Abstract:
An inverter apparatus includes a direct current to direct current converter (DC/DC converter), a direct current to alternating current converter (DC/AC converter), a primary-side control circuit and a secondary-side control circuit. The DC/DC converter is arranged for outputting a first DC power and a second DC power. The DC/AC converter is coupled to the DC/DC converter, and is arranged for receiving the first DC power. The primary-side control circuit is coupled to the DC/DC converter, and is arranged for controlling an operation of the DC/DC converter. The secondary-side control circuit is coupled to the DC/DC converter and the DC/AC converter, and is arranged for receiving the second DC power, and controlling an operation of the DC/AC converter according to the second DC power.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 61/944,587, filed on Feb. 26, 2014, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosed embodiments of the present invention relate to an inverter apparatus, and more particularly, to an inverter apparatus having a primary-side driver circuit and a secondary-side driver circuit driven by respective direct current to direct current (DC/DC) converters, and a related control method. 
     2. Description of the Prior Art 
     A conventional inverter apparatus uses a fly-back direct current to direct current (DC/DC) converter as an auxiliary power source shared by a primary-side driver circuit and a secondary-side driver circuit, wherein the primary-side driver circuit drives a DC/DC converter, and the secondary-side driver circuit drives a direct current to alternating current (DC/AC) converter. However, due to poor conversion efficiency (e.g. a large energy loss in a transformer), the fly-back DC/DC converter cannot provide a high efficiency auxiliary power source for the conventional inverter apparatus. 
     Thus, a novel inverter apparatus is needed to solve the problem of low performance of a conventional inverter apparatus due to poor conversion efficiency of a fly-back DC/DC converter. 
     SUMMARY OF THE INVENTION 
     It is therefore one objective of the present invention to provide an inverter apparatus having a primary-side driver circuit and a secondary-side driver circuit driven by respective DC/DC converters, and a related control method to solve the above problems. 
     It is therefore another objective of the present invention to provide overvoltage protection mechanism to effectively prevent an overvoltage condition from occurring on a DC bus of the inverter apparatus during an initial start-up period. 
     According to an embodiment of the present invention, an exemplary inverter apparatus is disclosed. The exemplary inverter apparatus comprises a DC/DC converter, a DC/AC converter, a primary-side control circuit and a secondary-side control circuit. The DC/DC converter is arranged for outputting a first DC power and a second DC power. The DC/AC converter is coupled to the DC/DC converter, and is arranged for receiving the first DC power. The primary-side control circuit is coupled to the DC/DC converter, and is arranged for controlling an operation of the DC/DC converter. The secondary-side control circuit is coupled to the DC/DC converter and the DC/AC converter, and is arranged for receiving the second DC power, and controlling an operation of the DC/AC converter according to the second DC power. 
     In one implementation, the exemplary inverter apparatus further comprises a guard circuit. The guard circuit is coupled to the DC/DC converter, and is arranged for detecting the first DC power and accordingly generating a protection signal to the primary-side control circuit. The primary-side control circuit further refers to the protection signal to control the operation of the DC/DC converter. 
     According to an embodiment of the present invention, an exemplary control method of an inverter apparatus is disclosed. The inverter apparatus comprises a DC/DC converter and a DC/AC converter. An output side of the DC/DC converter is coupled to an input side of the DC/AC converter. The exemplary control method comprises the following steps: outputting a first DC power and a second DC power from the output side of the DC/DC converter, wherein the first DC power is outputted to the input side of the DC/AC converter; and receiving the second DC power, and controlling an operation of the DC/AC converter according to the second DC power. 
     In one implementation, the exemplary control method further comprises: detecting the first DC power, and accordingly generating a protection signal; and controlling an operation of the DC/DC converter according to the protection signal. 
     The proposed inverter apparatus and control method thereof may not only provide a high efficiency auxiliary power source for a primary-side/a secondary-side circuit, but also provide overvoltage protection mechanism to prevent an overvoltage condition from occurring on a DC bus during an initial start-up period. Hence, the proposed inverter apparatus and control method thereof can be used in a variety of power conversion schemes. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an exemplary inverter apparatus according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a part of circuit elements of the DC/DC converter shown in  FIG. 1  according to an embodiment of the present invention. 
         FIG. 3  is an implementation of the inverter apparatus shown in  FIG. 1 . 
