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Evaluation and Application of 600V GaN HEMT in Cascode Structure

Fig. 1. Cascode structure of 600V normally-on GaN HEMT with low voltage silicon MOSFET.
Gallium nitride high electron mobility transistor (GaN HEMT) has matured dramatically over the last few years. More and more devices have been manufactured and fielded(?) in applications ranging from a low power voltage regulator 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 has the potential for high frequency application. In general, a 600V GaN HEMT is intrinsically normally-on device. To easily apply the depletion mode GaN HEMT in the circuit design, a low voltage silicon MOSFET is in series to drive the GaN HEMT, which is well known as cascode structure shown in Fig. 1. This paper studies the characteristics and operation principles of a 600V cascode GaN HEMT. Evaluations of the GaN HEMT performance based on a Buck converter under hard-switching and soft-switching conditions are presented. The key converter parameters are listed in table I. Experimental results illustrate that the GaN HEMT is superior to the silicon MOSFET but still needs soft-switching in high frequency operation due to considerable package and layout parasitic inductors and capacitors. The detailed loss breakdown is shown in Fig. 2. Then GaN HEMT is applied to a unregulated LLC converter with parameters listed in table II. Comparison of the experimental results with the state of art silicon MOSFET is shown in Fig. 3 to validate the advantages of GaN HEMT.

Fig. 2. Converter loss breakdown.

Fig. 3. Efficiency comparison between cascode GaN HEMT and state of art silicon MOSFET in 1MHz LLC converter.
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