turned ON, resulting in the activation of a quadrupler circuit which comprises a voltage doubler circuit consisting of capacitors C3 and C4, diodes D3 and D4, and the secondary winding 64 of the transformer 62 (Tri), all of which are stacked on top of another voltage doubler circuit consisting of capacitors C 1 and C2, diodes D 1 and D2, and the tertiary winding 65 of the transformer 62 (Tri).

[0047] When the input voltage exceeds 38Vdc, as shown in Fig. 6, the switch S5 is turned ON, and S6 is turned OFF, causing the voltage doubler circuit consisting of C3, C4, D3, D4 and the secondary winding 64 of the transformer 62 (Tri) to be cut off from the voltage doubler circuit consisting of Cl, C2, DI, D2 and the tertiary winding 65 of the transformer 62 (Tri). Thus, the input voltage only experiences a double gain, which is the normal operation mode when the solar micro-inverter operates with input voltage above 38Vdc up to 60Vdc. In this operation mode, when S6 is turned OFF, the secondary winding 64 which contributes to the voltage quadrupling will be disabled completely, and all the current flows to the tertiary winding 65 only. This nominal operation mode achieves a high efficiency as no current flows through C3, C4, D3, D4 and the secondary winding 64. As such these devices do not operate, and losses are reduced.

[0048] Referring to Fig. 7, the switches 67 (S5 and S6) perform a tapping function which selects either the doubler circuit when S5 is activated as shown in Fig. 6, or the quadrupler circuit when S6 is activated as shown in Fig. 5. In practice, S5 and S6 can be MOSFETs. The invention provides an LLC topology with a dual mode operation of double or quadruple voltage gain using a tertiary winding on the secondary side of the transformer together with a doubler or quadrupler circuit in order to maintain system efficiency and reduce power losses during nominal operation.

[0049] As shown in Fig. 8, the shortcoming of a conventional LLC converter with a quadrupler capability is that there are three power diode drops when it operates in the nominal operation, i.e. the doubler mode. In contrast, when the circuit is configured according to the invention, the power diode drop is reduced by nearly a third, i.e. to 1.1. This involves the introduction of a tertiary winding to the LLC transformer. Advantageously, introducing the tertiary winding does not increase the cost for copper as the tertiary winding is integrated with the secondary winding in a printed circuit board.

[0050] A power diode drop refers to the voltage drop of a power diode. The value of the power diode drop depends on the specification of the power diode. For instance, a power diode drop of 1 can be in the range of 1.4 to 1.5V. The power diode can be a power Silicon Carbide (SiC) diode.

[0051] When a micro-inverter operates at a power of 550W, each power diode drop causes efficiency reduction of about 0.9-1.1%. As such a reduction in the power diode drop from 3 to 1.1 allows the efficiency to be increased by about 1-2%. This is the key to achieving an efficiency above 98% for a high gain LLC converter, e.g. one which converts an input of 20V-60V to an output of 400V.

[0052] While the diodes could be replaced with MOSFETs such that the drop loss can be lowered, using MOSFETs in an active synchronous rectification mode is cumbersome, because the use of MOSFETs involves the use of an active gate driving and isolation circuitry. This is relatively costly and requires a higher component count compared to using the passive power diodes.

[0053] Table 1 : Comparison of component count between a conventional converter and the invention As shown in Table 1, the invention has the same component count as that of a conventional quadrupler converter. However, when the components are configured according to the invention, a diode drop nearly three times smaller than that of the conventional quadrupler converter is achieved.

[0054] Fig. 9 shows a comparison between the invention and a conventional quadrupler converter when both operates in a quadrupler mode. The improvement that the invention provides in terms of the power diode drop is small, i.e. a reduction of 0.1 from 2.2 to 2.1.

[0055] Table 2: Operation of LLC Converter

[0056] The LLC converter as provided by the invention operates according to that shown in Table 2. An extended operation is activated when the PV voltage drops to a boundary limit of 38V (with some hysteresis implemented for stability). It should be noted that with the right choice of PV panel, the micro-inverter functions mostly in the nominal operation. More specifically, the micro-inverter is estimated to function in the nominal operation, i.e. doubler mode, 90% of its operation time. The extended operation, i.e. quadrupler mode, although shorter, is critical to allow for a stable and continuous operation without the need to shut down the operation should the PV voltage drops below the boundary limit (due to a variation of the solar irradiation). This provides an increased capability of energy harvesting when the solar irradiation is small or irregular.

