SR vs Diode Rectification in LLC EV Chargers — PatSnap Eureka
Synchronous Rectification vs. Diode Rectification in LLC EV Onboard Chargers
A patent-backed technical comparison of secondary-side rectification methods in LLC resonant converters — covering conduction losses, V2X capability, control complexity, and key industry innovators from 50+ patent records spanning 2015–2026.
How Diode and Synchronous Rectification Work in LLC Converters
In traditional LLC resonant converter designs for EV onboard chargers (OBCs), the secondary side employs a full-bridge or center-tap diode rectifier. The transformer secondary winding feeds a diode bridge that passively converts the high-frequency AC signal into regulated DC output for battery charging. As described by Sungkyunkwan University (2019), a dual-integrated LLC resonant converter uses a six-pulse diode rectifier, enabling multiple switching patterns to cover wide output voltage ranges while keeping the rectification stage passive and straightforward.
The inherent limitation of diode rectification is conduction loss. Each diode introduces a forward voltage drop — typically 0.6–1.0 V for silicon diodes — which, at high output currents characteristic of fast EV charging, results in significant power dissipation. Mitsubishi Electric's 2018 patent specifically identifies the recovery of rectifying diodes on the secondary side as a source of surge voltage propagating to the primary resonance reactor, requiring dedicated surge suppression components. This reverse-recovery behavior is a well-documented source of electromagnetic interference (EMI) and switching losses.
Synchronous rectification replaces the passive diodes with actively controlled MOSFET switches. When a MOSFET is turned on at the correct moment, current flows through its low on-resistance channel (R_DS(on)) rather than across a diode junction, dramatically reducing conduction loss. The University of British Columbia's homopolarity-based SR patents ensure that the MOSFET channel — not its body diode — carries the current for the majority of the conduction interval, directly targeting the dominant loss mechanism. According to U.S. Department of Energy EV charging efficiency guidelines, secondary-side losses are among the most impactful factors in overall charger system efficiency.
Diode rectifiers are inherently unidirectional. Renault's OBC control patents reference a diode bridge linking the transformer secondary to the battery — an architecture that inherently precludes reverse power flow. Kia's OBC control patents explicitly reference detecting "the conduction time of a diode" in the rectifier as a timing reference for controlling the switching frequency of the LLC converter's primary side — a technique that leverages the diode's passive behavior as a diagnostic signal. Learn more about PatSnap's industry-specific analytics for power electronics innovation.
Loss Mechanisms and Patent Activity: By the Numbers
All data derived from patent filings and peer-reviewed publications in the PatSnap Eureka dataset spanning 2015–2026.
Conduction Loss: Diode vs SR Mechanisms
Diode rectification incurs a fixed 0.6–1.0 V forward drop; SR loss scales with R_DS(on) × I², which is substantially lower at typical EV charging currents.
Key Patent Assignees: LLC OBC Rectification (2015–2026)
Hyundai/Kia leads in LLC OBC control patents; UBC holds the most focused SR methodology portfolio; Nissan drives bidirectional charger IP.
Diode Rectification vs. Synchronous Rectification: Parameter Comparison
All parameters sourced directly from patent filings and peer-reviewed literature in the PatSnap Eureka dataset.
| Parameter | Diode Rectification | Synchronous Rectification |
|---|---|---|
| Conduction loss | High — junction forward drop 0.6–1.0 V Higher | Low — R_DS(on) × I² Lower |
| Control complexity | None — passive operation Simpler | High — timing-critical gate drive Complex |
| Cost | Low Lower | Higher — gate drivers, DSP, sensing Higher |
| Bidirectionality | No — inherently unidirectional None | Yes — native V2G/V2H/V2L support Native |
| Reverse recovery | Present — EMI and surge voltage EMI risk | Absent — controllable turn-off Eliminated |
| ZCS capability | Passive natural only | Active, controllable ZCS turn-on Active |
| Thermal stress | Higher at heavy load Higher | Lower, better distributed Lower |
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Why Synchronous Rectification Is Essential for V2G and V2X Operation
Diode rectifiers are fundamentally one-way devices. The shift to V2G, V2H, and V2L operation requires active secondary-side control that only synchronous rectification can provide.
Diode Bridges Cannot Support Reverse Power Flow
Diode rectifiers block reverse current by design. Renault's OBC patents reference a diode bridge linking the transformer secondary to the battery — an architecture that inherently precludes reverse power flow. The Laboratoire des Sciences du Numérique de Nantes (2021) confirms that V2X operation requires bidirectional capability in the DC-DC stage, making diode-only secondary stages incompatible with V2G/V2H/V2L without full hardware redesign. See PatSnap's EV technology solutions for sector-specific intelligence.
