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Pulse Power Switching Devices - An Overview
By John Pasley
24/9/96
Copyright John Pasley 1996. This document may be freely distributed via. Any
means in
part or in whole, however the authors name must be included and correctly
attributed.
The References Listed and The Disclaimer must also be included.
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Section 1.0: Introduction.
Before entering into a consideration of the individual devices that concern us,
it would be as well to explain some of the associated technology/terminology.
1.1 Switching basics and terminology.
The switch is possibly the most elementary device in the field of
electronics. A switch controls the flow of current in a circuit in a manner such
that either the current flows at a value determined by the other components in
series with it, or does not flow at all, as the case may be. However this ideal
behavior is actually never exactly what is seen in real life. A switch has it's
own parameters that determine how fast it can switch from open to closed, or how
rapidly it can interrupt the flow of current once it is has been opened. Also of
course there are more elementary considerations such as the current handling
capacity of the switch and the peak voltage it can cope with before damage or
other unwanted effects occur.
Mechanical switches such as are common in the home are in actuality far from
ideal in their behavior. The time taken to switch from off to on (the
commutation time) is typically in the millisecond range. Also spurious effects
such as bouncing may occur as the switch fluctuates rapidly from open to closed
in the process of being physically manipulated by the operator.
Electromagnetic relays and reed switches experience similar problems to those
seen in the humble light switch. Long commutation and switch bounce are standard
features of virtually all mechanical switching devices.
With the advent of transistors and similar devices such as thyristors one would
have thought that these slow switching problems would be things of the past.
This is in fact largely true. But semiconductors are limited in other ways, it
is very hard to find semiconductors capable of switching many kiloamperes
especially at potentials in the kilovolt region, and those devices that can
manage high currents such as the larger thyristors are troubled by overly high
commutation times. Whilst there are now semiconductors coming onto the market
capable of performing at these extremes of current and voltage there are some
requirements which put even these devices to shame. If you want to switch 50
kilo Amperes with a sub 20 nanosecond commutation time at 20kV you are going to
be in trouble if you are relying on semiconductor technology. However there is
an alternative class of devices that have been around long before the humble
transistor came on the scene. You might think that vacuum tubes and similar are
a thing of the past. But for problems of this magnitude they are the only things
on the market that will do the job.
1.2 Vacuum and Gas filled switching tubes, introduction and terminology.
There are a great many different types of vacuum tube in existence, however it
is possible to group tubes according to some fairly basic criteria. There are
two primary distinguishing features, the source of free electrons within the
device and the gaseous filling (or lack of it) within the tube envelope. The
later of these two concepts we have already introduced by implication. A vacuum
tube is a device with a vacuum (very low pressure gas) filling. And a gas filled
device is, as the name would suggest, filled with gas that might be at a
pressure somewhat above or below atmospheric. The type of gas used is also an
important feature, particularly in switching tubes where a wide variety of
fillings are encountered.
The source of the free conduction electrons in the device may be either thermal
such as a heated filament physically associated with the cathode of the device -
a hot cathode, or alternatively a simple consequence of a high voltage gradient
across the device, resulting in autoemission from the cathode. A device
employing this latter method is known as a cold cathode device. In high voltage
switching the presence of high voltages, and hence the possibility of large
voltage gradients within devices means that the cold cathode system, quite a
rarity in most other types of tubes, is the norm rather than the exception.
Other important terms encountered in gaseous state switching tubes:
Delay time
The delay time is the time taken between the application of a trigger pulse and
the commencement of conduction between the primary electrodes.
Jitter
Jitter is the variation of time delay from shot to shot given similar electrical
stimulus.
Commutation time
The commutation time is the time taken for the conduction to reach maximum once
it has commenced. (i.e. From the time from the end of the delay time to the time
at which the maximum level of conduction occurs.)
It should be pointed out that none of the switching tubes we are about to
consider look very much like the things in the back of an old radio set. Many
are large, some exceptionally so. Also glass has largely given way to ceramic in
the higher powered devices. Before you go down your local electronics shop or
radio shack it should also be pointed out that many of these devices besides
costing $100's (often $1000's) a piece, and are also largely unavailable to the
general public due to their application in advanced missile and nuclear weapon
technologies. Of these devices the most 'everyday' is the ignitron which finds
much application in industrial welding situations.
