Patent Publication Number: US-2023155521-A1

Title: Bootstrap start-up circuit for microinverter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 63/279,435, filed Nov. 15, 2021, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Field of the Disclosure 
     Embodiments of the present disclosure relate generally to methods and apparatus configured for use with microinverters, and for example, to methods and apparatus that use bootstrap start-up circuits for microinverters. 
     Description of the Related Art 
     Distributed energy resources are known. Distributed energy resources can include one or more photovoltaics (PVs) (solar panels). In some instances, a DC input is sometimes required to provide power to the one or more PVs, e.g., start-up at nighttime to provide grid support functions. Similarly, a DC input is sometimes required to provide power to a battery storage microinverter to start-up an AC battery (e.g., single-phase or three-phase) of the battery storage, e.g., when the AC battery is completely discharged. In such instances, the existing AC battery, typically, requires a separate AC:DC converter to enable a start-up of the microinverters if the AC battery is completely discharged. 
     Therefore, the inventors have provided herein improved methods and apparatus that use bootstrap start-up circuits for microinverters. 
     SUMMARY 
     Methods and apparatus configured for use with bootstrap start-up circuits for microinverters are provided herein. For example, a microinverter comprises DC side MOSFETs connected to an input side of the microinverter, AC side MOSFETs connected to an output of the microinverter, and a plurality of gate drivers connected to the AC side MOSFETs and configured to automatically drive the microinverter without a DC voltage being applied to the input side of the microinverter. 
     In accordance with at least some embodiments, a method for providing power to a microinverter comprises detecting an input signal at a main control ASIC to drive isolated gate drivers, determining a voltage across AC side MOSFETs, alternately switching gate drivers of the AC side MOSFETs for driving the voltage into an AC side of a main isolation transformer, and rectifying the voltage to charge up a DC side input capacitor to power up a from a DC voltage present across the DC side input capacitor. 
     These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG.  1    is a block diagram of a power conversion system, in accordance with one or more embodiments of the present disclosure. 
         FIG.  2    is a block diagram of a microinverter configured for use with the system of  FIG.  1   , in accordance with one or more embodiments of the present disclosure; 
         FIG.  3    is a schematic of a gate driver of AC side MOSFETS, in accordance with one or more embodiments of the present disclosure; and 
         FIG.  4    is a flowchart of a method for providing power to the microinverter of  FIG.  2   , in accordance with one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure generally relate to methods and apparatus that use bootstrap start-up circuits for microinverters. For example, a microinverter can comprise DC side MOSFETs connected to an input side of the microinverter. AC side MOSFETs can be connected to an output of the microinverter, and a plurality of gate drivers can be connected to the AC side MOSFETs and configured to automatically drive the microinverter without a DC voltage being applied to the input side of the microinverter. The methods and apparatus described herein provide an efficient and cost effective manner for starting a fully discharged microinverter without the need of a separate AC:DC converter. 
       FIG.  1    is a block diagram of a system  100  (e.g., power conversion system) in accordance with one or more embodiments of the present disclosure. The diagram of  FIG.  1    only portrays one variation of the myriad of possible system configurations. The present disclosure can function in a variety of environments and systems. 
     The system  100  comprises a structure  102  (e.g., a user&#39;s structure), such as a residential home or commercial building, having an associated DER  118  (distributed energy resource). The DER  118  is situated external to the structure  102 . For example, the DER  118  may be located on the roof of the structure  102  or can be part of a solar farm. The structure  102  comprises one or more loads and/or energy storage devices  114  (e.g., appliances, electric hot water heaters, thermostats/detectors, boilers, water pumps, and the like), which can be located within or outside the structure  102 , and a DER controller  116 , each coupled to a load center  112 . Although the energy storage devices  114 , the DER controller  116 , and the load center  112  are depicted as being located within the structure  102 , one or more of these may be located external to the structure  102 . 
     The load center  112  is coupled to the DER  118  by an AC bus  104  and is further coupled, via a meter  152  and a MID  150  (microgrid interconnect device), to a grid  124  (e.g., a commercial/utility power grid). The structure  102 , the energy storage devices  114 , DER controller  116 , DER  118 , load center  112 , generation meter  154 , meter  152 , and MID  150  are part of a microgrid  180 . It should be noted that one or more additional devices not shown in  FIG.  1    may be part of the microgrid  180 . For example, a power meter or similar device may be coupled to the load center  112 . 
