Hubbry Logo
Active rectificationActive rectificationMain
Open search
Active rectification
Community hub
Active rectification
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Active rectification
Active rectification
Active rectification
View on Wikipedia
from Wikipedia
Voltage drop across a diode and a MOSFET. The low on-resistance property of a MOSFET reduces ohmic losses compared to the diode rectifier (below 32 A in this case), which exhibits a significant voltage drop even at very low current levels. Paralleling two MOSFETs (pink curve) reduces the losses further, whereas paralleling several diodes won't significantly reduce the forward-voltage drop.

Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches, usually power MOSFETs or power bipolar junction transistors (BJT).[1] Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5 to 1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop.

Historically, vibrator-driven switches or motor-driven commutators have also been used for mechanical rectifiers and synchronous rectification.[2]

Active rectification has many applications. It is frequently used for arrays of photovoltaic panels to avoid reverse current flow that can cause overheating with partial shading while giving minimum power loss. It is also used in switched-mode power supplies (SMPS).[1]

Motivation

[edit]
Plot of power dissipated vs. current in four devices.

The constant voltage drop of a standard p-n junction diode is typically between 0.7 V and 1.7 V, causing significant power loss in the diode. Electric power depends on current and voltage: the power loss rises proportional to both current and voltage.

In low voltage converters (around 10 volts and less), the voltage drop of a diode (typically around 0.7 to 1 volt for a silicon diode at its rated current) has an adverse effect on efficiency. One classic solution replaces standard silicon diodes with Schottky diodes, which exhibit very low voltage drops (as low as 0.3 volts). However, even Schottky rectifiers can be significantly more lossy than the synchronous type, notably at high currents and low voltages.

When addressing very low-voltage converters, such as a buck converter power supply for a computer CPU (with a voltage output around 1 volt, and many amperes of output current), Schottky rectification does not provide adequate efficiency. In such applications, active rectification becomes necessary.[1]

Description

[edit]
Active full-wave rectification with two MOSFETs and a center tap transformer.

Replacing a diode with an actively controlled switching element such as a MOSFET is the heart of active rectification. MOSFETs have a constant very low resistance when conducting, known as on-resistance (RDS(on)). A typical modern MOSFET has an on-resistance of less than an ohm, with many high-performance devices having specifications of on the order of 10 mΩ or less.[3] The voltage drop across the transistor is then much lower, causing a reduction in power loss and a gain in efficiency. However, Ohm's law governs the voltage drop across the MOSFET, meaning that at high currents, the drop can exceed that of a diode. This limitation is usually dealt with either by placing several transistors in parallel, thereby reducing the current through each individual one, or by using a device with more active area (on FETs, a device-equivalent of parallel).

The control circuitry for active rectification usually uses comparators to sense the voltage of the input AC and open the transistors at the correct times to allow current to flow in the correct direction. The timing is very important, as a short circuit across the input power must be avoided and can easily be caused by one transistor turning on before another has turned off. Active rectifiers also clearly still need the smoothing capacitors present in passive examples to provide smoother power than rectification does alone.

Using active rectification to implement AC/DC conversion allows a design to undergo further improvements (with more complexity) to achieve an active power factor correction, which forces the current waveform of the AC source to follow the voltage waveform, eliminating reactive currents and allowing the total system to achieve greater efficiency.

MOSFET-based ideal diode

[edit]
Not to be confused with Ideal diode equation.

A MOSFET actively controlled to act as a rectifier—actively turned on to allow current in one direction but actively turned off to block current from flowing the other direction—is sometimes called an ideal diode.

Using these ideal diodes rather than standard diodes for solar electric panel bypass, reverse-battery protection, or bridge rectifiers reduces the amount of power dissipated in the diodes, improving efficiency and reducing the size of the circuit board and the weight of the heat sink required to deal with the power dissipation.[4][5][6][7][8][9]

Such a MOSFET-based ideal diode is not to be confused with an op-amp based super diode, often called a precision rectifier.

Construction

[edit]

See H-bridge.

