LLC Resonant Half-Bridge Basics & Variants

The LLC half-bridge is built from a switching half-bridge, a series resonant inductor Lr, resonant capacitor Cr and the transformer magnetizing inductance Lm, forming a boost-to-buck gain profile around the resonant frequency f_r. The shape of the gain curve and the presence of ZVS depend strongly on the Lm/Lr ratio, the tank Q-factor and the chosen operating point relative to resonance.

Two resonant frequencies govern behavior: the series resonance f_r determined by Lr and Cr, and the magnetizing branch frequency f_m. Their separation controls circulating current, ZVS margin and light-load behavior. Operating slightly above f_r minimizes tank current, providing high efficiency and stable regulation across wide line conditions.

Although full-bridge LLC and variants such as LCC or CLLC are used in higher-power systems, this page focuses strictly on the half-bridge class commonly used from 150 to 600 W. System-level structures such as PFC+LLC stacking or digital loop coordination are covered in other pages to maintain separation of design domains.

Gain Curves, Operating Regions & ZVS Window

The LLC gain characteristic is defined by the normalized frequency f_n = f_sw/f_r. Below the resonant point f_r, the stage enters a high-gain boost region where circulating current and magnetics stress increase sharply. Around resonance, the converter achieves maximum efficiency, reduced tank current and stable control behavior. Above resonance, the gain drops into a buck region where light-load operation tends to push the switching frequency higher, compressing ZVS margin.

ZVS depends on sufficient magnetizing current from Lm and appropriate dead-time. Heavy load and low-line conditions shift operation toward f_n < 1, improving ZVS but raising copper and core losses. Light load and high-line conditions shift operation into f_n > 1, reducing magnetizing current and risking ZVS loss, diode recovery stress and increased EMI.

Wide-range power supplies—such as universal 90–264 VAC inputs or adapters with flexible USB-C/PPS outputs—compress the safe operating envelope. Ensuring ZVS across all input and load points becomes a primary design constraint. This section explains how the gain curve, the Lm/Lr ratio and tank Q-factor shape the boundaries of the ZVS window.

Resonant Tank & Transformer Design Workflow

Designing the resonant tank requires selecting a realistic switching-frequency window based on power level, thermal constraints, magnetics capability, and efficiency targets. Mid-range values such as 70–140 kHz typically offer the best compromise between transformer size and switching loss.

The nominal operating point is then placed slightly above the resonant frequency f_r, where efficiency is high and ZVS margin is strong. From the required gain profile, the designer derives Lr, Cr, and the Lm/Lr ratio that determines magnetizing current and ZVS robustness.

Transformer design involves selecting the core, defining leakage inductance (sometimes integrated intentionally into Lr), and verifying magnetizing inductance levels. Final validation must confirm gain and ZVS margins at minimum and maximum line voltage, full load, mid load, and standby conditions, ensuring stable behavior across the full operating envelope.

Soft-Start, Burst and Hiccup for LLC Controllers

LLC resonant half-bridges cannot be started like simple PWM converters with an abrupt rise of duty cycle. The non-linear gain around the resonant point means that starting too close to the low-frequency, high-gain region can cause large inrush current, transformer saturation and repeated over-current trips. A controlled soft-start that begins at a much higher switching frequency and gradually ramps down toward the target operating band is essential to avoid overshoot and stress on switches, rectifiers and magnetics.

At light load, the controller must prevent the operating point from moving too far into the high-frequency region where magnetizing current falls and ZVS becomes marginal. Techniques such as burst mode and skip-cycle operation reduce average switching activity and standby input power while keeping the active bursts within a frequency band that still supports reliable resonant behavior. Proper thresholds, burst lengths and restart timing are required to balance acoustic noise, EMI and standby efficiency.

