How to reduce the leakage inductance of planar transformer? 0.5μH leakage inductance, winding interleaving and switching spike verification in 90W power supply | VOOHU

In a 90W isolated power supply, the MOSFET turn-off peak is relatively high, the rectifier node rings obviously, and the absorption circuit generates heat, which often draws attention to the transformer leakage inductance. How to reduce the leakage inductance of planar transformers? The real solution is not to pursue a smaller value alone, but to make the coupling of magnetic components, winding capacitance, insulation structure and PCB commutation circuit jointly serve the goals of efficiency, EMI and device stress. The earlier you specify these conditions during model selection, the easier it will be to find the right direction during sample debugging.

1. How to reduce the leakage inductance of a planar transformer: first look at why it forms a peak

Leakage inductance stored energy will be released during switching commutation

The magnetic flux that is not fully coupled between the primary side and the secondary side can be equivalent to leakage inductance. When the switching current changes rapidly, the energy in the leakage inductance will form ringing together with the MOSFET output capacitance, rectifier junction capacitance, winding capacitance and PCB stray parameters, and may also be absorbed by RCD, RC, active clamp and other loops. The larger the load current, the greater the peak and absorption losses are usually of concern, which is why the low-voltage high-current branch needs to handle the commutation path more carefully on the 90W platform.

Board-level loops that are too long will also amplify the visible peaks

Traces, vias, and pads from the transformer terminals to the MOSFETs, rectifiers, and local bypass capacitors introduce additional parasitic inductance. If the magnetic component itself has met the target, but the high di/dt loop still winds far away, replacing a transformer with a lower leakage inductance may not completely solve the problem. When laying out, the commutation end should be connected to the power device and local capacitance nearby, and the current loop between the primary side and the secondary side should be minimized.

Leakage inductance is not as low as possible

Tighter interleaving of planar transformer windings generally helps improve coupling, but increases primary and secondary adjacent area and distributed capacitance. The common mode current formed by high dv/dt through this capacitive path may bring new pressure to EMI; some LLC or soft switching solutions will also incorporate a certain series inductance into the commutation conditions. Therefore, a suitable goal would be to consider peaking, efficiency, common-mode noise, isolation, and topology requirements simultaneously, rather than just comparing who has the lowest leakage inductance number.

2. How to choose VOOHU WHPT-EQ200-90W series

These four products can be selected first according to the target output voltage and current branches. After entering the sample stage, the input range, switching frequency, rectification method, mechanical space and expected temperature rise are all brought into judgment. The parameter table below directly gives the core differences of each product, making it easier to quickly determine the appropriate product in the early stages of the project.

From 24V to 3.3V, the current path will become more and more critical

WHPT-EQ200-022Corresponding to 24V/3.7A, it is suitable to start with voltage stress, rectifier selection and auxiliary winding cooperation;WHPT-EQ200-021Corresponding to 12V/7.5A, you need to pay more attention to the secondary side RMS current and copper loss. After entering the 5V/18A and 3.3V/27A branches, the terminals, pads, vias, parallel copper layers and rectifier conduction losses will significantly affect the temperature rise. The PCB thermal diffusion capability should also be evaluated simultaneously at the sample stage.

Turns ratio is the starting point for selection, not the only determinant of the output of the whole machine.

The four products use turns ratios of 1:1:0.5, 1:0.5:0.5, 1:0.25:0.5 and 1:0.166:0.666 respectively. The actual output is also affected by the input voltage range, duty cycle or resonant gain, switching frequency, rectifier voltage drop, dead zone and losses. When selecting a specific product, it is recommended to provide the target topology and control method together to avoid making decisions based only on the two labels of ‘90W’ or ‘0.5μH’.

Products of the same series facilitate faster entry into sample verification

If you need to compare different output solutions on the same power platform, you can first establish a clear sample range from VOOHU's four 90W planar transformers. VOOHU also provides specification data, packaging libraries, reference circuit data and 3D models, and supports sample testing and customization communication. Giving input, output, topology, frequency, isolation and structural space to the technical team is often easier to match than just providing power numbers.

3. When reducing leakage inductance, windings, terminals and insulation should be considered together.

Winding staggering should take distributed capacitance into consideration

Staggering methods such as P-S-P can shorten the distance between the primary and secondary magnetomotive forces to cancel each other, thereby reducing the leakage magnetic field energy. However, as the interleaving level increases, the overlapping area of ​​the original and secondary edges will also increase. How to reduce the distributed capacitance of planar transformers requires a comprehensive approach that combines interlayer distance, copper layer overlap, shielding methods and grounding paths. If the number of interleaved layers is continuously increased just to reduce leakage inductance, the original peak problem may become a common mode noise problem.

The terminal position must obey the actual commutation path

No matter how well the internal coupling of the planar windings is done, if the high di/dt terminals are far away from the MOSFET or rectifier, board level parasitics will still offset the gains of magnetic optimization. Parallel copper layers must also maintain similar copper lengths, number of vias, and return paths to avoid current concentration on a certain layer. Simultaneously confirming the pin direction and the position of the PCB power device during model selection can reduce the number of subsequent board modifications.

