AN-2116

SolarMagic ICs in Microinverters Applications SolarMagic™ ICs in Microinverter Applications National Semiconductor Application Note 2116 Perry Tsao May 4, 2011 Introduction Microinverters are a growing and rapidly evolving part of the photovoltaic (PV) system. Modern microinverters are designed to convert the DC power from one PV module (solar panel) to the AC grid, and are designed for a max output power in the range of 180W to 300W. Compared to conventional string or central inverters, microinverters have advantages in ease of installation, localized max power point tracking, and redundancy that provides robustness to failure. Since this area of power electronics is seeing such rapid innovation, there are many different topologies and variations being developed. This article explores some of the prevalent topologies used in microinverters today, and the use of SolarMagic™ ICs in these demanding applications. In particular, the use of the SM72295 Photovoltaic Full-Bridge Driver will be highlighted. lifetime. This line of products includes MOSFET gate drives, PWM controllers with integrated switches, LDO regulators, amplifiers, and many other ICs necessary for photovoltaic electronics. All of the ICs recommended in this article are being made available as Renewable Energy Grade components. Single-Stage Microinverters There have been a multitude of microinverter topologies developed [1], and these topologies can be broken up into two broad categories. The first category depicted in the block diagram of Figure 1 employs a DC/DC converter and controls the converter output voltage to have the shape of a rectified sinusoid. This rectified sinusoid waveform is then inverted into a full sinusoidal waveform using an “unfolding bridge” that interfaces to the grid voltage. Though perhaps not the most accurate name, this category of microinverter topologies is often referred to as a “single-stage microinverter” because the boosting of the panel voltage and shaping of the AC waveform is accomplished in a single stage. A more formal categorization of microinverter topologies [2] refers to this as a PV-side decoupled topology because the input capacitors decouple the AC power variation. The most widespread topology of this category is a quasi-resonant interleaved flyback, however, there are other variants such as interleaved flyback (not quasi-resonant) and interleaved forward converter. The unfolding inverter is generally implemented with 4 SCR’s (silicon controlled rectifiers) that switch at the grid frequency. SolarMagic Renewable Energy Grade Components The environment for electronics in PV systems is a very demanding one due to the extremes in temperatures and requirements for long-lifetime. The ambient temperature behind a photovoltaic module can range from below freezing in the winter to over 90°C on a summer day. With this in mind National Semiconductor created the Renewable Energy Grade line of SolarMagic ICs that are all rated for operation from -40° C to +125°C and have all been screened and tested to standards appropriate for products that are designed for a 25 year 30150101 FIGURE 1. Block diagram of a microinverter using a single-stage topology. The DC/DC stage can be implemented as a quasi-resonant interleaved flyback or another topology. AN-2116 © 2011 National Semiconductor Corporation 301501 www.national.com AN-2116 30150102 FIGURE 2. Simplified schematic of a quasi-resonant interleaved flyback using the SM72295. Current sensing is implemented with high-side current sense resistors, and output overvoltage shutdown is implemented using voltage sense windings and the OVS pin. Figure 2 depicts a simplified schematic of a single-stage microinverter using a quasi-resonant interleaved flyback for the DC-DC stage. In the quasi-resonant interleaved flyback, the SM72295 provides the microinverter designer with a high level of integration and enables maximized power density, reduced component count, and reduced PCB space. The SM72295 combines four independent 3A MOSFET gate drives with signal conditioning, power good, and overvoltage sensing functionality. Gate drive signal inputs are compatible with both 3.3V and 5V logic. The integrated signal conditioning provides two channels optimized for using high-side current sense resistors with common-mode voltages up to 100V. A transconductance amplifier provides gain and is followed by a low-impedance buffer suitable for interfacing into an analog to digital converter (ADC). The use of current sense resistors is a lower cost alternative to commonly used current-sense transformers. In addition, the ability to put the current sense resistors on the high-side (positive voltage) current path as opposed to the low-side (ground) current return path can ease layout because it does not require segmenting the ground plane and also eliminates the need for a negative rail voltage in cases where the sense resistor voltages goes below ground. The