For fault conditions, the LLC controller combines fast current limiting with frequency increase and hiccup-based restart. Over-current, short-circuit, over-voltage and transformer or gate-drive anomalies each map to specific recovery strategies. Hiccup periods long enough to allow components to cool, but short enough to avoid unacceptable downtime, are essential for robust field operation. System-level hot-swap, eFuse and upstream protection are handled by dedicated pages; this section focuses on the internal soft-start, light-load and hiccup behavior of the LLC controller itself.

Control Modes and Modulation Strategies

LLC converters are controlled primarily through frequency modulation rather than duty-cycle control. The error between the regulated output and its reference drives a compensation network whose output defines a commanded switching frequency. Movement along the resonant gain curve takes the place of duty-cycle variation, allowing the converter to maintain ZVS over a wide input and load range while achieving the required voltage regulation.

Hybrid schemes that combine limited duty variation with frequency modulation are used in some high-performance supplies to extend the effective regulation range or improve transient behavior. However, any duty change must respect resonant-tank constraints and the timing required for ZVS and synchronous-rectifier conduction. Output-voltage feedback is often complemented by feedforward of bus or PFC voltage so that the frequency command reflects both load changes and input variations.

Traditional analog LLC controllers implement error amplification, compensation, soft-start, protection and frequency modulation in dedicated hardware. Digital and multi-loop solutions offload some of these tasks to a microcontroller or digital power controller, enabling profile switching, more complex dynamics and PMBus-based configuration and telemetry. Spread-spectrum frequency dither can be applied in moderation to reduce EMI peaks, but with care to avoid excessive gain modulation or conflict with burst-mode and SR timing.

Sensing, Protection and IC Hooks in LLC Stages

LLC controllers rely on a small set of critical measurement points to keep the resonant stage within its safe operating area. Primary current sensing through shunts or current transformers feeds cycle-by-cycle OCP comparators, while output-voltage feedback and bus-voltage sensing define the regulation target and line-dependent limits. Switch-node or ZCD sensing tracks the resonant waveform and helps maintain ZVS, and temperature inputs enable derating and over- temperature protection around hotspots.

Protection functions typically cover peak and average over-current, over-voltage and under- voltage on supply rails, over-temperature, and abnormal transformer or drive conditions. Many LLC controllers expose dedicated pins for CS, FB/COMP, VCC UVLO, bus OVP, OTP, DRV UVLO and FAULT or PGOOD signaling. Controllers such as TI UCC25640x, ST L6599C or ONsemi NCP1399 families integrate these comparators and fault-handling state machines so that the converter can limit current, shift frequency, enter hiccup mode and restart in a controlled way.

The LLC controller also acts as a timing and status hub for companion ICs. Synchronous rectifier controllers receive a clean, predictable switching pattern and dead time from the half-bridge, and in some architectures derive their own gating from the resonant timing. Upstream eFuse and hot-swap devices provide PG and FAULT signals that gate LLC start-up and trigger controlled shutdown when the input source is stressed. PFC stages expose PFC_OK and bus-voltage sense lines that the LLC controller uses to decide when to start, how to set frequency limits and how to react to bus collapse or brown-out events.

In a practical bill of materials, LLC controllers such as UCC25640x or NCP1399 pair with synchronous rectifier controllers like UCC24612 or NCP4306 and upstream protection devices like eFuses or hot-swap controllers. The sensing and protection hooks described here define how these devices share information through CS, FB, ZCD, PG and FAULT pins so that the overall power supply can meet efficiency, safety and reliability requirements across its operating range.

Magnetics, Layout and EMI Considerations for LLC

Magnetic design for LLC stages balances efficiency, repeatability and size. The resonant inductance can be implemented as a discrete inductor in series with the transformer or by deliberately using transformer leakage inductance. Discrete Lr offers tighter control of resonant frequency and easier tuning, while integrated leakage saves space and cost but makes the resonant point highly sensitive to winding geometry and production tolerances. The chosen implementation directly affects circulating current, transformer heating and gain stability.

