Why Voltage-Mode Control Falls Short for Telecom Rectifiers
Voltage-mode control of LLC series resonant converters is the traditional approach, and its core weakness is that it relies solely on output voltage feedback. An error amplifier compares the sensed output against a reference, and the resulting error modulates switching frequency via pulse-frequency modulation (PFM). This works tolerably at fixed operating points, but the LLC converter’s control-to-output transfer function is inherently nonlinear and operating-point-dependent — a property that becomes a serious liability in telecom service.
Telecom rectifiers must accommodate input voltages from 90 VAC to 264 VAC and operate across temperatures from −40 °C to full load — a wide envelope by any standard. As documented in research from the Power Electronics Team at LG Electronics (2015), “conventional voltage mode control only offers limited performance for LLC series resonant DC-to-DC converters experiencing wide variations in operational conditions.” The paper states explicitly that “a specific control design optimized at one particular operating point could become unacceptable when the operational condition is varied.” This is not a marginal degradation: the compensator can become under-damped or over-damped at operating corners it was not tuned for, degrading transient response and risking instability.
The LLC resonant converter’s control-to-output transfer function contains complex double poles that shift dramatically with load and input voltage, making a single voltage-mode compensator design unacceptable across the wide operating range required by telecom rectifiers (90–264 VAC input), as documented by LG Electronics (2015).
Great Wall Power Technology’s 2024 patent on current-mode LLC control confirms the structural nature of this problem: “voltage control mode currently has problems such as slow loop response speed and double poles in the control-to-output transfer function.” The double-pole structure creates a resonant peak in the open-loop gain that must be suppressed by the compensator — but because the peak frequency and Q-factor both vary with operating point, a fixed compensator cannot do this reliably across the full range.
In an LLC series resonant converter, the control-to-output transfer function contains a pair of complex conjugate poles — a “double pole” — whose frequency and damping ratio shift with load current and input voltage. Under voltage-mode control, a compensator tuned to suppress this resonance at one operating point may provide insufficient phase margin at other points, causing oscillation or sluggish response.
A further practical consequence is that voltage-mode control provides no direct visibility into the resonant tank current. Overcurrent detection is purely reactive, dependent on secondary protective circuits rather than cycle-by-cycle enforcement. Shenzhen Megmeet Electric’s 2024 patent on LLC startup control reinforces this: existing LLC resonant converters “generally have certain limitations in startup control, such as large startup current and poor monotonicity” — problems amplified when only voltage feedback is available at startup, before the output reaches its regulation band. According to IEEE standards for power conversion, cycle-by-cycle current limiting is a fundamental requirement for robust fault management in high-power converters.
How Current-Mode Control Solves the LLC Stability Problem
Current-mode control introduces an additional inner feedback loop that senses a current signal from the resonant tank network and uses it to directly modulate switching frequency — or, in peak current-mode implementations, to terminate each half-switching-cycle when the resonant current reaches a commanded reference. This two-loop architecture is the defining characteristic of peak current-mode LLC control, and it fundamentally changes the plant seen by the outer voltage compensator.
The foundational description comes from LG Electronics (2015), which proposes “an additional feedback from the current of the resonant tank network to overcome the limitation of the existing voltage mode control,” enabling the scheme to “consistently provide good closed-loop performance for LLC resonant converters for the entire operational range.” The key mechanism is linearisation: by feeding back resonant current, the inner loop makes the effective control-to-output characteristic far more predictable across operating points, because resonant current variations are suppressed within a single switching cycle rather than waiting for the output voltage to reflect them.
“Current control mode, for LLC resonant converters, directly controls the frequency of switch control signals through comparison of resonant current with peak current, significantly improving closed-loop stability and reliability, while also improving the dynamic performance of the system.”
Great Wall Power Technology’s 2024 patent elaborates on the telecom-specific advantages: “because current control mode LLC resonant converters have high low-frequency DC gain, the suppression effect on power-frequency ripple is better, and because the resonant current increment is directly controlled, output power is indirectly controlled — therefore in overcurrent and short-circuit problems, the reliability is better compared to directly controlling frequency control methods.” Higher low-frequency DC gain is directly linked to holdup time and mains immunity specifications that define ETSI-class telecom rectifiers, as referenced in standards published by bodies including ETSI.
Current-mode LLC resonant converters exhibit higher low-frequency DC gain than voltage-mode designs, providing better suppression of power-frequency (mains) ripple — a parameter directly linked to holdup time and mains immunity specifications in telecom power systems, as documented by Great Wall Power Technology (2024).