advantages of the single-stage topology microinverters are their lower component count, low switching frequencies of the unfolding bridge, and ease of implementing isolation. Disadvantages include high voltage ratings on both the primary side switches and the secondary side diode, and high amplitude 120Hz ripple current at the input. This input ripple current must be controlled to maintain an acceptable efficiency level due to the nature of the photovoltaic module. A photovoltaic module has a load curve with a specific maximum power point Pmp that occurs when its output voltage equals Vmp and output current equals Imp. To maximize energy harvest, the microinverter maintains the module output voltage and current as closely as possible to Vmp and Imp using a max power point tracking algorithm. Deviations from Vmp or Imp, such as those caused by input ripple current, would cause power loss. Therefore the input ripple current of the singlestage inverters must be reduced to minimize the power loss, and this necessitates large capacitors at the input of the microinverter. For practical cost and size purposes these capacitors can only be implemented with electrolytic capacitors. www.national.com 2 AN-2116 Two-Stage Microinverters The second category of microinverter topologies depicted in the block diagram of Figure 3 employ an intermediate high voltage DC-bus. These topologies use a DC/DC converter with a high boost ratio to boost from the PV module voltage to the intermediate DC-bus voltage, and then use a conventional PWM controlled MOSFET or IGBT full-bridge to invert the waveform to the grid. This type of microinverter is also referred to as a DC-link topology [2]. There are many different options being implemented for the DC/DC stage in the designs being developed today. Possibilities include: 1. Interleaved flyback 2. Push-pull converter (current-fed or voltage-fed, with passive or active clamp) 3. Full-bridge converter (voltage-fed, current-fed, or resonant) From a high-level perspective, all of these topologies are more complex and costly than the single-stage microinverter due to the additional high-frequency switching components. At first it may not be obvious why these topologies are being developed. However, for applications in microinverters, there’s an overriding focus on maximizing reliability, which puts emphasis on choosing a topology that enables the selection of the highest reliability components. All of the these topologies have much lower input ripple current at the PV input side, and therefore use lower capacitance values that make it practical to use higher reliability film capacitors in the place of electrolytic capacitors. Another benefit of the two-stage topologies is that it makes it possible to provide reactive power to the grid, whereas it is not possible with single-stage inverters with an SCR unfolding bridge. The ability to provide reactive power is a highly desirable feature for some commercial installations, and it is already a requirement for larger photovoltaic installations in some countries. 30150104 FIGURE 3. Block Diagram Of Two-Stage Inverter With A DC Bus. 3 www.national.com AN-2116 30150105 FIGURE 4. Simplified schematic of a two stage microinverter using the SM72295. The DC/DC stage is implemented as a voltage-fed full-bridge converter, and the DC/AC stage is implemented as a MOSFET Full-bridge. Shown in Figure 4 is a simplified schematic of a two-stage microinverter implemented with a voltage-fed full-bridge for the DC/DC stage and a MOSFET full-bridge for the DC/AC stage. In this application, the SM72295 provides gate drives for the four primary side MOSFETs. The SM72295 is ideally suited as a gate driver in many of these two-stage topologies, several of which use a MOSFET full-bridge on their primary side. As shown in Figure 4, the SM72295 is capable of driving all 4 MOSFETs in the primary side full-bridge. It provides 2 highside and 2 low-side gate drives, integrated bootstrap diodes, and is suitable for input voltages up to 100V. The additional integration of signal conditioning, undervoltage lockout, and overvoltage shutdown further reduce part count and conserve valuable PCB real-estate. Conclusion Microinverters are an exciting and growing application area for power electronics. This article gave a brief overview of some of the topologies being used in microinverters today, and described the SM72295 Photovoltaic Full-bridge Driver which integrates the key functions of MOSFET gate drives, signal conditioning, under-voltage lockout, and overvoltage shutdown. The SM72295 and other SolarMagic ICs support the most prevalent topologies used in microinverters today, and help microinverter designers maximize reliability, minimize complexity, minimize size, and minimize cost. References [1] B. Burger, B. Goeldi, S. Rogalla, H. Schmidt. “Module integrated electronics – an overview” 25th European Photovoltaic Solar Energy Conference and Exhibition. 