V2X requires hardware redesign with diode stagesTransistor-Diode-Capacitor Parallel Structures
Nissan's DC-DC Converter for Bi-Directional Charger patents (France, 2019 and 2021) illustrate the practical hybrid approach: each rectifier leg comprises a transistor, a diode, and a capacitor in parallel. The transistor enables synchronous rectification and bidirectional operation, the diode provides the passive freewheeling path, and the capacitor supports ZVS. A cancellation device eliminates the capacitor's effect in V2G mode — demonstrating the additional circuit complexity required for full bidirectionality while maintaining soft-switching performance in both directions.
Transistor + diode + capacitor per legPFM, PWM, and PSM Require Active Secondary Bridges
The Ecole Centrale de Nantes (2023) review covers Pulse Frequency Modulation (PFM), Pulse Width Modulation (PWM), and Phase-Shift Modulation (PSM) for bidirectional LLC converters. All of these advanced modulation strategies apply to both the primary and secondary active bridges — the synchronous rectifier side becomes an active inverter in V2X mode. A converter using passive diode rectification cannot participate in these control strategies. The IEC standards body increasingly addresses bidirectional EV charging interfaces.
SR side becomes active inverter in V2X modeTsinghua's CLLLC: Active Rectification on Both Sides
Tsinghua University (2023) confirms the trend: the CLLLC topology uses active rectification on both sides and achieves consistent working characteristics in both forward and reverse operations due to its symmetrical structure and good soft-switching characteristics — a configuration that would be incompatible with passive diode rectification on the secondary side. The symmetrical design enables the same resonant tank parameters to be leveraged in both charging and discharging directions. Explore how leading OEMs use PatSnap for power electronics R&D.
Incompatible with diode secondary stagesThe Non-Trivial Challenge of Synchronous Rectifier Timing in LLC Converters
LLC converters operate with variable frequency and sinusoidal-like resonant currents, making zero-crossing detection and gate timing the central engineering challenge for SR implementation.
UBC Homopolarity Concept
The University of British Columbia's SR patents use a homopolarity cycle concept: one rectifier switch is turned ON when both inverter and rectifier voltages are simultaneously positive, and the other when both are negative. This ensures the MOSFET channel — not its body diode — carries current for the majority of the conduction interval, directly reducing the dominant loss mechanism identified in their patent family (2020–2023).
Eaton's Shunt Resistor Approach
Eaton Intelligent Power Limited's 2024 WO patent generates driving signals for synchronous rectifier switches S1–S4 using current signals measured from shunt resistors — one per switch pair — to precisely determine when each switch should conduct and when it should turn off. This per-switch sensing eliminates timing ambiguity caused by variable-frequency resonant current waveforms in LLC converters, addressing the core shoot-through risk.
Key Patent Assignees Driving LLC Rectification Innovation
From automotive OEMs to academic research groups, these organisations define the state of the art in LLC converter rectification for EV onboard chargers.
Hyundai Motor Company / Kia
The most prolific patent assignee in LLC-based OBC control, with multiple active US patents covering topology switching (2023, 2024), startup behavior (2020), and resonance-frequency optimisation using diode conduction timing as feedback. Their work targets both unidirectional and efficiency-optimised OBC architectures. The PatSnap Analytics platform tracks their full IP portfolio in real time.
5+ active US patents in LLC OBC controlUniversity of British Columbia
Holds a family of patents specifically on synchronous rectification of LLC converters using the homopolarity concept (CA 2023; US 2021; US 2020), representing the most focused academic-originated IP portfolio on SR methodology for LLC converters in this dataset. Their primary motivation — explicitly stated in the patents — is conduction loss reduction through ensuring MOSFET channel conduction rather than body diode fallback.
4 patents — most focused SR methodology portfolioNissan Motor Company
Contributes a series of bidirectional LLC charger patents from France (2019, 2021), all centred on enabling reverse operation through transistor-diode-capacitor parallel structures in the rectifier bridge. Their designs demonstrate the practical compromise required for real-world bidirectional OBCs: multi-function circuit elements that simultaneously serve as synchronous rectifier, freewheeling path, and ZVS capacitor. The EPA's EV charging standards increasingly require bidirectional capability.
3 patents — bidirectional charger focusEaton Intelligent Power Limited
Represents the industrial power electronics supplier perspective, filing a WO patent (2024) on shunt-resistor-based SR timing control for full-bridge LLC converters. Their approach — one shunt resistor per switch pair — provides per-switch current sensing that precisely determines conduction windows, eliminating the timing ambiguity inherent to variable-frequency LLC operation. Renault S.A.S. separately focuses on system-level OBC control combining Vienna rectifier front ends with LLC DC-DC stages using diode bridge secondary rectification. Explore the PatSnap Open API to integrate this data into your own tools.
WO 2024 — shunt resistor SR timingLLC Rectification in EV OBCs — Key Questions Answered
Diode rectification produces higher conduction losses due to fixed forward junction voltage drops of 0.6–1.0 V. Synchronous rectification uses MOSFET R_DS(on) to significantly reduce these losses. Experimental results from National Taiwan University of Science and Technology demonstrated efficiency improvements averaging 0.4625% in charging mode and 1.097% in discharging mode using adaptive synchronous rectification.
