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Forward converter

A forward converter is an isolated DC-DC power converter topology that functions as a buck converter with galvanic isolation, converting the input voltage to a regulated output voltage (which can be lower or higher depending on the transformer turns ratio) while using a transformer to separate the input and output circuits electrically.^[1] It operates by applying the input voltage to the transformer's primary winding via a single switching transistor during the on period, inducing voltage in the secondary to deliver power to the output through a rectifier diode, filter inductor, and capacitor; during the off period, a tertiary (demagnetizing) winding resets the transformer core to prevent saturation.^[1]^[2] Developed around 1974, the forward converter is particularly suited for low- to medium-power applications, typically up to 200 W, such as in switching power supplies for telecommunications, computers, and auxiliary power units, due to its simplicity and low component count compared to more complex topologies like the full-bridge.^[2] Key advantages include continuous power transfer during the switch-on time, reduced voltage stress on the output diode relative to flyback converters, and the ability to handle higher output currents efficiently, though it is limited by a maximum duty cycle of 50% in single-ended designs to allow for transformer reset, resulting in voltage stress on the switch up to twice the input voltage.^[1]^[2] Variants, such as the two-transistor (double-ended) forward converter, mitigate some limitations by reducing switch stress to the input voltage and eliminating the need for a tertiary winding, enabling duty cycles up to 100% in certain configurations.^[2] In practice, the topology achieves high efficiency—often over 80%—through pulse-width modulation (PWM) control of the switch, with the output voltage regulated by adjusting the duty cycle according to V_o = D \cdot n \cdot V_g, where D is the duty cycle, n is the transformer turns ratio, and V_g is the input voltage.^[1] Despite its advantages, challenges include the need for careful transformer design to manage magnetizing currents and potential electromagnetic interference from high-frequency switching, typically in the range of 20–100 kHz.^[2] Modern implementations often incorporate synchronous rectification and active clamping to further improve efficiency and reduce losses, making it a foundational choice in isolated power conversion systems.^[1]

Introduction and Basics

Definition and Purpose

A forward converter is an isolated buck-derived switched-mode power supply (SMPS) that provides step-down voltage conversion while achieving galvanic isolation between input and output through a transformer.^[1]^[3] In this topology, energy is transferred from the input source to the output during the on-time of the primary switch, typically a single transistor, with the transformer's primary winding conducting the input voltage to magnetize the core and induce voltage on the secondary side.^[1] The primary purpose of the forward converter is to deliver a regulated DC output voltage suitable for applications demanding electrical isolation for safety, such as compliance with regulatory standards, and to minimize electromagnetic interference by separating input and output circuits.^[1]^[3] It excels in providing efficient power conversion at moderate levels, typically up to 200 W,^[4] where non-isolated buck converters fall short due to the need for isolation in higher-power or safety-critical systems like telecommunications equipment and industrial power supplies.^[1] Key characteristics include its single-ended topology, which employs one active switch on the primary side, and a secondary-side configuration featuring a rectifier diode to conduct during energy transfer followed by an inductive-capacitive (LC) filter to smooth the output current and voltage.^[1]^[3] The converter can operate in either continuous conduction mode (CCM), where inductor current flows continuously, or discontinuous conduction mode (DCM), depending on load conditions, offering flexibility for design optimization.^[1] This topology emerged in 1956 as an advancement over earlier non-isolated converters, enabling reliable isolated power delivery at higher power levels with the advent of suitable transistors.^[5]^[6]

Historical Development

The forward converter topology emerged in the mid-20th century as part of broader advancements in switched-mode power supplies (SMPS), with roots tracing to transformer-coupled converters developed during the 1950s. An early identifiable version of the circuit was introduced by D.A. Paynter in 1956, laying the groundwork for isolated DC-DC conversion techniques that addressed the limitations of linear regulators in efficiency and size.^[5] Subsequent refinements in the late 1950s and early 1960s involved innovations in core reset mechanisms and single-switch operation, contributed by engineers such as H.C. Dudley and teams at Philips Semiconductors, focusing on practical implementations for higher power handling.^[5] A key milestone occurred in the 1960s when patented variations proliferated, including Hewlett-Packard's US Patent 3,313,998 (1967) for a switching-regulator power supply incorporating an energy return circuit to demagnetize the transformer, which was instrumental in early computer power supplies like those for minicomputers. Other notable patents from this era, such as those by La Duca and Massey (US Patent 3,414,780, 1968) and Heinicke (US Patent 3,579,098, 1971), emphasized improved demagnetization and voltage regulation, enabling reliable operation at emerging switching frequencies. These developments solidified the forward converter's role in commercial applications requiring galvanic isolation.^[5] In the 1970s and 1980s, the topology gained widespread adoption alongside the evolution of semiconductor switches, transitioning from bipolar junction transistors (BJTs) to more efficient metal-oxide-semiconductor field-effect transistors (MOSFETs), which supported higher switching frequencies and reduced losses. This era saw its integration into distributed power architectures for telecommunication systems and industrial equipment, where its simplicity and cost-effectiveness outperformed alternatives like push-pull converters in medium-power ranges (50–200 W).^[4] The analytical frameworks established by R.D. Middlebrook, particularly in his 1976 paper on input filter considerations for switching regulators, profoundly influenced forward converter refinements by providing methods to ensure stability and mitigate interactions between the converter and its power source.^[7] By the 2000s, the forward converter evolved through integration with soft-switching techniques, such as zero-voltage switching (ZVS) and self-core reset methods, to minimize electromagnetic interference (EMI) and enable operation at frequencies above 100 kHz without excessive losses; exemplary work includes the active-clamp and resonant reset topologies proposed in early 2000s research.^[8] In the 2020s, it continues to hold relevance in renewable energy interfaces, particularly for step-down conversion in solar and wind systems where isolation and efficiency are critical.^[9]

