High-Frequency Transformer Manufacturing Process in Solar Inverters

High-Frequency Transformer Manufacturing Process in Solar Inverters

Among the core components of solar inverters, high-frequency transformers play a crucial role as the "energy conversion bridge." They not only provide electrical isolation between the input and output terminals but also directly impact the inverter's conversion efficiency, size, and long-term reliability. As the global photovoltaic industry moves towards higher efficiency and smaller size, the manufacturing process of high-frequency transformers has become a crucial component of inverter companies' core competitiveness. This article will systematically analyze the manufacturing process, key process challenges, and quality control points of high-frequency transformers based on technical principles, providing a comprehensive technical reference for photovoltaic industry practitioners.

I. The Technical Positioning of High-Frequency Transformers in Solar Inverters: Why is Process Critical?

Before delving into the manufacturing process, we must first clarify the core value of high-frequency transformers—they are more than just "voltage conversion components"; they are the "technical hub" that determines the upper limit of inverter performance. In the operating principle of a solar inverter, a high-frequency transformer performs three core tasks: electrical isolation (preventing direct electrical connection between the photovoltaic panel and the grid, ensuring system safety), voltage matching (converting the low-voltage DC power output of the Photovoltaic Modules into high-voltage AC power compatible with the grid), and electromagnetic interference suppression (reducing the impact of harmonics generated during high-frequency switching on the grid). To achieve these functions, the precision of the manufacturing process directly determines the transformer's losses, temperature rise, insulation performance, and lifespan:

Deviations in the winding process can increase copper losses and potentially reduce inverter conversion efficiency by 0.5%-1% (for large-scale photovoltaic power plants, this can result in tens of thousands of kWh of energy lost annually).

Inadequate flatness in the core lamination process can increase hysteresis losses. Long-term operation can cause excessive transformer temperature rise, triggering the inverter's protection mechanism and impacting system stability.

Substandard insulation treatment can lead to breakdown failures in humid, high-temperature outdoor environments, directly causing inverter shutdown.

Therefore, the manufacturing process of high-frequency transformers is not only a "production link" but also the "lifeline" of inverter reliability and cost-effectiveness.

II. Breakdown of the Full High-Frequency Transformer Manufacturing Process: 8 Core Steps from Raw Materials to Finished Product

The manufacturing of high-frequency transformers is a complex process that integrates materials science, precision machining, and electromagnetics. It requires rigorous control across eight key steps, and the process parameters of each step must be closely aligned with the inverter design requirements.

1. Raw Material Selection: The Starting Point of the Process Determines the Performance Limit

The choice of raw materials directly determines the basic performance of the transformer and must focus on the three core requirements of low loss, high insulation, and high-temperature resistance.

Core Material: The mainstream choices are nanocrystalline alloys or high-silicon steel sheets. Nanocrystalline alloys have higher magnetic permeability (reaching 10⁵-10⁶) and only one-third the high-frequency loss of high-silicon steel sheets, making them suitable for high-frequency inverters above 10kHz. High-silicon steel sheets offer better cost-effectiveness in the 5-10kHz frequency range and require a cold-rolled orientation process to enhance magnetic properties. The silicon content is typically controlled at 3.2%-3.5%. Winding conductors: Enameled copper wire (copper purity ≥ 99.95%) must be used. The wire diameter is calculated based on the inverter's rated current (typically, the current density is controlled at 4-6A/mm²). To reduce high-frequency skin effect (current concentration on the conductor surface, resulting in increased copper losses), "multi-strand" (Litz) wire is used. The number of strands is adjusted according to the frequency—for frequencies above 15kHz, use fine wire with at least 100 strands.

Insulation materials: This includes insulating paper between the windings (typically polyimide film, temperature resistance ≥ 180°C), a nylon 66 + glass fiber bobbin (aging resistance must meet requirements for outdoor use for at least 10 years), and a potting compound (silicone rubber or epoxy resin, with low dielectric loss and high thermal conductivity, thermal conductivity ≥ 0.8W/(m・K)). Key Processing Points: Raw materials must undergo incoming factory inspections, including core hysteresis loss testing (using a B-H hysteresis loop), conductor paint film thickness and adhesion testing (cross-cut method), and insulation material temperature resistance and dielectric loss testing (using a dielectric loss meter, with a dielectric loss value ≤ 0.005).