     The DER  118  comprises at least one renewable energy source (RES) coupled to power conditioners  122  (microinverters). For example, the DER  118  may comprise a plurality of RESs  120  coupled to a plurality of power conditioners  122  in a one-to-one correspondence (or two-to-one). In embodiments described herein, each RES of the plurality of RESs  120  is a photovoltaic module (PV module), although in other embodiments the plurality of RESs  120  may be any type of system for generating DC power from a renewable form of energy, such as wind, hydro, and the like. The DER  118  may further comprise one or more batteries (or other types of energy storage/delivery devices) coupled to the power conditioners  122  in a one-to-one correspondence, where each pair of power conditioner  122  and a battery  141  may be referred to as an AC battery  130 . 
     The power conditioners  122  invert the generated DC power from the plurality of RESs  120  and/or the battery  141  to AC power that is grid-compliant and couple the generated AC power to the grid  124  via the load center  112 . The generated AC power may be additionally or alternatively coupled via the load center  112  to the one or more loads (e.g., a solar pump) and/or the energy storage devices  114 . In addition, the power conditioners  122  that are coupled to the batteries  141  convert AC power from the AC bus  104  to DC power for charging the batteries  141 . A generation meter  154  is coupled at the output of the power conditioners  122  that are coupled to the plurality of RESs  120  in order to measure generated power. 
     In some alternative embodiments, the power conditioners  122  may be AC-AC converters that receive AC input and convert one type of AC power to another type of AC power. In other alternative embodiments, the power conditioners  122  may be DC-DC converters that convert one type of DC power to another type of DC power. In some of embodiments, the DC-DC converters may be coupled to a main DC-AC inverter for inverting the generated DC output to an AC output. 
     The power conditioners  122  may communicate with one another and with the DER controller  116  using power line communication (PLC), although additionally and/or alternatively other types of wired and/or wireless communication may be used. The DER controller  116  may provide operative control of the DER  118  and/or receive data or information from the DER  118 . For example, the DER controller  116  may be a gateway that receives data (e.g., alarms, messages, operating data, performance data, and the like) from the power conditioners  122  and communicates the data and/or other information via the communications network  126  to a cloud-based computing platform  128 , which can be configured to execute one or more application software, e.g., a grid connectivity control application, to a remote device or system such as a master controller (not shown), and the like. The DER controller  116  may also send control signals to the power conditioners  122 , such as control signals generated by the DER controller  116  or received from a remote device or the cloud-based computing platform  128 . The DER controller  116  may be communicably coupled to the communications network  126  via wired and/or wireless techniques. For example, the DER controller  116  may be wirelessly coupled to the communications network  126  via a commercially available router. In one or more embodiments, the DER controller  116  comprises an application-specific integrated circuit (ASIC) or microprocessor along with suitable software (e.g., a grid connectivity control application) for performing one or more of the functions described herein. For example, the DER controller  116  can include a memory (e.g., a non-transitory computer readable storage medium) having stored thereon instructions that when executed by a processor perform a method for grid connectivity control, as described in greater detail below. 
     The generation meter  154  (which may also be referred to as a production meter) may be any suitable energy meter that measures the energy generated by the DER  118  (e.g., by the power conditioners  122  coupled to the plurality of RESs  120 ). The generation meter  154  measures real power flow (kWh) and, in some embodiments, reactive power flow (kVAR). The generation meter  154  may communicate the measured values to the DER controller  116 , for example using PLC, other types of wired communications, or wireless communication. Additionally, battery charge/discharge values are received through other networking protocols from the AC battery  130  itself. 
     The meter  152  may be any suitable energy meter that measures the energy consumed by the microgrid  180 , such as a net-metering meter, a bi-directional meter that measures energy imported from the grid  124  and well as energy exported to the grid  124 , a dual meter comprising two separate meters for measuring energy ingress and egress, and the like. In some embodiments, the meter  152  comprises the MID  150  or a portion thereof. The meter  152  measures one or more of real power flow (kWh), reactive power flow (kVAR), grid frequency, and grid voltage. 