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Active rectification

View on Grokipedia
from Grokipedia
Active rectification, also known as synchronous rectification, is a technique that replaces passive in circuits with actively controlled switches, such as MOSFETs or other low-loss semiconductors, to emulate behavior while minimizing voltage drops and conduction losses during AC-to-DC conversion. This approach enables higher efficiency, particularly in low-voltage, high-current applications, by synchronizing the switch conduction with the direction of current flow, typically using control circuits that detect voltage or current thresholds to manage turn-on and turn-off timing. The adoption of active rectification gained prominence in the late as switching designers sought to meet demands for higher in compact, high-performance converters, building on advancements in technology that allowed for reduced on-resistance compared to forward voltages. Unlike passive rectification, which relies on the inherent properties of s and suffers from fixed voltage drops (typically 0.7 V for s), active methods can achieve effective forward drops as low as 50-100 mV, leading to gains of 1-5% in typical topologies like flyback or forward converters. Key challenges include precise gate driving to prevent body conduction or shoot-through currents, often addressed through self-driven, current-sense, or voltage-sense control schemes. Active rectification finds widespread application in modern power systems, including switch-mode power supplies (SMPS), DC-DC converters for and , resonant converters in , and high-efficiency adapters for . It is particularly valuable in multi-output flyback designs to improve cross-regulation by mitigating effects from parasitic inductances, and in mains rectification (e.g., 110/230 V AC at 50/60 Hz) where integrated GaN-based active rectifier diodes enable compact, cost-effective solutions with forward currents up to several amperes. Emerging implementations also extend to chargers and inverters. In modern EV charging stations supporting bidirectional Vehicle-to-Grid (V2G) functionality, Active Front-End (AFE) rectifiers are commonly used. These AFE rectifiers are voltage source converters (typically three-phase) employing PWM (Pulse Width Modulation) control to enable bidirectional power flow, unity power factor correction, low harmonic distortion, and grid support features. PWM techniques, such as sinusoidal or space vector modulation, control the active switches (e.g., IGBTs or SiC MOSFETs) to allow power transfer from grid to vehicle (G2V charging) and from vehicle to grid (V2G discharging). This topology is prevalent in research and emerging commercial bidirectional AC and DC chargers for V2G applications. In renewable energy inverters, bidirectional power flow and further loss reduction are critical.

Fundamentals

Definition and Principles

Active rectification, also known as synchronous rectification, is a power electronics technique that converts (AC) to (DC) by replacing traditional passive diodes with actively controlled switches, such as MOSFETs, to reduce conduction losses and improve overall efficiency. This method is particularly effective in applications requiring high efficiency at low output voltages, where the inherent limitations of diode-based rectification become significant. The fundamental principle of rectification involves transforming an AC input waveform into a pulsating DC output by allowing current to flow only during the positive half-cycles of the input signal, effectively blocking the negative half-cycles. In passive rectification using silicon diodes, this process incurs a forward voltage drop, typically around 0.7 V, which represents a fixed loss regardless of load current and contributes substantially to power dissipation, especially at high currents. Active rectification mitigates this by employing switches that are turned on precisely during the conduction periods of the positive half-cycles, presenting a very low on-state resistance (often in the milliohms range) and achieving a near-zero voltage drop across the device during forward conduction. The switches are turned off during non-conduction periods to prevent reverse current flow, ensuring unidirectional power transfer similar to a diode but with minimized losses. Practical implementation of active rectification became prominent in the late 1990s, enabled by advancements in power MOSFET technology that allowed for low-voltage, high-current switching devices to overcome the efficiency constraints of diode rectifiers in emerging low-voltage power supplies. A key metric for understanding the efficiency gains is the power loss during conduction, which for a diode rectifier is approximated by the equation Ploss=Vf⋅IavgP_{\text{loss}} = V_f \cdot I_{\text{avg}} where VfV_f is the forward voltage drop (e.g., 0.7 V for silicon diodes) and IavgI_{\text{avg}} is the average forward current. In contrast, active switches reduce VfV_f to a value determined primarily by the device's on-resistance multiplied by the current (I2RDS(on)I^2 R_{\text{DS(on)}}), resulting in significantly lower losses at moderate to high currents, though careful control is required to manage switching timing and avoid body diode conduction.