Operating Principle

Transformer-Based Isolation

The transformer in a forward converter provides galvanic isolation between the input and output sides, preventing direct current flow while enabling alternating current energy transfer through magnetic coupling between the primary and secondary windings.^[10] This isolation is crucial for safety and noise reduction in applications requiring separation of power stages, such as telecommunications and medical equipment.^[11] During the switch-on period, the input voltage is applied across the primary winding, generating a magnetic flux in the core that induces a proportional voltage in the secondary winding, allowing forward power delivery to the output rectifier and load.^[12] Simultaneously, a magnetizing current builds up in the primary due to the transformer's inherent inductance, storing energy in the core without contributing directly to the output.^[13] To prevent core saturation from the accumulating magnetizing flux, a reset mechanism is essential during the switch-off period, typically implemented via a tertiary winding connected to a diode that returns the stored magnetizing energy to the input or dissipates it safely.^[12] Alternatively, clamp circuits, such as active-clamp topologies, can recycle this energy more efficiently by reversing the voltage across the magnetizing inductance.^[10] The transformer's windings are configured to ensure proper polarity and unidirectional power flow, adhering to the dot convention where dots mark corresponding ends for positive voltage alignment during forward conduction.^[12] Common secondary configurations include a center-tapped winding with two diodes for full-wave rectification.^[14]

Switching Stages

The forward converter operates via a periodic switching cycle controlled by a primary switch, typically a MOSFET, which alternates between on and off states to transfer energy from input to output through the transformer while maintaining isolation. This cycle ensures the transformer core does not saturate by incorporating a reset mechanism during the off period. The operation is divided into distinct stages, with transitions managed to prevent issues like shoot-through currents. In the on-state, the primary switch closes, applying the input voltage V_{in} directly across the transformer primary winding. This induces a proportional voltage on the secondary winding, forward-biasing the output rectifier diode and allowing current to flow through the output inductor to charge it and supply the load. The primary current during this phase consists of the reflected secondary current plus the transformer's magnetizing current, which ramps up linearly. No energy is stored in the core for later release, unlike in flyback converters; instead, power is delivered continuously to the output during this interval. When the switch opens in the off-state, the secondary voltage collapses, reverse-biasing the output rectifier and halting direct power transfer from the input. The output inductor current, however, continues to flow through the freewheeling diode, providing a low-impedance path that maintains supply to the load while the inductor current decays linearly. Simultaneously, the transformer's magnetizing energy must be reset to prevent core saturation; this is achieved either by a tertiary (reset) winding that discharges the energy back to the input or through a clamp circuit, such as an RCD snubber or active clamp, which dissipates or recycles the stored energy. The duty cycle is limited to less than 0.5 to ensure sufficient reset time, balancing the on and off durations. Transition periods between states include brief dead times to avoid simultaneous conduction of the switch and any clamp elements, preventing shoot-through and voltage spikes. In continuous conduction mode, the inductor current never reaches zero, sustaining smooth operation; however, in discontinuous mode at light loads, the current falls to zero before the next cycle begins, introducing a third interval of zero current that can affect regulation. Qualitatively, the primary voltage waveform is a square wave, positive during the on-state and negative (or clamped) during reset in the off-state, while the switch voltage exhibits low levels when on and peaks up to approximately twice V_{in} when off. The output inductor current forms a triangular waveform, rising during the on-state and falling during the off-state, with the average value equaling the load current; secondary currents are pulsed, rectified to produce a relatively constant output voltage with minimal ripple. The switching is typically governed by pulse-width modulation (PWM) at a fixed frequency ranging from 20 to 100 kHz, where the duty cycle modulates the on-time to regulate output voltage while adhering to the reset constraint.