[0057] Since the micro-inverter functions mostly in the nominal operation, i.e. doubler mode, the invention provides significant advantages of a smaller power diode drop under the doubler mode, and a capability to function under the extended operation.

[0058] To illustrate the superior efficiency and reduced power loss attributed to the topology as provided by the invention, a simulation has been performed and the waveforms resulted from the simulation are shown in Fig. 10.

[0059] A voltage ramp is applied to the input of the micro-inverter from 0 to about 70Vdc- This input voltage is denoted as Vin. The voltage ramp simulates a gradual increase in the solar irradiance which starts up the micro-inverter at about 20Vdc, and continues to power the operation of the micro-inverter to about 60Vdc. The output voltage reaches 380Vdc when the input voltage is 20Vdc, increases and peaks at 400Vdc, and remains at peak until the input voltage is 60V de-

[0060] Within the interval of 20Vdc and 38Vdc (the threshold), the switch S5 is turned OFF and the switch S6 is turned ON as shown in Fig. 5, and the quadrupler circuit is enabled and the doubler circuit is disabled to provide a quadruple gain to the input voltage.

[0061] During this interval, current flows through the diodes D1-D4. Referring to Fig. 11, a snapshot of the current magnitudes for the input voltage of 35Vdc shows that the current flowing through DI and D2 comprises pulses with a peak amplitude of about 4.7A, while the current flowing through D3 and D4 comprises pulses with a peak amplitude of about 4.8A. These currents contribute to a voltage drop which results in a power loss.

[0062] At the input voltage of 38Vdc (the threshold), a switch transition occurs, whereby the switch S5 is turned ON and the switch S6 is turned OFF. This operation is depicted in Fig. 6 in which the quadruple circuit is disabled and the doubler circuit is enabled to provide a double gain to the input voltage.

[0063] Within the interval of 38Vdc to 60Vdc, current flows through the diodes DI and D2, but does not flow through the diodes D3 and D4. Referring to Fig. 12, a snapshot of the current magnitude for the input voltage of 40V de shows that the current flowing through D 1 and D2 comprises pulses with a peak amplitude of about 6A, and no current is detected at D3 and D4. As a result, there is a much lower power loss through the diodes compared to the power loss that occurs during the interval of 20Vdc to 38Vdc-

[0064] Since the nominal operation of the solar micro-inverter starts from the input voltage of 40Vdc onwards, the power saving of the topology as provided by the invention is substantial.

[0065] As shown in Fig. 13, a PV panel 1 terminal pair is connected to an LLC H-Bridge converter 2 which performs a DC-to-DC conversion. By means of a closed loop frequency control, the LLC H-Bridge converter adjusts the rate of power pumped into the primary side of an LLC transformer through the terminals 3, 4 of a primary winding 63.

[0066] As shown in Fig. 13 and 14, the primary winding 63 is wound onto a plurality of primary printed circuit boards in two distinguished ways. In the first way, the primary winding 63 is wound onto a primary printed circuit board in a contour which resembles a Greek alphabet of omega Q to form a first type of primary printed circuit board 5. In the second way, the primary winding 63 is wound onto a primary printed circuit board in a contour which resembles a Greek alphabet of alpha a to form a second type of primary printed circuit board 6. The first and second types of primary printed circuit boards 5, 6 are stacked in an alternate manner.

[0067] Each primary printed circuit board of the first and second types 5, 6 comprises a single turn of copper winding. The primary printed circuit board of the first type 5 is connected to that of the second type 6 by a set of half vias 8, 9 soldered on the sides of the primary printed circuit boards. Both types of primary printed circuit boards 5, 6 are stacked in an alternate manner until the number of turns for the copper winding reaches a predetermined number as specified by the design. The manner in which the primary printed circuit boards are stacked ensures that the current always flows through the winding in either a clockwise or anti-clockwise direction.

[0068] The half vias 8, 9 are soldered on the opposing sides of the primary printed circuit boards in an interleaved manner. This allows the number of turns to be increased without requiring additional length of copper for a loopback connection. Since each primary printed circuit board has a single turn of winding, the total number of primary printed circuit boards 10 is equivalent to the number of turns required.