     The MID  150 , which may also be referred to as an island interconnect device (IID), connects/disconnects the microgrid  180  to/from the grid  124 . The MID  150  comprises a disconnect component (e.g., a contactor or the like) for physically connecting/disconnecting the microgrid  180  to/from the grid  124 . For example, the DER controller  116  receives information regarding the present state of the system from the power conditioners  122 , and also receives the energy consumption values of the microgrid  180  from the meter  152  (for example via one or more of PLC, other types of wired communication, and wireless communication), and based on the received information (inputs), the DER controller  116  determines when to go on-grid or off-grid and instructs the MID  150  accordingly. In some alternative embodiments, the MID  150  comprises an ASIC or CPU, along with suitable software (e.g., an islanding module) for determining when to disconnect from/connect to the grid  124 . For example, the MID  150  may monitor the grid  124  and detect a grid fluctuation, disturbance or outage and, as a result, disconnect the microgrid  180  from the grid  124 . Once disconnected from the grid  124 , the microgrid  180  can continue to generate power as an intentional island without imposing safety risks, for example on any line workers that may be working on the grid  124 . 
     In some alternative embodiments, the MID  150  or a portion of the MID  150  is part of the DER controller  116 . For example, the DER controller  116  may comprise a CPU and an islanding module for monitoring the grid  124 , detecting grid failures and disturbances, determining when to disconnect from/connect to the grid  124 , and driving a disconnect component accordingly, where the disconnect component may be part of the DER controller  116  or, alternatively, separate from the DER controller  116 . In some embodiments, the MID  150  may communicate with the DER controller  116  (e.g., using wired techniques such as power line communications, or using wireless communication) for coordinating connection/disconnection to the grid  124 . 
     A user  140  can use one or more computing devices, such as a mobile device  142  (e.g., a smart phone, tablet, or the like) communicably coupled by wireless means to the communications network  126 . The mobile device  142  has a CPU, support circuits, and memory, and has one or more applications  146  (e.g., a grid connectivity control application) installed thereon for controlling the connectivity with the grid  124  as described herein. The application  146  may run on commercially available operating systems, such as  10 S, ANDROID, and the like. 
     In order to control connectivity with the grid  124 , the user  140  interacts with an icon displayed on the mobile device  142 , for example a grid on-off toggle control or slide, which is referred to herein as a toggle button. The toggle button may be presented on one or more status screens pertaining to the microgrid  180 , such as a live status screen (not shown), for various validations, checks and alerts. The first time the user  140  interacts with the toggle button, the user  140  is taken to a consent page, such as a grid connectivity consent page, under setting and will be allowed to interact with toggle button only after he/she gives consent. 
     Once consent is received, the scenarios below, listed in order of priority, will be handled differently. Based on the desired action as entered by the user  140 , the corresponding instructions are communicated to the DER controller  116  via the communications network  126  using any suitable protocol, such as HTTP(S), MQTT(S), WebSockets, and the like. The DER controller  116 , which may store the received instructions as needed, instructs the MID  150  to connect to or disconnect from the grid  124  as appropriate. 
       FIG.  2    is a block diagram of a microinverter  200  (e.g., power conditioners  122 ) configured for use with the system  100  of  FIG.  1   ,  FIG.  3    is a schematic of a gate driver of AC side MOSFETS, and  FIG.  4    is a flowchart of a method for providing power to the microinverter of  FIG.  2   , in accordance with one or more embodiments of the present disclosure. 
     The control circuitry that drives DC side MOSFETs  202  (e.g., four DC side MOSFETS on the input side of microinverter) and AC side MOSFETs  204  (four AC side MOSFETs on the output side of microinverter) is powered from a housekeeping power supply that derives power from a DC input  206 . The microinverter  200  requires a DC voltage to be applied to the input of the microinverter  200  before the microinverter  200  can start-up. With conventional microinverters, as noted above, however, in a PV application, microinverters shut down during nighttime and start up the following morning when the sun rises and causes the PV module to provide a DC voltage to the input of the microinverter. 
     The microinverter  200  is bi-directional from a power conversion perspective, i.e., DC→AC and AC→DC, which is central to the microinverter  200  being used in a battery energy storage microinverter. Additionally, the bi-directional functionality of the microinverter  200  allows for PV applications in that the microinverter is able to continue to run once the sun goes down. That is, if the power output from the PV module falls to zero (e.g., at nighttime) the microinverter  200  starts to operate in the AC→DC mode, thus allowing a housekeeping power supply to be powered from power that is derived from the AC side of the microinverter  200 . In this way the microinverter  200  is able to run indefinitely during the nighttime. 