2. Core Processing: The Key Step in Controlling Magnetic Losses
The core is the heart of the magnetic circuit of a high-frequency transformer. Processing techniques must avoid magnetic damage and uneven magnetic circuits.

Cutting and Lamination: A precision punching machine (accuracy ±0.02mm) is used to cut core sheets. Lamination methods include "staggered lamination" (staggering each sheet by 1/2 the thickness to reduce air gaps in the magnetic circuit) and "double lamination" (suitable for small transformers). The lamination factor (the ratio of the actual core volume to the total lamination volume) must be controlled between 0.92-0.95. A lower factor will increase magnetic resistance and losses. Annealing: After cutting, the core will experience "stress damage" (a 10%-15% decrease in magnetic permeability), requiring treatment in a vacuum annealing furnace. Process parameters are: heating to 700-800°C (nanocrystalline alloy) or 850-900°C (high-silicon steel), holding for 2-3 hours, cooling at a rate of ≤5°C/min, and maintaining a vacuum environment (vacuum pressure ≤10^-3Pa) throughout the process to eliminate internal stress and restore magnetic properties.
Core Assembly: Use a "non-magnetic clamp" (such as a stainless steel clamp) to secure the core to avoid using ferromagnetic materials that could cause magnetic shorting. After assembly, check the core's air gap (typically 0.1-0.3mm). Excessive air gap increases the excitation current, while too small may lead to magnetic saturation. Process Difficulties: During the lamination process, it is important to avoid scratching the edges of the cores. Scratches exceeding 0.05mm will increase local magnetic resistance and require full inspection using optical inspection equipment (resolution ≥ 0.01mm).

3. Winding: Key to Copper Losses and Insulation Reliability

The winding is the current transmission path, and the accuracy of the winding process directly affects copper losses, distributed capacitance, and insulation performance.

Winding Design: The number of turns and winding method are determined based on the inverter topology (e.g., forward, flyback, LLC resonant). For example, high-frequency transformers in LLC resonant topologies require a "sandwich winding" method (primary winding - insulation layer - secondary winding - insulation layer - primary winding) to reduce leakage inductance (the leakage inductance value must be within 5% of the primary inductance). Forward topologies often use a "layered winding" method to improve insulation reliability. Precision Winding: A fully automatic winding machine (positioning accuracy ±0.01mm) is used. Winding tension must be constant (typically 50-100g, adjusted according to wire diameter). Avoid excessive tension, which may stretch the wire (reducing cross-sectional area and increasing resistance), or too little tension, which may loosen the winding (increasing distributed capacitance). During the winding process, a layer of insulating paper should be placed every 5-10 layers, with an overlap of ≥5mm to ensure no gaps.

Winding Shaping: After winding, the winding undergoes "hot pressing" (temperature 120-150°C, pressure 0.5-1MPa) to ensure a tight fit and reduce vibration and noise. (Noise in high-frequency transformers primarily comes from vibration between the winding and the core. Shaping can reduce noise by 3-5dB.)
Quality Inspection: After winding, the winding's DC resistance (deviation ≤ 2% of the design value), insulation resistance (≥ 1000MΩ, tested with a 500V megohmmeter), and interturn withstand voltage (no breakdown when applying 1.5 times the rated voltage for 1 minute) must be tested.

4. Bobbin and Insulation Assembly: A "Barrier" to Ensure Electrical Safety

The bobbin is not only a supporting structure for the winding but also a key carrier of insulation. The assembly process must meet requirements for "breakdown prevention" and "high-temperature resistance":

Bobbin Injection Molding: Bobbins are produced using a precision injection molding machine (accuracy ±0.05mm). The injection temperature is adjusted according to the material (260-280°C for nylon 66 + glass fiber) to avoid air bubbles (which can cause localized electric field concentration and facilitate breakdown). The bobbin's creepage distance (the distance between the winding and the core) must comply with IEC 62109, typically ≥ 8mm (for inverters above 1000V). Insulation Assembly: Place an insulating spacer (polyimide film, thickness ≥ 0.2mm) between the bobbin and the core. Enclose the winding leads in insulating sleeves (silicone rubber sleeves, temperature resistant ≥ 150°C). Use crimping (crimping pressure ≥ 5MPa) to connect the leads to the terminals. Avoid welding (welds are prone to oxidation, increasing contact resistance).
Key Process Points: After insulation assembly, perform a partial discharge test (applying 1.2 times the rated voltage, partial discharge ≤ 10pC) to ensure the insulation system is free of defects.

