Ben Guo

Evaluation of SiC MOSFETs for a high efficiency three-phase buck rectifier

This paper presents the characteristics of a 1200 V, 33 A SiC MOSFET and a 1200 V, 60 A SiC schottky barrier diode (SBD). The switching characteristics of the devices are tested by a double pulse test (DPT) based on a current-source structure at voltage levels up to 680 V and current up to 20 A. In addition, based on these devices, a 7.5 kW, three-phase buck rectifier for a 400 Vdc architecture data center power supply is designed. The total loss of this rectifier is calculated full load. The results show that the SiC based buck rectifier can obtain low power loss and smaller weight and volume than a Si based rectifier.

Methodology for switching characterization evaluation of wide band-gap devices in a phase-leg configuration

Double pulse tester (DPT) is a widely accepted method to evaluate the switching behavior of power devices. Considering the high switching-speed capability of wide band-gap (WBG) devices, the test results become significantly sensitive to the alignment of voltage and current (V-I) measurement. Also, because of the shoot-through current induced by Cdv/dt, during the switching transient of one device, the switching losses of its complementary device in the phase-leg is non-negligible. This paper summarizes the key issues of DPT, including layout design, measurement considerations, grounding effects and data processing. Among them, the latest probes for switching waveform measurement are compared, the methods of V-I alignment are discussed, and the impact of grounding effects induced by probes on switching waveforms are investigated. Also, for the WBG devices in a phase-leg configuration, a practical method is proposed for switching loss evaluation by calculating the difference between the input energy supplied by a dc capacitor and the output energy stored in a load inductor. Based on a phase-leg power module built with 1200 V SiC MOSFETs, the test results show that regardless of V-I timing alignment, this method can accurately indicate the switching losses of both the upper and lower switches by detecting only one switching current.

Characterization of an enhancement-mode 650-V GaN HFET

GaN heterojunction field-effect transistors (HFETs) in the 600-V class are relatively new in commercial power electronics. The GaN Systems GS66508 is the first commercially available 650-V enhancement-mode device. Static and dynamic testing has been performed across the full current, voltage, and temperature range to enable GaN-based converter design using this new device. A curve tracer was used to measure Rds-on across the full operating temperature range, as well as the self-commutated reverse conduction (i.e. diode-like) behavior. Other static parameters such as transconductance and gate current were also measured. A double pulse test setup was constructed and used to measure switching loss and time at the fastest achievable switching speed, and the subsequent over-voltages due to the fast switching were characterized. Based on these results and analysis, an accurate loss model has been developed for the GS66508 to allow for GaN-based converter design and comparison with other commercially available devices in the 600-V class.

Design and performance of an all-SiC three-phase buck rectifier for high efficiency data center power supplies

This paper presents a 7.5 kW liquid cooled three-phase buck rectifier which will be used as the front-end rectifier in 400 Vdc architecture data center power supply systems. SiC MOSFETs and SiC Schottky barrier diodes (SBDs) are used in parallel to obtain low power semiconductor losses. Input and output filters are designed and inductor core material is compared to reduce passive component losses. A low-loss modulation scheme and 28 kHz switching frequency are selected to optimize the converter design for efficiency. A prototype of the proposed rectifier is constructed and tested, and greater than 98.5% efficiency is obtained at full load.

Compensation of input current distortion in three-phase buck rectifiers

An overlap time for two commutating switches is necessary to prevent current interruption in a three-phase buck rectifier, but it may cause input current distortion. In this paper, a modified pulse-based compensation method is proposed to compensate for the overlap time. In addition to the traditional method which places the overlap time based on the voltage polarity, this new method first minimizes the overlap time to reduce its effect and then compensates the pulse width according to the sampled voltage and current. It is verified by experiments that the proposed method has better performance than the traditional method, especially when the line-to-line voltage crosses zero. Another distortion comes from the irregular pulse distribution when two sectors change in a 12-sector space vector PWM. This paper proposes two compensation methods for that scenario as well, compensating the duty cycle and increasing switching frequency near the boundaries of two sectors. It is shown through experiments that both methods can reduce the input current distortion in the buck rectifier.