         FIG. 4  is an implementation of the guard circuit shown in  FIG. 3 . 
         FIG. 5  is another implementation of the guard circuit shown in  FIG. 3 . 
         FIG. 6  is a circuit diagram of the detection circuit shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     The proposed inverter apparatus includes a primary-side auxiliary power source and a secondary-side auxiliary power source which are disposed separately from each other, and provide overvoltage protection mechanism to avoid an overvoltage condition between a primary side and a secondary side of the inverter apparatus, thereby replacing a conventional inverter architecture where the primary side and the secondary side share an auxiliary power source implemented by a fly-back DC/DC converter. To facilitating an understanding of the present invention, the proposed inverter apparatus is implemented by a photovoltaic inverter in the following. However, the proposed inverter apparatus and related control method are not limited to be employed in a photovoltaic inverter. Further description is provided below. 
     Please refer to  FIG. 1 , which is a block diagram illustrating an exemplary inverter apparatus according to an embodiment of the present invention. The inverter apparatus  100  is coupled between a photovoltaic cell (PV cell)  102  and a grid  104 , and may include, but is not limited to, a DC/DC converter  110 , a DC/AC converter  120 , a primary-side control circuit  130  and a secondary-side control circuit  140 . The DC/DC converter  110  may receive an input power V PV  provided by the PV cell  102 , and accordingly output a DC power V BUS  (e.g. a DC bus voltage; a voltage drop across a DC bus capacitor C BUS ) and a DC power V AUX  from an output side S DO . The output side S DO  of the DC/DC converter  110  is coupled to an input side S AI  of the DC/AC converter  120 , wherein the DC/AC converter  120  may receive the DC power V BUS  from the input side S AI , and convert the DC power V BUS  to generate an AC power V AC  to an output side S AC . The primary-side control circuit  130  is coupled to the DC/DC converter  110 , and is arranged for generate a control signal S C1  to control an operation of the DC/DC converter  110 , wherein a required power of the primary-side control circuit  130  may be supplied by the PV cell  102 . The secondary-side control circuit  140  is coupled to the DC/DC converter  110  and the DC/AC converter  120 . The secondary-side control circuit  140  is arranged for receiving the DC power V AUX , and generating a control signal S C2  according to the DC power V AUX  to thereby control an operation of the DC/AC converter  130 . 
     By way of example but not limitation, the DC/DC converter  110  may be implemented by an LLC resonant converter, which may provide soft switching to increase conversion efficiency and reduce electromagnetic interference (EMI). The DC/AC converter  120  may be referred to as a DC/AC inverter. Additionally, in one implementation, the DC/DC converter  110  may use a transformer included therein to provide the DC power V BUS  and the DC power V AUX . Please refer to  FIG. 2 , which is a diagram illustrating a part of circuit elements of the DC/DC converter  110  shown in  FIG. 1  according to an embodiment of the present invention. In this embodiment, the DC/DC converter  110  may include a transformer TX, which may have a primary side and a secondary side. The primary side of the transformer TX may include a winding L 11 , and the secondary side of the transformer TX may include a plurality of windings L 21  and L 22 . The transformer TX may perform voltage conversion on an electrical power received at the primary side (e.g. the input power V PV ), and accordingly output the DC powers V BUS  and V AUX  from the windings L 21  and L 22  of the secondary side, respectively. 
     Please note that the generation of the DC powers V BUS  and V AUX  described above is for illustrative purposes only, and is not meant to be a limitation of the present invention. For example, the DC/DC converter  110  may use a voltage divider circuit to generate a divided voltage of the DC power V BUS  as the DC power V AUX . As long as the DC/DC converter  110  may convert the input power V PV  to generate the DC powers V BUS  and V AUX  to the output side S DO , other modifications and alternatives fall within the scope of the present invention. 
     In practice, the primary-side control circuit  130 /the secondary-side control circuit  140  may include a DC/DC converter which is used as a primary-side/secondary-side auxiliary power source, wherein the DC/DC converter may be implemented by a high-efficiency buck converter rather than a flyback converter. Please refer to  FIG. 3 , which is an implementation of the inverter apparatus  100  shown in  FIG. 1 . The inverter apparatus  300  may include the DC/DC converter  110  and the DC/AC converter  120  shown in  FIG. 1 , a primary-side control circuit  330  and a secondary-side control circuit  340 , wherein the primary-side control circuit  130  and the secondary-side control circuit  140  shown in  FIG. 1  may be implemented by the primary-side control circuit  330  and the secondary-side control circuit  340  respectively. 