Comparison to Passive Rectification

Passive rectification utilizes , such as or Schottky types, arranged in half-wave or full-wave bridge configurations to convert (AC) to (DC). diodes typically exhibit a forward of approximately 0.7 V, while Schottky diodes offer a lower drop of about 0.3 V. These voltage drops result in conduction losses proportional to the forward voltage times the current (V_f × I), which become substantial in high-current scenarios. In contrast, active rectification employs actively controlled devices, such as MOSFETs, to replace , reducing the effective to primarily ohmic losses given by I² × R_DS(on), where R_DS(on) is the on-resistance, often in the milliohm range. This approach yields significant improvements, particularly in low-voltage (under 5 V) and high-current applications, where losses can be reduced by 50–90% compared to passive methods. For instance, in a low-voltage AC-to-DC conversion at 1.5 A using Schottky , passive rectification incurs about 1.5 W of losses due to a 0.5 V drop per across two , whereas an active MOSFET-based configuration with 12 mΩ R_DS(on) limits losses to 54 mW—a 27-fold reduction. In higher-current automotive applications at 100 A, passive conduction losses reach 74 W, dropping to 37 W with active rectification using 2.8 mΩ MOSFETs, a roughly 50% improvement. Active rectification also provides superior performance in voltage regulation, as it can maintain stable output by actively managing current flow, unlike passive systems limited by fixed diode characteristics. Ripple is reduced in active setups through rapid blocking of reverse currents, halving RMS ripple current and easing demands on output capacitors compared to passive bridges. Thermally, the lower power dissipation in active rectification—such as a mere 13 mV forward drop—results in reduced heat generation and improved management, avoiding the excessive temperatures associated with diode losses. The primary limitations of passive rectification stem from its inefficiency at high currents and low voltages, where diode voltage drops represent a large fraction of the output voltage, leading to high power dissipation and necessitating active alternatives for modern high-efficiency designs. For example, at 1 kW output in an , passive losses total 152 W versus 32 W for active, highlighting a 75–79% reduction that underscores the shift toward active methods in demanding applications.

Operation and Control

Synchronous Switching Mechanism

In synchronous rectification, active switches such as replace traditional to achieve higher by providing a low-resistance conduction path during forward current flow while blocking reverse current. The mechanism operates by turning the switch on precisely during the period of forward current flow, allowing current to flow through the channel with minimal , and turning it off before current reversal to prevent reverse conduction. This approach minimizes the losses associated with the intrinsic body of the , which would otherwise conduct if the switch were not actively controlled. Timing synchronization is essential for effective operation, aligning the switch activation with the points of zero drain-source voltage or current reversal to ensure conduction only when current flows forward. Voltage sensing across the switch or in the circuit detects these points, triggering turn-on slightly before the forward conduction starts to account for delays and turn-off shortly before the current reverses. In full-bridge configurations, this precise timing prevents shoot-through, where simultaneous conduction of opposing switches could cause short circuits and excessive losses. In synchronous rectification, the rectifier processes the input waveform, typically a pulsed AC from the transformer secondary in SMPS topologies, converting it to a pulsating DC output. During the forward conduction phase, the low on-resistance of the switch maintains the output voltage close to the input voltage with minimal drop, reducing conduction losses; in the reverse phase, precise turn-off prevents unwanted conduction through the body diode. This results in higher average DC output voltage compared to passive methods, particularly beneficial in low-voltage applications. Switching losses, primarily from charging and discharging the MOSFET's output CossC_{oss}, are modeled as Pswitch=12CossV2fsw,P_{switch} = \frac{1}{2} C_{oss} V^2 f_{sw}, where VV is the drain-source voltage and fswf_{sw} is the switching . Accurate timing control minimizes these losses by reducing overlap between voltage and current during transitions, ensuring the switch operates near zero-voltage or zero-current conditions.

