The LLC converter

An LLC resonant converter is a highly efficient isolated DC-DC converter topology that changes one DC voltage to another by using a specialized resonant tank circuit. It is uniquely capable of "soft-switching," which means its transistors turn on and off when the voltage or current across them is near zero, eliminating the bulk of energy losses.

Here is how it works and how it differs from traditional alternatives:

The name LLC comes from the three specific reactive components that form its resonant tank:

  • L (First L): A series resonant inductor (Lr).
  • L (Second L): A parallel magnetizing inductor (Lm) built into the transformer.
  • C: A series resonant capacitor (Cr).

These components act like a high-frequency filter. By changing the switching frequency of the incoming square-wave voltage, the converter alters the impedance of the tank, thereby regulating the output voltage.

To understand what makes the LLC converter unique, it helps to compare it to the two most common alternative topologies: Hard-Switched Converters (like standard Buck or Full-Bridge) and Phase-Shifted Full-Bridge (PSFB) converters.

LLC vs. traditional hard-switched topologies

  • Switching Mechanism: Hard-switched converters force transistors to turn on and off while subjected to high currents and voltages simultaneously, creating massive heat and energy spikes. The LLC uses Zero Voltage Switching (ZVS) on the primary side, allowing the transistors to switch only when the voltage across them is already at zero.
  • Frequency Limits: Because hard-switching losses scale linearly with frequency, traditional converters are limited to sub-100 kHz operation to prevent overheating. LLC converters easily scale into hundreds of kHz or even multi-megahertz regions, shrinking the physical size of transformers and capacitors.

LLCvs. phase-shifted full-bridge (PSFB) topologies

  • Soft-Switching Range: PSFBs can achieve ZVS, but usually only under heavy or full load conditions; at light loads, they lose this ability and drop in efficiency. The LLC resonant tank maintains ZVS over a much wider load range, from completely empty to full capacity.
  • Secondary Diodes: PSFBs suffer from high voltage stress and reverse-recovery energy losses on the secondary-side output rectifiers. The LLC converter inherently achieves Zero Current Switching (ZCS) on the secondary side, turning off output diodes or transistors gently as the current naturally drops to zero, removing reverse-recovery spikes.
  • Control Method: PSFBs regulate voltage by changing the phase overlap (duty cycle) of the transistors at a constant frequency. LLC converters keep a fixed 50% duty cycle but dynamically vary the operating frequency to steer up or down the resonant curve.

The name LLC comes from the three specific reactive components that form its resonant tank:

  • L (First L): A series resonant inductor (Lr).
  • L (Second L): A parallel magnetizing inductor (Lm) built into the transformer.
  • C: A series resonant capacitor (Cr).

These components act like a high-frequency filter. By changing the switching frequency of the incoming square-wave voltage, the converter alters the impedance of the tank, thereby regulating the output voltage.

To understand what makes the LLC converter unique, it helps to compare it to the two most common alternative topologies: Hard-Switched Converters (like standard Buck or Full-Bridge) and Phase-Shifted Full-Bridge (PSFB) converters.

LLC vs. traditional hard-switched topologies

  • Switching Mechanism: Hard-switched converters force transistors to turn on and off while subjected to high currents and voltages simultaneously, creating massive heat and energy spikes. The LLC uses Zero Voltage Switching (ZVS) on the primary side, allowing the transistors to switch only when the voltage across them is already at zero.
  • Frequency Limits: Because hard-switching losses scale linearly with frequency, traditional converters are limited to sub-100 kHz operation to prevent overheating. LLC converters easily scale into hundreds of kHz or even multi-megahertz regions, shrinking the physical size of transformers and capacitors.