     In accordance with the instant disclosure, isolated gate drivers for the AC side MOSFETs  204  take the form of a gate driver  208  (integrated circuit (IC)) that includes special circuitry and logic to provide a bootstrap start up function. The bootstrap function requires a number of specific logical steps to be performed in a sequence so that the microinverter  200  is able to start up with no DC input applied to the microinverter (i.e., only AC voltage present). In at least some embodiments, the sequence is based on a double bootstrap concept. For example, in an embodiment, a first bootstrap concept can be used to power up the gate drivers  208  of the AC side MOSFETs  204 , which, in turn, run in a special start up mode that acts on a second bootstrap concept to start up the microinverter  200 . 
     For example, as illustrated in  FIG.  3   , the gate driver  208  uses a semiconductor based isolation barrier  301  comprising a differential pair of capacitors  303  to isolate a modulation signal from a modulator  305  across the semiconductor based isolation barrier  301 . In at least some embodiments, the gate driver  208  may also comprise or use structure that is configured to perform one or more other/different isolation methods which can include coreless transformer isolation, magnetic isolation, optical isolation, and so on. 
     The gate driver  208  can comprise or connect to a high-voltage bootstrap MOSFET (e.g., the AC side MOSFETs  204 , one of which is shown in  FIG.  3   ) that connects between a drain  302  &amp; V dd  connections of the gate driver  208  and is responsible for bootstrap charging of a gate driver power supply capacitor  304  when a gate driver power supply oscillator  306  is not operational, e.g., when a DC input power is not available to the microinverter  200 . 
     The gate driver  208  comprises mode detection logic  308  that connects to a diode input  310  of the gate driver  208  and determines if the gate driver power supply oscillator  306  is operational. For example, a presence of a high frequency signal (e.g., about 10 MHz to about 100 MHz) at the diode input  310  indicates that the gate driver power supply oscillator  306  is operational (i.e., DC input power is available to the microinverter  200 ), whereas an absence of any high frequency signal at the diode input  310  would indicate that the gate driver power supply oscillator  306  is not operational (i.e., DC input power is not available to the microinverter  200 ). 
     The gate driver  208  comprises a multiplexer  312  controlled by the mode detection logic  308 . For example, if DC input power is available to the input of the microinverter  200 , the multiplexer  312  connects the output of a demodulator  314  to a gate drive output buffer stage  316  resulting in the gate driver  208  operating in a conventional isolated gate driver fashion (e.g., a first mode of operation). Conversely, if DC input power is not available to the input of the microinverter  200 , the multiplexer  312  connects the output of a monostable oscillator  318  to the gate drive output buffer stage  316  resulting in the gate driver  208  operating in the bootstrap start-up mode of operation (e.g., a second mode of operation different from the first mode of operation), as described in greater detail below. 
     The gate driver  208  comprises a voltage comparator  320  that is configured to determine if a drain-to-source voltage across the main power MOSFET (e.g., the AC side MOSFETs  204  is above or below 250 Vdc. Thus, when the gate driver  208  turns on the main power MOSFET connected thereto, then the voltage comparator  320  expects that the drain-to-source voltage across the main power MOSFET will fall to approximately zero voltage. Accordingly, assuming the microinverter  200  is powered of 240 Vac, which has a peak voltage of 340 Vdc, the maximum voltage across the main power MOSFET would be approximately 170 Vdc (i.e., half the peak AC main voltage) if another of the complementary main power MOSFETs is turned on. Therefore, a voltage across the main power MOSFET exceeding 250 Vdc is a way for this gate driver  208  knowing that the complimentary main power MOSFET is turned on. 
     The monostable oscillator  318  generates a short duration gate drive signal (e.g., a predetermined value of about 1 μs long gate-on signal) when triggered by the voltage comparator  320  detecting a drain-to-source voltage in excess of a predetermined threshold (e.g., 250 Vdc). In at least some embodiments, the monostable oscillator  318  output signal can be delayed with respect to receiving the input signal from the voltage comparator  320  and once the monostable oscillator  318  has produced a single gate output on signal pulse (e.g., a 1 μs gate-on pulse), the monostable oscillator  318  imposes a blanking period (e.g., about 10 μs) in which time the monostable oscillator  318  will not produce another gate output signal. 
