Alon Blumenfeld, Alon Cervera, and Shmuel (Sam) Ben-Yaakov Power Electronics Laboratory, Department of Electrical and Computer Engineering Ben-Gurion University of the Negev P.O. Box 653, Beer-Sheva, 84105 Israel Emails: alonblu@bgu.ac.il ; cervera@bgu.ac.il ; sby@ee.bgu.ac.il Website: www.ee.bgu.ac.il/~pel/ Abstract – A circuit for operating high-side MOSFET transistors using a ground-referred low-side driver is proposed and investigated analytically and experimentally. The circuit is implemented solely with low-cost non-inductive passive components. The targeted application is for cases in which the source of the floating transistors (N or P type) can be connected to DC buses. Design guidelines are introduced, including design considerations that were developed and verified experimentally. The inexpensive implementation is shown to introduce negligibly small losses, making it energy-efficient and cost-effective, surpassing existing flotation solutions since neither active nor inductive devices are needed. Keywords – DC Restorer, High-side Drive, Floating Gate Drive I. INT RODUCT ION Driving gates of floating transistors in common power electronics systems, such as inductor-based converters and switched capacitor converters, requires designated driving systems. The driving circuitry needs to create proper isolation to overcome the voltage differences existing between the transistors’ sources and the drivers’ ground potential. Present solutions achieve isolation by means of transformers or opto-couplers. However, the latter requires a floating power supply mechanism, while the former solution is limited to the high switching frequency range and to MOSFET transistors that require relatively small gate charges. The objective of this study is to investigate a method that applies capacitive coupling in order to drive a high-side MOSFET transistor that is connected to a DC bus (such as the input or output voltage), where the latter is operated by a low- side ground-referred driver. This approach is examined and design guidelines and considerations for optimal operation are developed. The analytical predictions were verified by simulations and by experimental studies obtained from an implementation in a designated converter. II. G ATE DRIVE ISOLATION The employment of a transformer to achieve isolation of a high-side transistor’s gate from its low-side driver can be useful for high voltage applications, such as motor drives and inverter topologies [2,3]. However, this approach has its limitations. The average volt-second at the transformer needs to be zero in order to prevent core saturation, that is, high duty cycle ratios result in potentially high voltages, risking a gate-source breakdown or voltages lower than the gate’s threshold voltage. At low frequencies, the core size needs to be increased substantially to be able to withstand the magnetic flux without saturating. Creative solutions must be found to apply transformers to wideband high-side drives For instance, in [1] the no-saturation ‘taboo’ is broken when charging the high-side gate through a series diode and letting the core saturate, resulting in a voltage drop in the secondary, which, in turn, reverse biases the diode, leaving the gate high at float. The gate is later discharged via a small drive MOSFET during the off cycle. A second method commonly used is floating the whole driving circuitry. This requires galvanic isolation for both the drive signal and the driver’s power source. The drive signal can be isolated using an opto-coupler or a transformer and the high-side power supply can range from a conventional forward converter solution to designated topologies such as [5]. A common implementation that eliminates the need for a high-side supply is the use of an IC with a bootstrap capacitor to supply the isolated driver. The capacitor charges during off periods and supplies the needed charge independently during turn-on. This solution is applicable if the source is ground- referred during off-periods for proper charge. Using a bootstrap can be problematic in high voltages, mainly because the capacitor charging diode needs to withstand the high voltages and have a minimal reverse-recovery time to prevent discharge of the bootstrap capacitor. A different and more straightforward IC solution to the floating supply was developed in [4]. It consists of a low side driver, a high-side driver with an isolated input and a fully integrated DC-DC converter, capable of supplying additional auxiliary low-power components. This solution provides a one-chip solution at the expense of efficiency. An alternative way to optically deliver the needed drive power also mentioned in [1], is by a Photo-Voltaic Isolator (PVI). A PVI has an IR LED at its input that reflects on PV cells connected to the output, giving enough power for driving relatively low gate-charge MOSFET transistors at low frequencies, or in applications that demand constant-on times. Some application-specific solutions exist that do not include isolation, e.g. in Buck converters, a P-type MOSFET can be used, operated by a ground-referred inverted-output open collector driving circuit. This approach is limited to cases when the MOSFET’s source voltage is lower than the gate-to-source breakdown voltage. For higher voltage differences there is a need for voltage dividers to prevent breakdown, increasing the effective resistance of the drive and limiting the possible switching frequency. Analysis and Design of DC-Isolated Gate Drivers 1 978-1-4673-4681-8/12/$31.00 ©2012 IEEE 2012 IEEE 27 th Convention of Electrical and Electronics Engineers in Israel