Flyback converter with a stepped air-gap transformer

This paper presents the analysis and design of gallium nitride (GaN) devices based flyback converter. It is critical to optimize power loop and gate loop for flyback converter due to high dv/dt of GaN device. EPC devices with 200V/12A were used in flyback converter. Because GaN device has no avalanche voltage and is sensitive to gate voltage, one need pay more attention to design of gate driver and selection snubber circuits. Different snubber circuits for GaN based flyback converter were compared in this paper. Magnetics selection for high frequency (HF) transformer was discussed in view of size, frequency, turn ratio and loss. A detailed power loss breakdown has been executed with core loss, cooper loss, leakage inductance loss, conduction loss and switching loss. Performance, such as current/voltage stress, voltage spike, efficiency, ect., were analyzed and compared under continuous conduction mode (CCM), boundary conduction mode (BCM), discontinuous conduction mode (DCM), and BCM with valley switching. Experimental results show highest efficiency is achieved under BCM with valley switching.

Flyback converters exhibit the advantages of simple structure, low cost, small volume, isolation, and control simplicity. However, flyback converters demonstrate disadvantages like leakage inductance; hence, losses are incurred. Losses pertain to the requirement of an external cooling system and external costs. It also results in low efficiency and the use of flyback converters in low-power applications. The discontinuous conduction mode (DCM) is used for low-power applications to achieve efficiency. In flyback converters, the reverse recovery losses of the diode are eliminated with DCM. There are many applications of flyback converters in industrial systems, such as led drivers, battery charge systems, telecommunication systems. In this study, analysis and design of a flyback converter for universal input and wide load ranges was performed. The detailed theoretical analysis was conducted and verified with the implementation of a DCM flyback converter for 85 V-265 V RMS AC input voltage, 12 Vo output, 100 kHz switching frequency, and 48 W output power. The overall efficiency of the proposed converter at full load was measured as 81%. Additionally, external cooling systems were not required, which highlighted the development of a compact design with small volume and ease of application for an industrial system that requires the inclusion of more serial flyback converters. Cite this article as: F. Gökçegöz, E. Akboy, A. H. Obdan, “Analysis and Design of a Flyback Converter for Universal Input and Wide Load Ranges”, Electrica, vol. 21, no. 2, pp. 235-241, May, 2021.

The flyback converter is widely used in modern power conversion systems due to its simple topology, single switch, low component count, and inherent galvanic isolation. This paper investigates the performance characteristics of flyback converters by analyzing their operation in Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM), with particular emphasis on voltage spike behavior. The main contribution of this study is the development of a mathematical model that correlates the operating mode (CCM or DCM) with the voltage spike across the power switch. In addition, the study examines the impact of duty cycle variations on voltage spike behavior. The results show that, for comparable operating conditions and moderate duty cycles, CCM operation generally results in lower voltage spikes than DCM due to reduced peak magnetizing current, while at higher duty cycles and load levels, CCM may exhibit increased voltage stress. The proposed approach was validated through MATLAB/Simulink simulations and a scaled-down experimental prototype. A proportional integral (PI) controller is implemented in simulation to ensure regulated output voltage under practical operating conditions. The results demonstrate that optimizing the duty cycle and accounting for the influence of conduction mode can enhance the reliability of flyback converters, making them suitable for applications requiring galvanic isolation and cost-effective solutions.

The use of power factor correction (PFC) converters is widely discussed and considered for most off-line power supplies. For low power applications, boost PFC converters operated in discontinuous conduction mode (DCM) or boundary conduction mode (BCM) mode are usually employed. The main advantage of BCM mode is the reduction of reversal recovery loss of diode and simple control, especially at the high-line light-load condition. In this paper, a digital-controlled method for boost PFC operating under boundary conduction mode without input voltage sensing is proposed. The digital PI controller voltage is used to determine the constant on-time period and the comparator block inside the DSP controller is employed to detect the zero-crossing of inductor current for turning on the main switch. Therefore, the size and cost are reduced significantly. A 200W boost PFC prototype is implemented and tested to verify the feasibility of the proposed control scheme.