LLCvs. phase-shifted full-bridge (PSFB) topologies

  • Soft-Switching Range: PSFBs can achieve ZVS, but usually only under heavy or full load conditions; at light loads, they lose this ability and drop in efficiency. The LLC resonant tank maintains ZVS over a much wider load range, from completely empty to full capacity.
  • Secondary Diodes: PSFBs suffer from high voltage stress and reverse-recovery energy losses on the secondary-side output rectifiers. The LLC converter inherently achieves Zero Current Switching (ZCS) on the secondary side, turning off output diodes or transistors gently as the current naturally drops to zero, removing reverse-recovery spikes.
  • Control Method: PSFBs regulate voltage by changing the phase overlap (duty cycle) of the transistors at a constant frequency. LLC converters keep a fixed 50% duty cycle but dynamically vary the operating frequency to steer up or down the resonant curve.

Introducing the gallium nitride LLC converter

Modern power electronics demand higher power density, lower thermal profiles, and unprecedented efficiency. Traditional isolated DC-DC conversion architectures struggle to meet these strict requirements due to inherent switching losses and material limitations of silicon (Si) MOSFET switch technology.

The integration of Gallium Nitride (GaN) High-Electron-Mobility Transistors (HEMTs) into LLC resonant converter topologies provides a solution to this problem. The LLC converter inherently enables Zero Voltage Switching (ZVS) across a wide operating range, minimizing turn-on losses. By replacing silicon MOSFETs with GaN devices, engineers can reduce parasitic capacitances, eliminate reverse recovery losses, and dramatically increase switching frequencies.

Introduction to high-density power conversion

Power conversion design has entered an era defined by aggressive space constraints and stringent efficiency regulations. Industry standards such as 80 PLUS Titanium and Ruby for data center power supplies require over 96% peak efficiency, while electric vehicle (EV) onboard chargers must minimize weight and volume to maximize driving range.

Historically, hard-switched topologies like the phase-shifted full-bridge or interleaved buck-boost served as industry workhorses. However, these topologies suffer from significant switching losses that scale linearly with frequency. This restricts the operational frequency of silicon-based designs to under 100 kHz in most high-power applications, requiring large, heavy magnetic components and capacitors.

Soft-switching topologies, specifically the LLC resonant converter, overcome these operational frequency barriers. When combined with Gallium Nitride (GaN), the LLC resonant converter can operate efficiently in to the multi-megahertz range. This combination delivers a significant leap in power density and thermal management efficiency.

Fundamental operating principles of the LLC resonant converter

The LLC resonant converter is a member of the resonant tank DC-DC converter family. It is named for its three reactive tank components: a resonant capacitor (Cr), a series resonant inductor (Lr), and a parallel magnetizing inductor (Lm) typically integrated into the isolation transformer.

A standard isolated LLC resonant converter consists of four distinct operational blocks:

  • Square generator: A half-bridge or full-bridge (shown above) configuration — the latter often built as a GaN full-bridge converter in higher-power designs — that converts the DC input voltage into a high-frequency square wave.
  • Resonant Tank: Filters the high-frequency harmonics, presenting a sinusoidal current to the transformer primary based on the switching frequency.
  • Transformer: Provides galvanic isolation and steps the voltage up or down.
  • Rectifier: Converts alternating current (AC) into direct current (DC).
  • Filter: A capacitive filter that smoothes the rectified AC current into a stable DC output voltage.

Silicon power MOSFETs have approached the physical limits dictated by their material properties. As a wide bandgap semiconductor, Gallium Nitride (GaN) provides superior material characteristics that fundamentally alter soft-switching circuit design.

A standard isolated LLC resonant converter consists of four distinct operational blocks:

  • Square generator: A half-bridge or full-bridge (shown above) configuration — the latter often built as a GaN full-bridge converter in higher-power designs — that converts the DC input voltage into a high-frequency square wave.
  • Resonant Tank: Filters the high-frequency harmonics, presenting a sinusoidal current to the transformer primary based on the switching frequency.
  • Transformer: Provides galvanic isolation and steps the voltage up or down.
  • Rectifier: Converts alternating current (AC) into direct current (DC).
  • Filter: A capacitive filter that smoothes the rectified AC current into a stable DC output voltage.

Silicon power MOSFETs have approached the physical limits dictated by their material properties. As a wide bandgap semiconductor, Gallium Nitride (GaN) provides superior material characteristics that fundamentally alter soft-switching circuit design.