Circuit Components and Design

Core Components

The forward converter relies on a set of essential active and passive components to achieve isolated DC-DC power conversion through high-frequency switching. These include the primary switching element, various diodes for rectification and protection, an output LC filter for smoothing, a control integrated circuit for regulation, and auxiliary elements for noise suppression and stability. Each component is selected based on voltage, current, frequency, and efficiency requirements to ensure reliable operation typically in the 50-500 W power range.^[15] The switching element, positioned on the primary side, is typically a power MOSFET for high-frequency applications (up to 500 kHz) due to its low on-resistance and fast switching speed, though IGBTs may be used in lower-frequency designs for higher power handling. It controls the application of input voltage to the transformer primary, enabling pulse-width modulation (PWM) operation. Gate driver requirements include a high-current totem-pole output stage capable of sourcing and sinking up to 1 A peak to charge the MOSFET gate capacitance quickly, minimizing switching losses; selection considers gate charge (Q_g) and switching frequency (f_SW) such that driver current I_OUTPUT ≈ Q_g × f_SW.^[16]^[17] Diodes play critical roles in rectification, core reset, and freewheeling. The output rectification is achieved using a rectifier diode connected to the single secondary winding, which conducts during the switch on-state to deliver power to the output, along with a freewheeling diode across the output inductor that provides a path for the inductor current during the off-state, preventing voltage spikes and ensuring continuous DC output. The clamp diode, connected to a tertiary winding or across the primary, facilitates transformer core reset by recirculating magnetizing energy, avoiding saturation. The freewheeling diode requires a voltage rating exceeding V_out, while the rectifier diode requires exceeding V_out + n V_in (typically greater than 2 V_out), to withstand reverse bias during non-conduction, typically using fast-recovery or Schottky types for low forward drop and high efficiency.^[15]^[18] The output LC filter smooths the rectified voltage to produce a stable DC output. The inductor stores energy during the on-period and releases it during off, handling peak-to-peak ripple current (typically 20-40% of output current) to maintain continuous conduction mode (CCM) at loads above 10-20%. The capacitor filters residual ripple, reducing output voltage deviation (e.g., to <1% of V_out) by absorbing AC components; selection prioritizes low equivalent series resistance (ESR) and sufficient capacitance for hold-up time. Representative values include a 27 µH inductor for 10 A output and 2000 µF capacitors for low-ripple applications.^[15]^[18]^[17] The control IC generates the PWM signal to regulate output voltage or current. Integrated circuits like the UC384x series provide fixed-frequency operation (up to 500 kHz), an error amplifier with 2.5 V reference for feedback via optocoupler, and current-mode control for inherent line regulation and cycle-by-cycle limiting, or voltage-mode for simpler implementation. These features enable precise duty cycle adjustment (typically <50% to allow reset time) while including undervoltage lockout and soft-start protection.^[17] Auxiliary elements mitigate parasitic effects and ensure electromagnetic compatibility. Snubber networks, often RC type (e.g., 50 kΩ resistor with 10 nF capacitor across the switch), absorb voltage spikes from leakage inductance and switching transients, reducing electromagnetic interference (EMI). Input capacitors, such as 180 µF electrolytic types, filter high-frequency noise from the source and stabilize bulk voltage, selected for ripple current handling and low ESR to support the converter's input dynamics.^[17]

Transformer Specifications

The turns ratio of the transformer in a forward converter, defined as n = N_s / N_p where N_p is the primary turns and N_s is the secondary turns, is selected to achieve the desired voltage step-down while optimizing the duty cycle, typically following n = V_{out} / (V_{in_{min}} \cdot D_{max}), with D_{max} limited to 0.45-0.5 to ensure sufficient reset time and avoid excessive voltage stress.^[19] For example, in a design with V_{in_{min}} = 36 V, V_{out} = 12 V, and D_{max} = 0.45, a ratio of approximately 0.74 (e.g., 8:11) may be chosen.^[20] A tertiary (reset) winding is commonly incorporated with turns equal to the primary (N_t = N_p), connected via a diode to return magnetizing energy to the input during the off period, enabling duty cycles up to 0.5 and preventing core saturation.^[3]^[21] Ferrite cores are preferred for the transformer due to their low losses at high switching frequencies (typically 20-500 kHz) in forward converters, with common shapes including E or EE types (e.g., ETD34) that provide ample window area for windings.^[19] To avoid saturation, the maximum flux density B_{max} is kept below 0.3 T, often around 0.2 T for ferrite materials like Magnetics Type P, balancing core losses with size efficiency.^[22]^[23] Winding construction employs Litz wire for the primary (e.g., 100 strands of #42 AWG) to mitigate skin effect losses at high frequencies, while the secondary may use copper foil for high-current outputs; leakage inductance is controlled to less than 1% of the magnetizing inductance through interleaved layering to minimize voltage spikes and ringing.^[19] Transformer sizing is determined by the area product Ap = A_w \cdot A_e, where A_w is the window area and A_e is the effective core area, calculated as Ap = P_o / (f \cdot \Delta B \cdot K) with K \approx 0.014 for forward converters and \Delta B the flux swing (e.g., 0.2 T); an air gap may be introduced in discontinuous conduction mode to adjust the magnetizing inductance.^[19] Thermal management focuses on limiting total losses to ensure efficiency above 90%, with core losses (hysteresis and eddy current) dominant below 200 kHz (e.g., 0.84 W at 0.14 T and 200 kHz) and copper losses from winding resistance (e.g., 1.32 W), targeting a temperature rise under 40°C via thermal resistance modeling \Delta T = R_{th} \cdot P_{loss} where R_{th} \approx 19^\circC/W.^[19]

