Soft-switching Turn-on Research Articles

Analysis and Implementation of a Bidirectional Converter with Soft Switching Operation

This paper presents a soft switching direct current (DC) converter, with the benefits of bidirectional power conversion and wide-ranging voltage operation for battery charging and discharging capability. A series resonant circuit with variable switching frequency modulation is used to achieve the advantages of soft switching turn-on or turn-off of semiconductor devices. Therefore, the switching power losses in power devices can be reduced. A symmetric resonant circuit topology with a capacitor–inductor–inductor–capacitor (CLLC) structure is adopted to achieve a bidirectional power conversion capability for battery storage units in electric vehicle applications. Due to the symmetric circuit structure on both input and output sides, the converter has similar voltage gains for each power flow operation. In order to overcome the drawback of narrow voltage range operation in conventional resonant converters, a variable transformer turns ratio is adopted in the circuit, to achieve wide output voltage operation (150–450 V) for battery charging applications. To demonstrate the converter performance, a 1-kW laboratory prototype was constructed and tested. Experimental results are provided, to verify the effectiveness of the studied circuit.

Analysis of a Wide Voltage Hybrid Soft Switching Converter

A hybrid PWM converter is proposed and investigated to realize the benefits of wide zero-voltage switching (ZVS) operation, wide voltage input operation, and low circulating current for direct current (DC) wind power conversion and solar PV power conversion applications. Compared to the drawbacks of high freewheeling current and hard switching operation of active devices at the lagging-leg of conventional full bridge PWM converter, a three-leg PWM converter is studied to have wide input-voltage operation (120–600 V). For low input-voltage condition (120–270 V), two-leg full bridge converter with lower transformer turns ratio is activated to control load voltage. For high input-voltage case (270–600 V), PWM converter with higher transformer turns ratio is operated to regulate load voltage. The LLC resonant converter is connecting to the lagging-leg switches in order to achieve wide load range of soft switching turn-on operation. The high conduction losses at the freewheeling state on conventional full bridge converter are overcome by connecting the output voltage of resonant converter to the output rectified terminal of full bridge converter. Hence, a 5:1 (600–120 V) hybrid converter is realized to have less circulating current loss, wide input-voltage operation and wide soft switching characteristics. An 800 W prototype is set up and tested to validate the converter effectiveness.

Modular soft‐switching converter in DC micro‐grid system applications

Owing to environmental issue and global temperature rise, the growth of renewable energy sources such as photovoltaic arrays, fuel cells and wind turbine power has been developed. Therefore, the direct current (DC) micro-grid distributed systems are more and more important to integrate renewable energy sources, energy storage devices, local DC loads or alternative current (AC) loads and AC micro-grid through AC–DC converters to achieve high reliability and efficiency in power systems. For industry and transportation applications such as metro and light transit, the preferred voltage level on DC micro-grid system is recommended as 600–900 V. The proposed DC–DC converter contains three half-bridge circuits with a series–parallel connection at primary–secondary sides so as to decrease voltage rating of active devices and current rating of rectifier diodes. Asymmetric pulse-width modulation scheme is used to realise the soft-switching turn-on for active devices and the switching losses of power devices can be further reduced. To overcome input split voltage balance issue, flying capacitors are employed between two cascaded half-bridge circuits and the voltage rating of each active device is limited at V in/3. Finally, experiments with a laboratory prototype are provided to verify the theoretical analysis and circuit performance.

A Novel Soft Switching Bidirectional DC–DC Converter Using Magnetic and Capacitive Hybrid Power Transfer

This paper proposes a new bidirectional dc-dc converter capable of improving the convenience and modularity of the dc power grid system. By the unique operation of the $LC$ resonant transformer, the proposed converter achieves bidirectional power control with only two switches. Also, the resonant transformer drives the soft switching turn-on and turn-off over a wide power range. Hence, the circuit has high potential to downsize the converter by exploiting high frequency and simplicity. This paper provides the operating principle and the experimental verification of the proposed circuit. A prototype was fabricated by using a simple and high-density printed circuit board ($\geq$ 10 W/cc). It is verified that the circuit resonant operation successfully drives bidirectional power control with high efficiency ($\geq$92%). An application example is also provided for dc grid system integration.