Architectural analysis: silicon vs. GaN in LLC converters

When comparing identical LLC converter topologies designed with Silicon versus GaN, the differences in parasitic parameters directly influence key design considerations such as overall efficiency, driving dynamics, and maximum operating frequency.

The output capacitance (Coss) of a switch dictates the amount of energy stored in the device during its off-state (Eoss = 1/2 x Coss x Vin²). In an LLC converter, even though ZVS eliminates turn-on losses, Qoss still impacts performance:

  • Dead-Time Requirements: To achieve ZVS, the magnetizing current must fully discharge Qoss. Silicon devices have a high Qoss, requiring either a long dead-time or a high magnetizing current (low Lm). Long dead-times limit the maximum switching frequency and reduce the effective duty cycle, while low Lm values increase circulating currents and conduction losses.
  • The GaN Advantage: GaN transistors can provide a substantial reduction in Qoss — often in the range of 70–90%, depending on the specific silicon device used for comparison — for an equivalent RDS(on). This allows designers to shorten dead-times from several hundred nanoseconds down to tens of nanoseconds. Additionally, they can specify a much higher Lm, reducing circulating current losses in the resonant tank.

When a silicon MOSFET body diode conducts during the dead-time window or under synchronous rectification, it stores a minority carrier charge (Qrr). If the switch is hard-commutated or subjected to a rapid voltage transient (dv/dt), this charge must be cleared. This creates a large reverse current spike, leading to high power dissipation, voltage ringing, and electromagnetic interference (EMI).

GaN transistors have a near-zero (negligible) Qrr. Because they lack a physical p-n junction, reverse conduction relies entirely on majority carriers in the 2DEG channel. This removes reverse recovery losses, protects the system from dv/dt-induced failures, and simplifies synchronous rectification timing control on the secondary side.

The gate charge (Qg) represents the total charge required to fully turn on the switch channel. Silicon MOSFETs feature high gate charges, requiring significant drive current from the gate driver circuit (Pgate = Qg x Vgate x fs).

GaN transistors exhibit extremely small gate charges, typically several times lower than comparable Silicon equivalents, depending on the specific parts compared. This drastically lowers gate drive power consumption, permitting multi-megahertz operation without overheating the gate drive integrated circuit (IC). It also allows for extremely fast gate turn-on and turn-off transitions.

The output capacitance (Coss) of a switch dictates the amount of energy stored in the device during its off-state (Eoss = 1/2 x Coss x Vin²). In an LLC converter, even though ZVS eliminates turn-on losses, Qoss still impacts performance:

  • Dead-Time Requirements: To achieve ZVS, the magnetizing current must fully discharge Qoss. Silicon devices have a high Qoss, requiring either a long dead-time or a high magnetizing current (low Lm). Long dead-times limit the maximum switching frequency and reduce the effective duty cycle, while low Lm values increase circulating currents and conduction losses.
  • The GaN Advantage: GaN transistors can provide a substantial reduction in Qoss — often in the range of 70–90%, depending on the specific silicon device used for comparison — for an equivalent RDS(on). This allows designers to shorten dead-times from several hundred nanoseconds down to tens of nanoseconds. Additionally, they can specify a much higher Lm, reducing circulating current losses in the resonant tank.

When a silicon MOSFET body diode conducts during the dead-time window or under synchronous rectification, it stores a minority carrier charge (Qrr). If the switch is hard-commutated or subjected to a rapid voltage transient (dv/dt), this charge must be cleared. This creates a large reverse current spike, leading to high power dissipation, voltage ringing, and electromagnetic interference (EMI).

GaN transistors have a near-zero (negligible) Qrr. Because they lack a physical p-n junction, reverse conduction relies entirely on majority carriers in the 2DEG channel. This removes reverse recovery losses, protects the system from dv/dt-induced failures, and simplifies synchronous rectification timing control on the secondary side.