Analysis and Equations

Voltage Relationships

In the forward converter operating under ideal conditions and continuous conduction mode (CCM), the output voltage V_{out} relates to the input voltage V_{in} through the transformer's turns ratio and the duty cycle D (where $0 < D < 0.5). This relationship derives from the volt-second balance principle applied to the output inductor L, which states that the average voltage across an ideal inductor over one switching period must be zero to maintain steady-state operation. During the switch-on time D T_s (with T_s = 1/f_{sw} the switching period), the secondary voltage reflected to the output drives current through L with voltage V_{in} \cdot (N_s / N_p), where N_s and N_p are the secondary and primary turns, respectively. During the off-time (1 - D) T_s, the inductor sees -V_{out}. Balancing the volt-seconds yields [V_{in} \cdot (N_s / N_p) - V_{out}] D T_s + (-V_{out}) (1 - D) T_s = 0, simplifying to the ideal conversion ratio V_{out} = V_{in} \cdot (N_s / N_p) \cdot D.^[1] The transformer secondary voltage V_s during the on-time equals V_{in} \cdot (N_s / N_p), directly scaling the primary input voltage by the turns ratio and providing power transfer to the output stage. This follows from ideal transformer action, where the voltage is induced proportionally across windings when the primary switch is closed, assuming no leakage or magnetizing effects dominate.^[1] To prevent transformer core saturation, the magnetizing flux must reset during the off-time via volt-second balance on the primary winding's magnetizing inductance. In a typical configuration with a tertiary reset winding of turns N_{tertiary}, the clamp voltage V_{clamp} applied during reset is V_{in} \cdot (N_p / N_{tertiary}), ensuring the reverse volt-seconds match the forward accumulation. The derivation equates forward volt-seconds V_{in} \cdot D T_s to reset volt-seconds V_{clamp} \cdot D_{reset} T_s, where D_{reset} is the reset duty, yielding D_{reset} = D \cdot (N_{tertiary} / N_p). For complete reset within the available off-time, D + D_{reset} \leq 1, constraining D \leq 0.5 when N_{tertiary} = N_p.^[1]^[24] The output voltage ripple \Delta V_{out} arises from the LC filter's response to the rectangular secondary current waveform and approximates \Delta V_{out} \approx \frac{V_{out} (1 - D)}{8 L C f_{sw}^2} under the small-ripple assumption, where L and C are the output inductor and capacitor. This derives from the inductor current ripple \Delta I_L = \frac{V_{out} (1 - D)}{L f_{sw}}, which charges the capacitor as a triangular waveform; the peak-to-peak voltage excursion is then \Delta V_{out} = \frac{\Delta I_L}{8 C f_{sw}}, neglecting ESR.^[25]

Efficiency Calculations

The efficiency of a forward converter is defined as the ratio of output power to input power, given by

\eta = \frac{P_\text{out}}{P_\text{in}} = \frac{V_\text{out} I_\text{out}}{V_\text{out} I_\text{out} + P_\text{losses}},

where P_\text{losses} represents the total power dissipated in the converter.^[26] Typical efficiencies for forward converters range from 80% to 95%, depending on design parameters such as switching frequency, load conditions, and component selection.^[12] Power losses in a forward converter can be broken down into conduction losses, switching losses, and core losses. Conduction losses primarily arise from the resistive elements in the circuit, such as the on-resistance R_\text{DS(on)} of MOSFETs and the DC resistance of windings, calculated as P_\text{cond} = I^2 R integrated over the conduction intervals (e.g., P_{S1} = I_\text{MCP}^2 R_1 D for the primary switch, where I_\text{MCP} is the magnetizing current peak, R_1 is the switch resistance, and D is the duty cycle).^[27] Switching losses occur during the transition of semiconductor devices and are approximated by P_\text{sw} = \frac{1}{2} V_\text{in} I_\text{peak} t_r f_\text{sw}, where t_r is the rise time and f_\text{sw} is the switching frequency; these losses are minimized in active-clamp topologies through zero-voltage switching.^[27] Core losses in the transformer and output inductor follow the Steinmetz equation,

P_\text{core} = k_v f_\text{sw}^\alpha B^\beta V_\text{core},

where k_v, \alpha, and \beta are material-specific coefficients (typically \alpha \approx 1.6, \beta \approx 2.6 for ferrites), B is the peak flux density, and V_\text{core} is the core volume; for example, P_\text{Tcore} = 2.2 f^{1.6} B_M^{2.6} V_\text{tran} applies to transformer cores in PC40 material.^[27]^[28] The operating mode—continuous conduction mode (CCM) versus DCM—affects efficiency through trade-offs in current ripple and loss distribution. The CCM/DCM boundary is defined when the inductor current ripple \Delta I_L = 2 I_{out}, yielding the critical output inductance L_{crit} = \frac{ V_{out} (1 - D ) }{ 2 f_{sw} I_{out} }. In CCM, the inductor current never reaches zero, resulting in lower peak currents and reduced conduction losses at high loads, but potentially higher ripple-related losses if not managed. The current ripple is given by \Delta I_L = \frac{V_{in} D}{f_{sw} L} (adjusted for turns ratio in the forward topology). In contrast, DCM reduces switching losses at light loads due to natural zero-current switching but increases RMS currents and thus conduction losses, leading to efficiency peaks in CCM at full load (e.g., >90%) and in DCM at partial loads (<50%).^[27]^[29]^[25] Efficiency optimization in forward converters often involves synchronous rectification, where output diodes are replaced by MOSFETs to reduce forward voltage drops (from ~0.7 V to ~10-50 mΩ × I). This technique can improve efficiency by 2-3% at high currents, particularly in control-driven implementations that minimize body diode conduction through precise timing, achieving up to 88% at 12 A output for a 2.5 V design.^[30]

Advantages and Limitations

Key Benefits

The forward converter topology offers high efficiency, often exceeding 80% with up to 90% or more achievable in optimized designs at moderate loads, due to its transformer coupling that enables direct energy transfer from input to output during the switch-on period, minimizing conduction losses compared to topologies like the flyback. This efficiency is further enhanced by reduced peak currents in both primary and secondary windings, thereby lowering copper losses and allowing for smaller, cooler-running components.^[10]^[31]^[4] Galvanic isolation provided by the transformer's core structure ensures safety by preventing direct electrical connection between input and output, with isolation ratings commonly reaching several kilovolts (1–4 kV) to meet stringent standards in medical and telecommunications equipment. This isolation is critical for applications requiring protection against ground loops, noise, and high-voltage transients, while maintaining reliable power delivery.^[10]^[31]^[32] The topology excels in power handling, supporting output ranges typically from 50 W to 200 W, making it suitable for medium-power systems such as distributed power architectures. Scalability is achieved through parallel output configurations or multi-phase designs, allowing higher total power without compromising performance.^[10] Control implementation is simplified by the use of a single primary switch, which reduces the complexity of feedback loops and driver circuitry relative to multi-switch bridge topologies, facilitating easier design and lower overall system cost.^[10]^[4] Output voltage regulation benefits from an effective LC filter on the secondary side, which smooths the rectified waveform to achieve ripple levels below 1% at switching frequencies above 100 kHz, ensuring stable supply for sensitive loads.^[4]^[33]^[34]

