Comparisons on Different Innovative Cascode GaN HEMT E-Mode Power Modules and Their Efficiencies on the Flyback Converter

Abstract

The conventional cascode structure for driving depletion-mode (D-mode) gallium nitride (GaN) high electron mobility transistors (HEMTs) raises reliability concerns. This is because of the possibility of the gate to source voltage of the GaN HEMT surging to a negative voltage during the turn off transition. The existing solutions for this problem in the literature produce additional drawbacks such as reducing the switching frequency or introducing many additional components. These drawbacks may outweigh the advantages of using a GaN HEMT over its silicon (Si) alternative. This paper proposes two innovative gate drive circuits for D-mode GaN HEMTs—namely the GaN-switching based cascode GaN HEMT and the modified GaN-switching based cascode GaN HEMT. In these schemes, the Si MOSFET in series with the D-mode GaN HEMT is always turned on during regular operation. The GaN HEMT is then switched on and off by using a charge pump based circuit and a conventional gate driver. Since the GaN HEMT is driven independently, the highly negative gate-to-source voltage surge during turn off is avoided, and in addition, high switching frequency operation is made possible. Only two diodes and one capacitor are used in each of the schemes. The application of the proposed circuits is experimentally demonstrated in a high voltage flyback converter, where more than 96% efficiency is obtained for 60 W output load.

Flyback Converter Using a D-Mode GaN HEMT Synchronous Rectifier

“…The GaN HEMT brings power efficiency advantages when the parasitic capacitive behavior is thoroughly investigated. In order to achieve the soft switching, we can introduce the ZCS (Zero Current Switching) control to optimize the efficiency in the DCM operation, when in the CCM operation the ZVS (Zero Voltage Switching) control can be applied [12,18,19]. In Figure 2, under the gate signal of frequency f0, the entire cycle of switching can be divided into two parts.…”

“…The GaN HEMT brings power efficiency advantages when the parasitic capacitive behavior is thoroughly investigated. In order to achieve the soft switching, we can introduce the ZCS (Zero Current Switching) control to optimize the efficiency in the DCM operation, when in the CCM operation the ZVS (Zero Voltage Switching) control can be applied [12,18,19]. These five steps include the diode voltage rise to turn-on time t D,rise , the snubber reaction time t snubber , the battery charging time t BCH , the diode voltage falls to turn-off time t D,fall , and the inductor charging time t LCH .…”

“…GaN devices have some superior material properties resulting in low gate charge, small output capacitance, and zero reverse recovery characteristics that provide them the capability of fast switching and low loss [8,9]. However, there are several challenges needed to be overcome for using the D-mode GaN HEMT, such as the negative gate voltage needing to be applied to let normally-on device operate as normally-off [10][11][12], and the third quadrant loss of GaN FETs needs to be solved when it is used as a rectifier [9,13]. Few studies have presented several methods to let normally-on devices act as normallyoff [10][11][12], like using self-powered gate drives [10], biased charge pump gate drives [11], or cascode structure [12].…”

“…However, there are several challenges needed to be overcome for using the D-mode GaN HEMT, such as the negative gate voltage needing to be applied to let normally-on device operate as normally-off [10][11][12], and the third quadrant loss of GaN FETs needs to be solved when it is used as a rectifier [9,13]. Few studies have presented several methods to let normally-on devices act as normallyoff [10][11][12], like using self-powered gate drives [10], biased charge pump gate drives [11], or cascode structure [12]. For addressing the third quadrant loss of GaN FETs, optimizing the dead time can improve the overall system performance [9,13].…”

“…Previous works have already suggested implementing the developed P-Cascode GaN HEMT module with the biased charge pump gate drive [17] to synchronous buck converter [18]. Moreover, the GaN-switching based cascode GaN HEMT and the modified GaN-switching based cascode GaN HEMT with only two diodes and one capacitor are used on the flyback converter [12]. Based on these mentioned works, an insight of the CCM, DCM mechanism and zero current switching of flyback converter are implemented to yield higher power efficiency [19].…”