Entire Line Cycle Research Articles

A High-Frequency Power Factor Correction Stage With Low Output Voltage

Single-phase power factor correction (PFC) stages would ideally have three features—operation over the entire line cycle, soft switching at high frequency for all powers and voltages, and a reasonably low output voltage. These features permit high power factor, high combination of density and efficiency, and reduced stress on subsequent stages. Most PFC stages, including the most commonly used, achieve only a subset of these features. Here, we present control innovations to allow a PFC stage to operate from universal input (90–265 Vac) over the entire line cycle with zero-voltage switching (ZVS) with a reduced output voltage. These features are verified with a prototype with very high power factor (> 0.99), high efficiency (98%), and high density (80 W/in3). The PFC can provide an output of 200 Vdc from the entire universal input voltage range which greatly reduces the conversion stress required from subsequent stages. The proposed PFC control consists of a blended feedforward/feedback method that provides additional advantages like guaranteed soft switching, the ability to correct for input capacitance, and the ability to interface with a high-ripple output bus. Therefore, the proposed PFC provides advantages in efficiency and density both for the PFC stage and the overall system.

Improvement of ZVS Range and Current Quality of the Nine-Switch Single-Stage AC–DC Converter

The nine-switch single-stage three-phase ac-dc isolated converter integrates a three-phase active front-end ac-dc converter and a three-phase high-frequency galvanically isolated dc-dc converter in one stage, leading to a device count reduction of 25% compared to an equivalent two-stage configuration. The advantage of the nine-switch converter can be truly realized if it can achieve similar efficiency and input current quality as a two-stage converter. In this article, a design methodology is proposed, which ensures zero-voltage switching of the bottom and the middle switches of the nine-switch converter for the entire line cycle and also reduces its input current total harmonic distortion below 5%. By implementing space vector pulsewidth modulation, the dc-link voltage of the nine-switch converter is reduced to the level of an equivalent two-stage converter. The analytical claims are verified with a 3-kW laboratory prototype operating at 400-V ac input, 800-V dc-link voltage, and 380-V isolated dc output voltage.

A PWM ZVS High-Frequency-Link Three-Phase Inverter With T-Type NPC Unfolder

In this article, a pulsewidth modulated single-stage high frequency link three-phase dc–ac converter is proposed for grid integration of solar and fuel cell-based energy sources. On the dc side, the converter has three half-bridge legs which are zero-voltage switched over the entire line cycle without using any additional snubber circuit. On the ac side, two diode bridge rectifiers and one neutral point clamped (NPC) T-type three-level inverter are employed. The active switches in NPC inverter are switched either at line frequency or twice of it, incurring negligible switching loss. Modulation ensures soft commutation in the diode bridges. The intermediate three-level dc bus is pulsating and does not require any dc-filter capacitor and thus reduces overall filtering requirements. The high frequency galvanic isolation provides high power density, low-cost converter solution. The converter operation is analyzed in detail and verified on a 2-kW hardware prototype.

A Single-Stage Soft-Switched Isolated Three-Phase DC–AC Converter With Three-Phase Unfolder

This article proposes a unidirectional, single-stage, three-phase, and high-frequency-link dc–ac converter. The converter uses three high-frequency transformers to provide galvanic isolation. In the primary, the converter has three half-bridge legs. In the secondary, three diode-bridge rectifiers are used. Each rectifier is followed by a half-bridge (unfolder) leg. The modulation strategy proposed in this article ensures zero-voltage switching of all six primary side switches over the entire line cycle without additional snubber circuit. The secondary half-bridge legs are switched at line frequency, incurring negligible switching loss. The proposed converter does not require any interstage bulky dc-link filter capacitor. Thus, the overall filtering requirement is reduced, and the converter reliability is improved. The high-frequency galvanic isolation improves the converter power density. In this article, the modulation strategy and converter operation are described in detail. The operation is validated in a 3.7-kW hardware prototype. Key experimental results are presented in this article.

Three-Legged High-Gain Phase-Modulated DC–AC Converter for Mitigation of Device Capacitance Induced Ringing Voltage

In this paper, a method is proposed to eliminate the voltage ringing problem in a phase-modulated dc–ac converter, also known as a micro-inverter. The basic idea is to apply a time-critical two-stepped voltage waveform to the input of the converter bridge connected to the filter inductor. This method is implemented by a new topology that has one more switching leg added to the full bridge voltage fed converter. It produces a second voltage level and adds it to the first by using a transformer. The advantages of the proposed method are as follows. It almost eliminates the requirement of a voltage clamp, there is no circulating current in the freewheeling state, and the secondary side devices have less voltage and current stress compared to a conventional phase-modulated converter. The overall benefit is an improvement in converter efficiency even for a high-voltage conversion ratio application. The proposed topology operates with a secondary side modulation scheme that achieves zero-voltage switching of four devices on the primary side and either zero-voltage switching or zero-current switching of all the devices on the secondary side, over the entire line cycle of the ac output. Finally, the experimental results verify the proposed method.