Common Drawbacks

The forward converter requires a dedicated mechanism to reset the transformer core after each switching cycle, as the topology does not inherently provide symmetrical magnetization like bridge-based designs.^[10] This typically involves an additional tertiary winding, RCD clamp, or active clamp circuit, which increases the overall parts count and introduces potential failure points, such as extra diodes or capacitors prone to thermal stress.^[10] Consequently, the maximum duty cycle is limited to less than 50%, often practically around 0.45 to account for reset time and switching transitions, restricting the converter's step-down ratio and making it less suitable for applications with wide input voltage ranges without additional stages.^[35] The primary switch in a standard single-switch forward converter experiences high voltage stress, with drain-to-source voltage spikes reaching up to twice the input voltage (2×V_IN) during core reset, particularly in third-winding configurations.^[18] This necessitates the use of higher-rated MOSFETs with greater breakdown voltage, which in turn increases conduction losses and overall design complexity through added snubbers or zero-voltage-switching adaptations. Additionally, the transformer's design, including the extra reset winding, results in a bulkier core compared to simpler topologies, elevating material costs and making the forward converter less economical for low-power applications below 50 W.^[36] Due to its reliance on hard switching, the forward converter generates significant electromagnetic interference (EMI), primarily common-mode noise from rapid voltage transitions across the switches and parasitics in the transformer and PCB traces.^[37] This often requires additional shielding, filtering components, or layout optimizations to meet regulatory standards, further complicating the design and potentially increasing system size.^[38]

Applications and Variants

Typical Uses

Forward converters are widely employed in offline power supplies for personal computers and servers, where they facilitate efficient voltage conversion in distributed power architectures, such as stepping down from 48 V intermediate bus voltages to 12 V for point-of-load regulation. This topology provides the necessary galvanic isolation while handling moderate power levels typical in these systems, often up to several hundred watts per module.^[39] In industrial settings, forward converters power DC motor drives and programmable logic controllers (PLCs) that require robust isolation to protect sensitive control circuitry from high-voltage transients.^[40] For instance, they are integrated into motor drive systems to deliver stable, isolated DC supplies for brushless DC motors, ensuring reliable operation in harsh environments.^[41] Their ability to provide clean, regulated outputs makes them suitable for PLC modules in automation equipment.^[40] Telecommunications equipment, particularly isolated power supplies for base stations, commonly utilizes forward converters to manage wide input voltage ranges of 36-72 V while delivering outputs like 12 V at high currents. These converters support the demanding efficiency and isolation requirements of telecom infrastructure, enabling reliable powering of radio frequency amplifiers and signal processing units.^[10] In medical devices, forward converters are favored for patient monitoring equipment due to their galvanic isolation, essential for patient safety under standards like IEC 60601.^[42] They provide isolated low-voltage rails for sensors and displays in devices such as electrocardiographs and vital signs monitors, minimizing electromagnetic interference risks.^[42] The adoption of forward converters in automotive applications has grown post-2020, particularly in electric vehicles (EVs) for auxiliary systems that convert high-voltage battery outputs (e.g., 400 V) to 12 V for lighting, infotainment, and control modules. This trend aligns with the increasing electrification of vehicles, where forward converters offer compact, efficient isolation in onboard chargers and power distribution units.^[43]

Modified Topologies

The active clamp forward converter addresses the demagnetization challenges of the basic topology by incorporating an auxiliary switch across a clamp capacitor, which recycles the transformer's magnetizing energy and enables zero-voltage switching (ZVS) for the main switch, thereby reducing switching losses and allowing operation at higher frequencies while achieving efficiencies exceeding 90% at full load.^[44] This modification is particularly beneficial for medium-power applications where minimizing voltage spikes and improving thermal performance are critical.^[45] The two-switch forward converter enhances the basic design by adding a second primary-side switch, which balances the reset process and clamps the voltage stress on both main switches to approximately half the input voltage, making it suitable for high input voltage scenarios without requiring oversized components.^[46] This topology reduces the risk of switch failure under elevated input conditions and simplifies the design for robust operation in variable-voltage environments.^[47] Synchronous rectification in forward converters replaces the output rectifier diodes with low-resistance MOSFETs controlled to conduct during the freewheeling period, significantly lowering conduction losses in applications with low output voltages and high currents.^[48] Self-driven schemes, where gate signals are derived from the transformer windings, further simplify implementation and boost overall efficiency in high-current outputs.^[49] Interleaved forward converters employ multiple parallel phases with phase-shifted operation to handle power levels above 1 kW, distributing the current load and reducing input/output ripple currents, which is advantageous for server power supplies requiring stable, high-density delivery. This approach also enables smaller filter components and improved transient response in multi-phase configurations.^[50] Recent variants in the 2020s leverage gallium nitride (GaN) transistors in forward converters to support megahertz switching frequencies, enabling compact designs for chargers and adapters with reduced magnetic component sizes and efficiencies approaching 95% due to lower parasitic losses.^[51] These GaN-based implementations prioritize high power density for portable and electric vehicle applications, building on active clamp techniques for soft switching.