The derivation and implementation of natural switching surface (NSS) control for a flyback converter operating in boundary conduction mode (BCM) is the main focus of this paper. The flyback converter has been wildly utilized in the area of power electronics and BCM operation has been proven to be successful in attaining high efficiencies. The NSS presents many benefits for the control of non-linear systems such as fast transient response under load-changing conditions. The NSS control technique has previously been implemented in non-isolated (e.g., buck and boost converters) and isolated (dual active bridge) converter topologies demonstrating excellent performances. The analytical derivation of the proposed switching surfaces is presented and validated through simulations using MATLAB/Simulink™ and a 65W prototype was experimentally tested.

Primary side regulation (PSR) technology has the characteristics of lower cost and standby power losses. However, there is no algorithm suitable for discontinuous conduction mode (DCM) and continuous conduction mode (CCM) operations of primary side control for flyback converter because of the difficulty in estimating the output voltage correctly. In this paper, a PSR control algorithm suitable for DCM/CCM flyback converter is proposed and implemented with digital micro controller. The voltage drop on the secondary winding and output diode at CCM operation is compensated by sensing the secondary side current at the instant when primary side switch is being turned-on. The secondary side current is zero at the voltage sensing instant and equal to the current at the instant when primary side switch is being turned-on as flyback operated at DCM. In addition, the proposed algorithm can be used to regulate either the output voltage or current very well. An experimental prototype with 150 W rated power is built to verify the feasibility of the proposed control. Experimental results show that the output voltage can be controlled with error less than 0.25%, and the output current error is less than 5%. The conversion efficiency is up to 92.8%.

Due to the simple structure and low cost, the primary-side-regulation (PSR) flyback converter is widely used in small power applications. As the output voltage is sampled from auxiliary winding in PSR flyback converter, some sampling error exists, especially when continuous conduction mode (CCM) is introduced for higher power. To improve the constant voltage accuracy, a novel digital controlled sampling strategy is proposed. This scheme uses a digital ramp sampling method to sample output voltage in both discontinuous conduction mode (DCM) and CCM, and proposes a voltage adjustment strategy to adjust sampling errors in CCM without weakening the stability in the load-switch process. Finally, the experimental results validate the presented scheme and show that the output voltage error is less than 0.6% in a 20-V, 5-A PSR flyback prototype.

Pulse regulation, a fixed frequency control technique, is introduced and applied to flyback converter operating in discontinuous conduction mode (DCM). The control parameters are designed in a way that the converter operates as close as possible to the critical conduction mode. In contrast to the conventional pulsewidth modulation control scheme, the principal idea of pulse regulation is to achieve output voltage regulation using high and low-power pulses. Pulse regulation is simple, cost effective, and enjoys a fast dynamic response. The proposed technique is applicable to any converter operating in DCM. However, this work mainly focuses on flyback topology. In this paper, the main mathematical concept of the new control algorithm is introduced and simulations as well as experimental results are presented.

A non-isolated three-phase high step-up DC–DC converter suitable for renewable energy systems

This paper presents design considerations for a high frequency (≥1 MHz) Discontinuous Conduction Mode (DCM) flyback converter. As switching frequency increases, the problems such as switching loss and undesirable effects due to the parasitic capacitance of switching device are exacerbated. Considering these issues, design instructions are proposed about DC link capacitance, PCB transformer, snubber, magnetizing inductance, switch, output capacitance and turn ratio. Following the instructions, 1-MHz 65-W flyback converter was implemented. Its output voltage was regulated to 19 V in DC 100-373 V input voltage. The experimental results and loss breakdown analysis are given. To observe the effect of variation in switching device capacitance, Si and SiC MOSFET switches are tested. The switching waveforms and the efficiency of the two cases are compared.

