Screening Criteria for Core Components in Solar Inverters

Screening Criteria for Core Components in Solar Inverters

Amid the accelerating global energy transition, solar inverters, as the "heart" of Photovoltaic Systems, directly determine energy conversion efficiency, system stability, and service life. The core performance of an inverter is fundamentally determined by the quality and compatibility of its key components. According to the International Energy Agency (IEA), approximately 60% of inverter failures are due to improper component selection. Therefore, establishing scientific component screening criteria is crucial to the long-term value of photovoltaic systems. This article will examine the comprehensive screening process from laboratory testing to field application, focusing on four core categories: power devices, magnetics, capacitors and resistors, and control chips.

I. Power Devices: The "Power Engine" of the Inverter: Screening Logic

Power devices are the core of the inverter's DC-to-AC conversion, fulfilling the critical role of high-frequency switching and energy transfer. Their selection directly impacts conversion efficiency and thermal stability. Currently, mainstream power devices include IGBTs (insulated gate bipolar transistors) and MOSFETs (metal-oxide semiconductor field-effect transistors). Screening for these two devices should focus on the following four core dimensions:

1. Performance Parameter Compatibility

Voltage and Current Margin: Based on the inverter's rated power and input voltage range, allow for at least 20% voltage margin and 30% current margin. For a 10kW three-phase inverter, for a maximum input voltage of 1000V, select an IGBT with a rated voltage ≥ 1200V. The rated current must cover 1.3 times the rated output current (approximately 15A), meaning a current ≥ 19.5A.

Switching Loss and On-State Voltage Drop: Switching loss directly affects conversion efficiency under high-frequency conditions, while the on-state voltage drop determines static losses. When screening, the optimal balance should be determined based on the inverter topology (e.g., string or centralized). String inverters, due to their high-frequency switching requirements, prioritize devices with switching losses below 50mJ. Centralized inverters prioritize low on-state voltage drop (≤1.8V) to minimize full-load losses.

2. Thermal Stability and Heat Dissipation Compatibility

The junction temperature tolerance of power devices is a key screening criterion. Products with a maximum junction temperature of 150°C or higher should be selected to withstand the long-term high-temperature outdoor environment of the Pv System. The device's thermal resistance (Rth(j-c)) must also be matched to the inverter's heat dissipation design. For example, in microinverters with natural cooling, IGBT modules with a thermal resistance of 0.5K/W or less should be selected to ensure the junction temperature does not exceed the safety threshold of 120°C.

3. Reliability and Lifespan Verification
Avalanche Energy Withstand: Dynamic avalanche testing is required to verify the device's surge resistance. Products with an avalanche energy (EAS) of 500mJ or higher are selected to withstand grid fluctuations and lightning strikes.

Accelerated Lifespan Testing: Power Cycling Testing (PCT) simulates the device's long-term operating conditions. The requirement is a cycle life of 100,000 or more cycles at a junction temperature fluctuation of ΔTj = 50°C, ensuring the inverter's service life of at least 15 years.

II. Magnetic Components: The "Conduction Hub" of Energy Conversion - Selection Criteria

Magnetic components (transformers and inductors) serve as the "energy bridge" of the inverter. Their core material, winding process, and structural design directly impact efficiency, noise, and size. Screening should focus on three key dimensions: core performance, electrical parameters, and mechanical stability:

1. Core Material Compatibility

High-frequency transformer cores: String inverters typically operate at a frequency of 10-20kHz. Ferrite cores with low high-frequency losses (such as PC44) should be selected. Their specific loss at 20kHz and 200mT should be ≤300mW/cm³. For centralized inverters using a medium- to low-frequency design (5-10kHz), amorphous alloy cores can be used to improve permeability.

Filter inductor cores: The selection should be based on current ripple characteristics. For continuous current operation, iron powder cores (such as Kool Mμ) are preferred, as they offer low hysteresis losses and strong resistance to saturation. For pulsed current operation, ferrite cores are suitable to ensure magnetic saturation is avoided at peak currents.

2. Precise Matching of Electrical Parameters

Inductance and Tolerance Control: The inductance of the filter inductor must be designed based on ripple suppression requirements. For example, the output filter inductor for a 10kW inverter is typically 0.3-0.5mH. During screening, the inductance tolerance must be controlled within ±5% to avoid excessive ripple due to excessive tolerance.

Winding Copper Loss and Insulation Grade: Winding conductors must be oxygen-free copper wire. The wire diameter is determined by the rated current density (≤5A/mm²). The insulation layer must also meet temperature resistance requirements. Class H insulation (180°C) is suitable for high-temperature environments, while Class N (200°C) is suitable for harsh climates.

3. Structural and Reliability Verification
Vibration Resistance: Magnetic components must pass the IEC 60068-2-6 vibration test, demonstrating no structural loosening at 2g acceleration for 30 minutes within a frequency range of 10-500Hz. Temperature Rise Control: Temperature rise testing verifies that under rated load, the core surface temperature rise must be ≤40K, and the winding temperature rise must be ≤60K to prevent insulation aging and magnetic performance degradation caused by high temperatures.

