4kW LLC Transformer Design: Levane Power HLF4K

4kW LLC Transformer Design Deep Dive:

From Insulation Challenges to Levane Power's Airborne Power Supply Solution

I. Industry Demand for 4kW Power Systems

With the rapid development of tethered drones, industrial telemetry, smart grids, and other fields, 4kW DC‑DC power supplies are becoming a critical power node. In tethered drone applications, the onboard DC/DC power supply plays a key role in converting high‑voltage DC power (typically 600V–1000Vdc) sent from the ground into the voltages required by different loads such as flight control systems, sensor payloads, and communication equipment. The global drone battery and power supply market was valued at expected ro reach approximately   1.672 billion in 2025 and is expected to reach approximately 4.497 billion by 2035. The 4kW power level, especially in lighting drones and tethered drones, has become the "performance sweet spot" for industrial applications – sufficient to support most professional missions while avoiding the weight penalty of higher‑power solutions.

Behind these applications, the performance of DC‑DC power supplies heavily depends on a core component – the LLC resonant converter transformer. Designing an LLC transformer at the 4kW power level faces extreme challenges in three dimensions: high voltage, high current, and small size.

II. Three Core Challenges in 4kW LLC Transformer Design

1. High‑Voltage Insulation Challenge

With the widespread adoption of 800V automotive high‑voltage platforms and tethered drone architectures (800V–1000V high‑voltage input), transformer insulation design has become the primary factor affecting reliability. To meet the insulation requirements of an 800V high‑voltage platform, the design must ensure: adequate interlayer insulation of windings, creepage distances and clearances that satisfy safety standards, triple‑insulated wire (TIW) between primary and secondary windings to increase withstand voltage, strict compliance with IEC 61558 or equivalent safety standards, and segmented or multi‑winding series structures when necessary to reduce interlayer electric field gradients.

2. Thermal Management Challenge

Under 4kW full‑load conditions, heat generated by copper and core losses, if not conducted away effectively, directly affects system stability and conversion efficiency. The design must address the following difficulties: high‑frequency winding losses due to skin and proximity effects must be suppressed using multi‑strand Litz wire; core material selection must balance flux density and losses; and for high‑power heat dissipation, forced air cooling or even liquid cooling must be employed within a compact space. Current industry‑leading efficiency can reach 97%, which imposes very high demands on transformer loss control.

3. Power Density Challenge

Achieving 4kW output in a limited space means the transformer design must strike a delicate balance among high voltage, high current, and high frequency: a higher switching frequency (typically around 100kHz) reduces magnetic component size but increases core losses; low DC resistance winding structures are needed to handle high currents, but they occupy more window area; and the 800V high‑voltage output requires stricter insulation, further crowding the transformer window. These conflicting constraints constitute the main design challenge.

III. Levane Power HLF4K Series Airborne Power Supply: Design Validated by Product Practice

Among 4kW‑class power products, Levane Power’s HLF4K series tethered drone onboard DC/DC power supply – with measured efficiency ≥97%, ultra‑high power density, and an automotive‑grade reliability verification system – provides an excellent reference for 4kW LLC transformer design.

Key parameters of the HLF4K‑D60‑800:

How the HLF4K Series Addresses the Three Challenges

High‑Voltage Insulation Strategy: The HLF4K‑D60‑800 is optimised for 800V high‑voltage platforms, with an input voltage range covering 680–800Vdc, fully compatible with tethered drone high‑voltage DC transmission architectures. The series follows automotive‑grade development standards. All products undergo component stress analysis, thermal cycling, vibration shock, damp heat, safety compliance, EMC testing, and other full‑type tests. Reliability validation methods such as MTBF analysis, FMEA, accelerated ageing, and HALT are also introduced, ensuring end‑to‑end quality control from design to delivery.