5. Overall Assembly: Achieving Precise Matching of the Magnetic and Electrical Circuits

Overall assembly integrates the core, windings, and bobbin into a single unit. Key elements include magnetic alignment and mechanical fastening:

Core and Winding Assembly: Insert the wound winding (with bobbin) into the core. Apply magnetic permeability matching adhesive to the core joints (to reduce air gap losses). After assembly, secure with non-magnetic bolts (such as titanium alloy bolts). Bolt torque should be uniform (typically 2-5 N·m, adjusted based on transformer size) to prevent core deformation caused by uneven torque.

Lead Wire Securing: The lead wires are secured to the transformer housing with wire clamps. Insulation pads should be placed between the clamps and the lead wires to prevent vibration-induced wear. The lead wires should be of uniform length (≤5 mm) to facilitate subsequent connection to the inverter mainboard.
Testing Items: After assembly, the transformer's inductance (primary-secondary inductance deviation ≤ 5% of the design value, using an inductance tester) and leakage inductance (leakage inductance deviation ≤ 10% of the design value) must be tested.

6. Potting Process: A "Protective Layer" to Improve Environmental Adaptability

Solar inverters are often installed outdoors, and high-frequency transformers require potting to enhance their waterproof, dustproof, and vibration-resistant properties.

Pre-potting Preparation: Place the assembled transformer into the mold. The mold must be preheated to 60-80°C (to avoid bubbles caused by a large temperature difference between the potting compound and the components). The potting compound must be mixed in the correct proportions (for example, a 10:1 ratio of epoxy resin to curing agent). After mixing, vacuum degassing (vacuum ≤ 10^-2 Pa, degassing time 20-30 minutes) is required to ensure that no bubbles are present.

Potting and Curing: A "vacuum potting" process is used (the mold is kept vacuum-free, and the potting compound is injected under pressure) to prevent air ingress. The curing process consists of two steps: initial curing (80°C, 2 hours) and complete curing (120°C, 4 hours). After curing, the potting compound must reach a Shore D hardness of 80-85 to ensure mechanical strength.

Process Difficulties: The potting compound must have moderate fluidity (viscosity 2000-3000 mPa·s at 25°C). Poor fluidity will result in an incomplete potting, while excessive fluidity may allow the compound to seep into winding gaps, impairing heat dissipation.

7. Enclosure Assembly: The Final "Protective Barrier"

The enclosure not only protects the transformer from external forces but also provides heat dissipation.

Enclosure Material: Aluminum alloy (such as 6061-T6) is die-cast, with wall thickness adjusted according to transformer power (1.5-2mm for transformers below 500W, 2-3mm for transformers above 1000W). The housing surface must be anodized (oxide film thickness ≥ 10μm) to improve corrosion resistance (meeting 500-hour salt spray test without rust).

Heat dissipation design: The housing must be designed with heat dissipation fins. The fin height and spacing are calculated based on transformer losses (generally, a heat dissipation area of ​​10cm² is required for every 10W of loss). Thermal grease (thermal conductivity ≥ 1.5W/(m・K)) must be applied between the housing and the transformer to ensure rapid heat transfer.

Assembly and fixing: The potted transformer must be secured to the housing with bolts. Lock washers (such as spring washers) must be added to the bolts to prevent loosening due to long-term vibration. The housing must meet IP65 protection (completely dustproof and water jet-proof) requirements for outdoor use.