Comparison with Other Converters

Versus Flyback Converter

The forward converter and flyback converter are both single-ended isolated DC-DC topologies, but they differ fundamentally in energy transfer mechanisms. In a forward converter, power is delivered continuously from the primary to the secondary side of the transformer during the switch on-time, utilizing the transformer as a direct energy transfer device without storing energy in the core.^[52] In contrast, the flyback converter operates by storing energy in the transformer's magnetizing inductance during the on-time and releasing it to the output during the off-time, resulting in discontinuous power delivery and higher output ripple.^[53] This direct transfer in the forward topology leads to better transformer utilization and lower peak currents compared to the flyback's storage-based approach.^[54] Regarding component count and design simplicity, the flyback converter typically requires fewer components, using the transformer itself as an inductor and needing only a single primary switch, which makes it more compact and cost-effective for low-power applications.^[34] The forward converter, however, demands an additional output inductor for energy storage and filtering, along with reset mechanisms such as a clamp circuit or diodes, increasing complexity and component count.^[52] Both topologies provide galvanic isolation through the transformer, but the forward converter's output inductor enables a filtered, continuous conduction mode output, reducing ripple.^[54] In terms of efficiency and power handling, the forward converter excels at higher power levels, typically above 50 W, where it achieves efficiencies up to 95% due to lower conduction losses and reduced ripple, making it suitable for outputs up to 200-300 W.^[54] The flyback converter is more efficient for powers below 50-60 W but suffers from higher losses at elevated powers owing to energy storage inefficiencies and increased peak currents.^[52] Additionally, the forward converter imposes lower voltage stress on the primary switch—often around Vin or 2Vin with clamping—compared to the flyback's higher stress of Vin + nVout plus leakage spikes, which can exceed 1.5 times Vin.^[34] For use cases, the forward converter is preferred in regulated, high-power applications requiring high efficiency and low output ripple, such as telecom power supplies and medium-power DC-DC modules.^[53] The flyback converter, with its simplicity and lower cost, is ideal for cost-sensitive, wide-input, low-power designs like auxiliary supplies or adapters under 50 W.^[34]

Versus Push-Pull Converter

The forward converter operates as a single-ended topology utilizing one primary switch, which simplifies the control circuitry compared to the push-pull converter's balanced configuration employing two switches on the primary side for alternating operation.^[55]^[56] This single-switch design in the forward converter enables straightforward gate driving but restricts it to unidirectional power flow during the switch-on period.^[55] In contrast, the push-pull topology achieves balanced excitation by driving the two switches out of phase, allowing the transformer core to operate in both positive and negative B-H quadrants for improved utilization.^[56]^[57] Regarding power capability, the forward converter is typically limited to medium-power applications up to approximately 200-300 W due to challenges with transformer reset mechanisms that constrain the duty cycle to less than 50% and increase voltage stress on the switch.^[55]^[56] The push-pull converter, however, supports higher power levels exceeding 500 W—often up to 1 kW—through more efficient core usage and reduced ripple currents, making it suitable for applications requiring greater output capacity without proportional increases in component size.^[55]^[56] This enhanced power handling in push-pull stems from its ability to interleave power transfer across both halves of the switching cycle, minimizing the need for oversized magnetics.^[57] Transformer usage differs significantly between the two. The forward converter requires a reset mechanism, such as a tertiary winding or active clamp, to demagnetize the core and prevent saturation, which adds complexity and potential losses from clamping circuits.^[55]^[56] Conversely, the push-pull topology employs a center-tapped primary winding without a dedicated reset winding, enabling full core flux swing and smaller overall transformer size for equivalent power ratings, though it demands precise current balancing to avoid flux walking.^[56]^[57] In terms of efficiency, the push-pull converter generally achieves higher performance in high-power scenarios due to the absence of reset-related clamp losses and steadier input currents that reduce conduction losses and filter requirements.^[55]^[57] However, its dual-switch driving introduces greater control complexity, often necessitating current-mode techniques to mitigate imbalances.^[56] The forward converter, while simpler and efficient for medium-power levels with non-pulsating output currents, may suffer minor efficiency penalties from reset energy dissipation.^[55] Comparing drawbacks, the forward converter offers easier control and implementation for its power range but can generate higher electromagnetic interference (EMI) from its single-ended operation and pulsating input current.^[57] The push-pull converter excels in providing balanced operation suitable for bipolar outputs and exhibits lower EMI due to its interleaved nature, yet it is susceptible to flux imbalance from mismatched switch timings or transformer asymmetries, potentially leading to core saturation and requiring additional sensing circuitry.^[56]^[57]