Common-mode and differential-mode noise in LLC converters originate mainly from high dv/dt at the half-bridge node and from capacitive coupling between windings and chassis. LLC soft- switching reduces some hard-switching spikes compared to PWM converters, but poorly controlled loops and parasitics still create ringing and broadband emissions. Simple measures such as minimizing the high-frequency loop between the half-bridge, resonant capacitor and bus capacitors, and keeping the Lr–Cr–transformer loop compact, are critical to controlling both EMI and stress on semiconductors.

Layout should separate power paths from sensitive sensing nodes. The half-bridge and resonant loop require short, wide copper and tightly coupled traces to reduce loop inductance. Sense resistors, feedback dividers and controller ground pins should connect to a quiet sense ground that meets the power ground at a single, well-chosen point. This prevents large tank and rectifier currents from modulating CS and FB references, which would otherwise degrade regulation and protection thresholds.

EMI optimization at the LLC stage uses modest frequency dithering, snubbers and termination networks. Spread-spectrum modulation reduces peak emissions but must be limited to a small frequency window to avoid large gain modulation or conflicts with burst mode and SR timing. RC or RCD snubbers at the half-bridge node and rectifier nodes damp high-frequency ringing, while carefully chosen RC terminations on long traces or secondary loops further reduce overshoot. Detailed EMI filter design, surge and EFT protection are handled in the AC Input & EMI Front-End page; this section focuses on the magnetics and local layout decisions that make those upstream filters effective.

IC Role Mapping for LLC Resonant Stages

An LLC power stage is rarely driven by a single controller IC. It relies on a coordinated set of devices: a resonant controller, half-bridge or GaN gate drivers, current and voltage sensing amplifiers or isolated converters, protection devices such as eFuses or hot-swap controllers, and system-level supervisors for temperature, fan control and power-good or fault aggregation. This section summarizes these roles and the key selection points for each category.

For the LLC controller, important criteria include supported switching-frequency range and modulation method, soft-start behavior and current limiting, burst or skip-cycle modes for standby efficiency, integrated protection types, frequency dither capability and how easily it interfaces to synchronous rectifier controllers, PFC stages and digital supervisors. Half-bridge and GaN gate drivers are chosen for their CMTI robustness, adjustable dead time, Miller clamp strength, supported gate-voltage levels and whether isolation is integrated or provided externally.

Sense and protection devices cover shunt or CT current monitors, voltage-sense amplifiers, isolation amplifiers and sigma-delta modulators, along with eFuses, hot-swap controllers and fast comparators. Selection focuses on bandwidth, common-mode range, offset and drift, fault response time, SOA enforcement and the way PG and FAULT outputs interface to the LLC controller. System monitors and digital power controllers add temperature monitoring, fan drive, rail supervision and PMBus communication on top of the analog control loop.

Across the industry, LLC controllers are offered by at least seven major vendors. Example device families include TI UCC25640x and UCC25630x, ST L6599A and L6599C, Infineon ICE2HS01G and ICE1HS01G, ON Semiconductor NCP1399 and NCP13992, Renesas R2A20114 and related resonant controllers, NXP TEA171x PFC+LLC combo ICs and Microchip digital implementations based on dsPIC33 or MCP19xxx analog front ends. Complementary half-bridge and GaN drivers span TI LM5113 and UCC2753x, Infineon 1EDN and 1EDI families, ON Semiconductor NCP5153x and NCP518x, ST L639x series, Renesas, NXP and Microchip high-side/low-side gate drivers, as well as SR controllers such as TI UCC24612 or UCC24624, ON Semiconductor NCP4306 and ST SRK families.

Current-sense and isolation roles can be covered by shunt monitors such as TI INA19x and INA21x families, sigma-delta modulators and isolation amplifiers from multiple vendors, combined with digital isolators. Protection roles map to eFuse and hot-swap controllers from TI, Infineon, ON Semiconductor, NXP, Renesas, ST and Microchip, which provide programmable current limits, dv/dt control, power-good and fault signaling. System supervisors, fan controllers and digital power managers tie these elements together, exposing PMBus or SMBus interfaces to higher-level control while keeping the LLC stage within safe operating limits.