III. Capacitors and Resistors: Key Selection Points for "Precision Regulators" for System Stability

Although capacitors and resistors are passive components, they perform critical functions such as filtering, voltage stabilization, and voltage division. Their performance degradation is a major cause of inverter failure. Screening should focus on three key factors: stability, lifespan, and environmental adaptability.

1. Capacitors: From Dielectric Type to Lifespan Prediction

Electrolytic Capacitors: As the core of DC bus filtering, solid electrolytic capacitors should be preferred over liquid electrolytic capacitors. Their temperature resistance (≥105°C) and lifespan (≥10,000 hours at 105°C) far exceed those of liquid products. When selecting capacitors, calculate the lifespan at actual operating temperatures. According to the Arrhenius model, if the operating temperature drops to 85°C, the lifespan can be extended to over 40,000 hours, meeting the long-term use requirements of the inverter.

Film capacitors: Suitable for high-frequency filtering and resonant circuits, select metallized polypropylene film capacitors with an insulation resistance ≥ 10,000 MΩ and a dielectric loss tangent (tanδ) ≤ 0.001. They must also pass a 1,000-hour humidity and heat test (40°C, 90% RH) without performance degradation.

2. Resistors: Accuracy and stability are prioritized.

Power resistors: Used for current sampling and load matching, select metal film or wirewound resistors with a power margin ≥ 2 times the actual power consumption, a temperature coefficient ≤ ±50 ppm/°C, and a resistance change of ≤ 1% within the -40°C to 85°C temperature range. Chip resistors: Suitable for miniaturized inverters, they must meet AEC-Q200 automotive-grade standards and pass temperature cycling (-55°C to 125°C) and humidity bias testing, with no pin detachment or resistance drift.

IV. Control Chip: The Inverter's "Intelligent Brain" - Key to Selection

The control chip is responsible for maximum power point tracking (MPPT), grid-connected control, and fault protection. Its computing power, interference immunity, and integration directly determine the system's intelligence level. Selection should focus on three key dimensions: performance, reliability, and compatibility:

1. Computing Performance and Functional Integration
Processor Performance: Select a 32-bit ARM architecture MCU with a main frequency of ≥100MHz and a floating-point unit (FPU) to support complex MPPT algorithms (such as the perturbation-and-observe method and the incremental conductance method) and ensure MPPT tracking efficiency ≥99.5%. Peripheral Integration: Prioritize chips that integrate an ADC (12-bit or higher, sampling rate ≥1MSPS), a PWM module (resolution ≥16-bit), and a CAN communication interface to reduce external circuit complexity and improve system stability.

2. Interference Immunity and Environmental Adaptability

Electromagnetic Compatibility (EMC): Chips must pass the IEC 61000-4 series of tests, operating normally under 8kV contact discharge and 15kV air discharge conditions, with radiated emissions ≤30dBμV/m (30-1000MHz).

Wide Operating Temperature Range: The chips must meet industrial standards, with an operating temperature range of -40°C to 125°C and a storage temperature range of -55°C to 150°C, suitable for complex outdoor environments such as high altitudes and extreme cold and heat.

3. Development Support and Supply Chain Assurance

Select chip solutions with a comprehensive development toolchain (compiler, debugger) and technical documentation to shorten the R&D cycle. Suppliers' production capacity stability and delivery cycles must also be evaluated, prioritizing brands with the top three market shares (such as TI, ST, and Microchip) to avoid production interruptions due to chip shortages.

V. Implementation of Screening Criteria: A Verification System from Lab to Field

Scientific screening criteria must be implemented through a comprehensive verification process; otherwise, it's easy to fall into the trap of "excellent specifications but actual failure." A complete verification process should include three steps:

Laboratory Performance Testing: Build a test platform simulating photovoltaic operating conditions to verify component performance stability under varying light intensities (200-1000W/m²), temperatures (-40°C to 85°C), and grid fluctuations (voltage ±10%, frequency ±2Hz).

Accelerated Life Testing: Through accelerated aging tests such as high temperature and humidity (85°C, 85% RH), temperature cycling (-40°C to 125°C), and vibration and shock, simulating component usage scenarios over 15 years, screen for products with a failure probability of ≤0.1%. Field Operational Verification: Pilot projects are established in different climate zones (tropical, plateau, and frigid regions) to continuously monitor component parameters such as temperature, resistance, and loss. Actual operating data is accumulated to optimize screening criteria.

Conclusion: Component Quality Builds a Solid Foundation for PV Systems

The performance competition in solar inverters is essentially a competition in the quality of core components. From the thermal stability of power devices to the anti-interference capabilities of control chips, from loss control of magnetic components to the lifespan assurance of capacitors and resistors, the implementation of each screening standard responds to the core requirements of PV systems: "efficiency, reliability, and longevity." As the PV industry upgrades towards higher efficiency and intelligent technology, only by establishing a refined, comprehensive component screening system can we create high-quality inverter products that stand the test of the market and provide solid technical support for the global energy transition.