Insulation, temperature rise and leakage inductance must be balanced

Increasing the inter-layer distance will help reduce the primary and secondary side capacitance and meet the isolation design, but it may weaken the coupling; excessive compression of the insulation space will bring safety boundary risks. The final structure should be determined based on the operating voltage, insulation level, ambient temperature and heat dissipation method. The VOOHU technical team also needs these working conditions when communicating on samples, so that they can give suggestions on leakage inductance, temperature rise and structural dimensions that are closer to the needs of the complete machine.

4. What conditions are provided to VOOHU to help it match suitable products faster?

Input range, topology and switching frequency

For the same 90W output, the magnetics in LLC, phase-shifted full-bridge or other isolation topologies work differently. Entering the minimum value and maximum value, normal operating frequency and frequency variation range will affect the turns ratio selection, magnetic flux density and loss judgment. The more complete the information, the easier it is for the sample direction to converge in the first round.

Output voltage, current and rectification method

The target output determines which of the four products to start evaluating first. For 5V/18A or 3.3V/27A low-voltage and high-current solutions, the diode rectification or synchronous rectification method, continuous load and peak load should also be explained, because these conditions are directly related to the secondary current, copper loss and thermal design.

Isolation requirements, mounting height and pad space

One of the advantages of planar transformers is their compact structure, but magnetic components cannot be directly placed on the board as long as the electrical parameters are close. Before placing the board, you must also confirm the available height, pad direction, creepage and electrical clearance requirements, as well as the layout of surrounding power devices and heat dissipation copper surfaces, so that magnetic component selection and PCB layout can be completed simultaneously.

Specific issues you want to improve

If the project already has a prototype, it can provide the shutdown peak, ringing frequency, absorption loop temperature rise, overall machine efficiency or EMI phenomenon, and explain the test input, load and probe connection methods. If there is already a planar transformer leakage inductance test method and target, the measurement terminal, short-circuit terminal, frequency and test level can also be provided. In this way, technical communication can be directed around issues that need improvement.

5. When there is a problem with the prototype, how to determine where to change it next?

Peak height: first reduce the commutation loop, and then compare the magnetic samples

First check the high di/dt path between the MOSFET, rectifier and transformer to confirm whether the local capacitor location and absorption loop are reasonable. Keeping the same PCB and the same absorption parameters, and then comparing different transformer samples, it is easier to see whether the improvement comes from winding coupling or board-level layout.

Temperature rise: Don’t just focus on the core

The temperature rise of low-voltage high-current branches may come from winding copper losses, terminal and via current density, rectifier conduction losses, or insufficient heat dissipation on the board. When testing samples of 5V and 3.3V products, the temperature distribution of the magnetic components, rectifier, solder joints and PCB copper surface should be observed at the same time, and then decide whether to adjust the magnetic components or the current path.

EMI becomes worse after the leakage inductance is reduced: check the original and secondary capacitor paths

If the spikes decrease and the common mode noise increases after interleaving adjustment, it is usually necessary to recheck the primary and secondary distributed capacitance, shielding or Y capacitor path, and chassis grounding relationship. At this time, it may not be effective to continue to reduce leakage inductance in one direction. A more appropriate approach is to re-find a balance between leakage inductance, common mode current and insulation.

6. Frequently Asked Questions (FAQ)

1. How to reduce the leakage inductance of a planar transformer, and what is the most worthwhile thing to do first?

First determine whether the spike mainly comes from magnetic parts or PCB commutation circuit. If the board-level path is too long, shorten the high di/dt loop first; if the leakage inductance of the magnetic parts is high, then adjust the winding stagger, interlayer distance and terminal position, and review the distributed capacitance and insulation.

2. Is the lower the Lk maximum of 0.5μH, the better?

no. Lower leakage inductance can reduce part of the peak energy, but excessive interleaving will increase the primary and secondary side capacitance, and topologies such as LLC may also require a certain series inductance. It should be judged based on efficiency, EMI, device stress and temperature rise.

3. Can the four 90W planar transformers be replaced with each other?

Not directly interchangeable. 24V/3.7A, 12V/7.5A, 5V/18A and 3.3V/27A correspond to different turns ratios, current and rectification requirements, and also need to match the input range, topology, frequency, pins and PCB space.

4. How to control the leakage inductance of LLC planar transformer?

First determine whether the leakage inductance is a controlled parasitic quantity or participating in the resonant inductance. The maximum value of 0.5μH cannot be directly regarded as the resonance design value. It should be combined with the gain, frequency range, excitation inductance, resonance capacitance and tolerance setting window, and then confirmed with samples.

5. What information is recommended to be prepared when applying for VOOHU samples?

It is recommended to provide input range, topology, switching frequency, target output, rectification method, isolation requirements, height and pad space, as well as spikes, temperature rise or EMI issues that you want to improve, so that the technical team can recommend suitable products.

結論

Planar transformer selection should go back to the overall machine goal.VOOHU WHPT-EQ200-90WシリーズCovering 24V, 12V, 5V and 3.3V output, you can first select the appropriate product according to voltage and current, and thenVOOHU sample applicationSubmit topology, frequency, isolation and waveform requirements.