8. Finished Product Testing: The Final Check on Process Quality

Finished product testing is the final step to ensure that the high-frequency transformer meets the inverter design requirements. It must cover electrical performance, environmental adaptability, and reliability.
Electrical performance testing: includes DC resistance, insulation resistance, turn-to-turn withstand voltage (1.5 times the rated voltage, no breakdown for 1 minute), temperature rise test (2 hours under rated load, temperature rise ≤ 60K, tested using thermocouples), and efficiency test (≥ 98.5% efficiency at rated power).
Environmental adaptability testing: includes high and low temperature cycle test (-40°C to 85°C, 50 cycles, no performance degradation), damp heat test (40°C, 95% humidity, no insulation failure for 1000 hours), and vibration test (10-500Hz, 10g acceleration, 2 hours in each of three axes, no mechanical damage). Reliability testing includes: lifespan testing (continuous operation at 1.2 times the rated load for 1000 hours, performance degradation ≤5%), and short-circuit testing (secondary side short-circuited for 1 second, with normal performance after recovery).
Only high-frequency transformers that pass all tests are eligible for solar inverter assembly.

III. Optimization Directions for High-Frequency Transformer Manufacturing Processes: Adapting to Technological Trends in Photovoltaic Inverters

As photovoltaic inverters evolve toward higher frequencies, higher efficiency, and smaller sizes, the manufacturing process for high-frequency transformers is also undergoing continuous innovation, primarily demonstrating three key trends:

1. High-Frequency Processes: Improving Inverter Power Density
Current inverter switching frequencies have increased from the traditional 5kHz to over 20kHz, with some high-end products even reaching 50kHz. This requires optimizing the manufacturing process for high-frequency transformers toward low high-frequency losses:

Core Processing: Using amorphous alloys instead of traditional nanocrystalline alloys. The high-frequency losses of amorphous alloys (at 20kHz) are only half of those of nanocrystalline alloys. However, the annealing process needs to be optimized (increasing the annealing temperature to 450-500°C and shortening the holding time to 1 hour) to prevent crystallization of the amorphous structure. Winding Process: Flattened wire is used instead of traditional round wire. Flat wire has a larger cross-sectional area (increasing conductive area by 30% within the same space) and minimizes skin effect (reducing copper loss by 15% at 20kHz). A fully automatic flat wire winding machine is used for winding to ensure uniform winding tension (deviation ≤ 5%).

2. Integrated Process: Reducing Inverter Size

In traditional inverters, the high-frequency transformer, inductor, and filter are separate components. Integrated processes can combine these three components into an integrated magnetics component, reducing the size by over 30%.

Core Integration: Using a multi-magnetic core (such as an ETD core), the transformer and inductor windings are wound on the same core. Voltage conversion and filtering functions are achieved by adjusting the turns ratio. The position of each winding must be precisely controlled during manufacturing to avoid magnetic interference (leakage inductance deviation ≤ 3%). Process Integration: Integrating winding, potting, and heat dissipation design, such as using "embedded winding" (windings directly embedded in thermally conductive potting compound, eliminating the need for a bobbin), improves heat dissipation efficiency (reducing temperature rise by 10-15K).

3. Intelligent Processing: Improving Manufacturing Precision and Consistency

The manufacturing of high-frequency transformers has gradually incorporated "Industry 4.0" technologies, using intelligent equipment to improve process stability:

Automated Equipment: Utilizing a "robot + visual inspection" system fully automates core lamination, winding, and finished product testing, reducing manual errors (e.g., reducing winding tension fluctuations from ±10% to ±2%).

Data-Driven Management: Establishing a "process parameter database" links raw material parameters, process parameters at each stage, and finished product test data. Using AI algorithms, process deviations are analyzed (e.g., if the magnetic loss of a batch of cores is too high, the annealing temperature can be automatically traced to determine if it is abnormal), enabling real-time process optimization.

IV. Process Innovation is the Core of PV Inverter Competitiveness

The manufacturing process of high-frequency transformers may seem like a "back-end production step," but in reality, it is the "front-end driving force" for technological breakthroughs in solar inverters. It not only determines the efficiency and reliability of the inverter, but also impacts the cost per kilowatt-hour of Photovoltaic Systems. As the global photovoltaic industry enters the "evolution of power parity," inverter performance requirements will continue to rise, and high-frequency transformer manufacturing processes will also require continuous iteration. From the meticulous selection of raw materials, to automated upgrades in precision machining, to innovative exploration of integrated and intelligent processes, the optimization of every process detail will drive the development of photovoltaic inverters towards greater efficiency, reliability, and miniaturization.