20 key questions and answers for PCB planar transformer design, covering basic concepts, core selection, winding layout, parasitic parameter control, thermal design, and process implementation.

Original: Expert in Magnetic Components

Flat transformers are special transformers that use PCB copper foil as windings, and their design requires repeated trade-offs between electrical performance, thermal management, and manufacturing costs. The following are 20 key questions and answers for PCB planar transformer design, covering basic concepts, core selection, winding layout, parasitic parameter control, thermal design, and process implementation.

1. Question: What is a planar transformer? What is the core difference between it and traditional wound transformers?
Answer: A flat transformer is a type of transformer that uses flat copper foil on a multi-layer printed circuit board (PCB) as the winding. The core difference is that traditional transformers use enameled wire wound around the skeleton, while the windings of flat transformers are spiral copper foils etched on the PCB board, and the magnetic core (usually ferrite) is directly clamped on the PCB component. This structure gives it the characteristics of low height (low profile), high power density, and excellent consistency.

2. Question: What are the main advantages of using PCB planar transformers?
Answer: The main advantages include:
1. High efficiency and low leakage inductance: The winding coupling is tight, and the leakage inductance can usually be controlled below 0.2%.
2. Good heat dissipation performance: The flat structure has a larger surface area/volume ratio, shorter heat channels, and is easy to dissipate heat.
3. Good consistency: Parasitic parameters are determined by PCB manufacturing accuracy, and product performance can be repeated, making it very suitable for automated production.
4. Low profile: The overall height is significantly reduced, making it suitable for surface mount (SMT) and highly sensitive module power supplies.

3. Question: What are the main design challenges or drawbacks of planar transformers?
Answer: The main challenge is:
1. Large distributed capacitance: Due to the large parallel area and small spacing between flat copper foils, the parasitic capacitance (CPS) between the primary and secondary sides is usually larger than that of traditional transformers, which may affect EMI and high-frequency characteristics.
2. Limited number of turns: The number of PCB layers and process limits the total number of turns that can be achieved, which is usually suitable for situations with relatively small turns (such as half bridge topology).
3. Low window utilization: The PCB substrate (epoxy resin) occupies a considerable portion of the space in the magnetic core window, and the copper filling coefficient is relatively low (about 30%).

4. Question: What frequency range does a planar transformer typically operate in?
Answer: Flat transformers are particularly suitable for high-frequency working environments, typically operating at frequencies ranging from tens of kHz to several MHz. Due to its flat conductor, which can effectively reduce the skin effect, it has a significant efficiency advantage at high frequencies.

Magnetic Core and Material Selection
5. Question: What are the commonly used magnetic core shapes for planar transformers? How to choose?
Answer: Common magnetic cores include E-type, RM type, and ER/ETD type.
·E-type (such as EI, EE): Low cost, good heat dissipation, large window area, suitable for high current applications, but poor shielding performance.
·RM type (can type): The circular center column can shorten the winding turn length (reduce copper loss), has good self shielding effect, small leakage inductance, but the window is relatively small.
·ER/ETD type: Between the two, it combines the advantages of the E-type large window and the RM type circular center column.

6. Question: What material is usually used for the magnetic core of a planar transformer?
Answer: Almost all of them use high-frequency power ferrite soft magnetic materials, such as Philips’ 3F3, 3F4 or TDK’s PC40/PC95. These materials have low magnetic core losses (hysteresis and eddy current losses) at high frequencies.
7. Question: What is the window utilization coefficient of a magnetic core? Why is the flat transformer lower?
Answer: The window utilization coefficient refers to the proportion of copper conductors actually occupied in the window area of the magnetic core. Traditional transformers are about 0.4, while flat transformers are usually only 0.25~0.3. This is because in addition to copper foil, there are also a large number of epoxy resin insulation layers (PP and Core) occupying the window space in the PCB board.

Winding Design and Layout
8. Question: How can the windings of a planar transformer be connected in series or parallel on a PCB?
Answer: Inter layer interconnection is achieved through through through holes (vias), buried holes, or blind holes on the PCB.
·Series connection: Use vias to connect the spiral coils of different layers end-to-end to increase the number of turns.
·Parallel connection: Connecting multiple layers of coils in parallel to increase current carrying capacity, commonly used in secondary windings for low voltage and high current output.

Question: What is “interleaving” or “insertion” technology? Why do we have to do this?
Answer: Interlocking refers to placing the primary winding (P) and the secondary winding (S) alternately in layers, such as using the P-S-P-S or S-P-S structure. The benefits of doing so are: 1 Reduce leakage inductance: Enhance primary and secondary magnetic coupling.
2. Reduce AC resistance: make the high-frequency current more evenly distributed in the conductor and reduce the loss caused by proximity effect.

