I. Why High-Power-Density Power Supplies Are Increasingly Focusing on Planar Transformers

1. AI Servers and Data Center Power Supplies
AI servers place extremely high demands on the power system. Current mainstream directions include:

• 48V bus power distribution

• 48V-to-12V intermediate bus conversion

• Fixed-ratio DC/DC modules

• Board-level high-power-density power modules

• Future 800V HVDC to 48V/12V stage power conversion
  Such applications typically require power modules with low profile, high efficiency, high power density, good thermal performance, and excellent consistency. Therefore, planar transformers are receiving growing attention in AI server power architectures.

2. New Energy Vehicle OBC and DC/DC Converters
Automotive power systems demand not only efficiency from magnetic components but also:

• High-voltage isolation

• Wide-temperature operation

• Long lifetime

• Vibration resistance

• Thermal shock resistance

• High reliability validation
  In on-board chargers (OBC), on-board DC/DC converters, and high-voltage platform power systems, planar transformers can enhance product consistency and power density through structured winding design and more stable manufacturing processes.

3. Telecom Power Supplies, Industrial Power Supplies, and Fast-Charging Power Supplies
In 5G communications, industrial power supplies, and high-power fast-charging applications, switching frequencies often reach several hundred kHz, and in some scenarios even enter the MHz range. As frequency increases, winding AC losses, core losses, leakage inductance spikes, and EMI issues become more pronounced.
Compared to traditional wire-wound structures, planar transformers make it easier to control leakage inductance, distributed capacitance, and thermal paths through structural design, making them more suitable for high-frequency, high-density power design requirements.

II. Four-Step Planar Transformer Selection Method

1. Topology and Operating Frequency
The first step in selecting a planar transformer is to examine the application topology and operating frequency.
Planar transformers are commonly found in the following topologies and applications:

• LLC / CLLC resonant converters

• Fixed-ratio DC/DC modules

• Intermediate bus converters

• On-board OBC

• High-power-density telecom and industrial power supplies

• Certain forward, flyback, half-bridge, and full-bridge topologies

2. High-Frequency Losses
For planar transformers, it is essential to focus on three types of losses:
a. Copper Loss: Copper loss includes DC loss and AC loss. DC loss is primarily determined by winding resistance, while AC loss is related to frequency, copper thickness, stacking structure, winding arrangement, and current distribution. In high-frequency applications, AC loss can become a significant factor affecting efficiency and temperature rise.
b. Core Loss: Core loss is related to core material, operating frequency, flux density, and temperature. In high-frequency scenarios, one should not only look at core size but also pay attention to the material’s loss performance at the target frequency.
c. Leakage Inductance-Related Losses and Spikes: It is worth noting that lower leakage inductance is not always better. In some hard-switching topologies, low leakage inductance helps reduce voltage spikes and snubber circuit stress. However, in resonant topologies such as LLC and CLLC, leakage inductance may participate in resonant parameter design, so what matters more is controllable leakage inductance rather than simply pursuing the lowest possible value.

During selection, it is recommended to confirm with the supplier:

• Whether they can provide electromagnetic and thermal simulations

• Whether they have AC loss evaluation

• Whether there is a design target for leakage inductance

• Whether sample test data is available

• Whether efficiency curves and temperature rise data under different operating conditions are provided

For high-frequency applications using GaN or SiC devices, leakage inductance, distributed capacitance, and AC losses will directly affect system efficiency, voltage spikes, and EMI performance. Therefore, joint evaluation with the supplier should be conducted early in the design phase.

3. Thermal Management Path
Planar transformers typically offer larger thermal contact areas and shorter heat conduction paths, which is a key advantage over traditional wire-wound structures. However, this does not mean planar transformers are inherently immune to heat. Therefore, when evaluating planar transformers, it is necessary to further identify where the hot spots are, how heat is dissipated, and whether the thermal path is stable. Key areas to focus on include:

• Core and winding hot spot temperatures

• Solder joint temperature

• PCB interlayer temperature

• Via temperature rise

• Bottom-side heat dissipation and chassis thermal path

• Thermal interface material (TIM) contact condition

For AI server power modules, on-board OBC, and high-power industrial power supplies, it is recommended to request temperature rise data or thermal simulation results from the supplier under different loads and ambient temperatures.

4. Insulation and Reliability
In high-voltage, high-frequency applications, the design of the insulation system is critical. Especially in on-board OBC, on-board DC/DC, industrial power supplies, and future HVDC supply scenarios, planar transformers must not only meet basic withstand voltage requirements but also demonstrate insulation reliability under long-term operation. Key items to confirm include:

• Withstand voltage test

• Insulation resistance

• Partial discharge

• Creepage distance

• Clearance

• Insulation material class

• Interlayer insulation structure

• Insulation stability after high temperature and high humidity

• Structural reliability after temperature cycling

• Electrical performance retention after vibration and shock

For automotive applications, verification should also be combined with AEC-Q200, customer automotive specifications, and specific product categories.

III. Selection Recommendations for Typical Application Scenarios

IV. Conclusion
For AI servers, new energy vehicles, telecom power supplies, and high-end industrial power supplies, magnetic components are no longer just passive components but key parts that impact system efficiency, thermal management, reliability, and overall power density. A truly excellent planar transformer solution should be the result of the combined efforts of electromagnetic design, thermal design, structural design, material technology, and manufacturing processes. This is also the fundamental reason why planar transformers are gaining increasing attention in the continuous upgrade of AI servers, new energy vehicles, and high-end power modules.