Advantages and Disadvantages of External and Integrated LLC Resonant Inductors

In an LLC power supply, there is a resonant inductor and a resonant capacitor. However, in many practical PCB designs, only the LLC transformer and the resonant capacitor are visible, with no separate resonant inductor. This is because the resonant inductor is integrated into the transformer, utilizing the leakage inductance between the primary and secondary windings as the resonant inductor. In hard-switching circuits, leakage inductance is undesirable and should be minimized because the energy stored in it requires additional circuitry to handle. However, in resonant converters, leakage inductance is beneficial and can serve as the resonant inductor, eliminating the need for a separate inductor. This is known as an integrated resonant inductor transformer. In contrast, some power supplies employ a separate resonant inductor alongside the transformer, which is referred to as an external resonant inductor circuit. Below, we discuss the advantages and disadvantages of these two approaches.

I. Advantages of External Resonant Inductors:

An external resonant inductor is a standalone component separate from the main transformer, offering high design flexibility. Although the inductance value is determined by specific design requirements, different materials can be used to optimize losses on the inductor, enabling higher power density. Additionally, there are more options for selecting the magnetic core and bobbin of the main transformer. Adjustments to the primary winding of the transformer, which may affect the leakage inductance, have minimal impact on the external resonant inductor because the transformer is designed with very low leakage inductance compared to the external inductor. Thus, fine-tuning the number of primary turns has a negligible effect on the resonant inductance.

II. Disadvantages of External Resonant Inductors:

The cost of an external resonant inductor is higher than that of an integrated one due to the additional materials required, such as the magnetic core, bobbin, and copper wire for winding, as well as the labor costs associated with manufacturing the inductor and the additional assembly cost for inserting it into the power supply. Since the resonant inductor operates with alternating current, the magnetic flux density operates in both the first and third quadrants. To minimize core losses, the maximum flux density (Bmax) is typically kept low, which often requires a large air gap in the ferrite core when using it. This can lead to significant eddy current losses in the resonant inductor, resulting in higher temperatures.

III. Advantages of Integrated Resonant Inductors:

Integrated resonant inductors can save on material and labor costs. However, they require a specialized bobbin for winding because they rely on leakage inductance as the resonant inductor. Typically, the calculated resonant inductance accounts for about 20% of the primary inductance, meaning the leakage inductance needs to reach 20% of the primary inductance, which implies that the coupling between the primary and secondary windings does not need to be very tight.

To increase the leakage inductance, specialized bobbins are often used. These bobbins offer the advantage of high leakage inductance and reduced parasitic capacitance between the primary and secondary windings, which is beneficial for LLC converters. Generally, integrated bobbins separate the primary and secondary windings into different slots, significantly improving safety compliance compared to separate components. This allows both primary and secondary windings to use enameled wire without major safety concerns, simplifying the winding process.

IV. Disadvantages of Integrated Resonant Inductors:

The drawbacks of integrated resonant inductor transformers are also evident. The overall size of the transformer is larger than that of a conventional one due to the slot-winding design, requiring more winding space. Additionally, the specialized bobbins needed to achieve the desired leakage inductance are typically custom-made, increasing costs. However, if production volumes are large, these additional costs can be negligible. Since the resonant inductor is essentially the primary leakage inductance, the number of primary turns has a significant impact on the leakage inductance. Once the resonant leakage inductance is fixed, adjusting the number of turns becomes difficult. Therefore, integrated resonant inductors are generally used in lower-power applications. For power levels above a kilowatt, separate inductors are typically used as resonant inductors.

In summary, whether to use an integrated resonant inductor or a separate one in a design depends on the specific circumstances of each company. For high-volume, lower-power applications, integrated resonant inductors offer significant advantages because the resonant inductance and magnetizing inductance are typically larger. Using an external inductor with ferrite cores would result in a large number of winding turns, high winding resistance, and significant copper losses. Integrated resonant inductors do not suffer from this issue. However, at higher power levels, eddy current losses in the magnetic core of integrated inductors become significant, making external inductors the preferred choice.