[0069] Referring to Fig. 15 and 16, the secondary and tertiary windings 64, 65 are integrated into a plurality of secondary printed circuit boards. Each secondary printed circuit board comprises N turns, i.e. at least four turns of the secondary winding 64 and at least four turns of the tertiary windings 65. The secondary and tertiary winding 64, 65 are wound in a way such that the secondary winding 64 always encloses the tertiary winding 65. The secondary winding 64 includes terminals 12, 13, and the tertiary winding 65 includes terminals 14, 15. Said terminals 12, 13, 14, 15 are connected to a power electronic topology of rectifier.

[0070] The secondary winding 64 forms a secondary ring 18 on a secondary printed circuit board and the tertiary winding 65 forms a tertiary ring 19 thereon. The annular width of the tertiary ring 19 is at least three times smaller than that of the secondary ring 18.

[0071] The terminal 14 is connected to a resonant capacitor 29 and the terminal 13 is connected to another resonant capacitor 28. Two separate capacitors 28, 29 are used for the secondary and tertiary windings 64, 65 so that the current stress of the resonant capacitors 28, 29 are relatively low when the converter operates in the quadrupler mode. The use of two capacitors 28, 29 also allows the secondary and tertiary windings 64, 65 to be tuned resonantly to compensate for any leakage mismatch.

[0072] The secondary printed circuit boards having the secondary and tertiary windings 64, 65 stack on one another to increase the number of turns of the secondary and tertiary windings 64, 65. There are two types of secondary printed circuit boards 22, 23, the first type 22 of which has the input and output ports configured in a reverse manner to those of the second type 23.

[0073] The secondary winding 64 of the first type of secondary printed circuit board 22 is connected to that of the second type 23 by side soldering of half vias 24, 25. The tertiary winding 65 of the first type of secondary printed board 22 is connected to that of the second type by side soldering of half vias 26, 27.

[0074] The secondary winding inter-board soldering connection 16 is configured in an ascending zigzag manner on one side of the printed circuit board, while the tertiary winding inter-board soldering connection 17 is configured in an ascending zigzag manner on the opposing side of the printed circuit board.

[0075] The intra-layer connection between the turns of the secondary winding is done through an array of vias 20 while the intra-layer connection between the turns of the tertiary winding is done through an array of vias 21. The intra-layer vias 20, 21 allow both the secondary and tertiary windings 64, 65 to have N number of turns per printed circuit board. Referring to Fig. 17, the stack of primary printed circuit boards is connected to the LLC H-Bridge 2, and the stack of secondary printed circuit boards is connected to the rectifier structure 11 through the tuning resonant capacitors 28, 29. The stack of primary printed circuit boards is stacked on top of the stack of secondary printed circuit boards . Both the stacks of primary and secondary printed circuit boards have : a hole through which a pair of magnetic cores 30, 31 is capable of passing in opposing directions and abutting each other; and an exterior contour capable of accommodating two pairs of magnetic limbs of said magnetic cores, facilitating one pair of said magnetic limbs to abut the other pair thereof. It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications which does not affect the overall functioning thereof.