The inner current loop also directly benefits protection. Telecom rectifiers must withstand sustained short-circuit events. Under voltage-mode control, a short circuit collapses output voltage and forces switching frequency to maximum, producing high resonant current peak-to-average ratios and large switching losses. Research from Nanjing University of Aeronautics and Astronautics (2019) documents this precisely: “in short-circuit mode, the output voltage drops to zero…the switch frequency needs to be raised to several times the rated operating frequency to reduce the resonant current peak value,” and “resonant current will have a relatively large peak-to-average ratio, bringing greatly increased switching losses.” Current-mode control with a resonant-current-reference clamp directly limits the current waveform, providing controlled short-circuit current that voltage-mode cannot achieve.
Under voltage-mode control, an LLC converter in short-circuit mode must raise switching frequency to several times the rated operating frequency to reduce resonant current peak value, producing a high peak-to-average current ratio and greatly increased switching losses — a problem that current-mode control with a resonant-current-reference clamp directly eliminates, as established by Nanjing University of Aeronautics and Astronautics (2019).
Explore the full patent landscape for LLC resonant converter control strategies in PatSnap Eureka.
Search LLC Converter Patents in PatSnap Eureka →Wuhan University’s 2018 paper provides the damping mechanism at a modelling level: the rectifier current inner loop “increase[s] the control system damping, improving dynamic performance.” Specifically, it converts what is an oscillatory second-order system under voltage-mode control into a well-damped system under current-mode control — a result validated on a 200 W prototype with measured improvements in load step response relative to a voltage-mode baseline. This modelling work, aligned with power electronics research published in journals indexed by IEEE, demonstrates that the advantage is not merely empirical but structurally grounded in control theory.
Resonant Tank Sensing, Slope Compensation, and Startup
Implementing current-mode control in an LLC converter presents circuit challenges that differ substantially from the current-mode implementations familiar from non-resonant PWM converters. In a conventional PWM buck or flyback, the peak inductor current is a straightforward sawtooth that resets each cycle. In an LLC resonant converter, the primary current is approximately sinusoidal — sensing it and using it as the control variable requires careful attention to timing and slope compensation.
Slope-Based Sensing for Adaptive Frequency Control
ON Semiconductor’s 2024 patent describes a specific peak current-mode implementation in which “the value indicating the current through the primary winding of the LLC converter’s transformer is measured during the first on-time of the first switching cycle of the electrically controlled switch,” and the switching frequency is then controlled “based on the slope of the current waveform.” This slope-based approach is important because in an LLC converter the resonant current slope varies with operating frequency, load, and input voltage. By computing slope rather than using a fixed ramp, the controller implicitly adapts its timing to the actual resonant state — effectively implementing a predictive algorithm that can anticipate resonant tank behaviour over the next half-cycle, providing faster dynamic response than any purely output-voltage-driven scheme.
Zhejiang Yaeneng Energy Technology (2016) documents that in large-power LLC converters above 300 W, “the resonant current is a waveform similar to a sinusoid, and compared to the approximately trapezoidal wave of a regular half-bridge at equal power, the peak value of this sinusoidal current is much higher — the higher the power, the more pronounced the effect.” Voltage-mode control at these power levels “is prone to losing control after disturbance, which can destroy the power switches.”
Startup: Two-Stage Slope-Compensated Current Control
Startup is a particularly demanding scenario for LLC converters, where voltage-mode control is at its weakest. Shenzhen Megmeet Electric’s 2024 patent introduces a two-stage approach: the first startup phase uses a fixed-frequency open-loop signal, and the second startup phase transitions to current-mode control with a ramp-compensated feedback using a slope compensation gradient that is initially negative and incrementally increases each switching cycle. This solves the large inrush current and poor monotonicity characteristic of voltage-mode startup, which relies entirely on the outer voltage loop and cannot enforce cycle-by-cycle current limits during the initial transient.
Light-Load and DCM Management
Sanken Electric’s 2026 patent implements current detection via a zero-crossing signal and a ramp voltage generator that charges a capacitor with a feedback current proportional to output load. The ramp slope becomes steeper near no-load, enabling accurate detection of very low current levels — a key capability for telecom rectifiers that must efficiently handle the full range from standby to full load. This addresses a fundamental limitation of voltage-mode control at light load: switching frequency rises to impractically high values, whereas current-mode control can terminate switching cycles earlier and naturally reduce average power. Infineon Technologies Austria’s 2023 patent on synchronous rectifier sensing further closes this loop by detecting discontinuous conduction mode (DCM) on the secondary side, providing the information needed for comprehensive current-mode management across all load conditions — an approach aligned with efficiency standards tracked by organisations including IEC.
In LLC resonant converters above 300 W, the resonant current waveform is approximately sinusoidal with a peak value much higher than the trapezoidal waveform of a standard half-bridge at equal power. Voltage-mode control at these power levels is prone to losing control after disturbance, which can destroy the power switches — a risk that cycle-by-cycle current-mode control eliminates, as documented by Zhejiang Yaeneng Energy Technology (2016).
