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Weixin Official Accounts Platform Simulation Core World | How to use an ideal diode controller in solar application bypass circuits and extend its input voltage range Original Texas Instruments Texas Instruments Texas Instruments Texas Instruments Semiconductor Technology (Shanghai) Co., Ltd. Texas Instruments is a global semiconductor company dedicated to designing, manufacturing, testing and selling analog and embedded processing chips for markets such as industrial, automotive, personal electronics, communication devices, and enterprise systems. We will share with you the new developments and technological innovations of TI here. 871 original content 2024-07-20 12:01 Shanghai Welcome back to our technical column – Simulation Core World. In the last issue, we discussed solutions for AC voltage drop and recovery issues based on lab validation data from a variable-frequency, ZVS, 5kW, GaN-based two-phase totem-pole PFC reference design. This time we bring you ‘How to use an ideal diode controller in solar application bypass circuits and extend its input voltage range.’ This article will introduce a scalable input bypass circuit solution using a floating gate ideal diode controller. This circuit addresses challenges in solar power applications such as solar power optimizers, rapid shutdowns, and PV junction boxes that require broad voltage support for bypass switches. Introduction In solar photovoltaic (PV) systems, module-level power electronics (MLPE) can improve power generation performance under certain conditions (especially shaded conditions). MLPE was once considered a high-cost specialty application, but now it is one of the fastest-growing segments in the solar industry. A solar power optimizer is an MLPE used to optimize the power output of PV panels and enhance efficiency. Traditional solar power optimizers use p-n junction diodes or Schottky diodes as bypass circuits. When high current flows through the diode, the high power dissipation caused by the relatively high forward voltage drop of the diode can lead to serious thermal issues. An improved method is to use a MOSFET with a lower voltage drop than the diode to overcome the problem of high power loss. Additionally, solar optimizers now support higher input voltages (up to 150V transient voltage in the case of two PV panels in series) thanks to efficiency improvements achieved through lower conduction losses at given power levels and lower system costs. In this article, we will discuss a scalable input bypass circuit solution using a floating gate ideal diode controller. This circuit addresses challenges in solar power applications such as solar power optimizers, rapid shutdowns, and PV junction boxes that require broad voltage support for bypass switches. What is a solar power optimizer? Figure 1 shows a PV system where the solar power optimizer is installed on a single PV panel. Figure 1: PV system with solar power optimizer installed The power optimizer can be considered a compromise between a microinverter and a string inverter. Like a microinverter, the power optimizer is installed on a single solar panel, but it does not function to convert DC power to AC power. The power optimizer tracks the maximum power of each solar panel in real-time, regulates the output voltage, and then transmits it to the inverter. Therefore, the inverter can handle more power, allowing each solar panel’s power generation performance to be optimized, regardless of the panel’s angle toward the sun, shading conditions, or damage to one or more panels. Compared to solar systems without individual panel-level optimizers, solar systems with power optimizers installed on each PV panel can see efficiency improvements of 20% to 30%. Solar power optimizer output bypass function For high-power photovoltaic inverters, connecting multiple PV panels in series can achieve high DC input voltage at the inverter input. Deploying optimizers to the corresponding PV panels can achieve ultra-high efficiencies, as shown in Figure 2. The PV panel string is connected through the optimizers’ output terminals. Since all PV panels are connected in series, if any one solar panel fails, the voltage of the PV panel string collapses. The output bypass circuit provides a parallel path for string current around the malfunctioning optimizer. Figure 2 shows how the bypass function works when one of the PV panels is disconnected. Figure 2: Output bypass function of the solar power optimizer Output bypass circuit solutions There are generally two solutions for bypass circuits. A common method to achieve bypass functionality is to use p-n junction diodes or Schottky diodes, as shown in Figure 3. This method is low-cost, easy to use, and can achieve very high reverse voltages depending on the selected diode. However, there are also some disadvantages, such as high forward voltage drop (0.5V to 