The following devices are considered herein:
2.0 Vacuum and Gaseous State Switching devices
Introduction to Cold Cathode Trigger Tubes
The Krytron
The Sprytron
The Thyratron
The Over Voltage Spark Gap
The Triggered Spark Gap
2.7 The Ignitron
In addition I will include a short section on some of the solid state devices
that are finally beginning to fill the shoes of the above gaseous state device
(to a very marginal extent in most cases).
2.0 Vacuum and gaseous state switching devices
Most of the devices in this section switch by inducing an arcing process in a
gaseous medium. I have included in the triggered spark gap section some mention
of devices that actually use a liquid or solid substitute for the gaseous
material that is the norm in triggered spark gaps.
The process of arc formation is actually quite complex physically, and it will
not be gone into in any depth. Anyone who wishes to look more deeply into this
aspect of device operation may contact the author for some suggestions as to
suitable text books for use in such study.
2.1 An Introduction to Cold Cathode Switching Tubes.
Cold cathode trigger tubes are physically small devices designed to switch
impulse currents and voltages of relatively small amplitude. Usually they are
intended, as their name suggests, to trigger other larger devices.
Typically cold cathode trigger tubes are designed to switch pulses of a few
hundred volts and a few hundred milliamperes. Most trigger tubes have three or
four electrodes, anode, cathode (+ve and -ve terminals respectively), a
trigger/control electrode and sometimes a priming electrode.
A trigger tube performs in a very simple manner akin to that of a triggered
spark gap, excepting that usually the conduction is not by an arcing but glow
discharge. The glow discharge is initiated when all of the following factors are
present:
i) A sufficiently high voltage is present across the device(between anode and
cathode)
ii) A trigger pulse of sufficient amplitude is present at the trigger electrode.
iii) The gas in the tube is primed.
Cold cathode trigger tubes rely upon some external or internal source to ionize
the gas suitably for conduction to commence (This is called priming). This means
that in theory some of these tubes will only switch a minute or so after the
application of a suitable triggering voltage to the appropriate terminal of the
device when some natural source of ionizing radiation ionizes the gas (forming a
plasma) and hence causes conduction to commence.
The triggering is basically random- it is subject to huge statistical variation
even in apparently similar environments. Some devices incorporate a suitably
ionizing source to reduce the maximum possible time delay after trigger
application considerably. This source may be an electronic, radioactive or
photon source of some form or other. However even the standard commercial
devices often display a large variation (up to and above an order of magnitude
different) between devices fired in sunlight and darkness, a standard commercial
tube Z900T for instance displays a 20us delay in day light and a 250us delay in
darkness.
2.2 The Krytron:
Krytrons are a highly specialized variety of cold cathode trigger tube. They
were one of the first products developed by the US based company EG&G. The
Krytron has 4 electrodes, and is filled with a gas at low pressure. A Krytron is
distinguished among cold cathode trigger tubes for a variety of reasons.
The Krytron is designed to switch moderately high impulse currents (up to around
3kA) and voltages (Up to around 5kV) in an arc discharge mode, compare this with
the usual glow discharge of the standard trigger tube. Also, and perhaps more
importantly, the Krytron is able to turn on this arc discharge very rapidly, the
reason being that it relies on an already present plasma to support the
conduction, rather than waiting for the plasma to be formed as a result of
priming etc. This plasma is created and sustained by a keep-alive current
between the keep-alive electrode and the cathode of the device. When the trigger
is applied under the conditions of a high anode to cathode voltage, this plasma
forms an easy path for the main conduction between anode and cathode.
The fact that a conduction path is already established prior to triggering makes
a huge difference in the commutation time of these devices compared to standard
cold cathode trigger tubes. Commutation times below 1 nanosecond are achievable
with Krytrons and the time lag between application of trigger and the
commencement of switching may be less than 30ns with an optimized driver
circuit. (Note this delay is largely due to the fact that the ionized path will
need to spread from the keep alive terminal to the anode of the device) Compare
this delay time to that seen in the standard trigger tube which is dependent
upon many environmental factors and typically 3 or 4 orders of magnitude
greater. Note that the variation in time delay exhibited by the krytron is
almost totally independent of environment, however the time delay may be reduced
up to a point with increasing trigger voltage. Likewise the commutation time is
generally decreased if the rise time of the trigger pulse is also decreased.