Claims

CLAIMS1. An LLC DC-DC converter for use in a micro-inverter system, to provide a double or a quadruple gain of voltage, comprising: a full-bridge circuit (2); a resonant transformer (62) including: a primary winding (63) on one end thereof, first end of said primary winding (63) being connected to output of said full-bridge circuit (2); and a secondary winding (64) on the other end thereof; and a rectifier circuit (11) connected to said other end of said resonant transformer (62), characterised in that: said resonant transformer (62) also includes a tertiary winding (65) on the other end thereof, wherein said secondary and tertiary windings (64, 65) are integrated into a plurality of secondary printed circuit boards, each of said secondary printed circuit board being wound by said secondary and tertiary windings (64, 65) in a way such that said secondary winding (64) encloses said tertiary winding (65), and the plurality of secondary printed circuit boards being stacked; said rectifier circuit (11) is connected to said other end of said resonant transformer (62) in a way such that: first end of said secondary winding (64) is connected to a first node (71) diverging to a first set of two diodes, and second end of said secondary winding (64) is connected to a second node (72) diverging to a first set of two capacitors, wherein input ends of said first sets of diodes and capacitors converges at a third node (73), and output ends of said first sets of diodes and capacitors converges at a fourth node (74); and first end of said tertiary winding (65) is connected to a fifth node (75) diverging to a second set of two diodes, and second end of said tertiary winding (65) is connected to a sixth node (76) diverging to a second set of two capacitors, wherein input ends of said second sets of diodes andcapacitors converges at a seventh node (77), and output ends of said second sets of diodes and capacitors converges at said third node (73); and the LLC DC-DC converter also comprises: a first tapping switch (68), input of which is connected to said third node (73); and a second tapping switch (69), input of which is connected to output of said first tapping switch (68) at an eighth node (78), and output of which is connected to said fourth node (74); wherein said seventh and eighth nodes (77, 78) are connected to a DC-AC inverter in a way such that: when said first tapping switch (68) is switched on, the LLC DC-DC converter provides a double gain of voltage; and when said second tapping switch (69) is switched on, the LLC DC- DC converter provides a quadruple gain of voltage.

2. The LLC DC-DC converter as claimed in claim 1, wherein a photovoltaic (PV) panel (1) is connected to said full bridge circuit (2) to provide an input voltage to the LLC DC-DC converter, said LLC DC-DC converter being configured in a way such that: when the input voltage is within a first range, said first tapping switch (68) is switched off and said second tapping switch (69) is switched on; and when the input voltage exceeds the maximum of the first range, but is within a second range, said first tapping switch (68) is switched on and said second tapping switch (69) is switched off.

3. The LLC DC-DC converter as claimed in claim 1, said tertiary winding (65) is of an annular ring width of at least three times smaller than that said secondary winding (64) is.

4. The LLC DC-DC converter as claimed in claim 1, further comprising a resonant capacitor (61) connected between the output of said full-bridge circuit (2) and the first end of said primary winding (63).

5. The LLC DC-DC converter as claimed in claim 1, further comprising: a first resonant capacitor (28) connected between said secondary winding(64) and said first node (71); and a second resonant capacitor (29) connected between said tertiary winding(65) and said fifth node (75).

6. The LLC DC-DC converter as claimed in claim 1, said primary winding (63) being wound onto a plurality of primary printed circuit boards in two ways, wherein: in a first way said primary winding (63) is wound onto a primary printed circuit board in an fl-like contour such that a first type of primary printed circuit board (5) is formed; and in a second way said primary winding (63) is wound onto a primary printed circuit board in an a-like contour such that a second type of primary printed circuit (6) board is formed.

7. The LLC DC-DC converter as claimed in claim 6, wherein the first and second types of primary printed circuit boards (5, 6) is stacked in an alternate manner, each primary printed circuit board being connected to an adjacent primary printed circuit board by a plurality of half vias (8, 9) configured on the sides thereof.

8. The LLC DC-DC converter as claimed in claim 7, wherein the plurality of half vias (8, 9) is configured on opposing sides in an interleaved manner.

9. The LLC DC-DC converter as claimed in claim 1, wherein said plurality of secondary printed circuit boards comprises two types of secondary printed circuit boards (22, 23), a first type (22) of which has input and output ports configured in a reverse manner to those of a second type (23).

10. The LLC DC-DC converter as claimed in claim 9, wherein said first and second types of secondary printed circuit boards (22, 23) stack in an alternate manner.

11. The LLC DC-DC converter as claimed in claim 9, wherein: said secondary winding (64) of said first types of secondary printed circuit board (22) is connected to that of said second types of secondary printed circuit board (23) by a plurality of half vias (24, 25) configured on a side thereof; and said tertiary winding (65) of said first types of secondary printed circuit board (22) is connected to that of said second types of secondary printed circuit board (23) by a plurality of half vias (26, 27) configured on the opposing side thereof.

12. The LLC DC-DC converter as claimed in claim 9, wherein adjacent secondary printed circuit boards are connected by a plurality of inter-board solders (16, 17) configured on two opposing sides thereof, adjacent interboard solders (16, 17) being spaced transversely in an alternate manner.

13. The LLC DC-DC converter as claimed in claim 2, wherein the first range is20-38Vdc, and the second range is 38-60Vdc-

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