1V), leading to higher power dissipation and requiring a larger printed circuit board. To overcome the drawbacks of the bypass diode solution, an N-channel MOSFET with lower voltage drop and power loss (due to lower R_DS(on)) can be selected. But this method also has the following drawbacks: A MOSFET is not a standalone solution; it requires control circuits (usually with discrete MOSFET driver circuits and microcontroller (MCU)) to act as a switch. The MCU needs to be powered by the PV panel. If the PV panel is severely damaged or completely covered by shade or an object, the MCU cannot operate, and the MOSFET cannot turn on. In the event of MCU failure, the MOSFET cannot turn on, and the bypass path will flow through the MOSFET’s body diode. But the MOSFET’s body diode cannot handle large currents and will generate high heat due to accumulated thermal, leading to a fire risk. Figure 3: Typical solution using bypass switches in solar optimizers To overcome the drawbacks of MCU-based on/off control schemes, an intelligent method is to use a standalone MOSFET controller that operates autonomously without any external intervention. Texas Instruments’ LM74610-Q1 series floating gate ideal diode controllers simulate the behavior of a series diode by controlling an external N-channel MOSFET, providing a standalone low-loss bypass switch solution. These controllers feature a floating gate drive architecture that can operate at input voltages as low as the MOSFET body diode’s forward voltage drop (about 0.5V). However, as the power levels of photovoltaic inverters increase and higher voltage PV panel applications gain traction, the bypass circuit must meet certain requirements to outperform traditional solutions. It needs to work with PV panels with a voltage range of 20V to 150V to be scalable across multiple platforms, and it should be independent of other circuits. A scalable bypass switch solution using a low-voltage ideal diode controller The bypass circuit solution using a floating gate drive architectureHigh temperature, resulting in fire risk. Figure 3: Typical solution using bypass switches in solar optimizers. To overcome the shortcomings of MCU-based on/off control schemes, an intelligent approach is to use a standalone MOSFET controller that works autonomously without any external intervention. Texas Instruments’ LM74610-Q1 series floating gate ideal diode controllers simulate the behavior of a series diode by controlling an external N-channel MOSFET, providing an independent low-loss bypass switch solution. These controllers have a floating gate drive architecture and can operate with an input voltage as low as the MOSFET body diode forward voltage drop (approximately 0.5V). However, as the power level of photovoltaic inverters increases and the application of higher voltage PV panels increases, the bypass circuit needs to meet some requirements to outperform traditional solutions. It needs to be used with PV panels with a voltage range of 20V to 150V so that it can scale across multiple platforms, and it should be independent of other circuits. Scalable bypass switch solution using low-voltage ideal diode controller. The bypass circuit solution uses an ideal diode controller with a floating gate drive architecture (such as LM74610-Q1) to drive an external MOSFET and simulate an ideal diode as a bypass circuit, making it independent of other circuits. The floating gate drive architecture enables a universal input range because the gate drive is not referenced to ground. Moreover, the unique advantage of this mechanism is that it is not referenced to ground, resulting in zero quiescent current. When the solar panel and solar equipment operate normally, the bypass MOSFET is off, and a reverse voltage equal to the maximum panel voltage appears from the cathode to the anode pin of the ideal diode controller. However, the reverse voltage from the cathode to the anode pin of the ideal diode controller (PV+ to PV-) can be very high, reaching the transient voltage of the PV panel and panel string. When using multiple PV panels in series with a large input voltage range, designing the maximum input voltage range for the bypass circuit can be very challenging. The maximum reverse voltage limit of the LM74610-Q1 is 45V transients. Therefore, currently available ideal diode controller devices are not suitable for solar panels with rated input voltages of 80V or 125V. By adding a depletion-mode MOSFET QD in the sense path to extend the reverse voltage range of the ideal diode controller, the voltage level can be maintained for any range, as shown in Figure 4. The drain of QD is connected to the output PV+. The source and gate are connected to the cathode and anode of the ideal diode controller, respectively. Figure 4: Scalable bypass switch solution LM74610-Q1 reverse voltage range expansion principle. Depletion-mode MOSFETs are by default on when MOSFET VGS is 