Given identical trigger pulses however a krytron will have a very similar time
delay from one shot to the next. This variation is known as jitter and may be
less than 5ns in optimal circumstances.
A Krytron contains a source of Beta radiation, Ni-63. The quantity in each
device is less than 5 microcuries and presents no significant hazard. Usually
the source is pulse welded to a piece of Nickel wire that is in turn welded to
one of the electrode supports. The purpose of this source is to increase the
reliability of the krytron by aiding the formation of the initial glow discharge
between the keep alive and the cathode. This initial keep alive current is very
much subject to environmental factors such as are seen in the formation of the
glow discharge in standard trigger tubes. It is for this reason that a
radioactive priming element is used, much as in the priming source employed in a
standard trigger tube (which is also occasionally a radioactive source).
Krytrons typically come in a small glass envelope somewhat similar to a neon
indicator bulb with more leads.
Krytrons require a high voltage pulse (500V to 2kV) to be applied to the trigger
electrode to fire successfully. This pulse is almost always generated by a pulse
transformer fired by a capacitor discharge in the primary (rather like a simple
strobe tube firing circuit).
The krytron often has only a short life expectancy if used regularly (often is
few as a couple of hundred shots) However when used within the appropriate
parameters and well within the expected life time they are extremely reliable,
requiring no warm up and being immune to many environmental factors to a large
extent (e.g. vibration, temperature, acceleration).
These properties, combined with the small size make the krytron ideal for use in
the detonating circuitry of certain types of missiles and smart bombs. The
krytron may be used directly to fire a high precision exploding wire, or
alternatively as part of the triggering circuitry for a triggered spark gap or
similar ultra high current triggering device as used in exploding foil slapper
type detonators and larger EBW circuits.
Krytrons are used in firing circuits for certain lasers and flash tubes and also
in some pulse welding applications, often as triggering devices for other larger
devices such as Thyratrons and spark gaps.
2.3 The Sprytron
The Sprytron, otherwise known as the Vacuum Krytron, is a device of very similar
performance to the Krytron. Though it generally exhibits a somewhat lower time
delay after triggering. The Sprytron is designed for use in environments were
high levels of radiation are present. The sprytron is a hard vacuum 'filled'
device unlike the krytron which, as noted above contains a low pressure gas.
The Sprytron has only three leads, (no keep alive), but is otherwise very
similar in outward construction to the Krytron. The reason for the use of a
vacuum filling is almost certainly that there is no medium present for radiation
from the external environment to ionize (such ionization could promote spurious
triggering effects.)
The Sprytron requires a more powerful trigger pulse than the Krytron, as the
device works by forming an arc directly between the trigger and the cathode,
which causes the tube to breakdown (go into conduction) by disrupting the field
between the anode and cathode.
A Sprytron is triggered in a similar fashion to Krytron, but as mentioned
requires a higher energy trigger pulse and therefore a more powerful trigger
transformer etc. EG&G makes trigger transformers optimized for use with their
various tubes, and also make devices named Krytron-Pacs which incorporate a gas
filled krytron and trigger transformer in a single housing.
One final point. It is interesting to note that in application
circuits(references 1 and 4) the sprytron is always shown directly switching a
load (an Exploding bridge Wire.) and a Krytron is always shown triggering a
secondary device such as a triggered spark gap.
2.4 Thyratrons:
Thyratrons come in several varieties. All work similarly to the emiconductor
Thyristor, one difference being that in many designs (Hydrogen Thyratrons are a
common exception) the gate must be biased highly negative in the off state and
then biased positive to achieve switching. Like Thyristors, Thyratrons operate
like a latching switch, ie. once you have turned them on you can only turn off
by cutting the supply to the main circuit. Mercury filled Thyratrons are the
slowest, least useful type and are much more restricted environmentally than
other types due chiefly to problems with the mercury condensing . They are
rarely used as they have few advantages of the thyristor. Hydrogen Thyratrons
are *much* faster switching than Thyristors. Some can achieve commutation in
under 20ns. Inert gas fillings tend to offer superior performance compared to
mercury filled devices, without matching the speed of the Hydrogen filled
devices.
