Power Electronics ♦ news digest offer the greatest potential for GaN.
These applications typically require fast, efficient switches, where the higher carrier mobility inherent to GaN transistors is advantageous. While silicon devices are widely regarded to be at or approaching their performance limits, GaN offers opportunity for improvement, particularly in form factor and electrical efficiency. A key figure of merit used in power management applications is the combination of on-resistance and switching speed. There is a lot of development underway to improve the performance of GaN FET devices. Figure 2 shows the Ron performance for two GaN devices from EPC Corporation. It is evident from the curves that the GaN devices maintain lower on-resistance for a given breakdown voltage versus the silicon MOSFET devices listed. The chart also shows that the theoretical maximum performance for GaN devices is superior to SiC. Figure 2: Resistance versus Breakdown Voltage
Source: EPC Corporation EPC Corporation is one of the leading proponents of GaN for power management applications, and they make a compelling case for this technology. In Figure 4, they illustrate the theoretical performance comparison for GaN technology versus the incumbent silicon MOSFET technology and SiC technology. Figure 4: Theoretical “RQ” Performance
The advantage goes beyond the on-resistance, however. The gate charge required to switch a GaN FET is much lower than that of a silicon device. The product of on- resistance and gate charge is a useful figure of merit for power transistors. A lower figure indicates a low resistance to turn-on, combined with a fast switching speed. As shown in Figure 3, EPC Corporation claims their GaN devices have an RQ product more than 10 times lower than silicon alternatives. Figure 3: “RQ” Product Comparison: GaN vs. Silicon MOSFET
Source: EPC Corporation In practice, a low RQ figure of merit means that circuit designers do not have to sacrifice low on-resistance for fast switching, as is normally the case. A side benefit of this performance is that GaN DC-DC converter devices will operate at higher bandwidths. Silicon power MOSFETs have difficulty with pulse widths below 100ns (corresponding to a 250 KHz bandwidth), while GaN FETs can be turned on or off in as little as 4ns. Figure 5 summarizes these metrics with the performance of an EPC GaN device versus some representative silicon MOSFET devices. Note that the “RQ” product of the GaN device is almost an order of magnitude lower than best MOSFET. Figure 5: “RQ” Product Comparison: GaN vs. Silicon MOSFET
October 2013
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