Inductor and Transformer Selection in LLC Converter Design

This inductor is one of many components you’ll need in your LLC converter design

All electronics need power supplies and power regulation, whether they are large industrial systems or small wearables. If you’re designing automotive power management units, power converters for appliances, or industrial power systems, then you’ll likely be using an LLC converter design for DC-DC conversion and regulation. This topology provides high power conversion efficiency with a simple regulation scheme, but it all depends on selecting the right components.

The two primary components that are critical in an LLC converter design are inductors and transformers. These two components govern power conversion and collectively determine the resonant behavior on the primary side of the converter stage. When designed alongside a PFC circuit and control loops, you’ll have a high efficiency power converter for a range of applications.

Components for an LLC Converter Design

An LLC resonant converter is a switching DC-DC converter where the output voltage is controlled by adjusting the driving frequency of a half-bridge or full-bridge circuit with power MOSFETs. The output current from the bridge circuit flows into a resonant LC tank connected in series with a transformer. The transformer then steps up or steps down the pulsed voltage onto the secondary side.

The secondary side of the converter contains a bridge rectifier circuit (built from diodes or MOSFETs), which rectifies the output voltage/current to a stable DC level. A bank of capacitors further stabilizes the output voltage and reduces ripple to a low-noise DC value. These converters have a complex topology, but the control method is quite simple and the required functional blocks in the converter can be built from a range of commercially available components. The basic topology of an LLC converter design is shown below.

Topology of a half-bridge LLC converter design.

Unlike other switching converters, which require changing the duty cycle of a PWM signal to control the output voltage, an LLC converter requires changing the frequency (pulse frequency modulation, or PFM) to set the output voltage to the required value. Transistors Q1 and Q2 in the above half-bridge LLC converter design are switched 180 degrees out of phase. When the high-side transistor is switched on, current flows to capacitor Cr and charges it. Once Q1 turns off and Q2 turns on, Cr discharges. Both cycles induce a current in the secondary side, which is then rectified to a DC voltage. A capacitor bank on the output side then smooths the output voltage to a stable value.

By exploiting gain in the resonant tank section (the LLC circuit in the above diagram), the voltage on the primary side can be adjusted and maintained at the desired value. The voltage measured through the feedback loop is used to adjust the PFM signal from the gate driver circuit, which exploits more or less gain in the primary side LLC network. This is where the transformer and inductor become critical and need to be selected to provide the right gain range.

Inductor and Transformer Selection Criteria

The inductor and transformer need to meet particular criteria in order to function properly in an LLC converter design:

• Coil inductance value: The inductance values of the two coils in the LLC converter stage depend on the value of capacitor Cr. A typical capacitor value is Cr ~ 100 nF to 1 uF, so the primary side inductor (Lr) will be ~0.1 mH for operation near 100 kHz (typical gate driver frequencies). The inductance of the transformer’s primary side coil (Lm) is normally about 5-10% of the value of Lr. 

• Coil resistance and current rating: The resistance of the conductor influences the current rating in the transformer/inductor. Ideally, the coil resistance should be as low as possible to reduce heat generation. 

• Winding capacitance. This parasitic will determine susceptibility to noise, which becomes important if the converter is powering other high speed digital circuit blocks. Physically smaller inductors and transformers will have larger winding capacitance. 

• Additional primary or secondary side coils: The transformer may have multiple coils on the primary side to step down the voltage to ~5 V or ~3.3 V to power peripherals in the feedback loop and other components in the system. 

• Footprint: An LLC converter designed for high power operation will have bulky components. Footprints can be ~5-10 cm in size in ~1 kW converters. 

One example inductor that is common in high power LLC converter designs is the 2300HT series of power inductors from Bourns. These inductors have small footprint (up to 1.28 inches in diameter) and can withstand currents from 2.9 up to 38.7 A. They are specifically designed to withstand the high temperatures that can arise in high power converters in extreme environments, with a rated operating temperature from -55 to +200 °C. They also come in vertical or horizontal mounting styles, giving designers a low profile option if needed.

2300HT series inductor photograph and mechanical drawings. From the 2300HT datasheet.

Würth Elektronik’s line of pulse transformers is one option for use in high power LLC converter designs. These transformers provide the low inductance and high current rating required in moderate power systems. They also have a range of terminals for selecting output voltage/current values in a relatively small package. 

Example footprint data for the 750311591 pulse transformer from Würth Elektronik. From the 750311591 datasheet.

Other Components for LLC Resonant Converter Design

By combining the transformer’s coil inductance and primary inductor value, you can select the range of usable frequencies in your LLC converter design. Some other components will need to appear in the feedback loop in the LLC resonant stage shown above, as well as in the feedback and control loop used in your PFC circuit. Some of these components include:

• Precision sensing amplifiers for voltage/current measurement

• Inductors and common-mode chokes

• Low voltage regulators for powering peripherals

Whether you’re working on an LLC converter design or another power converter topology, you can find the parts you need thanks to the advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to distributor data and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our power management integrated circuits page to find the components you need.

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