Thermal Management Design: The HLF4K series achieves industry‑leading conversion efficiency of ≥97%, greatly reducing the transformer’s own losses and heat generation. The product incorporates a comprehensive temperature protection mechanism – including over‑temperature protection and intelligent power derating – which actively reduces output power when the power supply temperature becomes too high. Meanwhile, the efficient heatsink design allows operation over a wide temperature range of -20℃ to +55℃.

Ultra‑High Power Density: The HLF4K‑D60‑800 delivers 4kW continuous output within compact dimensions of 187×95×70 mm, weighing only 1050g. This high power density fully validates the feasibility of compact transformer design, imposing high engineering requirements on winding layout optimisation, core miniaturisation, and loss control at high frequencies. As industry analysis points out, modular design is becoming an important trend in power supply systems; high‑efficiency, high‑power‑density power modules can be flexibly combined to meet diverse power requirements.

Intelligent Monitoring: All Levane Power products are equipped with a CAN 2.0B communication interface, supporting real‑time monitoring of input/output voltage and current, internal temperature readings, fault diagnosis and status reporting, and remote parameter configuration – aligning with the “intelligent” development trend highlighted in the 2026 drone power supply market.

Reliability Verification: The HLF4K series undergoes rigorous temperature cycling, vibration shock testing, humidity exposure, HALT (Highly Accelerated Life Testing), and other complete reliability verifications to ensure zero‑failure performance in harsh operating environments.

IV. Detailed Design and Calculation Process for a 4kW LLC Transformer

This section uses a full‑bridge LLC topology as an example. The input is approximately 540Vdc after three‑phase rectification (corresponding to 380Vac industrial power) or 600–800Vdc after single‑phase voltage doubling (common for tethered drones). The output is 800Vdc / 5A (4kW), switching frequency 100kHz, target efficiency 96%. The design follows the Fundamental Harmonic Approximation (FHA) method and provides reproducible engineering steps.

4.1 Basic Design Parameters

4.2 Turns Ratio Design (Near Resonant Point)

At nominal input voltage, set the resonant gain Nnom = 1 (i.e., the secondary voltage directly reflects to the primary). Then the transformer turns ratio $n$ is:

n =   Vinnom/Vo =540/800 = 0.675

Considering secondary rectification (full‑bridge rectifier with voltage utilisation factor of 1), the actual turns ratio Np : Ns = n : 1, i.e., 0.675:1. Integer ratios such as 2:3 or 3:4 are often used. To ensure rated output voltage even at minimum input voltage, the actual turns ratio may be slightly lower than the calculated value, compensated by gain boost. Here we taken =$0.667$, i.e., primary turns Np =$20$, secondary turns Ns =$30$.

4.3 Equivalent Load Resistance Rac

The AC equivalent resistance reflected from the secondary load to the primary side is:

Rac = 8/(π *π) * n * n * Vo* Vo/ Po   = 57.7 Ω

4.4 Resonant Parameter Design Lr ,Cr

Choose a quality factor Q=0.5 (balancing gain capability and switching frequency range). Then:

Cr= 1/(2πQfr * Rac) = 55.2 nF

Take the standard value Cr=56 nF(630V high‑voltage film capacitor).

The resonant inductor:

Lr = 1/（(2πfr) * (2πfr) C r）= 45.2μH

Take Lr =47 μH.

4.5 Magnetising Inductance Lm Design

Define the inductance ratio Ln =Lm /Lr, typically between 3 and 7. A larger Ln = 5

gives lower peak gain but a wider operating frequency range. Here we take Ln = 5:

Lm =Ln ×Lr =5×47=235 μH

4.6 Core Selection and Turns Calculation

Choose a nanocrystalline core or low‑loss ferrite. Example: EE65 ferrite core with Ae =535 mm2, maximum flux density Bmax =0.15 T (at 100kHz).

