Reader Q&A of the Month: Alex Lidow Answers Your Questions

Who is Alex Lidow?
Prior to founding EPC, Dr. Lidow was CEO of International Rectifier Corporation. A co-inventor of the HEXFET power MOSFET, Dr. Lidow holds many patents in power semiconductor technology and has authored numerous publications on related subjects. Lidow earned his Bachelor of Science degree from Caltech in 1975 and his Ph.D. from Stanford in 1977.
Below is a recent question submitted by a Power Electronics Reader, answered by Alex.Q. How do GaN transistors compare with power MOSFETs for power switching applications in terms of efficiency? Alex Lidow - To effectively compare the potential performance of a power MOSFET and an enhancement mode GaN transistor in a power conversion circuit, some figures of merit (FOM) need to be defined first. A FOM that has been used by MOSFET manufacturers to show both generational improvements and to compare their products to other competitive devices is the product of the gate charge, QG, and the RDS(ON) for a given device. What makes this so useful is that no matter the size of the die, this FOM is almost constant for a given technology or ‘Generation’ of device. This FOM is related to device performance and can be useful in predicting power loss improvements with improved technologies, but it is less sensitive to differences when a device is used more as a switching element than as a conducting element. We will therefore discuss three distinct FOMs.  The first of these is the traditional FOM.  We will call that the “Rectifier FOM” because it is most applicable when a FET is used as a rectifier element such as in the lower transistor of a buck converter.  The second FOM we will call the “Switching FOM” because it best describes relative performance of devices used mostly as switching elements such as in the upper transistor in a classic buck converter. Of these two FOMs, the switching performance is more important in ‘hard switching’ converter circuits. The third FOM we will call the “Resonant FOM” because it best describes relative performance of devices used in resonant circuits where the switching occurs when there is zero current through the device (ZCS).

Fig. 3 plots RDS(ON) vs. QG+ QOSS (Resonant FOM) for eGaN FETs as well as for various equivalent silicon MOSFETs.  When a device is switched at zero current, the major elements of switching loss come from charging and discharging the device output (QOSS) and the input gate (QG).  We can see that, based on the resonant FOM, eGaN FETs again offer a distinct advantage over any equivalent voltage rated silicon device. Whether the designer of a power conversion system is using a hard switched topology or a soft switched  topology, GaN transistors can reduce the power losses and increase power density.  The higher the conversion frequency, the greater the gain by upgrading to the new generation of power transistor made with a remarkable material, GaN.GlossaryParameterDescriptionRDS(ON)Drain- source on-resistanceQGTotal gate chargeQOSSOutput chargeQGDGate-to-drain chargeFOMFigure-of-meriteGaNEnhancement mode GaN transistorFiguresRectifier FOM (RDS(ON)vs. QG) for eGaN FETsSwitching FOM (RDS(ON) vs. QGD) for various eGaN FETs and MOSFETs Resonant FOM (RDS(ON) vs. QG+ QOSS) for eGaN FETsIf you would to submit your questions to Alex anonymously, please post any questions you have in the comments section below.