Note that Hydrogen Filled Devices employ a hot cathode.
The actual Physical construction/ operation of the thyratron is quite omplicated
compared to the other devices we have looked at and no attempt will be made to
explain it's operation. The reader is advised to consult a wide range of books
as devices employing different fillings, or electrode heating methods operate
ifferently. It is not considered to be especially important to consider all
these variations here as this is merely an overview of these devices and is not
intended to be the final word on the subject. However, in order to differentiate
the thyratron form other similar devices and to define it in at least some
physical manner here follows Frungel's (Ref.4) definition of the device:
'By the term 'thyratron' there is meant a discharge chamber in which are
arranged a cathode, one or several grids, and an anode, and which is filled with
an inert gas or metal vapor.'
Some Thyratrons can handle up to 50kV(double gap types) switch thousands f
Amperes and handle very high power outputs( e.g. CX 1154 can handle peak powers
of 40MW). Typical applications are Radar pulse modulators, Particle
accelerators, Lasers and high voltage medical equipment. Another variety of
thyratron is filled with Deuterium. These Deuterium filled devices are similar
to their Hydrogen filled counterparts but the sparking potential for Deuterium
is higher thus allowing even higher voltages to be handled.
E.g. E3213 can switch 70kV (double gap type). Specialist Thyratrons with ceramic
and metal bodies are encountered. These are designed to be used in extreme
environmental conditions. There is a wide variety of grid configurations seen in
Thyratrons, it would be impractical to consider them all here. Manufacturers of
Thyratrons Include EG&G, GEC, English Electric Valve Co. Ltd, M-O Valve co. Ltd.
Big Thyratrons often require you to get a big box full of driver/control
circuitry. Prices vary from a couple of dollars to thousands. Hot and cold
cathode type devices are encountered.
Note these ratings are the exception rather than the rule in Thyratron devices,
devices designed for sub kilovolt voltages and only capable of handling a few
tens of amps pulsed are common enough.
Thyratrons typically come in either small multi pin base type packages such as
are common in other vacuum tubes or in the case of the higher current devices
large tubular packages with hefty end connectors.
2.5 The Over Voltage Spark Gap
The Over voltage spark gap is essentially just two electrodes with a gap
between. When the voltage between the two electrodes exceeds the breakdown
voltage of the gas, the device arcs over and a current is very rapidly
established. The voltage at which arcing occurs in these devices is given by the
Dynamic Breakdown Voltage, which is the voltage at which the device will
breakdown for a fast rising impulse voltage. Note that this voltage may be as
much as 1.5 times greater than the static breakdown voltage (breakdown voltage
for a slowly rising voltage.) how much greater than the static breakdown voltage
the actual breakdown voltage is will be depends almost entirely on how rapidly
the voltage rise, a shorter rise time means a higher breakdown voltage.
Commutation times for these devices are exceptionally low (sometimes less than
1nanosecond).
Overvoltage gaps are primarily used for protection. But in combination with the
other devices mentioned here they are commonly used to sharpen the output pulses
(decrease the rise times) of very high current pulses form triggered switching
devices e.g. Thyratrons.
The size of these devices is almost entirely dependent upon how much
current/voltage they are intended to switch, There is really no limit as to the
size of these devices they can be as small as krytrons, however they can also be
very big, and devices intended to switch MA will be just that.