Control Strategies

Control strategies for active rectification, also known as synchronous rectification, focus on precisely timing the switching of active devices to minimize conduction losses while preventing cross-conduction between complementary switches. These strategies rely on feedback mechanisms to synchronize the rectifier's operation with the input , achieving improvements over passive rectification by reducing voltage drops during forward conduction. Sensing methods are essential for detecting optimal switching instants in active rectifiers. Voltage sensing across the switch, particularly the drain-to-source voltage (V_DS), enables zero-voltage detection by monitoring body conduction, allowing the to turn on when the voltage drops below a threshold, typically around -0.7 V, to initiate synchronous operation. Current complements this by measuring current through the secondary-side switch, primary-side , or resonant , providing data for adaptive timing adjustments, though it introduces additional complexity and resistive losses. Self-oscillating approaches, often employed in resonant converters, generate switching signals intrinsically from circuit oscillations without external sensors, simplifying but limiting adaptability to varying conditions. Control topologies vary based on the drive mechanism for the synchronous switches. Self-driven topologies utilize auxiliary transformer windings to generate gate signals, offering simplicity and no need for external power, but they produce non-ideal waveforms that can lead to suboptimal timing under load variations. Controller-driven topologies employ dedicated integrated circuits (ICs), such as the UCC24610, which use V_DS sensing and adaptive algorithms to precisely control gate drive, supporting frequencies up to 600 kHz and automatic light-load management for broader applicability. Adaptive control extends these by dynamically adjusting switching based on load conditions, using digital signal processing (DSP) or microcontrollers to optimize performance across operating ranges. Dead-time management is critical in these strategies to avoid cross-conduction, where both switches conduct simultaneously, potentially causing shoot-through currents. Typical dead times range from 20 to 100 ns, optimized through algorithms that monitor switch voltages or currents to insert minimal delays, ensuring the body conducts briefly only when necessary. Adaptive dead-time control, often implemented via or threshold-based regulation, adjusts this interval in real-time to balance efficiency and safety, minimizing body losses that can otherwise reduce overall performance. Advanced techniques, emerging in the 2000s for high-power applications, incorporate predictive control using DSP to anticipate switching events and reduce harmonics. These methods employ (MPC) schemes, such as cancellation predictive control in three-phase PWM rectifiers, which extract fundamental components via low-pass filtering and compensate for delays, achieving low (THD) under unbalanced conditions without phase-locked loops. Such DSP-based approaches enable harmonic mitigation and improved power quality in demanding systems.

Circuit Implementations

MOSFET-Based Ideal Diode

The MOSFET-based ideal diode serves as a common implementation in active rectification, where an N-channel or P-channel replaces a conventional to achieve a significantly lower forward while maintaining reverse blocking capability. The basic circuit employs a single MOSFET connected in series with the load, accompanied by a dedicated (IC), such as the LM74700-Q1, to control switching. The MOSFET's intrinsic body provides initial reverse current blocking when the gate is off, preventing unintended conduction in the reverse direction. For enhanced protection in automotive or industrial applications, back-to-back MOSFET configurations can be used to isolate the body during reverse bias. In OR-ing setups for power supply redundancy, multiple ideal diode circuits are paralleled, allowing the highest input voltage source to supply the load while others remain isolated, with switchover times as low as 0.75 µs to minimize disruptions. MOSFET selection for these circuits prioritizes low on-state drain-source resistance, R_{DS(on)}, typically below 10 mΩ, to ensure high by limiting conduction losses—for instance, at 10 A, a 10 mΩ R_{DS(on)} results in only 1 W of loss compared to several watts with a . The drain-source voltage rating, V_{DS}, must exceed the system's maximum operating voltage by a safety margin, often at least twice the nominal value to accommodate transients like load dumps in automotive systems. ruggedness is critical, as the MOSFET must dissipate energy from inductive spikes without failure; devices rated for ISO 7637-2 pulses (e.g., up to 150 V) are preferred to enhance reliability in harsh environments. Examples include the DMT6007LFG , which offers low R_{DS(on)} and robust characteristics suitable for such applications. Operationally, the applies a voltage of 10-20 V to fully enhance the channel, minimizing R_{DS(on)} and achieving a forward drop of 20-50 mV, far superior to the 300-600 mV of passive diodes. Turn-on is initiated by an internal sensing the V_{DS} voltage; when the exceeds the by the threshold (e.g., 50 mV), the gate is driven high to conduct current with near-zero loss. In reverse conditions, the driver rapidly turns off the (recovery time <1 µs), relying on the body diode or back-to-back setup for blocking. This synchronous control mechanism, often integrated in the driver IC, ensures minimal leakage (<1 µA) and fast response to polarity changes. A precision ideal diode circuit can utilize an operational amplifier (op-amp) for gate control, particularly in low-voltage scenarios where tight regulation of the forward drop is needed. In this setup, the op-amp (e.g., configured as a comparator or regulator) monitors the voltage across the —input versus output—and adjusts the gate voltage to maintain a drop of 10-20 mV, effectively emulating an ideal diode with high accuracy. The circuit typically features a P-channel for positive rail protection, with the op-amp powered from the input supply and including feedback resistors for offset adjustment. This approach, while more component-intensive than IC-based drivers, offers flexibility for custom thresholds and has been noted in designs for alternator rectification to achieve voltage drops as low as 0.1 V at 30 A.