References

[1]
[PDF] DC-DC Power Converters
The forward converter, a single-transistor isolated buck converter. Forward converter. The forward converter is illustrated in Fig. 11. This transformer ...
[2]
[PDF] Forward Converter, Transformer Design, and Output Inductor Design
A description of a forward converter is that when current is flowing in the primary, there is current flowing in the secondary, and in the load. The push-pull ...
[3]
[PDF] Lecture 13: Isolated DC/DC Converters, Part 1 - MIT OpenCourseWare
Isolation in a direct converter: Single-ended forward converter. This is ... This is poor compared to the non-isolated buck where Vpk = Vin! How much ...
[4]
The History Of The Forward Converter - How2Power.com
The forward converter is a power supply topology whose history reaches back to the 1950s with the introduction of an identifiable circuit by D.A. Paynter, which ...
[5]
[PDF] A precisely regulated multiple output forward converter topology
The forward converter topology has long been used in designing the distributed power supplies in tele- communication and computer systems, because of its.Missing: history | Show results with:history
[6]
[PDF] f ,"'e,o'OI'. ;' - Ridley Engineering
R. D. Middlebrook, "Measuree.ent of Loop Cain in Feedback Systems," Int. J. Electronics,. Vol. 36, No.4, pp. 465-512, April 1975. Supplied by The British ...
[7]
[PDF] A self core reset and zero voltage switching forward converter topology
To solve these problems, soft switching techniques are normally used. Several modified forward topologies have been developed in recent years [1]–[17]. Among ...
[8]
Future Trends in DC-DC Conversion for Renewable Energy Systems
Forward converter: Offers better efficiency at higher power ... DC–DC converter for simultaneous power management of multiple different renewable energy sources.
[9]
[PDF] Operation and Benefits of Active-Clamp Forward Power Converters
Power converters based on the forward topology are an excellent choice for applications where high efficiency and good power handling capability is required in ...Missing: ended | Show results with:ended
[10]
Forward Converter Circuit Design and Analysis
Oct 14, 2019 · Compared to a bust-boost switching regulator, a forward converter takes advantage of a transformer for galvanic isolation to suppress conducted ...
[11]
[PDF] FORWARD CONVERTER
The forward-converter transformer works like a normal power transformer where both primary and secondary windings conduct simultaneously with opposing magneto ...
[12]
A Guide to Forward-mode Transformers | Coilcraft
### Summary of Forward-mode Transformers
[13]
Analysis and experimentation of a forward converter with active ...
Aug 10, 2025 · This paper proposes a study of a forward DC/DC power converter topology where the isolating transformer has its core reset by active voltage ...<|control11|><|separator|>
[14]
None
Summary of each segment:
[15]
[PDF] Fundamentals of MOSFET and IGBT Gate Driver Circuits
Fundamentals of MOSFET and IGBT Gate Driver Circuits. Figure 2. Power MOSFET Models ... the forward converter. The reset time interval limits the operating duty ...
[16]
[PDF] UCx84x Current-Mode PWM Controllers datasheet (Rev. H)
Based on calculated inductor value and the switching frequency, the current stress of the MOSFET and output diode can be calculated. The peak current in the ...
[17]
[PDF] An Improved 2-Switch Forward Converter Application
The NCP1252 offers everything needed to build a cost-effective and reliable ac-dc switching power supply. ▫ Adjustable soft start duration. ▫ Internal ramp ...
[18]
[PDF] Section 4 – Power Transformer Design - Texas Instruments
This Section covers the design of power trans- formers used in buck-derived topologies: forward converter, bridge, half-bridge, and full-wave center- tap.
[19]
https://www.ti.com/lit/ml/slup126/slup126.pdf
[20]
[PDF] maxrefdes1206-reference-design.pdf - Analog Devices
The transformer turns ratio for the forward converter topology is: IN(MIN) ... N. N. 25T. 12.5T. V. 24. = ×. = ×. = 12 turns are used for the tertiary winding.
[21]
Transformer Design with Magnetics Ferrite Cores - Mag Inc.
Bmax = Flux Density (gauss) selected based upon frequency of operation. Above 20kHz, core losses increase. To operate ferrite cores at higher frequencies, it is ...
[22]
[Forward Converter][Calculations of transformer inductances]
Sep 23, 2022 · Assume maximum flux density of 0.2T for a ferrite core. You can work out the number of primary turns from: Np=V*ton/(Bmax*Ae) Core losses ...
[23]
[PDF] Design of HF Forward Transformer Including Harmonic Eddy ...
Dec 16, 2010 · Applying volt-second balance to the magnetizing inductance voltage, the value of maximum duty cycle of the forward converter is given by [1].
[24]
[PDF] Understanding Buck Power Stages In Switchmode Power Supplies
The exact power rating of the forward converter power stage, of course, is dependent on the input voltage/output voltage combination, its operating environment ...
[25]
[PDF] Calculating Efficiency (Rev. A) - Texas Instruments
This application report explains how to calculate the dissipated power at any output voltage and thereby plot the efficiency of the converter at any output.
[26]
Power loss analysis of active clamp forward converter in continuous ...
Jul 1, 2013 · The power losses of SR MOSFETs (S3 and S4) also increase and account for a large proportion in the percentage of the converter's power loss.Missing: breakdown | Show results with:breakdown
[27]
[PDF] Improved Core-Loss Calculation for Magnetic Components ...
The parameters k, α, and β are the same parameters as used in the Steinmetz equation (1). By the use of the iGSE, losses of any flux waveform can be calculated, ...
[28]
None
