This paper presents a comprehensive study on the influences of parasitic elements on the MOSFET switching performance. A circuit-level analytical model that takes MOSFET parasitic capacitances and inductances, circuit stray inductances, and reverse current of the freewheeling diode into consideration is given to evaluate the MOSFET switching characteristics. The equations derived for emulating MOSFET switching transients are assessed graphically, which, compared to results obtained merely from simulation or parametric study, can offer better insight into where the changes in switching performance lie when the parasitic elements are varied. The analysis has been successfully substantiated by the experimental results of a 400 V, 6 A test bench. A discussion on the physical meanings behind these parasitic effect phenomena is included. Knowledge about the effects of parasitic elements on the switching behavior serves as an important basis for the design guidelines of fast switching power converters.

Characterization and Experimental Assessment of the Effects of Parasitic Elements on the MOSFET Switching Performance

Gallium nitride high electron mobility transistor (GaN HEMT) has matured dramatically over the last few years. A progressively larger number of GaN devices have been manufactured for in field applications ranging from low power voltage regulators to high power infrastructure base-stations. Compared to the state-of-the-art silicon MOSFET, GaN HEMT has a much better figure of merit and shows potential for high-frequency applications. The first generation of 600 V GaN HEMT is intrinsically normally on device. To easily apply normally on GaN HEMT in circuit design, a low-voltage silicon MOSFET is in series to drive the GaN HEMT, which is well known as cascode structure. This paper studies the characteristics and operation principles of a 600 V cascode GaN HEMT. Evaluations of the cascode GaN HEMT performance based on buck converter at hard-switching and soft-switching conditions are presented in detail. Experimental results prove that the cascode GaN HEMT is superior to the silicon MOSFET, but it still needs soft-switching in high-frequency operation due to considerable package and layout parasitic inductors and capacitors. The cascode GaN HEMT is then applied to a 1 MHz 300 W 400 V/12 V LLC converter. A comparison of experimental results with a state-of-the-art silicon MOSFET is provided to validate the advantages of the GaN HEMT.

Evaluation and Application of 600 V GaN HEMT in Cascode Structure

This paper presents an accurate analytical model to calculate the power loss of a high voltage Gallium Nitride high electron mobility transistor (GaN HEMT) in cascode configuration. The proposed model considers the package and PCB parasitic inductances, the nonlinearity of the junction capacitors, and the transconductance of the cascode GaN transistor. The switching process is illustrated in detail, including the interaction of the low voltage Si MOSFET and the high voltage GaN HEMT in cascode configuration. The switching loss is obtained by solving the equivalent circuits during the switching transition. The analytical results show that the turn-on loss dominates in hard-switching conditions while the turn-off loss is negligible, due to the intrinsic current source driving mechanism. The accuracy of the proposed model is validated by numerous experimental results. The results of both the analytical model and experiments suggest that soft-switching is critical for high voltage GaN in high-frequency high-efficiency applications.

Analytical Loss Model of High Voltage GaN HEMT in Cascode Configuration

Based on a survey on over 1400 commercial LED drivers and a literature review, a range of LED driver topologies are classified according to their applications, power ratings, performance and their energy storage and regulatory requirements. Both passive and active LED drivers are included in the review and their advantages and disadvantages are discussed. This paper also presents an overall view on the technical and cost aspects of the LED technology, which is useful to both researchers and engineers in the lighting industry. Some general guidelines for selecting driver topologies are included to aid design engineers to make appropriate choices.

A Survey, Classification, and Critical Review of Light-Emitting Diode Drivers

The high-brightness white-light-emitting diode (LED) has attracted a lot of attention for its high efficacy, simple to drive, environmentally friendly, long lifespan, and compact size. The power supply for LED also requires long life, while maintaining high efficiency, high power factor, and low cost. However, a typical power supply design employs an electrolytic capacitor as the storage capacitor, which is not only bulky, but also with a short lifespan, thus hampering performance improvement of the entire LED lighting system. In this paper, a novel power factor correction (PFC) topology is proposed by inserting the valley-fill circuit in the single-ended primary inductance converter (SEPIC)-derived converter, which can reduce the voltage stress of the storage capacitor and output diode under the same power factor condition. This valley-fill SEPIC-derived topology is, then, proposed for LED lighting applications. By allowing a relatively large voltage ripple in the PFC design and operating in the discontinuous conduction mode (DCM), the proposed PFC topology is able to eliminate the electrolytic capacitor, while maintaining high power factor and high efficiency. Under the electrolytic capacitor-less condition, the proposed PFC circuit can reduce the capacitance of the storage capacitor to half for the same power factor and output voltage ripple as comparing to its original circuit. To further increase the efficiency of LED driver proposal, a twin-bus buck converter is introduced and employed as the second-stage current regulator with the PWM dimming function. The basic operating principle and analysis will be described in detail. A 50-W prototype has been built and tested in the laboratory, and the experimental results under universal input-voltage operation are presented to verify the effectiveness and advantages of the proposal.

A Novel Valley-Fill SEPIC-Derived Power Supply Without Electrolytic Capacitor for LED Lighting Application

The piecewise linear model has traditionally been used to calculate switching losses in switching mode power supplies due to its simplicity and good performance. However, the use of the latest low voltage power MOSFET generations and the continuously increasing range of switching frequencies have made it necessary to review this model to account for the parasitic inductances that it does not include. This paper presents a complete analytical switching loss model for power MOSFETs in low voltage switching converters that includes the most relevant parasitic elements. It clarifies the switching process, providing information about how these parasitics, especially the inductances, determine switching losses and hence the final converter efficiency. The analysis presented in this paper yields two different types of possible switching situations: capacitance-limited switching and inductance-limited switching. This paper shows that, while the piecewise linear model may be applied in the former, the proposed model is more accurate for the latter. Carefully-obtained experimental results, described in detail, support the analytical results presented.

An Insight into the Switching Process of Power MOSFETs: An Improved Analytical Losses Model

This paper presents a new control strategy for power factor correctors (PFCs) that are used to drive high-brightness LEDs. This control strategy is extremely simple and is based on the use of a conventional peak-current-mode controller with a suitable selection of the compensation ramp waveform. Neither an analog multiplier nor an input voltage sensor is needed to achieve quasi-sinusoidal line waveforms at nominal conditions and full load. If the converter belongs to the flyback family (flyback, buck-boost, SEPIC, Cuk and Zeta), the line waveform appears notably distorted if the compensation function is a linear ramp, but becomes almost sinusoidal if the linear ramp is substituted by a properly chosen exponential function. The line waveform is slightly distorted when the load varies or when the converter works under either overvoltage or undervoltage conditions. However, the waveform maintains a very high power factor (PF) even under these conditions. Moreover, the line current is cycle-by-cycle-controlled due to the peak-current-mode control, and hence, the input-current feedback loop is extremely fast, thereby allowing this type of control to be used with high-frequency lines (above 400 Hz).

Javier Sebastián Zúñiga

Papers

An Insight into the Switching Process of Power MOSFETs: An Improved Analytical Losses Model

A Very Simple Control Strategy for Power Factor Correctors Driving High-Brightness LEDs