Alternative Device Configurations

Active rectification can employ devices other than MOSFETs to address specific requirements in high-voltage, medium-to-high power, or specialized applications. Insulated-gate bipolar transistors (IGBTs) are commonly used for medium- to high-power levels, typically from 1 kW to 10 kW or more, where they provide robust performance in synchronous rectification setups. In full-bridge synchronous rectifier configurations, six IGBTs form a PWM rectifier that replaces diode bridges, enabling active power factor correction and harmonic mitigation while handling medium voltages effectively. IGBTs support voltage ratings up to 1.7 kV, making them suitable for industrial and utility-scale systems, though they incur higher switching losses compared to unipolar devices due to tail current during turn-off. For high-frequency applications exceeding 100 kHz, wide-bandgap semiconductors such as gallium nitride (GaN) and silicon carbide (SiC) transistors offer superior performance over traditional silicon devices. GaN-based synchronous rectifiers, often integrated as active rectifier diodes, achieve efficiencies above 95% by minimizing conduction and switching losses, allowing for compact designs with reduced passive component sizes. Similarly, SiC transistors enable operation at elevated frequencies in boost or interleaved configurations, supporting applications like resonant converters where fast switching reduces electromagnetic interference. Hybrid configurations, combining diodes with transistors such as IGBTs or SiC MOSFETs, reduce costs by leveraging passive diodes for freewheeling paths while using active switches only where necessary, as seen in matrix rectifiers for bidirectional power flow. Since the early 2010s, wide-bandgap semiconductors have emerged as key enablers in active rectification for electric vehicle (EV) chargers, delivering peak efficiencies exceeding 98% in totem-pole or Vienna rectifier topologies. As of 2025, advancements in SiC and GaN technologies have pushed efficiencies beyond 98% in advanced topologies like hybrid Vienna rectifiers for EV charging, enabling higher power densities.

Applications and Benefits

Power Conversion Systems

Active rectification, particularly through synchronous rectification, plays a crucial role in enhancing the efficiency of DC-DC converters by replacing passive diodes with actively controlled switches such as MOSFETs on the secondary side. In buck converters, this technique minimizes conduction losses during the output rectification phase, allowing for higher power densities in point-of-load applications. Similarly, boost and flyback topologies benefit from secondary-side synchronous rectification, where the MOSFET is timed to conduct only during the freewheeling period, reducing voltage drops and enabling operation at low output voltages like 3.3V or below. Overall, these implementations can improve converter efficiency by 1-2% compared to diode-based rectification, especially under high-load conditions. In AC-DC adapters, active rectification is integral to power factor correction (PFC) stages, where full-bridge or boost configurations with synchronous MOSFETs ensure compliance with harmonic standards while boosting efficiency. For instance, in laptop chargers rated at 90W, active PFC bridges rectify and shape the input current waveform, achieving power factors near unity and efficiencies exceeding 90% across universal input ranges of 85-265V AC. This approach not only reduces input current distortion but also lowers thermal dissipation in compact designs. Integration of active rectification in isolated converters often relies on opto-couplers to provide galvanic isolation for gate drive signals, preventing noise coupling between primary and secondary sides while enabling precise synchronous switching. A representative case is in 48V telecom power supplies, where forward or LLC topologies with synchronous rectification on the output stage deliver peak efficiencies of 96%, supporting high-reliability distributed power architectures in data centers. The widespread adoption of synchronous rectification in computing power supplies began in the late 1990s, driven by the need for higher efficiencies in desktop and server systems to handle increasing processor power demands without excessive heat generation. By the early 2000s, it had become standard in isolated DC-DC modules for these applications, often incorporating brief voltage-mode control strategies to optimize timing.