Paralleled Three-Phase Current-Source Rectifiers for High-Efficiency Power Supply Applications

This paper presents the paralleling operation of three-phase current-source rectifiers (CSRs) as the front-end power conversion stage of data center power supply systems based on 400-Vdc power delivery architecture, which has been proven to have higher efficiency than traditional ac architectures. A control algorithm of paralleled three-phase CSRs is introduced to achieve balanced outputs and individual rectifier module hot swap, which are required by power supply systems. By using silicon carbide (SiC) power semiconductors, SiC MOSFETs, and Schottky diodes, the power losses of the front-end stage are reduced, and the power supply system efficiency can be further increased. The prototype of a 19-kW front-end rectifier to convert 480 Vac,rms to 400 Vdc, based on three paralleled three-phase CSRs, is developed. Each CSR is an all-SiC converter and designed for high efficiency, and the front-end stage full-load efficiency is greater than 98% from experimental tests. The balanced outputs and individual converter hot swap are realized in the hardware prototype too.

High efficiency data center power supply using wide band gap power devices

The energy efficiency of typical data centers is less than 50% because more than half of the power is consumed during power conversion, distribution, cooling, etc. In this paper, a combination of two approaches to improve power supply efficiency is implemented and experimentally verified. One approach uses a high voltage DC architecture, designed to reduce distribution loss and remove unnecessary power conversion stages. The other approach employs wide band gap (WBG) power devices, including silicon carbide (SiC) and gallium nitride (GaN) FETs and diodes, which helps to increase converter efficiency and power density. Scaled down prototypes of all power conversion stages in the data center power supply chain are designed, built, and tested. The advantages of utilizing WBG power devices are illustrated through simulations and then verified by experiment.

Methodology for Wide Band-Gap Device Dynamic Characterization

The double pulse test (DPT) is a widely accepted method to evaluate the dynamic behavior of power devices. Considering the high switching-speed capability of wide band-gap devices, the test results are very sensitive to the alignment of voltage and current (V–I) measurements. Also, because of the shoot-through current induced by Cdv/dt (i.e., cross-talk), the switching losses of the nonoperating switch device in a phase-leg must be considered in addition to the operating device. This paper summarizes the key issues of the DPT, including components and layout design, measurement considerations, grounding effects, and data processing. Additionally, a practical method is proposed for phase-leg switching loss evaluation by calculating the difference between the input energy supplied by a dc capacitor and the output energy stored in a load inductor. Based on a phase-leg power module built with 1200-V/50-A SiC MOSFETs, the test results show that this method can accurately evaluate the switching loss of both the upper and lower switches by detecting only one switching current and voltage, and it is immune to V–I timing misalignment errors.

Modulation scheme analysis for high efficiency three-phase buck rectifier considering different device combinations

The three-phase buck-type rectifier has advantages as front-end converter for high efficiency power supplies in telecommunication and data centers. In this paper, the different commutation types of a three-phase buck rectifier with a freewheeling diode are analyzed through experiments using different semiconductor devices. Further, the switching loss of the converter is modeled and calculated for four space vector modulation schemes. It is shown that when the switches include minority carrier devices, such as Si PiN diode, IGBT and Reverse Blocking IGBT (RB-IGBT), more switching loss will occur in the commutation between two switches than between a switch and the freewheeling diode. This difference can be reduced if majority carrier devices, such as SiC Schottky diodes, are used in series with the switches. The modulator can be arranged to eliminate the specific transition which has the most switching loss. According to the analysis, each modulation scheme has its own field for high efficiency application. The advantageous modulation scheme is given for different device combinations in this paper.

Control of three-phase buck-type rectifier in discontinuous current mode

In the three-phase buck-type rectifier, the current on the dc-link inductor becomes discontinuous under light load condition, at which point the current ripple is larger than the dc current value. Traditional control algorithms and modulation schemes do not work consistently well in discontinuous current mode (DCM), causing input current distortion and output voltage ripple. In this paper, the three-phase buck-type rectifier is modeled and analyzed in DCM. The DCM transfer function is derived and compared with the one for continuous current mode (CCM). It is shown that the pole and gain of the DCM transfer function changes significantly compared to that of CCM. A new modulation scheme for DCM is then proposed, which places the space vectors in such way to keep the dc-link current continuous during the active states. A digital controller is then used to eliminate the sampling error caused by the large current ripple, successfully controlling the rectifier in DCM. Simulation and experimental results are used to verify that the input current distortion and the output voltage ripple are dramatically reduced under the proposed DCM modulation and control strategy.