Application Mini-Stories: LLC in Real PSUs

LLC resonant stages are deployed in a wide range of power supplies, from compact adapters to multi-kilowatt server units. The same design concepts appear with different priorities: adapters emphasize low standby power and high efficiency at medium load, server PSUs require digital manageability and tight coordination with PFC stages, and LED or TV power systems combine power-factor, dimming and acoustic-noise constraints. This section gives short application stories that show how LLC controllers, drivers and companion ICs are used in practice.

Case 1 · 250 W Notebook Adapter

A 250 W notebook adapter with a 90–264 VAC input and a single 19–20 V output uses a front-end PFC stage followed by an LLC half-bridge and synchronous rectification. The LLC controller operates near resonance at low line and full load for high efficiency and good ZVS margin, then shifts to higher frequency at high line and lighter load. Soft-start ramps frequency from a high value toward the target band to avoid overstressing the MOSFETs and transformer during adapter plug-in, while burst or skip-cycle modes reduce switching activity to meet stringent standby power limits.

Over-current and short-circuit events are handled through cycle-by-cycle current sensing and hiccup-based restart, which protects both the adapter and the notebook during overload or connector faults. Typical IC roles include an LLC controller with integrated burst and protection logic, a half-bridge gate driver or GaN driver, a synchronous rectifier controller and an upstream eFuse or hot-swap device. Multi-vendor solutions can combine LLC controllers from TI, ST, Infineon, ON Semiconductor, Renesas, NXP or Microchip with SR controllers and protection devices from the same vendors to meet adapter efficiency and standby targets.

Case 2 · 1–2 kW Server / CRPS PSU

A 1–2 kW server or CRPS power supply targets 80+ Titanium efficiency, redundant operation and full PMBus manageability. The front end typically uses an interleaved PFC followed by a full-bridge LLC stage. The LLC may be controlled by a dedicated resonant controller or integrated into a digital power controller that also manages PFC, secondary regulation, fan control and protection thresholds. Frequency modulation, soft-start, burst and fault handling remain in the LLC domain, while the digital controller supervises mode changes, margining and telemetry.

During normal operation the LLC stage maintains ZVS over wide load and line conditions, while the digital controller uses PMBus to adjust output voltage, set current limits and log events. For brown-out, fan faults or thermal alarms, the digital manager can step down power limits or command a controlled shutdown. Typical IC combinations pair LLC controllers and high-performance gate drivers from TI, Infineon, ON Semiconductor, ST, Renesas, NXP and Microchip with digital power controllers and PMBus managers from the same vendors, enabling coherent control from the rack level down to the individual LLC tank.

Case 3 · High-Power LED Driver / TV PSU

In high-power LED drivers or TV power supplies, the LLC stage often feeds a current-regulated secondary or downstream DC-DC converters that set LED string current or panel rails. Dimming or brightness changes alter the effective load on the LLC stage, shifting the operating point on the gain curve. The controller must limit frequency excursions to preserve ZVS and maintain acceptable efficiency across brightness levels, while preventing audible noise from burst or skip-cycle operation.

Thermal behavior in enclosed housings and compatibility with EMI limits further shape the choice of LLC controller, gate driver, sensing and protection ICs. Multi-vendor portfolios from TI, ST, Infineon, ON Semiconductor, Renesas, NXP and Microchip allow designers to combine resonant controllers, isolated drivers, current-sense amplifiers and protection devices that are tuned for lighting and consumer PSU requirements, without locking into a single ecosystem.

LLC Resonant Half-Bridge – FAQs

These questions summarize typical decisions and pitfalls when moving to an LLC resonant half-bridge: when to migrate from flyback, how to choose tank parameters, verify ZVS margin, manage soft-start and burst behavior, place sensing and protection, and scale controller choices from adapter-level to kilowatt server PSUs.