Renewable Energy Integration

Active rectification enhances the performance of solar photovoltaic (PV) systems by employing active diodes, such as MOSFET-based ideal diode configurations, to prevent reverse current flow in panel arrays under partial shading or mismatch conditions. This approach mitigates hot spots, which occur when shaded cells are reverse-biased and dissipate power as heat, potentially damaging modules or reducing overall array output. Distributed power electronics integrated with active rectification dynamically isolate underperforming sections, maintaining forward current paths and improving system reliability without the voltage drop limitations of passive bypass diodes. In microinverters for solar PV, synchronous rectification replaces traditional diode bridges, reducing conduction losses and achieving efficiency improvements, particularly under varying irradiance levels. This is especially beneficial in distributed PV setups where microinverters optimize power extraction at the module level. For instance, dual-mode rectifiers in PV microinverters enable seamless transitions between synchronous and diode modes, boosting peak efficiencies to over 98% while minimizing thermal stress. As of 2024, gallium nitride (GaN)-based synchronous rectification in microinverters has further improved efficiency in high-voltage applications. Bidirectional active rectification is essential in wind turbine systems and electric vehicle (EV) applications, facilitating controlled power flow in grid-tie inverters and regenerative braking. In wind energy, active front-end rectifiers convert variable-frequency AC from generators to stable DC, enabling maximum power point tracking (MPPT) and bidirectional operation for grid synchronization or energy storage. Similarly, in EVs, these rectifiers support regenerative braking by reversing power flow from motors to batteries, with topologies like back-to-back converters achieving high efficiency in bidirectional modes. Integration of MPPT algorithms with synchronous rectifiers in PV charge controllers optimizes energy harvest from intermittent sources, dynamically adjusting duty cycles to track the maximum power point while minimizing switching losses. Post-2015 developments have led to PV charge controllers achieving high efficiencies through advanced techniques including multiphase synchronous rectification, ANN-based tracking, and SEPIC converters for enhanced PV utilization. Active front-end rectifiers in renewable systems can support compliance with IEEE 1547 standards for grid interconnection, helping to ensure low harmonic distortion, voltage ride-through, and stable power injection during faults.

Electric Vehicle Charging and Vehicle-to-Grid (V2G)

In modern EV charging stations supporting bidirectional Vehicle-to-Grid (V2G) functionality, Active Front-End (AFE) rectifiers are commonly used. These AFE rectifiers are voltage source converters (typically three-phase) employing PWM (Pulse Width Modulation) control to enable bidirectional power flow, unity power factor correction, low harmonic distortion, and grid support features. PWM techniques, such as sinusoidal or space vector modulation, control the active switches (e.g., IGBTs or SiC MOSFETs) to allow power transfer from grid to vehicle (G2V charging) and from vehicle to grid (V2G discharging). This topology is prevalent in research and emerging commercial bidirectional AC and DC chargers for V2G applications.