The gate charge (Qg) represents the total charge required to fully turn on the switch channel. Silicon MOSFETs feature high gate charges, requiring significant drive current from the gate driver circuit (Pgate = Qg x Vgate x fs).

GaN transistors exhibit extremely small gate charges, typically several times lower than comparable Silicon equivalents, depending on the specific parts compared. This drastically lowers gate drive power consumption, permitting multi-megahertz operation without overheating the gate drive integrated circuit (IC). It also allows for extremely fast gate turn-on and turn-off transitions.

System-level benefits of GaN in LLC converters

Using GaN in an LLC converter provides system-level advantages that extend beyond individual component efficiency.

Because GaN devices can switch cleanly in the megahertz range with minimal penalty, the physical size of the passive elements can be scaled down aggressively. The volume of both the resonant inductor (Lr) and resonant capacitor (Cr) decreases inversely with frequency.

Furthermore, transformer core volume shrinks at higher frequencies, enabling planar transformer configurations integrated directly into the multi-layer application PCB. This can compress the overall power supply volume by 50% to 70%, pushing power densities beyond 100 W/in³.

GaN-based LLC converters exhibit flat efficiency curves. At light loads, traditional converters suffer from disproportionately high fixed switching and gate drive losses. GaN's low gate charge and reduced Eoss minimize these fixed losses, keeping light-load efficiency high. At heavy loads, the reduction in circulating currents (enabled by a larger Lm) keeps conduction losses (I²R) in check.

Lower overall power dissipation means less heat needs to be rejected from the system. Heatsinks can be reduced in size or eliminated entirely, relying instead on surface-mount thermal vias within the PCB. Operating at lower internal junction temperatures also reduces thermal stress on surrounding components, extending the mean time between failures (MTBF) of the power supply unit.

Because GaN devices can switch cleanly in the megahertz range with minimal penalty, the physical size of the passive elements can be scaled down aggressively. The volume of both the resonant inductor (Lr) and resonant capacitor (Cr) decreases inversely with frequency.

Furthermore, transformer core volume shrinks at higher frequencies, enabling planar transformer configurations integrated directly into the multi-layer application PCB. This can compress the overall power supply volume by 50% to 70%, pushing power densities beyond 100 W/in³.

GaN-based LLC converters exhibit flat efficiency curves. At light loads, traditional converters suffer from disproportionately high fixed switching and gate drive losses. GaN's low gate charge and reduced Eoss minimize these fixed losses, keeping light-load efficiency high. At heavy loads, the reduction in circulating currents (enabled by a larger Lm) keeps conduction losses (I²R) in check.

Lower overall power dissipation means less heat needs to be rejected from the system. Heatsinks can be reduced in size or eliminated entirely, relying instead on surface-mount thermal vias within the PCB. Operating at lower internal junction temperatures also reduces thermal stress on surrounding components, extending the mean time between failures (MTBF) of the power supply unit.

Applicable target applications

The combination of GaN switching performance and LLC soft-switching dynamics is ideal for applications requiring high efficiency, compact size, and low weight.

Hyperscale data centers and high-power AI server clusters require highly efficient power infrastructure to minimize operating expenses and cooling requirements. The industry open standard (e.g., Open Compute Project OCP ORV3) dictates a transition toward a 48V DC power distribution architecture.

GaN LLC resonant converters are widely used to step down 400V DC input from the primary power factor correction (PFC) stage to a regulated 48V or 12V intermediate bus. This architecture delivers the 97%+ efficiency needed to achieve 80 PLUS Titanium and Ruby compliance.

Onboard chargers must convert residential or commercial AC grid power to charge the high-voltage traction battery pack (typically 400V or 800V). Space inside vehicles is limited, and every kilogram added directly reduces the vehicle's driving range.

For designs not implementing a single-stage topology with GaN bidirectional switches (BDS), a high-frequency GaN LLC converter reduces the size of the isolation transformer and EMI filter components. This helps downsize the OBC housing, lowering vehicle weight while ensuring efficient thermal dissipation to the vehicle's liquid cooling loop.