0V, unlike enhancement-mode MOSFETs which require VGS to be greater than the threshold voltage of the MOSFET to turn on. To turn off a depletion-mode MOSFET, VGS needs to be less than 0V (typically in the range of -1V to -4V). To analyze the role of depletion-mode MOSFETs in the ideal diode sense path, we look at the device operation under the following conditions: When VPV- is greater than or equal to VPV+: The ideal diode controller is in the forward conduction state, keeping the power MOSFET Q1 and the depletion-mode FET QD on. Under these operating conditions, you can calculate the output voltage VOUT = VIN – (ID_Q1 RDS(on)_Q1), approximately equal to VPV+. When VPV- is less than VPV+: The ideal diode controller is in the reverse current blocking state, and MOSFET Q1 is off. MOSFET QD is in the regulation mode as a source follower, maintaining VCATHODE higher than VANODE, and VCATHODE = VIN(VANODE) + (VGSMAX). Therefore, the voltage between VCATHODE and VANODE is within the absolute maximum rated value VGS of QD (typically less than 5V), much less than the 45V maximum transient reverse voltage of the LM74610-Q1. The high reverse voltage (VOUT – VIN) is maintained by the drain-source voltage (VDS) of QD and Q1. Choosing the correct depletion-mode MOSFET and power MOSFET depends on the following points: When selecting Q1 and QD, their VDS rating must be greater than the maximum peak input voltage. When choosing RDS(on), ensure ultra-low power dissipation on the power path MOSFET. The drain current (ID) of the FET should be higher than the maximum peak current required by the output load. Initially, you can select a depletion-mode MOSFET that allows a voltage drop across the power MOSFET of 50mV to 100mV at full load current. RDS(on) can be selected in the range of several hundred ohms (the floating gate drive architecture of the LM74610-Q1 has a large cathode-pin to ground impedance, and the ICATHODE of the controller is in the microampere range). Figure 5 shows the test results of the 60V bypass switch solution using the 40V LM74610-Q1 controller. Figure 5: Test results of the 60V bypass circuit using the LM74610-Q1 and depletion-mode MOSFET. Using MOSFETs (Q1 and QD) with appropriate specifications, the input voltage range can be extended to the VDS rating of the FET. This allows high voltage designs to be implemented using the same low-voltage controller. Furthermore, extending the input voltage range is also very useful in enterprise, communication, power tools, and high voltage battery management applications. Conclusion: If PV panels or solar equipment connected in series are damaged or fail, an appropriate design must be adopted to avoid hotspots and/or voltage supply interruptions. This responsibility is usually borne by solar power optimizers or rapid shutdown devices. Although using standard rectifier diodes or Schottky diodes is the simplest solution to bypass damaged panels, they are not preferred solutions due to poor thermal efficiency. Compared with bypass switch solutions, floating gate ideal diode controllers paired with N-channel MOSFETs can achieve less independent loss, and further system solutions by adding depletion-mode MOSFETs can provide a fully scalable input range solution to address the wide input voltage range requirements of PV panels. Stay tuned to our column, or explore more possibilities in analog design by browsing the Texas Instruments ‘Analog Design Journal’ e-zine through ‘Read Original’! Click ‘Read Original’ to browse the ‘Analog Design Journal’ e-zine, more related knowledge waiting to be unlocked!In addition to pressure design, the expanded input voltage range is also very useful in enterprise, communication, power tools, and high-voltage battery management applications. Conclusion If there is damage or failure in PV solar panels or solar equipment connected in series, appropriate design must be used to avoid hotspots and/or voltage supply interruptions. This responsibility is generally undertaken by a solar power optimizer or rapid shutdown device. Although using standard rectifier diodes or Schottky diodes is the simplest solution to bypass damaged panels, they are not the preferred solution due to low thermal efficiency. Compared to bypass switch solutions, floating gate ideal diode controllers paired with N-channel MOSFETs achieve fewer independent losses, and further system solutions by adding depletion-mode MOSFETs can provide a fully scalable input range solution to meet the broad input voltage range requirements of PV solar panels. Stay tuned to our column, or click ‘Read the original’ to browse the electronic version of the Texas Instruments ‘Analog Design Journal’ and explore more possibilities in analog design together! Click ‘Read the original’ to browse the electronic version of ‘Analog Design Journal,’ more related knowledge waiting to be unlocked! 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