6-10 Sept. 2010. pp. 3700 – 3707. [2] Haibing Hu; Harb, S.; Kutkut, N.; Batarseh, I.; Shen, Z.J. “Power decoupling techniques for micro-inverters in PV systems-a review” Energy Conversion Congress and Exposition (ECCE), 2010. pp. 3235 – 3240. Housekeeping Power and Other Applications In addition to the SM72295, there is a broad range of SolarMagic ICs suitable for application in other areas of the microinverter. As shown in Figure 5 and Table 1, this includes temperature sensors, voltage references, precision amplifiers for current and voltage sensing, and switchers and LDOs for housekeeping power. These ICs have application in both single-stage and two-stage microinverters of all topologies. www.national.com 4 AN-2116 30150106 FIGURE 5. Block Diagram Of SolarMagic Ics In A Microinverter Application 5 www.national.com AN-2116 TABLE 1. Gate Drives SM72295 SM72482 SM74101 SM74104 Description Photovoltaic Full-Bridge Driver Dual 5A Compound Gate Driver Tiny 7A MOSFET Gate Driver (LLP-6 package) High Voltage Half-Bridge Gate Driver with Adaptive Delay High Voltage Switching Regulators with Description Integrated Switch SM72485 SM74301 SM74304 Low Dropout Voltage Regulators SM74501 SM72238 SM74503 Amplifiers for current sensing, voltage sensing, and buffering SM72501 SM73301 SM73302 SM73303 SM73304 SM73305 SM73306 Comparators SM72375 SM73402 SM73403 Reset and Supervisory SM72240 SM74601 Thermostats and Temperature Sensors SM72480 SM73710 100V, 150mA Step-Down (Buck) Converter 100V, 350 mA Constant On-Time Buck Switching Regulator 80V, 500mA Step Down Swithching Regulator Description 50mA Low Dropout Voltage Regulator (3.3V, 5.0V), max Vin 40V 100mA Low Dropout Voltage Regulator (3.3V, 5.0V), max Vin 30V 800mA Low-Dropout Regulator (3.3V, 5.0V), max Vin 15V Description Precision, CMOS Input, Rail-to-Rail Input Output, Wide Supply Range Amplifier Rail-to-Rail Input Output, High Output Current & Unlimited Cap Load Op Amp 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Op Amp 5 MHz, Low Noise, Rail-to-Rail Output, Dual Operational Amplifier with CMOS Input Dual 17 MHz, Low Noise, CMOS Input Amplifier 17 MHz, Low Noise, CMOS Input Amplifier Dual CMOS Rail to Rail Input and Output Operational Amplifier Description Dual Micro-Power CMOS Comparator Low Power Low Offset Quad Comparators Single General Purpose Voltage Comparator Description 5-Pin Microprocessor Reset Circuit (3.08V, 4.63V thresholds) Precision Micropower Series Voltage Reference (2.5V) Description 125°C, 120°C, and 105°C Thermostat ±4°C Accurate, Temperature Sensor www.national.com 6 AN-2116 7 www.national.com SolarMagic ICs in Microinverters Applications Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage References PowerWise® Solutions Temperature Sensors PLL/VCO www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Applications & Markets Mil/Aero PowerWise® Design University Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. NATIONAL MAKES NO REPRESENTATIONS OR WARRANTIES WITH RESPECT TO THE ACCURACY OR COMPLETENESS OF THE CONTENTS OF THIS PUBLICATION AND RESERVES THE RIGHT TO MAKE CHANGES TO SPECIFICATIONS AND PRODUCT DESCRIPTIONS AT ANY TIME WITHOUT NOTICE. NO LICENSE, WHETHER EXPRESS, IMPLIED, ARISING BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS IS GRANTED BY THIS DOCUMENT. TESTING AND OTHER QUALITY CONTROLS ARE USED TO THE EXTENT NATIONAL DEEMS NECESSARY TO SUPPORT NATIONAL’S PRODUCT WARRANTY. EXCEPT WHERE MANDATED BY GOVERNMENT REQUIREMENTS, TESTING OF ALL PARAMETERS OF EACH PRODUCT IS NOT NECESSARILY PERFORMED. NATIONAL ASSUMES NO LIABILITY FOR APPLICATIONS ASSISTANCE OR BUYER PRODUCT DESIGN. BUYERS ARE RESPONSIBLE FOR THEIR PRODUCTS AND APPLICATIONS USING NATIONAL COMPONENTS. PRIOR TO USING OR DISTRIBUTING ANY PRODUCTS THAT INCLUDE NATIONAL COMPONENTS, BUYERS SHOULD PROVIDE ADEQUATE DESIGN, TESTING AND OPERATING SAFEGUARDS. EXCEPT AS PROVIDED IN NATIONAL’S TERMS AND CONDITIONS OF SALE FOR SUCH PRODUCTS, NATIONAL ASSUMES NO LIABILITY WHATSOEVER, AND NATIONAL DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY RELATING TO THE SALE AND/OR USE OF NATIONAL PRODUCTS INCLUDING LIABILITY OR WARRANTIES RELATING TO FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS PRIOR WRITTEN APPROVAL OF THE CHIEF EXECUTIVE OFFICER AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness. National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other brand or product names may be trademarks or registered trademarks of their respective holders. Copyright© 2011 National Semiconductor Corporation AN-2116 For the most current product information visit us at www.national.com National Semiconductor Americas Technical Support Center Email: support@nsc.com Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Technical Support Center Email: europe.support@nsc.com National Semiconductor Asia Pacific Technical Support Center Email: ap.support@nsc.com National Semiconductor Japan Technical Support Center Email: jpn.feedback@nsc.com