Challenges and Considerations

Efficiency Analysis

Active rectification enhances power conversion efficiency by replacing passive diodes with actively controlled MOSFETs, which exhibit lower conduction voltage drops and negligible reverse recovery charge. The primary loss components in synchronous rectifier systems include conduction losses, switching losses, and residual reverse recovery effects. Conduction losses dominate at high currents and are calculated as Pcond=Irms2RDS(on)P_{cond} = I_{rms}^2 R_{DS(on)} per MOSFET, where IrmsI_{rms} is the root-mean-square current through the device and RDS(on)R_{DS(on)} is the drain-source on-resistance, typically in the range of 1-10 mΩ for low-voltage applications. In a full-bridge rectifier configuration, with two MOSFETs conducting per half-cycle, the total conduction loss approximates Pcond,total=2Iout2RDS(on)P_{cond,total} = 2 I_{out}^2 R_{DS(on)} (assuming sinusoidal or DC output currents and negligible transformer effects), significantly lower than the VFIoutV_F I_{out} drop (0.5-1 V) in diode-based systems. Switching losses arise from gate driving and parasitic capacitances, given by Pgate=fswQgVgP_{gate} = f_{sw} Q_g V_g for gate charge dissipation and Poss=12CossVin2fswP_{oss} = \frac{1}{2} C_{oss} V_{in}^2 f_{sw} for output capacitance energy, where fswf_{sw} is the switching frequency, QgQ_g the total gate charge (often 10-50 nC), VgV_g the gate drive voltage (4-12 V), CossC_{oss} the output capacitance (100-500 pF), and VinV_{in} the input voltage. Reverse recovery losses, a major issue in passive rectification due to diode stored charge QrrQ_{rr} (up to 100 nC), are minimized in active systems as the MOSFET channel bypasses body diode conduction with precise timing, reducing this term to near zero under ideal control. The overall efficiency for a full-bridge active rectifier is derived as η=PoutPout+Pcond,total+Psw,total+Pother\eta = \frac{P_{out}}{P_{out} + P_{cond,total} + P_{sw,total} + P_{other}}, where Pout=VoutIoutP_{out} = V_{out} I_{out}, Psw,totalP_{sw,total} sums gate and capacitance losses across four devices (approximately 4fsw(QgVg+12CossVin2/2)4 f_{sw} (Q_g V_g + \frac{1}{2} C_{oss} V_{in}^2 / 2)), and PotherP_{other} includes minor leakage and magnetics losses; this yields η>95%\eta > 95\% at moderate loads when conduction dominates. Empirical measurements in low-output-voltage applications (<5 V, >10 A) demonstrate 2-5% gains over passive rectification, attributed to the reduced conduction drop; for instance, synchronous schemes achieve 93% at 5 V output compared to 88-90% with Schottky diodes, with peak performance at 50-80% load current where conduction and switching losses balance. curves versus load current show synchronous rectifiers maintaining >90% across wider ranges, peaking 2-3% higher than diode equivalents at full load. In one server power supply example, replacing Schottky diodes with 3.3 mΩ MOSFETs yielded a 3.5% improvement at 3.3 V standby output. Key influencing factors include and . RDS(on)R_{DS(on)} exhibits positive temperature dependence, increasing 0.35-0.5% per °C, often doubling from 25°C to 125°C (e.g., from 2.75 mΩ to 4.13 mΩ), which elevates conduction losses by 50-100% and degrades by 1-2% at high temperatures without cooling. Switching frequency impacts losses linearly through Psw∝fswP_{sw} \propto f_{sw}, favoring lower frequencies (<500 kHz) for in conduction-limited designs, though higher fswf_{sw} (1-2 MHz) reduces component size at the cost of 1-3% drop.

Design Trade-offs

The addition of gate drivers, controllers, and associated components in active rectifier designs significantly increases the bill of materials (BOM) cost, often by 20-50% compared to passive diode-based systems, due to the need for power switches like MOSFETs and supporting circuitry. This added complexity is generally justified only in low-voltage applications below 10 V combined with high currents, where the efficiency improvements—such as reduced conduction losses—provide substantial benefits that offset the higher upfront expenses. Reliability in active rectifiers is challenged by arising from rapid switching transitions, which can induce and require networks to dampen transients and protect components. Critical modes include shoot-through events, where simultaneous conduction of complementary switches leads to destructive short circuits, underscoring the importance of precise timing control to insert adequate dead time and prevent overlap. These issues demand robust practices to ensure long-term operation in demanding environments. A primary lies in selecting self-driven versus fully controlled rectification: self-driven schemes leverage windings or auxiliary sources for gate signals, offering simplicity and reduced component count but limited adaptability to load or input variations, whereas controlled approaches enable dynamic optimization at the expense of greater design effort and cost. Another key consideration balances power rating against switching performance, with MOSFETs favored for low-power, high-frequency applications due to their low on-resistance and fast switching, while IGBTs are preferred in high-power systems for superior voltage and current handling despite higher switching losses. Mitigation strategies include adopting integrated synchronous rectification ICs, which emerged prominently in the 2000s and consolidate gate drive, sensing, and control functions into single chips, thereby minimizing external components, PCB space, and overall system complexity. These ICs also enhance reliability by incorporating built-in protections against overcurrent and adaptive dead-time adjustments.