2.6 Triggered spark gaps
The triggered spark gap is a simple device, a high voltage trigger pulse applied
to a trigger electrode initiates an arc between anode and cathode. This trigger
pulse may be utilized within the device in a variety of ways to initiate the
main discharge. Different spark gaps are so designed to employ one particular
method to create the main anode to cathode discharge. The different methods
areas follows…
Triggered spark gap electrode configurations:
i) Field distortion: three electrodes; employs the point discharge (actually
sharp edge) effect in the creation a conducting path
ii) Irradiated: three electrodes; spark source creates an illuminating plasma
that excites electrons between the anode and cathode.
iii) Swinging cascade: three electrodes; trigger electrode nearer to one of the
main electrodes than the other.
iv) Mid plane: three electrodes; basic triggered spark gap with trigger
electrode centrally positioned.
v) Trigatron: trigger to one electrode current forms plasma that spreads to
encompass a path between anode and cathode.
The triggered Spark gap may be filled with a wide variety of materials, the most
common are…
1) Air
2) SF6
3) Argon
4) Oxygen
Often a mixture of the above materials is employed. However a few spark gaps
actually employ liquid or even solid media fillings. Solid filled devices are
often designed for single shot use (they are only used once then they are
destroyed) Some solid filled devices are designed to switch powers of 10TW (10
000 000 000 000 Watts) such as are encountered in extremely powerful capacitor
bank discharges. Except (obviously) in the case of solid filled devices, the
media is usually pumped through the spark gap. Some smaller gaps do not use this
system though.
Usually Gas filled spark gasp operate in the 20-100kV / 20 to 100kA range though
much higher power devices are available. I have one spec for a Maxwell gas
filled device that can handle 3 MA - that's 3 Million Amperes! But then it is
the size of a small car!! More commonly gas filled devices have dimensions of a
few inches. Packages are often shaped like large ice pucks though biconical,
tubular and box like structures are also seen.
Sparkgaps are often designed for use in a certain external environment(eg. they
might be immersed in oil). A system for transmitting the media to the
appropriate part of the device may sometimes be included. Common environments
used are:
a)Air
b)SF6
c)Oil
Typical spark gap device no.'s are: TG7, TG113, TG 114 etc. etc.
Spark gaps are damaged by repeated heavy discharge. This is an inevitable
consequence of such high discharge currents. Electrode pitting being the most
common form of damage. Between 1 and 10 thousand shots per device is usually
about what is permissible before damage begins to severely degrade performance.
EG&G make miniature triggered spark gaps specially designed for defense
applications. These devices are physically much smaller than normal spark gaps
(few cm typical dimensions) and designed for use with exploding foil slapper
type detonators.
Laser switching of spark gaps. The fastest way to switch a triggered spark gap
is with an intense pulse of Laser light which creates a plasma between the
electrodes with extreme rapidity. There have been quite a few designs employing
this method, chiefly in the plasma research area.
Triggered spark gaps tend to have long delay times than Thyratrons (their chief
competitor, at least at lower energies) However once conduction has started it
reaches a peak value exceptionally rapidly (couple of nanoseconds commutation.)
2.7 Ignitrons
The ignitron is mercury vapor rectifier in which an arc is switched between a
(usually graphite) anode and a mercury pool cathode. The discharge is initiated
by an ignitor electrode which dips into the mercury pool cathode.
On application of a suitable impulse current/voltage to this ignitor an electron
emitting source is formed at the point at which the ignitor contacts the pool.
This initiates the arcing between the anode and cathode.
It is important that the ignitor should be triggered correctly. The ignitor
requires a certain energy for successful ignition and also an 'ignitor
characteristic' application of this energy in terms of current and voltage with
respect to time. Misfiring or ignitor damage will otherwise occur. It is also
vital that no significant negative voltage should appear at the ignitor with
respect to the cathode else ignitor destruction will be the inevitable result.
There are two main ways by which the trigger can be biased:
Anode excitation: common in resistance welding applications here the anode bias
is connected to the ignitor by means of a switch (thyristor, thyratron etc.) and
a resistor/fuse network. The ignitor current drops rapidly on ignition as the
anode-cathode voltage drops very low during conduction.
Separate excitation: as the name suggests, here the ignitor circuit is largely
independent of the main circuit.
Ignitrons are often used in parallel for AC power control applications.
Ignitrons must often be cooled when used continuously (ie. Not single shot as in
capacitor discharge) Water cooling is commonly employed. It is vital that
Ignitrons must be used in the correct temperature range to hot or to cold can be
very bad news for these devices- (cold leads to mercury vapor condensing on the
anode.)