Single-Stage Isolated Electrolytic Capacitor-Less EV Onboard Charger With Power Decoupling

In this paper, a new single-stage single-phase isolated AC-DC converter derived from a differential boost AC-DC converter is proposed. This converter eliminates the need to use the bulky electrolytic capacitor from the system and at the same time provides DC charging by employing the AC Power Decoupling waveform control method, effectively addressing the power density and reliability related issues commonly associated with the bulky electrolytic capacitor. As half of the switches of this converter act as synchronous rectifier during half grid cycle they are inherently ZVS Turned On while the remaining switches achieve ZVS Turn On as they act as synchronous rectifier during other half grid cycle. However, all diodes at the secondary side achieve ZCS Turn Off during the entire line cycle. A conventional controller is implemented for output voltage regulation and PFC control whereas a power decoupling controller is added to compensate second harmonic ripple power. Besides, an interleaving technique is applied to provide high-frequency links for transformers' connection while at the same time increasing the power range and effectively reducing the size of the input filter. Finally, the operating principle of the converter is validated through simulation, and the experimental results are provided.

An Isolated High-Frequency Link Microinverter Operated with Secondary-Side Modulation for Efficiency Improvement

This paper discusses the operation of a single-stage, isolated, high-frequency ac-link-based single-phase dc–ac converter, suitable for photovoltaic microinverter applications, controlled using phase modulation of the secondary-side cycloconverter devices. A detailed analysis is presented explaining the impact of prominent circuit nonidealities such as device capacitance and transformer leakage inductance on circuit behavior. The proposed extended-dead-time based modulation scheme helps in achieving zero-voltage-switching of the primary devices over the entire line cycle of the ac output and also offers the benefit of reduced transformer rms current due to absence of circulating current in the zero state. A flyback-based regenerative clamp circuit is used for mitigating voltage spikes on the secondary devices arising due to leakage inductance induced oscillations. A generic commutation strategy is also discussed, which makes operation with nonunity power factor loads also possible. Experimental results on a 260-W laboratory prototype under different load conditions are presented to illustrate the discussed principles and highlight the performance improvements.

Design of GaN-Based MHz Totem-Pole PFC Rectifier

The totem-pole bridgeless power factor correction (PFC) rectifier has recently been recognized as a promising front-end candidate for applications like servers and telecommunication power supplies. This paper begins with a discussion of the advantages of using emerging high-voltage gallium-nitride (GaN) devices in totem-pole PFC rectifiers rather than traditional PFC rectifiers. The critical-mode operation is used in the totem-pole PFC rectifier in order to achieve both high frequency and high efficiency. Then, several high-frequency issues and detailed design considerations are introduced, including extending zero-voltage-switching operation for the entire line-cycle, a variable on-time strategy for zero-crossing distortion suppression, and interleaving control for ripple current cancellation. The volume reduction of differential-mode electromagnetic interference filters is also presented, which benefits greatly from MHz high-frequency operation and multiphase interleaving. Finally, a dual-phase interleaved GaN-based MHz totem-pole PFC rectifier is demonstrated with 99% peak efficiency and 220 W/in $^{\mathrm {{3}}}$ power density.

Input-Current Distortion of CCM Boost PFC Converters Operated in DCM

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> When power-factor correction (PFC) converters designed for operation in continuous-conduction mode (CCM) at full power are operated at reduced load, operation in discontinuous-conduction mode (DCM) occurs in a zone that is close to the crossover of the line voltage. This zone will gradually expand with decreasing load to finally encompass the entire line cycle. Whereas, in CCM, the parasitic capacitances of the switches only cause switching losses, in DCM, they are a source of converter instability, resulting in significant input-current distortion. In this paper, this source of input-current distortion is analyzed, and a solution is proposed. Experimental results are obtained using a digitally controlled boost PFC converter, which is designed to operate in CCM for 1 kW. </para>

Operation of high-frequency resonant converters on the utility line with improved characteristics

Operation and characteristics of Class-D-type resonant converters on the utility line are presented. Two types of resonant converters (LCC- and LCL-type) operating with variable frequency control as well as variable pulse width (fixed frequency) are considered. Analysis and simple design procedures (with design examples) are given. SPICE simulation and experimental results obtained for the designed converters (rated at 250 W) are presented to verify the theory. It is shown that high-line power factor (PF) (>0.95) and line-current total harmonic distortion (THD) of <25% are obtained for the LCC-type converter (with a capacitor ratio of one) for a wide-load range (from full load to 10% rated load) without any active control, and the switch-peak current decreases with the load current. Although the LCL-type converter gives about 20% THD at full load, it increases to about 29% at lighter loads. However, the LCL-type resonant converter can operate in lagging PF mode for the entire line cycle and for a wide-load variation.