Shad L. Holt

Failure Analysis of 1200-V/150-A SiC MOSFET Under Repetitive Pulsed Overcurrent Conditions

SiC MOSFETs are a leading option for increasing the power density of power electronics; however, for these devices to supersede the Si insulated-gate bipolar transistor, their characteristics have to be further understood. Two SiC vertically oriented planar gate D-MOSFETs rated for 1200 V/150 A were repetitively subjected to pulsed overcurrent conditions to evaluate their failure mode due to this common source of electrical stress. This research supplements recent work that demonstrated the long term reliability of these same devices [1]. Using an RLC pulse-ring-down test bed, these devices hard-switched 600 A peak current pulses, corresponding to a current density of 1500 A/cm2. Throughout testing, static characteristics of the devices such as BVDSS, RDS (on), and VGS(th) were measured with a high power device analyzer. The experimental results indicated that a conductive path was formed through the gate oxide; TCAD simulations revealed localized heating at the SiC/SiO2 interface as a result of the extreme high current density present in the devices JFET region. However, the high peak currents and repetition rates required to produce the conductive path through the gate oxide demonstrate the robustness of SiC MOSFETs under the pulsed overcurrent conditions common in power electronic applications.

High-Mobility Stable 1200-V, 150-A 4H-SiC DMOSFET Long-Term Reliability Analysis Under High Current Density Transient Conditions

For SiC DMOSFETs to obtain widespread usage in power electronics their long-term operational ability to handle the stressful transient current and high temperatures common in power electronics needs to be further verified. To determine the long-term reliability of a single 4H-SiC DMOSFET, the effects of extreme high current density were evaluated. The 4H-SiC DMOSFET has an active conducting area of 40 mm2, and is rated for 1200 V and 150 A. The device was electrically stressed by hards-witching transient currents in excess of four times the given rating (>600 A) corresponding to a current density of 1500 A/cm2. Periodically throughout testing, several device characteristics including RDS(on) and VG S(th) were measured. After 500 000 switching cycles, the device showed a 6.77% decrease in RDS (on), and only a 132-mV decreased in VG S(th). Additionally, the dc characteristics of the device were analyzed from 25 to 150 °C and revealed a 200-mV increase in on-state voltage drop at 20 A and a 2-V reduction in VG S(th) at 150 °C. These results show this SiC DMOSFET has robust long-term reliability in high-power applications that are susceptible to pulse over currents, such as pulsed power modulators and hard-switched power electronics.

A compact 5kV battery-capacitor seed source with rapid capacitor charger

Many pulsed power applications have demanding system requirements. Power systems for these applications are expected to provide high energy, high pulsed power and long standby times without recharge, all in a very compact package. The different properties of batteries and capacitors make them most suitable for different uses. When selecting a prime power source for compact pulsed power systems a hybrid system often provides the optimal solution, utilizing a battery for prime energy storage during standby and a capacitor for intermediate energy storage before and between operations. This system takes advantage of the best characteristics of both sources to fulfill the system requirements. The design and testing of such a compact system is discussed. The system utilizes a solid-state converter to charge a 50 µF polypropylene capacitor to 5 kV in under 500 ms from lithium-ion polymer (LiPo) batteries. Battery selection and testing is also covered. The battery and charger assembly occupies 1.25 L while the capacitor occupies an additional 1.4 L.

Theoretical performance of a GPS linked Pulsed Ring Down Array

Current research at Texas Tech University is focused on the development of a High-Power Pulsed Ring-Down Source (PRDS) Antenna Array. Previously, a Monte Carlo based analysis was conducted in order to predict the array performance based upon the estimated switching jitter between elements [1]. This analysis showed good performance for jitter times between 0 to 2 periods of the ringing frequency. Therefore, for ringing frequencies up to 500 MHz, jitter times up to 4 nanoseconds can be tolerated. Subsequently, we have shown practical switching solutions capable of the sub-nanosecond switching performance needed for the frequencies of interest [2]. Taking the analysis a step further, we introduce the uncertainty of the absolute position of each antenna element. To implement a randomly distributed array, where the position of elements is not fixed, a method of accurately resolving element positions relative to each other and the target location is required. The use of a variety of GPS technologies and techniques is explored as a method for position and timing resolution. The relative accuracy between elements and the absolute accuracy of each element is discussed. A Monte Carlo based analysis is conducted to predict array performance based upon GPS positional error, GPS timing error, and switch jitter.

COMSED 1 — A compact, gigawatt class microwave source utilizing helical flux compression generator based pulsed power

Recent progress in the development of a compact, portable, explosively-driven high power microwave source is presented. The envelope to which the system must fit has a 15 cm diameter, which means each sub-system fits within this dimension, with an optimized overall length. The system includes an autonomous prime energy source, which provides the initial energy for a two-stage, flux-trapping helical flux compression generator (FCG). Typical output from the FCG is several kilojoules into a 3 μH inductor. The amplified energy from the generator, after pulse conditioning, is used to drive a virtual cathode oscillator (vircator). Recorded voltages at the vircator with this arrangement were greater than 200 kV in experiments, where radiated output powers of greater than 100 MW have been measured. Voltages of at least 300 kV, with an electrical output power of 4 GW or greater, were generated by the FCG driven pulsed power source into a water resistor load with an impedance similar to the operating impedance of the vircator. A description of each component of the compact microwave source will be given, along with waveforms from tests performed with the components independent of the rest of the system. Data from experiments with the fully integrated microwave system will be shown, and analysis will be offered to detail the performance of the system in its present state.