Primary turns:

Np = Vin,nom/（4fr * Bmax *Ae）≈16.8turns

Round to Np =17 turns. From n=Np/Ns=0.667, we get Ns =Np /0.667=17/0.667≈25.5, so take Ns=26 turns. The actual turns ratio n=17/26=0.654, close to the original design.

4.7 Winding Structure and High‑Voltage Insulation Design

• Primary winding: 17 turns, using 200 strands of 0.1mm Litz wire to reduce skin effect losses at 100kHz.

• Secondary winding: 26 turns, output 800V, voltage per turn about 30.8V – interlayer voltage is manageable. However, triple‑insulated wire (TIW) must be used, with a rated withstand voltage ≥1250Vpk. Wind in a segmented multi‑layer structure (6‑7 turns per segment), adding polyimide insulation film (0.05mm) between segments to ensure interlayer withstand voltage.

• Primary‑secondary insulation: Add insulating barriers in the bobbin, creepage distance ≥14mm (for 800V system).

• Auxiliary measures: Vacuum impregnation to eliminate internal air gaps and prevent partial discharge.

4.8 Loss Estimation and Thermal Design

• Core loss: PC95 material at 100kHz, 0.15T gives about 300 mW/cm³. EE65 volume ≈ 48 cm³, core loss ≈ 14.4W.

• Copper loss: Primary Litz wire DC resistance ≈ 0.02Ω, RMS current ≈ 8A (calculation omitted), copper loss ≈ 1.3W; secondary TIW (AWG24) resistance ≈ 0.08Ω, RMS current ≈ 5A, copper loss ≈ 2W.

• Total loss ≈ 17.7W. Transformer temperature rise must be controlled within 85°C using thermally conductive potting (thermal conductivity 2W/m·K) and forced air cooling (air velocity 2m/s).

Through the above calculations, the main parameters of the LLC transformer are determined. Subsequent simulation verification (using Ansys Maxwell or PSIM) and hardware tuning are required to fine‑tune $L_r$ and the resonant capacitor to satisfy ZVS conditions.

Note: This is a typical design flow. In practice, iteration based on the specific core and system requirements is necessary.

V. From Theory to Product: Levane Power’s LLC Transformer Design Practice

From the successful practice of the HLF4K series, several core technical dimensions of 4kW LLC transformer design can be distilled.

Core Material Selection: High Saturation Flux vs. Low High‑Frequency Loss

A 4kW LLC transformer requires a core material that offers both high saturation flux density (to reduce size) and low high‑frequency losses (to minimise heat). Current mainstream choices include: nanocrystalline cores (e.g., FeCuNbSiB) with saturation flux density as high as 1.2T – far superior to conventional ferrite’s 0.39T – and Curie temperature ≥200°C, excellent electrical insulation characteristics, suitable for high‑end applications pursuing ultimate power density. Although cost is higher, the overall cost‑performance is good. Low‑loss ferrites such as PC95 are mature and widely documented; although their lower B requires larger cores for the same power, they are cost‑effective for price‑sensitive designs. In industry‑leading solutions, precise flux and winding distribution control has reduced the volume of a 4kW converter to 0.76L.

Winding Structure and High‑Voltage Insulation

For high‑voltage scenarios such as 800V output, TIW (rated withstand voltage >1250Vpk) must be used, together with segmented or multi‑winding series structures to reduce voltage stress on each segment. Interlayer insulation thickness should be increased appropriately, and creepage distances and clearances must meet standards to avoid partial discharge. Primary dual‑windings in series and secondary multiple windings in parallel are also common methods to reduce insulation stress and increase power density.

Thermal Design and Loss Balancing

In the thermal design of a 4kW LLC transformer, copper losses are reduced by using Litz wire to minimise skin effect losses. For core losses, the operating flux density must be chosen appropriately to prevent local overheating. For heat dissipation, a combination of thermally conductive potting, heatsinks, and forced air cooling is used to extract heat. Industry benchmark solutions show that the weight of a 4kW converter can be as low as 1.4kg, which imposes high demands on thermal management efficiency.