Avoiding Divergent Oscillation of a Cascode GaN Device Under High-Current Turn-Off Condition

The cascode structure is widely used for high-voltage normally-on GaN devices. However, the capacitance mismatch between the high-voltage GaN device and the low-voltage normally-off Si MOSFET may induce several undesired features, such as Si MOSFET reaches avalanche during turn-off and the high-voltage GaN device loses zero-voltage switching turn-on condition internally during a soft-switching turn-on process in every switching cycle. This paper presents another issue associated with the capacitance mismatch in the cascode GaN devices. Divergent oscillation could occur at high-current turn-off condition and, eventually, destroys the device. The intrinsic reason of this phenomenon is analyzed in detail in this paper. A simple solution is proposed by adding an additional capacitor whose position is critical and should be optimized. Experimental results validate the theoretical analysis and show that the proposed method improves device performance significantly under high-current turn-off condition.

DCM Analysis of Single-Switch-Based ZVZCS Converters With a Tapped Inductor

The analysis of a novel single-active-switch-based zero voltage and zero current switching (ZVZCS) tapped boost converter in the discontinuous conduction mode (DCM) is proposed in this paper. The ZVZCS converter includes a lossless snubber composed of three diodes and two capacitors and exhibits novel ZVZCS operation for wide operating ranges of duty cycles, load currents, and input voltages. The voltage stress of the active switch is always less than the load voltage, and soft switching turn-on and turn-off are achieved without cumbersome current or voltage sensing, which could not have been obtained from quasi-resonant converters that also have an active switch. Detailed analyses of the DCM and the component stresses of the proposed converter are presented. Experiments for a 450-W prototype exhibited 99.0% of the maximum efficiency with a SiC JFET at the switching frequency of 50 kHz, and the ZVZCS operation is guaranteed, even for the DCM under the experimental conditions. Moreover, the proposed ZVZCS principle has been applied to a tapped buck–boost converter case, which was also experimentally verified.

A Novel Single-SiC-Switch-Based ZVZCS Tapped Boost Converter

A new single-active-switch-based zero voltage and zero current switching (ZVZCS) tapped boost converter is proposed. For the ZVZCS operation of the converter, three diodes and two capacitors were added to an ordinary tapped boost converter. When turned ON, the active switch is in zero current switching with the help of leakage inductance of a tapped inductor. When turned OFF, the energy of the leakage inductance is transferred to a snubber capacitor through a diode, which results in zero voltage switching of the active switch. Then the energy of the snubber capacitor is retrieved by a recovery capacitor connected to the tapped inductor. Different from most conventional soft switching converters that need at least two active switches, the proposed converter requires only one active switch such as JFET and MOSFET to guarantee ZVZCS operation for wide operating ranges of duty cycle, load current, and input voltage. The voltage stress of the active switch is always less than the load voltage, and soft switching turn-on and off is achieved without cumbersome current or voltage sensing, which could not be obtained from quasi-resonant converters that also have an active switch. Furthermore, the dc voltage gain of the proposed converter is quite robust against load change and is determined only by duty cycle like a canonical boost converter; hence, it can be even open-loop controlled. All the diodes are in ZVZCS as well, and all the components except an output diode have the voltage stress less than the load voltage. In this paper, the voltage stress of the output diode was selected to have two times the load voltage. A detailed analysis for the continuous conduction mode and the design procedures of the proposed converter is fully established, and experimentally verified for a 450-W prototype over the input voltage of 100-250 V, achieving 98.1% of maximum efficiency at the switching frequency of 100 kHz with a SiC JFET. Due to its versatile soft switching characteristics, the proposed scheme can be generally applied to high-frequency and high-efficiency dc-dc converters as well as power factor correctors.

Analysis, Design, and Performance Evaluations of an Edge-Resonant Switched Capacitor Cell-Assisted Soft-Switching PWM Boost DC–DC Converter and Its Interleaved Topology

This paper presents a soft-switching pulsewidth modulation (PWM) nonisolated boost dc–dc converter embedding an edge-resonant switched capacitor (ER-SWC) cell and its interleaved circuit topology. The conceptual boost dc–dc converter treated herein can achieve high-frequency zero-current soft-switching turn-on and zero-voltage soft-switching turn-off operations in the active switches and minimization of a reverse recovering current in the freewheeling diode under discontinuous conduction mode partially including critical conduction mode in the input current. Those advantageous properties enable a wide range of soft-switching operations together with a high-voltage step-up conversion ratio with a reduced current stress. Circuit design guideline based on the soft-switching range is introduced; then, a theoretical analysis is carried out for investigating the step-up voltage conversion ratio. For demonstrating the effectiveness of the ER-SWC soft-switching PWM boost dc–dc converter and its newly developed interleaved topology, laboratory prototypes are evaluated in experiments; then, their performances are discussed from a practical point of view.