References

  1. https://www.ti.com/lit/wp/snva595a/snva595a.pdf
  2. https://ieeexplore.ieee.org/document/9954160
  3. https://www.ti.com/lit/pdf/SSZTD98
  4. https://www.powerelectronicsnews.com/how-synchronous-rectification-enhances-power-converter-performance-and-efficiency/
  5. https://www.st.com/resource/en/application_note/an604-calculation-of-conduction-losses-in-a-power-rectifier-stmicroelectronics.pdf
  6. https://www.vishay.com/docs/98280/howtocalculatepowerlossesingen5diodes.pdf
  7. https://www.electronics-tutorials.ws/diode/schottky-diode.html
  8. https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics
  9. https://www.ti.com/lit/pdf/sluab04
  10. https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/joe.2018.8241
  11. https://www.ti.com/lit/pdf/slvaf88
  12. https://www.analog.com/en/resources/design-notes/synchronous-rectification-aids-lowvoltage-power-supplies.html
  13. https://www.ti.com/lit/pdf/slup175
  14. https://www.infineon.com/dgdl/Infineon-MOSFET_OptiMOS_device_selection_for_synchronous_rectification-ApplicationNotes-v01_00-EN.pdf?fileId=8ac78c8c85ecb34701865afe0f7d3f05
  15. http://power.eecs.berkeley.edu/publications/acker_sullivan_sanders_adaptive_timing.pdf
  16. https://vtechworks.lib.vt.edu/bitstream/handle/10919/30316/chapter2.pdf
  17. https://fscdn.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/buck_converter_efficiency_app-e.pdf
  18. https://www.mdpi.com/1996-1073/14/8/2298
  19. https://vtechworks.lib.vt.edu/bitstream/handle/10919/103384/Yu_ON_D_2021.pdf
  20. https://ieeexplore.ieee.org/document/162670
  21. https://www.ti.com/product/UCC24610
  22. https://www.onsemi.com/download/data-sheet/pdf/ncp4318-d.pdf
  23. https://doi.org/10.1109/IECON.2006.347994
  24. https://www.ti.com/lit/an/slvae57b/slvae57b.pdf
  25. https://www.diodes.com/assets/App-Note-Files/AN1193_App-Note_Ideal-Diode-controller-FET-selection.pdf
  26. https://www.analog.com/media/en/technical-documentation/data-sheets/4357fd.pdf
  27. https://core.ac.uk/download/pdf/188647148.pdf
  28. https://www.researchgate.net/publication/4357172_Toward_the_Ideal_Diode_using_power_MOSFET_in_full_wave_synchronous_rectifiers_for_low_voltage_power_supplies
  29. https://ieeexplore.ieee.org/document/1179309/
  30. https://ieeexplore.ieee.org/document/7309444/
  31. https://ieeexplore.ieee.org/document/7369258/
  32. https://ieeexplore.ieee.org/document/10988539
  33. https://www.ti.com/download/trng/docs/seminar/Topic_1_Cohen_Lu.pdf
  34. https://www.mouser.com/pdfdocs/enDM00044787.pdf
  35. https://www.analog.com/en/resources/technical-articles/reducing-the-size-and-complexity-of-an-isolated-synchronous-gate-driver.html
  36. https://www.powerelectronicsnews.com/gan-brings-improved-efficiency-to-solar-microinverters/
  37. https://www.mdpi.com/1996-1073/14/21/7173
  38. https://ieeexplore.ieee.org/document/9446820
  39. https://etd.ohiolink.edu/acprod/odb_etd/ws/send_file/send?accession=wright152721348332911&disposition=inline
  40. https://www.researchgate.net/publication/252053085_Full_bridge_converter_with_synchronous_rectifiers_for_low_output_voltage_application
  41. https://pdfs.semanticscholar.org/41c2/e7da0c3ad723e4f5e21c2d2ef7ae0302bb5c.pdf
  42. https://www.digikey.com/en/blog/synchronous-rectifiers-improve-power-supply-efficiency
  43. https://www.diodes.com/assets/App-Note-Files/dn94.pdf
  44. https://www.analog.com/en/resources/technical-articles/mosfet-power-dissipation-calculation-in-highpower-supply.html
  45. https://eepower.com/technical-articles/hybrid-anpc-inverter-with-synchronous-rectification/
  46. https://lutpub.lut.fi/bitstream/10024/168744/1/mastersthesis_heikkila_teemu.pdf
  47. https://www.edn.com/trends-in-synchronous-rectification-for-chargers-and-adapters/
  48. https://www.st.com/en/power-management/synchronous-rectification-controllers.html
Add your contribution
Related Hubs
User Avatar
No comments yet.