Zimmar Physics Accelarator Notes-278.pdf - Diode Opening...

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Figure 8: Cross section and perspective view of power MOSFET [22]. drive circuitry, power MOSFETs can be turned on in as little as 10 ns. The limitation on pulse length is the average power dissipated in the de- vice’s channel, which is determined by the load. If the impedance requires more current than can be provided by a single MOSFET, the devices can be parallelized. MOSFETs have a positive tem- perature coefficient, which indicates that their on- resistance increases with temperature. This makes parallelization easier because the parallel devices will self-regulate their current draw as the average dissipated power increases. Commercial power MOSFETs are typically rated for drain-source hold-off between 50 V and 1.2 kV and average currents as high as 40 A. There are exceptions, including MOSFETs that are rated to hold-off 4 kV at no more than 1 A average current. The nominal on-resistance of Si power MOSFETs is typically about 0.5 Ω , which is higher than the on-resistance of an IGBT. SiC MOSFETs have recently been commercialized by CREE, Inc. These devices feature 1.2 kV hold- off, 20 A average current, and an on-resistance of 18.7 m Ω [26]. The low on-resistance provided by SiC enables faster switching operation and faster risetimes. When properly cooled, these devices have been demonstrated to operate continuously at switching frequencies of 1 MHz. Diode Opening Switch Diode opening switches, which are typically a type of drift step recovery diode (DSRD), are used to switch inductively stored energy on nanosecond and sub- nanosecond timescales. Their theory of operation was first described in 1985 by Igor Grekhov. In this and subsequent papers, Grekhov describes how a properly graded silicon pn junction can

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