10. Question: What are the effects of different winding layouts (such as P/S separation vs interleaving) on leakage inductance and parasitic capacitance?
Answer: This is a typical compromise relationship.
·Separate layout: large leakage inductance, but small interlayer parasitic capacitance.
·Simple sandwich (such as P-S-P): leakage inductance is significantly reduced, but parasitic capacitance increases.
·Deep interleaving (such as P-S-P-S): Leakage inductance can be minimized, but parasitic capacitance is maximized. Designers need to make trade-offs based on circuit requirements, such as LLC utilizing leakage inductance and hard switching controlling capacitance.
11. Question: What should be noted in PCB winding design for high voltage or high current applications?
Answer: High current: Thick copper foil (such as 2oz-4oz), multi-layer parallel connection, and the use of multiple parallel vias are required to carry the current, and external heat dissipation is utilized.
·High voltage: Sufficient insulation distance (creepage distance and electrical clearance) must be ensured. For example, IEC60950 requires that the insulation thickness between the primary and secondary edges should usually be above 400 μ m.

Parasitic Parameters and High Frequency Characteristics
Question: Why is the leakage inductance of planar transformers important? How to control?
Answer: Leakage inductance can cause voltage spikes when the switch is turned off and limit the high-frequency cutoff frequency. In resonant topologies such as LLC, leakage inductance can be utilized as a part of the resonant inductance. The methods for controlling leakage inductance include: using staggered windings, reducing the thickness of the insulation layer between windings, and aligning the original and secondary windings completely.
13. Question: How to optimize the large distributed capacitance of planar transformers to reduce EMI?
Answer: Methods to reduce distributed capacitance include increasing the thickness of the insulation layer between the primary and secondary windings (but increasing leakage inductance), inserting a grounding shielding layer between the primary stages, and optimizing the winding layout to reduce the overlapping area between layers.

14. Question: What are skin effect and proximity effect? How to deal with flat transformers?
Answer: At high frequencies, the current tends to flow towards the surface of the conductor (skin effect), and the magnetic field of adjacent conductors will further distribute the current unevenly (proximity effect), leading to an increase in AC resistance. Flat transformers use flat and thin copper foil as conductors, with a thickness typically designed to be less than the skin depth at that frequency, effectively reducing these high-frequency losses.
Thermal Design and Technology
15. Question: What is the main source of heat for planar transformers? How to dissipate heat?
Answer: Heat mainly comes from magnetic core losses (hysteresis losses) and winding losses (copper losses, especially losses caused by AC resistors). The advantage of heat dissipation is that the flat structure has a large surface area, and heat can be directly dissipated from the surface of the magnetic core and the outer copper foil of the PCB; Usually, transformers can be attached to aluminum substrates or heat sinks, and thermal conductive adhesive can be used to enhance heat dissipation.

16. Question: How do the copper thickness and line width of PCB affect the design? What is the recommended current carrying capacity?
Answer: The thickness of copper determines the current carrying capacity per unit width. The common copper thickness is 1oz (about 35 μ m) and 2oz (about 70 μ m). The current density is usually selected between 20~50A/mm ². The line width needs to be determined based on the effective current value, allowable temperature rise, and PCB manufacturing capability (such as minimum line width/line spacing).
17. Question: Why does PCB stack design emphasize symmetry?
Answer: The symmetrical laminated structure (with uniform thickness and copper distribution) can balance the thermal and mechanical stresses of the PCB during the lamination process, effectively preventing the PCB board from warping (bending deformation) after processing, ensuring the assembly yield of transformers and the tight fit of magnetic cores.

18. Question: How is the magnetic core fixed? Why can’t we stick it to the bonding surface with glue?
Answer: Magnetic core fixation usually uses clips (with slot magnetic cores) or epoxy resin adhesives. Special attention: Adhesive must never be applied to the bonding surface (center pillar) of the magnetic core, otherwise it will form unnecessary air gaps, leading to a decrease in magnetic permeability and inductance. The glue should be applied around the outer edge of the magnetic core.

Answer: 1 Specification determination: Determine the turn ratio, inductance, power, and frequency based on the topology.
2. Magnetic core selection: Use the AP method (area product method) to estimate the size of the magnetic core and select the appropriate magnetic core material and shape.
3. Calculation of turns: Calculate the number of turns on the primary and secondary sides to prevent magnetic saturation
4. Winding layout: Arrange the windings in PCB software to determine the stacked structure (whether staggered, how to parallel/series).
5. Loss and temperature rise accounting: Estimate copper and iron losses to ensure that the temperature rise is within the allowable range.
6. Parasitic parameter extraction: Evaluate whether the leakage inductance and distributed capacitance meet the requirements through simulation or calculation.
7. PCB engineering drawing

20. Question: What are the differences in the design focus of using planar transformers in forward and flyback converters?
Answer:
Forward/Bridge Converter: Transformers mainly function to transmit energy and isolate. The design focus is on reducing leakage inductance (avoiding spikes) and minimizing losses. The low leakage inductance characteristic of planar transformers is an absolute advantage here.
Flyback converter: The “transformer” here is actually a coupled inductor that needs to store energy. Therefore, the magnetic core needs to have an air gap to prevent saturation. The focus of the design is to precisely control the size of the air gap to obtain the desired sensitivity, while addressing the issue of increased losses in the vicinity caused by opening the air gap.