Below is a merged summary of the "Forward Converter," "DCM Operation," and "Power Transfer Formulas" based on all provided segments from the DC/DC Book of Knowledge. To retain all information in a dense and organized manner, I’ll use a combination of narrative text and tables in CSV format where applicable. The information is synthesized from the various summaries, ensuring no details are lost, and cross-referenced where multiple sources provide overlapping or complementary data.
[29]
[PDF] Efficiency Study of Isolated DC-DC Converters - Chalmers ODR
Jun 14, 2019 · ... Steinmetz's equation. Phys = kfa s Bb. (2.37) where B is the magnetic flux density peak and k, a and b are material constants em- pirically ...
[30]
https://www.ti.com/lit/ml/slup175/slup175.pdf
[31]
Isolated DC-DC converters: Key features & selection tips - XP Power
More efficient than flyback converters, forward converters transfer energy directly from the input to the output through the transformer during the switch-on ...Ground Loop Elimination · Flyback Converters · Forward Converters
[32]
None
### Summary of Advantages of Forward Converter from Design Note AN 2013-03 V 1.0
[33]
Isolation Transformers: Types, Applications and Benefits
These isolation transformers offer high dielectric strength with voltage ratings ranging from 1000V to 4000V between windings, featuring robust insulation that ...
[34]
[PDF] Switch−Mode Power Supply Reference Manual - onsemi
The output filter (the output inductor and capacitor) in forward−mode topologies is used for energy storage. In boost−mode topologies, the transformer is used ...
[35]
Forward or Flyback? Which is Better? | Coilcraft
### Key Differences Between Forward and Flyback Converters
[36]
Comparison of Two-Switch and Voltage-Clamp Forward Converters
Dec 1, 2000 · Disadvantages include a limited duty ratio (less than 50%, normally ... forward converter and (b) the voltage-clamp forward converter.Missing: drawbacks | Show results with:drawbacks
[37]
How to Choose a Forward vs. a Flyback Converter - Analog Devices
May 2, 2024 · Efficiency: In general, forward converters tend to be more efficient than flyback converters, as they have less losses due to core saturation ...
[38]
https://ieeexplore.ieee.org/document/6328932
[39]
https://ieeexplore.ieee.org/document/8742542
[40]
[PDF] Isolated Supply Overview and Design Trade-Offs - Texas Instruments
The push-pull converter is essentially an inter- leaved forward converter whose primary winding is made up of a center-tapped trans- former. The push-pull ...
[41]
[PDF] Side, Digital Output Module for PLC - Texas Instruments
Nov 9, 2017 · This eight-channel, parallel, 1-A, high-side, digital output reference design for PLC shows the diagnostic ... A forward converter transformer ...<|control11|><|separator|>
[42]
[PDF] Voltage Controlled PFC Forward Converter Fed PMBLDCM Drive for ...
The proposed PFC buck forward converter is designed for a. PMBLDCM drive with main considerations on PQ constraints at AC mains and allowable ripple in DC link ...
[43]
LT8315 Datasheet and Product Info - Analog Devices
Applications. Isolated Telecom, Automotive, Industrial, Medical Power Supplies; Isolated Off Line Housekeeping Power Supplies; Electric Vehicles and Battery ...
[44]
Analysis and implementation of an active clamp ZVS forward converter
This paper presents a ZVS forward converter using an auxiliary switch and clamp capacitor to reduce voltage stress and achieve zero voltage switching.Missing: topology | Show results with:topology
[45]
High-Efficiency Active-Clamp Forward Converter With Transient ...
In this paper, an active-clamp forward converter with transient current build-up zero-voltage switching (ZVS) technique is proposed.Missing: topology | Show results with:topology
[46]
A New Soft-Switching Two-Switch Forward Converter - IEEE Xplore
Especially, voltage stress across main switches can be clamped at 1/2 Vin, voltage stress across auxiliary switch can be clamped at Vin. In addition, due to ...
[47]
Two-switch flyback-forward PWM DC-DC converter with reduced ...
This paper introduces a two-switch flyback-forward pulse-width modulated (PWM) DC-DC converter along with the steady-state analysis, simplified design ...
[48]
https://ieeexplore.ieee.org/document/4460226/
[49]
https://ieeexplore.ieee.org/document/1216781/
[50]
Design and Implementation of an Interleaved Forward Converter ...
In this paper, an interleaved forward converter with magnetizing energy recycled is implemented. The interleaved forward converter used for hardware ...Missing: higher | Show results with:higher
[51]
Design and Implementation of an Active-Clamp Forward Converter ...
A dc-to-dc converter using GaN power device is proposed. Active clamped forward technique is adopted in conjunction with synchronous rectification.
[52]
High Frequency, High Efficiency, and High Power Density GaN ...
Oct 15, 2025 · Using Gallium-Nitride (GaN)-based power switches in this converter merits more and more switching frequency, power density, and efficiency.
[53]
When the Flyback Converter Reaches Its Limits - Analog Devices
Nov 1, 2020 · For the same power, a forward converter needs a smaller transformer than a flyback converter does. This makes the forward converter ...Missing: differences | Show results with:differences
[54]
https://www.ti.com/lit/pdf/ssztc11
[55]
[PDF] Excuse me for being a little forward... with my topology
The key advantages that the forward converter has over a flyback include better transformer utilization, lower peak currents and a filtered output.Missing: differences | Show results with:differences<|control11|><|separator|>
[56]
AC/DC Converter Topologies - RECOM Power
Here is an excerpt from our AC/DC Book of Knowledge covering flybacks, snubbers, clamps, resonant modes, forward converters, and poly-phase supplies.
[57]
Two- and four-switch power conversion topologies
Dec 9, 2021 · The two-switch design caps the maximum voltage stress on the switches and eliminates clamping losses, reducing thermal stress and boosting ...