Analysis and Optimization of Common Issues in LLC Resonant Converter Transformer Design

LLC resonant converters have become a mainstream topology in the field of high-efficiency, high-power power supplies due to their excellent characteristics such as zero-voltage switching (ZVS) and low device voltage stress. With their widespread application, engineers often encounter some common problems when designing their core component - the LLC transformer. This article will summarize and analyze these typical issues, providing corresponding design optimization ideas for reference.

1. High No-load or Light-load Output Voltage

Phenomenon: The output voltage is significantly higher than the design value under no-load or light-load conditions.

Cause Analysis: This problem is caused by various factors. One key reason is parasitic oscillation. When the transformer's secondary winding has a large number of turns or layers, the interlayer/interturn parasitic capacitance and the secondary leakage inductance form a resonant circuit. Under light loads, the ringing amplitude generated by this circuit can become abnormally high, thereby boosting the output voltage.

Solutions:

1. Reduce Parasitic Capacitance: Adding a layer of insulating tape after each completed layer of the secondary winding can effectively reduce interlayer capacitance.
   
2. Optimize Winding Process: Avoid using the traditional parallel winding method for forward and reverse windings. Instead, adopt a layered winding method, separating windings of different directions to suppress parasitic oscillation.
   

2. Excessive Winding Temperature Rise

Phenomenon: Excessively high transformer winding temperature is measured during aging tests.

Cause Analysis: LLC transformers operate at high frequencies, where conductors are subjected to alternating magnetic fields and are significantly affected not only by the well-known Skin Effect but also by the Proximity Effect.

• Skin Effect: This refers to the phenomenon where the alternating magnetic field generated by the conductor's own current forces the current to flow towards the surface of the conductor.
  
• Proximity Effect: This refers to the influence of the alternating magnetic field generated by currents in adjacent conductors on the current distribution in the target conductor.
  

Unlike flyback transformers, the primary windings of LLC transformers are typically concentrated and wound on the same side, with current flowing in the same direction. As the number of winding layers increases, the Proximity Effect intensifies sharply, leading to a significant increase in the AC resistance of the conductor and thus causing overheating.

Solutions: Use multiple strands of thinner wire, such as Litz wire or self-wound multi-strand wire, instead of a single thick conductor. This effectively increases the conductor's surface area, suppressing the skin and proximity effects and reducing high-frequency AC resistance.

3. Core Abnormal Heating and Saturation Risk

Phenomenon: The transformer's designed operating magnetic flux density is not high, but the core temperature is very high, posing a saturation risk.

Cause Analysis: The LLC transformer operates in an LC resonant state. The inherent Quality Factor (Q factor) of its resonant circuit is typically greater than 1. This means the actual resonant voltage applied across the transformer terminals is higher than the DC input voltage. If this factor is not considered in the design, the actual magnetic flux density at which the transformer operates will far exceed the design value.

Specifically, at high input voltages, the switching frequency is higher, and the resonant circuit gain is lower, so the saturation problem is less prominent. However, at low input voltages, the switching frequency decreases, the resonant circuit gain increases, and it becomes very easy to cause transformer core saturation, leading to a sharp increase in core loss and temperature rise.

Solutions:
When calculating the minimum number of turns required for the transformer, it must be multiplied by the gain factor determined based on the operating conditions. Furthermore, to be conservative, the influence of leakage inductance should also be considered by multiplying the result by the reciprocal of the coupling coefficient, ensuring the core does not saturate across the entire operating range.

4. Large Deviation Between Actual and Designed Operating Frequency

Phenomenon: There is a significant discrepancy between the power supply's actual operating frequency and the theoretical design frequency point.

Cause Analysis: The causes for this issue are complex, but a common design pitfall lies in the method of rounding the number of turns. The usual design process is to first determine the primary turns, then calculate the secondary turns based on the turns ratio. However, the calculated secondary turns are often non-integer. Simply rounding this value to the nearest integer introduces a significant error. Because the number of secondary turns itself is small, even rounding half a turn results in a considerable error ratio.

Solutions:
It is recommended to use the Reverse Design Rounding Method:

1. Calculate the secondary turns based on the turns ratio (usually a non-integer).
   
2. First, select a reasonable integer value for the secondary turns.
   
3. Based on this integer number of secondary turns, reverse-calculate the primary turns, and then round this result.
   Since the number of primary turns is larger, the error ratio introduced by the same rounding operation will be much smaller than that from rounding the secondary turns.