Telecom base stations and other communications equipment require isolated DC-DC bricks capable of converting high-voltage distribution lines down to standard -48V DC telecom buses. These modules operate in harsh ambient outdoor environments where active fan cooling is prone to failure.

GaN LLC converters operate reliably under conduction-cooled configurations, using minimal potting material and space while tolerating high ambient operating thresholds.

From ultra-slim OLED televisions to high-wattage USB-PD 3.1 laptop chargers (140W–240W), consumers demand pocket-sized chargers. GaN LLC topologies enable these adapters to run at high frequencies (100–500 kHz), turning bulky power bricks into sleek, compact travel accessories.

Hyperscale data centers and high-power AI server clusters require highly efficient power infrastructure to minimize operating expenses and cooling requirements. The industry open standard (e.g., Open Compute Project OCP ORV3) dictates a transition toward a 48V DC power distribution architecture.

GaN LLC resonant converters are widely used to step down 400V DC input from the primary power factor correction (PFC) stage to a regulated 48V or 12V intermediate bus. This architecture delivers the 97%+ efficiency needed to achieve 80 PLUS Titanium and Ruby compliance.

Onboard chargers must convert residential or commercial AC grid power to charge the high-voltage traction battery pack (typically 400V or 800V). Space inside vehicles is limited, and every kilogram added directly reduces the vehicle's driving range.

For designs not implementing a single-stage topology with GaN bidirectional switches (BDS), a high-frequency GaN LLC converter reduces the size of the isolation transformer and EMI filter components. This helps downsize the OBC housing, lowering vehicle weight while ensuring efficient thermal dissipation to the vehicle's liquid cooling loop.

Telecom base stations and other communications equipment require isolated DC-DC bricks capable of converting high-voltage distribution lines down to standard -48V DC telecom buses. These modules operate in harsh ambient outdoor environments where active fan cooling is prone to failure.

GaN LLC converters operate reliably under conduction-cooled configurations, using minimal potting material and space while tolerating high ambient operating thresholds.

From ultra-slim OLED televisions to high-wattage USB-PD 3.1 laptop chargers (140W–240W), consumers demand pocket-sized chargers. GaN LLC topologies enable these adapters to run at high frequencies (100–500 kHz), turning bulky power bricks into sleek, compact travel accessories.

Key benefits of GaN in LLC resonant converters

The GaN-driven LLC resonant converter represents a major evolution in isolated DC-DC power management and resonant converter design. By marrying the soft-switching characteristics of the LLC topology with the low parasitic charges and high electron mobility of Gallium Nitride transistors, engineers can bypass the traditional performance trade-offs dictated by Silicon MOSFET limitations.

The elimination of reverse recovery loss, combined with massive reductions in output charge (Qoss) and gate charge (Qg), enables reliable high-efficiency operation at switching frequencies multiple times higher than legacy designs resulting in system-level benefits such as downsized passive magnetics, reduced thermal footprints, and exceptional power densities — make this combination the preferred architecture for modern high-performance power supply design.

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Frequently asked questions about GaN-based LLC resonant converters

What is an LLC resonant converter?

It is a high-efficiency DC/DC converter topology that uses a resonant circuit made of two inductors and one capacitor to shape voltage and current, enabling soft switching.

It consists of primary-side power switches, a resonant tank, a high-frequency isolation transformer, output rectifier, and filter.

It is a switching technique where the transistor turns on or off when the voltage or current across it is zero, drastically reducing switching losses.

ZVS occurs when the voltage across a switching device drops to zero before the device is turned on, eliminating switching losses.

ZCS happens when the current through a switching device drops to zero before it turns off, preventing large current spikes and reducing electromagnetic interference (EMI).

It maintains high efficiency over wide line and load variations and eliminates the need for bulky output inductors, shrinking board space.

It stands for the two inductive elements (the resonant inductor (LR) and magnetizing inductor (LM)) and one capacitive element (resonant capacitor (CR)) in the resonant tank.

It is the energy-storing, wave-shaping portion of an LLC resonant converter located between the primary-side switching bridge and the isolation transformer.