Ignitrons are very limited with regards their physical orientation. This reason
being simple that they rely upon a pool of liquid at one end of the device that
must be correctly positioned for the ignitor to function correctly. Positioning
the device so that it leans over at an angle of more than 2 or 3 degrees from
the vertical is fatal.
Most ignitrons operate at most currents between 5 Amps and 100kA and may be
suitable for voltages from a couple of hundred to 20 000 Volts.
Thyratrons and Krytrons are sometimes used in ignitron triggering circuits along
with the familiar thyristor.
Ignitrons are suited to applications were power control of high voltages or
currents is required. Welding is probably the most common application.
3.0 Solid State Devices.
(Note this section may well be considerably
expanded following further research by the author.)
There are now a few commercially available transistors on the Market which can
switch many tens of kV. There are also a few transistors about that can handle
pulsed currents above 5kA. These devices may match for exampleKrytrons and
Sprytrons in terms of electrical performance, but not in terms of size and (in
the case of the Sprytron) radiation hardness.
Thyristors are widely available in designs that can handle upwards of 10kA
pulsed at several kV. They are however very slow switching devices and are not
capable of achieving even low microsecond switching speeds.
A new class of devices is at present showing great promise in the R&D sector.
These devices are optically (usually LASER) switched devices employing GaAs or
Diamond film technologies. The reader is advised to
consult the appropriate reference below for more information relating to these
devices.
Final note to the reader:
Some of the devices I have mentioned are subject to strict control due to their
military applications. Non of the above information is however in any way
restricted or controlled. For clarity switching devices that are restricted by
dual use guidelines are as follows: (courtesy Oak Ridg National Laboratory)
(a) Cold-cathode tubes (including gas krytron tubes and vacuum sprytron tubes),
whether gas filled or not, operating similarly to a spark gap, containing three
or more electrodes, and having all of the following characteristics:
1.Anode peak voltage rating of 2500 V or more;
2.Anode peak current rating of 100 A or more;
3.Anode delay time of 10 microsecond or less; and
(b) Triggered spark-gaps having an anode delay time of 15 microsecond or less
rated for a peak current of 500 A or more;
(c) Modules or assemblies with a fast switching function having all of the
following characteristics:
1.Anode peak voltage rating greater than 2000 V;
2.anode peak current rating of 500 A or more; and
3.turn-on time of 1 microsecond or less.
Acknowledgements:
I would like to thank the following for their help and assistance:
Carey Sublette for providing a great deal of help and encouragement.
Roy Schmaus for providing the original site for this information.
References: (in alphabetical order by title)
EG&G Catalogues/ Material. (RE: Components)
2) Exploding Wires Vol. 4, Proc. of 4th Conf. on the Exploding Wire Phenomena.
Ed. Chace and Moore -Plenum Press (RE: EBW's)
3) High Power Optically Activated Solid State Switches,
ed. Rosen And Zutavern- Artech House (RE: Solid state devices)
4) High Speed Pulse Technology by Frank Frungel -Academic Press.(RE: EBW's,
FCG's, components)
5) High Velocity Impact Phenomena by Ray Kinslow-Academic Press.(RE: Foil
Slappers)
IEEE publications (please contact author for more details).
Maxwell Catalogues. (RE: spark gaps)
8) Mullard Valves and Tubes Book 2 Part 3 (RE: components)
FURTHER INFORMATION PERTAINING TO THE SUBJECT MATTER WILL BE MUCH WELCOMED BY
THE AUTHOR.
Information regarding the author: I am not an expert in any of the above
technologies and I will welcome any corrections. However please could anyone
providing information also provide references to either the material they
present or as to themselves so that their contribution may be given due weight.
Anyone who would like to contact me (the author) for whatever reason should
mail:
John Pasley
Disclaimer: I the author assume no responsibility for anyone who injures/kills
themselves trying to implement any of the above technologies.
High voltages are generally exceptionally dangerous, and none of the above is
intended or should be used to provide instruction in the correct procedures for
building or constructing high voltage circuitry of any description. High voltage
is used here to describe any voltage which may cause death i.e. anything above
50V.