Resonant Parameter Optimisation

Iteration of the LLC resonant parameters ($L_r$Lr, Cr, Lm) is at the core of the design process. By choosing appropriate values for the normalised inductance coefficient Lₙ(3–7) and quality factor $Q$ (0.3–0.8), the design ensures sufficient voltage gain to maintain stable output over a wide input voltage range, and guarantees ZVS (zero‑voltage switching) of the primary switches over the entire load range, achieving both high efficiency and low EMI.

Industry Certification and Reliability Assurance

High‑quality automotive‑grade power modules must comply with the IATF 16949 quality management system. IATF 16949 adds automotive industry technical specifications on top of ISO 9001, emphasising defect prevention and reduction of variation. Levane Power’s power products follow IATF 16949 automotive‑grade development standards for testing and quality control. The products also fully use automotive‑grade components, meeting not only AEC‑Q100 (automotive integrated circuit stress test) but also AEC‑Q200 (passive component stress test) certification standards.

All Levane Power onboard DC/DC converters undergo, according to national standards, component stress analysis, thermal cycling, vibration shock, damp heat, safety compliance, EMC testing, and other full‑type tests. Reliability validation methods such as MTBF analysis, FMEA, accelerated ageing, and HALT are also introduced. Every product delivered has passed complete automotive‑grade reliability verification.

VI. Typical Application Scenarios

Tethered Drone Long‑Endurance Missions

The HLF4K series is designed specifically for tethered drone platforms, efficiently converting high‑voltage DC power from the ground supply into the voltages required by various drone subsystems. It is widely used in complex scenarios such as emergency communications, emergency lighting, and disaster relief.

Heavy‑Lift Drone Payload Power

When heavy‑lift drones carry advanced sensor suites, multispectral cameras, LiDAR systems, and communication relay equipment, they often require continuous power exceeding 3kW. The HLF4K series provides ample power margin to accommodate payload expansion.

Industrial Inspection and Agricultural Mapping

In industrial inspection drones (high‑resolution cameras and gas sensors), agricultural mapping platforms (multispectral payloads), and medium‑sized communication relay drones, the 4kW power level offers the best balance between performance and weight.

Onboard Auxiliary Power

Leveraging Levane Power’s more than twenty years of technical accumulation in new energy vehicles, the underlying LLC transformer and power stage design of the HLF4K series can also be applied to onboard auxiliary power, electrification retrofits, range extender systems, and other fields, meeting the stringent reliability requirements of new energy vehicles.

VII. Conclusion

Designing a 4kW LLC transformer is a systematic engineering task involving electromagnetics, thermodynamics, materials science, and system reliability. Optimising each dimension – from high‑voltage insulation and thermal management to power density – has a profound impact on the final product’s performance and stability.

The successful practice of Levane Power’s HLF4K series shows that solid fundamental technical accumulation and a complete product verification system are the keys to moving a 4kW power product from design to commercialisation. Benefiting from more than twenty years of power electronics expertise, Levane Power offers ground power equipment from 5kW to 150kW and onboard power supplies covering the full range from 3kW to 120kW, building a complete product matrix across the entire industry chain.

For engineers currently working on 4kW power supply design, the design of an LLC transformer is not just a matter of selecting and calculating electromagnetic components; it is a comprehensive test of technical depth and engineering rigour.

About Levane Power

Levane Power Technology Co., Ltd. is a high‑tech enterprise focusing on DC/DC power products in the new energy field, dedicated to the R&D and manufacturing of high‑performance, high‑efficiency power conversion systems. Leveraging more than twenty years of technical accumulation and automotive‑grade development standards, Levane Power provides customised power solutions characterised by high efficiency, high power density, and high reliability for tethered drones, new energy vehicles, industrial equipment electrification, and other fields.

For more product information, technical specifications, or customisation, please visit the Levane Power official website or contact the technical support team directly.