Rapid charging seed source with integrated fire set for flux compression generator applications

The design and testing of an integrated front-end power and control system for helical flux compression generators (HFCG) is presented. A current up to 12 kiloamps needs to be pushed into the 5.8 microhenry field coil of the HFCG to establish the necessary seed flux for generator operation. This current is created with the discharge of a 5 kilovolt, 50 microfarad metalized polypropylene film capacitor using a single-use semiconductor closing switch. Once peak current/flux is obtained in the seed coil an exploding bridge wire (EBW) detonator is initiated with a discharge from a 1 kilovolt, 500 millijoule capacitor array contained in the compact fire set. Both capacitances, seed and fire set, are charged using a rapid capacitor charger system. The rapid capacitor charger is a solid state step up converter supplied by lithium-ion polymer (LiPo) batteries. It provides the 5 kilovolts and 1 kilovolt dual output voltages required for the compact seed source and compact fire set, respectively. The rapid capacitor charger operates at an average output power of 3 kilowatts and charges both capacitances simultaneously in under 250 milliseconds. The rapid capacitor charger is reusable if protected from the explosive detonation. All components in the system are fiber-optically controlled by a battery powered microcontroller that is fully optically isolated from the system. This controller provides the precise timing required to maximize performance of the HFCG system. The entire front-end system including batteries, capacitors, power electronics and control circuitry but excluding the HFCG occupies a volume of less than 4 L and fits in a 15 cm diameter package.

Real Time Feedback Control System for an Electromagnetic Launcher

The design and implementation of a real time feedback control system for a distributed energy, bench top, electromagnetic launcher is presented. The feedback control system provides optimum pulse shaping by real time control of solid state switches. Advantages of pulse shaping control include increased energy efficiency and control of armature exit velocity. Lab VIEW 8.0 software1 is used to program a National Instruments CompactRIO programmable automation controller (PAC). This provides real time processing by use of the reconfigurable I/O (RIO) FPGA technology. The program controls switch timing from analog feedback signals supplied by B-dot probes placed along the rail length. Through signal analysis, real time armature position is derived. The program uses this data to control pulse shape and width. A dedicated B-dot probe is placed at the beginning of each stage which is the desired triggering location. A flux ruler sensor along the bore length provides a secondary velocity calculation excluded from the control system. This sensor provides velocity measurements for every centimeter of bore travel. Collected data is used to characterize the system under test for different load conditions.

Design of Explosive-Driven Ferroelectric Pulse Generators with Outputs Exceeding 200 kV

The design and testing of explosive-driven ferroelectric generators (FEGs) with output voltages exceeding 200 kV is presented. The generator design is aimed at achieving high voltages in a compact device by explosively compressing stacks of ferroelectric ceramic discs. The ferroelectric used in this application is EC -64 lead zirconate- titanate (PZT), a hard ferroelectric ceramic with a high dielectric constant and a good piezoelectric coupling coefficient in the direction of polarization. The pressure impulse is generated by high explosive detonating cord and travels longitudinally along the polarization vector in the PZT. The effect of variations in the rise-time, width, and magnitude of the pressure pulse on the peak voltage has been experimentally studied by modifying the ferroelectric stack and explosive driver geometries. Different dielectric insulations are experimentally evaluated for good compatibility with the ferroelectrics and maximum hold-off voltage.

Testing of a low inductance stacked mosfet switch for Pulsed Ring Down Sources

An inexpensive and mobile array of Pulsed Ring Down Sources (PRDS) were required to verify previous simulation results. Initial attempts to use a stacked MosFET circuit as a closing switch were unsuccessful due to the additional series inductance of the MosFET stack lowering the frequency of the oscillation on the coaxial radiator. Experimental results showed that by reducing the parasitic inductance due to the geometry of the MosFET stack, the frequency of the oscillation could be increased. Increased series resistance due to the stacked MosFETs was also a concern. In order to minimize the parasitic inductance of the stack and allow for multiple stacks to be connected in parallel, a printed circuit board was designed. Results from testing of the original switch stacks and board assembly are presented and compared with PSPICE simulations. Using the simulation results, the reduction in parasitic inductance can be estimated.

Theoretical performance of a mobile GPS linked pulsed ring down array

The development of mobile Pulsed Ring Down Source (PRDS) arrays requires the ability to accurately determine the relative positions of array elements at distances, and in situations, where discrete measurements are not practical. At the frequencies of interest, centimeter level accuracy is required for the array to localize radiated energy at a given target location. Global Positioning System (GPS) devices and techniques are evaluated for the purpose of position acquisition. Previously a Monte Carlo simulation was developed that takes into account the position error, the GPS timing error, and the switch jitter of the element. The error sources are combined and used a metric to evaluate and predict the array performance. Results of the GPS device testing, as well as previous work, are used as the input parameters of the simulation to determine their viability for use in the implementation of PRDS arrays capable of radiating at frequencies of up to 500 MHz.