This article describes a general method for studying the behavior of converters with three discontinuous conduction modes as automatic power factor correctors (PFCs). Some converters with a single discontinuous conduction mode (such as Buck-Boost, Flyback, SEPIC, Ćuk, and Zeta converters) can behave as automatic PFCs when operating in that discontinuous conduction mode, with a nearly constant duty cycle during each cycle of the line voltage. There are also dc/dc converters that, due to multiple diodes and inductors, can operate in different discontinuous conduction modes. A systematic method for studying these three discontinuous conduction modes was recently presented. This article extends that method to using the aforementioned dc/dc converters as part of an ac/dc converter with low harmonic injection in the line, identifying when the converters behave as “ideal automatic resistor emulators” (i.e., configuring ideal automatic PFCs) and when they behave as “quasi-ideal automatic resistor emulators” (i.e., as quasi-ideal automatic PFCs). As an example, the article examines a SEPIC converter used as a resistor emulator. In this case, three discontinuous conduction modes are possible, if the inductance of the input inductor is low enough to allow the input bridge rectifier diodes to stop conducting when the transistor is <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small>. The study allows us to determine of the line current waveform when the converter operates in any of the possible discontinuous conduction modes. The results show that quasi-ideal automatic PFC behavior can be achieved in discontinuous conduction modes with low inductance values. Moreover, the results for the SEPIC can be easily extended to the Ćuk converter operating in the same manner. Finally, the theoretical predictions for the line current from the proposed study were verified through simulation using PSIM (in the case of both the SEPIC and Ćuk topologies), and through experimentation (in the case of SEPIC).

The derivation and implementation of the natural switching surfaces (NSS) considering parametric uncertainties for a flyback converter operating at boundary conduction mode (BCM) is the main focus of this paper. The NSS presents many benefits for the control of non-linear systems; for example fast transient response under load-changing conditions. However, the performance considerable worsens when real parameters of the converter are slightly different from the designed ones. Therefore, a novel control strategy that considers the effect of parameter uncertainties is presented. This control law can estimate and adapt the control trajectories in one switching action to obtain excellent performances even under extreme parameter uncertainties. The analytical derivation of the proposed adaptive switching surfaces are presented.

Flyback converter with variable-frequency peak-current-mode (VFPCM) control is widely used in low-power adaptor and recently proposed universal serial bus power delivery application. The control scheme achieves high efficiency for the entire load range by reducing the switching frequency at light load. In the present study, an accurate unified small-signal model for VFPCM control in continuous conduction mode (CCM) and discontinuous conduction mode (DCM) is proposed. Based on the model, a compensation strategy is proposed to overcome the unstable problem in a wide range of operations and give engineers insight. Experimental and simulation results verify the proposed model and compensation strategy.

에너지 효율이 낮은 백열등이나 형광등에서 에너지 효율이 높은 LED로 대체하면서 관련 규격에서는 LED 드라이버에 대하여 역률과 왜곡률에 대한 요구를 하고 있다. 이에 따라 본 논문에서의 주회로 출력측 리플 저감을 위해 1차측에 Coupled Inductor와 출력 커패시터 용량을 줄이기 위해 2차측에 LC 공진을 적용한 Coupled LC 공진 Flyback 컨버터를 사용하였으며, 역률보상(PFC, Power Factor Correction)으로 불연속전도모드(DCM, Discontinuous Conduction Mode)이 널리 사용되고 있지만, 경계전도모드(BCM, Boundary Conduction Mode)보다 스위치에 흐르는 전류의 피크값이 높아 소자들의 스트레스가 증가하는 단점이 있다. 이를 보완하기 위해서 1차측과 2차측의 피크 전류를 저감하기 위하여 듀티비 분할 방식을 제안하였다. 제안한 방식의 검증을 위해 PSIM을 이용한 시뮬레이션을 통하여 기존 제어 방식(무분할 방식)과 제안한 듀티비 분할 방식을 지연시간 및 분할 수에 따른 스위치에 흐르는 전류 피크값을 비교하였다. 시뮬레이션 결과를 비교해보면 듀티비 분할 방식이 기존 방식에 비해 전류 피크값과 출력 리플이 1차측 전류 16%, 2차측 전류 22%, 출력 리플 4%로 저감 하는 것을 확인 하였다.

This paper presents the analysis and design of an isolated single-stage converter achieving high-power-factor correction and fast regulation. By using the technique suggested by Wu et al., a buck-boost converter and a flyback converter can be integrated to form the discussed converter. The buck-boost semistage working in the discontinuous conduction mode (DCM) functions as a power-factor corrector, and the flyback semistage operating in the DCM is a voltage regulator which is controlled, theoretically, to be independent of load variation. An approximated small-signal model of the converter operating in the DCM is developed. Design of a peak-current feedback loop with an optimal proportional integral controller is also presented. A prototype is implemented to verify that the analysis and design are effective and feasible.