     The primary-side control circuit  330  may include a primary-side auxiliary power source  332  and a primary-side driver circuit  336 , and the secondary-side control circuit  340  may include a secondary-side auxiliary power source  342  and a secondary-side driver circuit  346 . The primary-side auxiliary power source  332  may provide a primary-side auxiliary power signal S A1  to the primary-side driver circuit  336 , wherein a required power of the primary-side auxiliary power source  332  may be supplied by the PV cell  102 . The primary-side driver circuit  336  is coupled between the primary-side auxiliary power source  332  and the DC/DC converter  110 , and drives the DC/DC converter  110  according to at least the primary-side auxiliary power signal S A1 . The secondary-side auxiliary power source  342  may receive the DC power V AUX  to generate a secondary-side auxiliary power signal S A2 . The secondary-side driver circuit  346  is coupled between the secondary-side auxiliary power source  342  and the DC/AC converter  120 , and is arranged for driving the DC/AC converter  120  according to the secondary-side auxiliary power signal S A2 . In this implementation, at least one of the primary-side auxiliary power source  332  and the secondary-side auxiliary power source  342  may be implemented by a high efficiency DC/DC converter (e.g. a buck converter) instead of a flyback power converter. 
     It should be noted that the proposed inverter apparatus may further provide protection mechanism to prevent an overvoltage condition from occurring in a secondary-side circuit. For example, in the embodiment shown in  FIG. 3 , during an initial start-up period of the inverter apparatus  300 , the primary-side auxiliary power source  332  may activate the primary-side driver circuit  336  according to an electrical power supplied by the PV cell  102 . Hence, the primary-side driver circuit  336  may enable the DC/DC converter  110  to convert the input power V PV , thereby gradually increasing an energy level (e.g. a voltage level) of the DC power V BUS /V AUX . As the frequency and amplitude detection of the grid  104  takes a period of time, it may occur that the energy level of the DC power V BUS  is greater than a predetermined level while the secondary-side driver circuit  346  remains turned off. In other words, during the initial start-up period of the inverter apparatus  300  (the frequency and amplitude detection of the grid  104  has not yet been completed), the secondary-side driver circuit  346  may be unable to drive the DC/AC converter  120  to control the DC power V BUS  (a DC bus voltage). This may cause an overvoltage condition on the DC power V BUS . 
     In order to avoid the overvoltage condition which may damage the internal circuitry of the inverter apparatus  300 , the inverter apparatus  300  may further include a guard circuit  350 , which is coupled to the DC/DC converter  110  and is arranged for detecting the DC power V BUS  and accordingly generating a protection signal S P  to the primary-side control circuit  330 . Hence, the primary-side control circuit  330  may control the operation of the DC/DC converter  110  according to the electrical power supplied by the PV cell  102  and the protection signal S P . For example, the guard circuit  350  may compare the energy level of the DC power V BUS  with a predetermined level, and accordingly generate the protection signal S P  to control the operation of the DC/DC converter  110 . In one implementation, when the guard circuit  350  detects that the energy level of the DC power V BUS  is greater than the predetermined level, the primary-side control circuit  330  may turn off the DC/DC converter  110  according to the protection signal S P  so as to protect the circuit elements on the secondary side of the inverter apparatus  300 . In another implementation, when the guard circuit  350  detects that the energy level of the DC power V BUS  is less than the predetermined level, the primary-side control circuit  330  may turn on the DC/DC converter  110  according to the protection signal S P . In the embodiment shown in  FIG. 3 , the protection signal S P  generated by the guard circuit  350  may be received by the primary-side driver circuit  336 , and the primary-side driver circuit  336  may drive the DC/DC converter  110  according to the primary-side auxiliary power signal S A1  and the protection signal S P . 