Head-to-Head: Voltage-Mode vs. Current-Mode LLC Control
The table below consolidates the attribute-by-attribute comparison derived from the patent and literature dataset, covering the dimensions most relevant to telecom rectifier design engineers. Every entry is traceable to a specific source document.
| Attribute | Voltage-Mode (PFM) | Current-Mode (Peak/Slope) |
|---|---|---|
| Feedback signals | Output voltage only | Output voltage (outer) + resonant/primary current (inner) |
| Transfer function | Complex double-pole, operating-point-dependent | Inner loop linearises plant; outer loop sees simplified characteristic |
| Dynamic response | Limited by compensator bandwidth; degrades off design point | Faster; inner loop rejects resonant tank disturbances within one switching cycle |
| Overcurrent protection | Reactive (frequency clamp or external OCP) | Cycle-by-cycle enforcement via peak current reference |
| Short-circuit behaviour | High resonant current peak-to-average ratio; high switching losses | Direct current limiting; controlled short-circuit current waveform |
| Light-load operation | Frequency rises to impractically high values | Early cycle termination; natural power reduction |
| Startup | Large inrush possible; monotonicity poor | Slope-compensated ramp enables smooth, current-limited startup |
| Mains ripple rejection | Limited; relies on output capacitor | Higher low-frequency DC gain provides better mains-ripple rejection |
| Implementation cost | Lower; single sensing point | Higher; requires tank/primary current sensing and signal processing |
| Wide-range stability | Degrades significantly across operating range | Consistent closed-loop performance across entire operational range |
The Wuhan University paper (2018) provides the theoretical underpinning for the stability row: reducing the full seven-state LLC large-signal model to a tractable second-order representation using the rectifier current state shows that the inner current loop adds damping to the resonant poles, converting an oscillatory second-order system into a well-damped one. This is not a marginal improvement — it is a structural change in the closed-loop pole locations that makes the system robust to operating point variation in a way that compensator tuning alone cannot achieve under voltage-mode control.
Need to map the competitive patent landscape for LLC converter control? PatSnap Eureka surfaces prior art, assignee trends, and claim analysis in minutes.
Analyse LLC Patents in PatSnap Eureka →Key Players and the Industry Shift Toward Current-Mode LLC
The patent and literature dataset — spanning over 40 documents from more than a dozen assignees — reveals a clear migration from pure voltage-mode PFM control toward current-mode schemes, driven by tightening efficiency and dynamic performance requirements of ETSI-class telecom rectifiers and growing adoption of digital controllers that can implement slope computation and adaptive ramp compensation.
ON Semiconductor / Semiconductor Components Industries
ON Semiconductor appears with multiple filings (2021, 2024) covering slope-based peak current-mode LLC controllers, indicating sustained investment in this control paradigm for high-volume power supply markets including telecom and server power supplies. Their approach of computing primary current slope during the switch on-time — rather than relying on a fixed ramp — represents a step toward predictive current-mode control that adapts to resonant state changes in real time.
Infineon Technologies Austria AG
Infineon is heavily represented with patents on synchronous rectifier control (2018, 2023) covering burst-mode synchronous rectifier management and full-bridge LLC voltage regulation. Their synchronous rectifier controller addresses current sensing on the secondary side to detect DCM, which is directly relevant to current-mode control implementations that must handle both CCM and DCM conditions across the load range typical of telecom rectifiers.
Texas Instruments
Texas Instruments’ multiple filings (2018–2024) focus on primary-side current monitoring for burst-mode control. Their burst-mode controller monitors primary-side current to control off-time thresholds — an approach that brings current sensing into the burst-mode regulation loop that voltage-mode controllers typically handle with only output voltage hysteresis, representing a hybrid path toward full current-mode operation.
Chinese Telecom and Server Power Ecosystem
Great Wall Power Technology (2024) and Shenzhen Megmeet Electric (2024) represent the Chinese telecom and server power supply manufacturing ecosystem, with patents that directly implement and productise current-mode LLC control for server and telecom applications. Zhejiang Yaeneng Energy Technology’s 2016 patent on using the UC3846 current control chip for LLC converters above 300 W demonstrates that current-mode LLC control has been pursued in this ecosystem for nearly a decade, predating the more recent digital implementations.
Academic institutions — LG Electronics’ Power Electronics Team, Wuhan University, Northwestern Polytechnical University, and Sungkyunkwan University — provide the theoretical foundations, modelling frameworks, and comparative performance validation that underpin the industrial implementations. The trend visible across the data is a clear migration from pure voltage-mode PFM control toward current-mode schemes, coupled with growing adoption of digital controllers (DSP, MCU) that can implement slope computation and adaptive ramp compensation that analog current-mode circuits approximate. This trajectory aligns with broader power electronics research trends documented by organisations including IEEE and efficiency frameworks from IEC.