Solid state Pulsed Power Systems ( switches ) (fwd)

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Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Mon, 5 Jun 2006 12:16:31 +0200
From: Lyonel Baum <lbaum001@xxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Solid state Pulsed Power Systems ( switches )

A Pulsed Power System is characterized by its energy storage system, 
the energy can be released in the form of a high power pulse to the 
load, by means of a switching device. The energy storage is usually 
either of an inductive or a capacitive nature. The limiting device in a 
Pulsed Power System is often the switch, which limits the pulse peak 
power and the repetition rate. The conventional approach in Pulsed 
Power designs is to use a gas switch such as a thyratron, ignitron or 
spark gap. However these devices have limited lifetime, high cost, low 
repetition rate and high losses. Thanks to the continued improvement of 
the high power semiconductors in switching speed, voltage and current 
rating, solid-state semiconductors have become the device of choice for 
pulsed power systems.

The first semi-conductors used as high power switches were the 
Thyristors and GTOs, they can handle several kilovolts and kilo-amps, 
however their low switching speed is a serious limitation.

Today, the semiconductor devices of choice  for high power switching 
are the MOSFETs (Metal Oxide Field Effect Transistor) and the IGBTs 
(Insulated Gate Bipolar Transistor). The MOSFETs are substantially 
faster than IGBTs (typical switching time: 200ns for an IGBT, 20ns for 
a MOSFET), but IGBTs are more efficient at high voltage (less losses), 
cheaper per kW switched, and are being manufactured at higher voltage 
ratings (up to 6500V), where the MOSFETs are limited to 1200V. So 
generally, MOSFETs are used only when a high switching speed is 
required. However sometimes MOSFETs are preferred even for lower speed 
switching because they are more easy to stack in a series/parallel 
assembly than IGBTs.

Another type of solid-state high power switch is the saturable core 
inductor. This switch can perform nanosecond switching times of almost 
unlimited power. It is by nature extremely robust, yet it is not a 
gated switch. It is used in conjunction with a semiconductor switch to 
obtain faster switching time. The switching time of a saturable core 
inductor is defined by the time it takes to saturate. In the close 
state the impedance of the switch is the one of the saturated inductor 
(the relative permeability drops to 1, i.e. as if there was no core). 
In the open state the impedance of the switch is the one of the 
unsaturated inductor, which is typically about 10000 times the 
saturated impedance. Saturable inductor switches are usually used in 
series with a semiconductor switch to sharpen the pulse shape, or in 
magnetic compression designs, to obtain pulse power in the hundreds of 
GW range. The use of saturable core switches leads to generators with 
fixed pulse width.

More about this subject ?  Take a look on the paper of  Dr S. Roche at  
http://www.physiqueindustrie.com/Publi/solid_state_pulsed_power.pdf

http://www.physiqueindustrie.com/Graphics/pulse_generator_1.jpg