No. Diode rectifiers are inherently unidirectional and block reverse current. Architectures using diode-only secondary stages cannot support reverse energy transfer without hardware redesign. Synchronous rectifiers, by contrast, are fully controllable in both directions, enabling V2G, V2H, and V2L operation natively.
LLC converters operate with variable frequency and sinusoidal-like resonant currents, making the secondary current zero-crossing difficult to detect precisely. Turning on a synchronous rectifier switch too early or too late causes either cross-conduction (shoot-through) or reversion to body-diode conduction, negating the efficiency benefit. Eaton's 2024 patent solves this via per-switch shunt resistor current sensing, while NTUST's adaptive approach uses V_DS sensing.
The reverse-recovery behavior of standard p-n junction diodes is a well-documented source of electromagnetic interference (EMI) and switching losses. Mitsubishi Electric's 2018 patent specifically identifies the recovery of rectifying diodes on the secondary side of the transformer as a source of surge voltage that propagates to the primary resonance reactor, necessitating dedicated surge suppression diodes to protect the transformer.
Nissan's bidirectional charger patents illustrate a practical hybrid approach: the rectifier is formed of a full bridge of four parallel structures, each comprising a transistor, a diode, and a capacitor in parallel. The transistor enables synchronous rectification and bidirectional operation, the diode provides the passive freewheeling path, and the capacitor supports ZVS. A cancellation device is included to eliminate the capacitor's effect when operating in V2G mode.
Hyundai Motor Company and Kia emerge as the most prolific patent assignees in LLC-based OBC control. The University of British Columbia holds a focused family of patents on synchronous rectification using the homopolarity concept. Nissan Motor Company contributes bidirectional LLC charger patents. Renault focuses on system-level OBC control, and Eaton Intelligent Power Limited represents the industrial power electronics supplier perspective with shunt-resistor-based SR timing control.
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References
- Synchronous rectification of LLC converters based on homopolarity — The University of British Columbia, 2021
- Synchronous rectification of LLC converters based on homopolarity — The University of British Columbia, 2020
- Synchronous rectification of LLC converters based on homopolarity — The University of British Columbia, 2023
- Synchronous rectification of LLC converters based on homopolarity — The University of British Columbia, 2020
- An Adaptive Synchronous Rectification Driving Strategy for Bidirectional Full-Bridge LLC Resonant Converter — National Taiwan University of Science and Technology, 2021
- Method for synchronous rectification of a DC-to-DC converter — Eaton Intelligent Power Limited, 2024
- In-vehicle charger and surge-suppression method for in-vehicle charger — Mitsubishi Electric Corporation, 2018
- DC-DC Converter for Bi-Directional Charger — Nissan Motor Co. Limited, 2019
- DC-DC Converter for Bi-Directional Charger — Nissan Motor Co. Limited, 2019
- DC-DC Converter for Bi-Directional Charger — Nissan Motor Co. Limited, 2021
- Method and system for controlling on-board charger of vehicle — Hyundai Motor Company, 2020
- Method and system for controlling on-board charger of vehicle — Hyundai Motor Company, 2018
- Apparatus and method for controlling LLC resonance converter — Hyundai Motor Company, 2024
- Apparatus and method for controlling LLC resonance converter — Hyundai Motor Company, 2023
- LLC resonance converter and charging system having the same — Hyundai Motor Company, 2020
- Method for controlling a charging device on board an electric or hybrid vehicle — Renault S.A.S., 2021
- Method for controlling a charging device on board an electric or hybrid vehicle — Renault S.A.S., 2019
- LLC DC-DC Converter Performances Improvement for Bidirectional Electric Vehicle Charger Application — Laboratoire des Sciences du Numérique de Nantes, 2021
- Review on Modeling and Control Strategies of DC–DC LLC Converters for Bidirectional Electric Vehicle Charger Applications — Ecole Centrale de Nantes / LS2N, 2023
- A Novel Dual Integrated LLC Resonant Converter Using Various Switching Patterns for a Wide Output Voltage Range Battery Charger — Sungkyunkwan University, 2019
- System and method for improving efficiency of EV chargers with different open circuit voltages — Indian Institute of Technology Delhi, 2021
- Bidirectional CLLLC Resonant Converter Based on Frequency-Conversion and Phase-Shift Hybrid Control — Tsinghua University, 2023
- LLC resonant converter topologies and industrial applications — A review — Guangdong University of Technology, 2020
- IEEE — Power Electronics Standards and Publications — Institute of Electrical and Electronics Engineers
- U.S. Department of Energy — Electric Vehicle Charging Efficiency Guidelines — DOE Office of Energy Efficiency & Renewable Energy
- IEC — International Electrotechnical Commission EV Charging Standards — IEC TC 69
- EPA — Electric Vehicle Charging Infrastructure Standards — U.S. Environmental Protection Agency
All data and statistics on this page are sourced from the references above and from PatSnap's proprietary innovation intelligence platform.
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