     Continuing with reference to  FIGS.  2 - 4   , a full bootstrap start-up sequence (e.g., the first bootstrap concept and the second bootstrap concept) can comprise one or more high-voltage MOSFETs that can be added to the power circuit of the microinverter  200  and used as linear voltage regulators to directly derive current from an AC port  210  of the microinverter  200  and power up output stages of the isolated the gate drivers  208  of the AC side MOSFETs  204 . During a linear mode of operation (e.g., during a bootstrap period), an efficiency of the voltage regulator is relatively low (e.g. 2% efficiency), however, when the microinverter  200  is finally powered up, the AC side MOSFETs  204  are no longer operated as a linear regulators, thus allowing for a higher efficiency, normal power supply to power the gate drivers  208 . 
     At  402 , the method  400  comprises detecting an input signal at a main control ASIC to drive the isolated the gate drivers  208 . For example, the output stages of the gate drivers  208  of the AC side MOSFETs  204  (with bootstrap start-up logic) are powered up and logic in the output stage is configured to determine if the input stages to the isolated the gate drivers  208  are not powered up (e.g., if there is no input signal being provided to the main control ASIC to drive the isolated the gate drivers  208 ). The gate driver output stage logic interprets the no signal condition as a start-up condition, which requires the gate drivers  208  to generate valid gate drive signals to bootstrap the microinverter  200 . 
     Next, at  404 , the method  400  comprises determining a voltage across AC side MOSFETs. For example, the output stages of the gate drivers  208  (with bootstrap start-up logic) includes circuitry which allows the gate drivers  208  to determine a voltage across the AC side MOSFETs  204 . In at least some embodiments, the circuitry can be based on a voltage comparator set to a voltage of about 250 V. If the output stage of the gate drivers  208  detects that the start-up condition is present and the voltage across the AC side MOSFETs  204 , to which the output stage is connected, is above 250 V, the output stage generates a short duration (e.g. about 1 μs) high output to momentarily turn the AC side MOSFETs  204  on. In at least some embodiments, once the gate drivers  208  have provided the 1 μs signal to turn on the AC side MOSFETs  204 , a blanking period of about 10 μs can be applied during which time the gate drivers  208  are not allowed to turn on irrespective of the voltage across the AC side MOSFETs  204 . 
     Next, at  406 , the method  400  comprises alternately switching gate drivers of the AC side MOSFETs for driving a voltage into the AC side of a main isolation transformer. For example, the inventors have found that an effect of the above start-up logic being implemented in all of the output stages of the isolated gate drivers  208  results in the gate drivers  208  alternately switching at a frequency of about 100 kHz. The exact frequency that such switching occurs can be adjusted by degerming the blanking period applied in the gate drivers  208  start-up logic. The alternate of the gate drivers  208  switching drives a voltage into the AC side of a main isolation transformer  212 , which, in turn drives a voltage to be generated across the DC side winding of the main isolation transformer  212 . 
     Next, at  408 , the method  400  comprises rectifying the voltage to charge up a DC side input capacitor to power up a DC power supply from a DC voltage present across a DC side input capacitor. For example, the 100 kHz voltage generated across the DC side of the main isolation transformer  212  can be rectified by the body-diodes  214  of the DC side MOSFETs  202 . The body-diodes  214  of the DC side MOSFETs  202  rectify the 100 kHz voltage and charge up a DC side input capacitor  216 . The housekeeping power supply is powered up from the DC voltage present across the DC side input capacitor  216 . Moreover, the main control ASIC can be powered up and the main control ASIC generates gate drivers  208  signals (valid gate drivers signals) to drive both the DC side MOSFETs  202  and AC side MOSFETs  204 . In at least some embodiments, during 408, the method  400  can comprise isolating a modulation signal from the modulator  305  disposed across the semiconductor based isolation barrier  301 . 
     The output stages of the gate drivers  208  (with bootstrap start-up logic) determines if the input stages to the gate drivers  208  are now powered up (i.e., if there is an input signal provided by the main control ASIC to drive the isolated gate drivers  208 ). The gate drivers  208  output stage logic interprets such a condition as the end of the start-up condition and the start-up logic is disabled leaving the main control ASIC responsible for generating gate drivers  208  (valid gate drivers) to run the microinverter  200  in the normal operational mode. 
     With the housekeeping power supply up and operational and the main control ASIC providing the gate drivers  208  signals, the gate drivers  208  are provided with the normal auxiliary power supply. The DC side MOSFETs  202  and the AC side MOSFETs  204  (e.g., high voltage) used as linear regulators to bootstrap power up the gate drivers  208  are now deactivated, i.e., they now longer function as linear regulators. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.