     In a case where the guard circuit  350  detects a voltage level of the DC power V BUS  to generate the protection signal S P , the guard circuit  350  may be implemented with an overvoltage protection structure. Please refer to  FIG. 4 , which is an implementation of the guard circuit  350  shown in  FIG. 3 . In this implementation, the guard circuit  350  may include an overvoltage protection circuit  452  and a controller  456 . The overvoltage protection circuit  452  may compare the voltage level of the DC power V BUS  with a predetermined level V REF  to generate a comparison result DR. The controller  456  is coupled to the overvoltage protection circuit  452 , and is arranged for generating the protection signal S P  according to the comparison result DR. By way of example but not limitation, a detection circuit  455  may compare a voltage V D  generated by a voltage divider circuit  453  with the predetermined level V REF  to generate a detection result, and an optical coupler circuit  454  may generate the comparison result DR according to the detection result. When the voltage V D  is too high (i.e. the voltage level of the DC power V BUS  is too high), the optical coupler circuit  454  may couple a voltage V M  to ground (i.e. a resistor R 3  is grounded). Hence, the controller  456  may generate the protection signal S P  according to the comparison result DR (the voltage V M ), thereby instructing the primary-side control circuit  330  shown in  FIG. 3  to turn off the DC/DC converter  110 . 
     In the embodiment shown in  FIG. 4 , the voltage divider circuit  453  may be implemented by a resistor R 1  and a resistor R 2 , the detection circuit  455  may be implemented by a comparator CP and a switch SW, and the optical coupler circuit  454  may be implemented by a photodiode D 1  and a transistor M 1 . Voltages V S1  and V S2  may be used as required powers of the transistor M 1  and the photodiode D 1  respectively. The comparator CP may compare the voltage V D  with the predetermined level V REF . When the voltage V D  is greater than the predetermined level V REF  (e.g. 2.5 volts), meaning that the voltage level of the DC power V BUS  is too high, the switch SW turns on, and the photodiode D 1  turns on accordingly. Hence, the transistor M 1  turns on to couple the voltage V M  to ground. 
     The circuit structure of the guard circuit  350  shown in  FIG. 4  is for illustrative purposes only, and is not meant to be a limitation of the present invention. For example, at least one of the voltage divider circuit  453 , the optical coupler circuit  454  and the detection circuit  455  may be implemented by other circuit topologies. Please refer to  FIG. 5 , which is another implementation of the guard circuit  350  shown in  FIG. 3 . In this alternative design, the architecture of the guard circuit  550  is based on that of the guard circuit  350  shown in  FIG. 4 , wherein the main difference is that a detection circuit  555  included in an overvoltage protection circuit  552  may be implemented by a three-terminal adjustable precision shunt regulator (AS431). A plurality of connection terminals N 1 -N 3  may be coupled to the optical coupler circuit  454 , ground and the voltage V D  respectively, wherein the details of the detection circuit  555  are shown in  FIG. 6 . Please note that the three-terminal adjustable precision shunt regulator is merely one example of the proposed detection circuit and is not intended to suggest any limitation as to the scope of use or functionality of the invention. Additionally, as a person skilled in the art should understand that the operations associated with the comparator CR, the transistor M 2 , the diode D 2  and the predetermined level V REF  shown in  FIG. 6 , further description is omitted here for brevity. 
     In the embodiment shown in  FIG. 4 , the overvoltage protection circuit  452  uses the voltage divider circuit  453  (implemented by the resistors R 1  and R 2 ), the optical coupler circuit  454  (implemented by the photodiode D 1  and the transistor M 1 , whose voltage source is the voltage V S1 ) and the detection circuit  455  (implemented by the comparator CP and the switch SW) to detect the DC power V BUS . However, this is for illustrative purposes only, and is not meant to be a limitation of the present invention. In an alternative design, at least one of the voltage divider circuit  453 , the optical coupler circuit  454  and the detection circuit  455  may be implemented by other circuit topologies. In another alternative design, it is possible to directly use a comparison circuit to compare the voltage level of the DC power V BUS  with the predetermined level V REF  to generate the comparison result DR. Further, the overvoltage protection mechanism shown in  FIG. 3 / FIG. 4 / FIG. 5  may be employed in the inverter apparatus  100  shown in  FIG. 1 . 
     To sum up, the proposed inverter apparatus and control method thereof may not only provide a high efficiency auxiliary power source for a primary-side/a secondary-side circuit, but also provide overvoltage protection mechanism to prevent an overvoltage condition from occurring on a DC bus during an initial start-up period. Hence, the proposed inverter apparatus and control method thereof can be used in a variety of power conversion schemes. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.