The design of a high voltage scr pulse generator
for ultrasonic pulse echo applications
J.G.
OKYERE
and A.J.
COUSIN
This paper presents the design of a high voltage scr pulse generator
that is useful for
testing highly attenuative
materials
in the pulse echo mode. Electrical
and ultrasonic
considerations
are discussed in detail followed
by an example
of a practical
circuit
design that can operate from a supply voltage of 1000 V at a pulse repetition
frequency
of 1000 Hz.
Introduction
There are two different methods commonly used for
exciting ultrasonic transducers in the pulse echo mode. The
first involves the generation of a medium voltage, pulsed,
radio frequency sinusoid using a gated oscillator, and the
second involves the generation of a high voltage spike by a
sudden discharge of a capacitor charged to several hundreds
of volts. Comparatively, the first method is the more
complex and the maximum pulsed voltage is typically
limited to tens of volts. On the other hand, the second
method, referred to as shock excitation, has the virtue of
being s’mple, being easier to build and producing a maximum
pulsed voltage capability of many hundreds of volts.
A well-known shock excitation curcuit that is still in common
use constists of a capacitor initially charged to a high
voltage which is suddenly applied to a transducer using a
thyratron as a switch.’
The advantages of using a thyratron as a switch are the
following: rise time less than 20 ns, current capability in the
many tens of amperes and breakdown voltage in excess of
I kV. On the other hand, the thyratron consumes a
significant amount of power and is bulky and quite fragile.
In applications such as testing highly attenuative materials
where high voltage pulses are required, avalanche
transistors” 3 or silicon controlled rectifiers (scr)4, ’ can be
used, since they can withstand hundreds of volts (singly or
in cascade), pass switching currents in the tens of amperes
and have fast turn-on time (generally less than 10 ns). As
the attenuation coefficient of the material to be tested
increases, higher value voltage pulses are required for
adequate penetration. This constraint implies either higher
breakdown voltage capability of the switching element or
circuit techniques that employ series connected elements.
Since scr’s are able to withstand higher voltages before
breakdown occurs than avalanche transistors, they are more
readily employed for applications involving highly
attenuative materials.
The authors are at the Department
of Electrical Engineering,
University of Toronto, Toronto, Ontario M5.S 1 A4, Canada.
Paper received 10 August 1978.
ULTRASONICS.
MARCH
1979
Although shock excitation circuits have appeared previously
in the literature (for example references ‘-‘), the design
considerations of high voltage pulse generators have not
been reported. In this paper, the electrical and ultrasonic
considerations for and the design of a practical scr pulse
generator are presented.
Circuit operation
The basic circuit used to generate a high voltage pulse is
shown in Fig. 1. It consists of a dc voltage source V,,
resistance R,,storage capacitor C, a damping or load
resistance R and an scr THl . An understanding of the
operation of the circuit is important for a proper
appreciation of the analysis that follows. Assuming that the
scr is initially off, capacitor C is charged to the supply
voltage V, through resistances R, and R.When a trigger
signal is applied to the gate of the scr, the latter turns on.
Storage capacitor C then discharges through the scr and the
resistance R,and a pulse voltage appears at the output of
the load resistance. When the charge stored in capacitor C
has dissipated, the scr turns off since its current has become
less than the required holding current. The storage capacitor
is then recharged to the supply voltage Vs.
Analysis of the basic circuit
The discharge operation of the storage capacitor is initiated
by triggering the ser. At this instant, the anode of the scr
does not return to ground potential due to the finite turn-on
0
TO
transducer
R
Trigger
SIgnal
Fig. 1
0041-624X/79/020081-04/$02.00
Basic scr pulse generator
0
1979
circuit
IPC Business
Press
81
time of the ser. If the storage capacitor was initially charged
to the supply voltage, V,, and if t,, is the turn-on time of
the scr, it can be shown that the output voltage across
resistance R during the turn on period is given by
VO
=_!!i~c(I
_,-*lRC)
(1)
ton
Equation (1) shows that as the turn-on time, t,, , increases,
the voltage at the output of the load resistance decreases.
From the frequency domain viewpoint, the shorter the
turn-on time of the scr, the more higher frequencies are
contained in the falling edge of the output pulse voltage and
the easier it is to excite proportionately
higher frequency
transducers.
During turn-on, an scr can be subjected to a very steep rise
of current, the rate of which may be faster than the rate of
activation of the gate junction so that current concentration
occurs in an area of the junction forming a hot spot.6 To
prevent formation of hot spots that might result in the
failure of the component, the critical rate of rise of the
on-state anode current (&/dt) must not be exceeded. Using
(1) and the fact that the output pulse current is the same as
the anode curent of the scr during turn-on, it can be shown
that the maximum rate of rise of anode current is given by:
die
dt
,,,=
-- v,
= Rt,,
(2)
There are two stages in the recharging operation of the scr
pulse generator. During the first stage, the output voltage,
vo, that was at a maximum at time t = t, returns to zero
exponentially. This is attained by the switching current
decaying towards zero through the R, C and TH 1 circuit.
The second stage begins with the scr turned off and is
followed by the recharging of the storage capacitor to the
supply voltage, VS.
An important feature of the pulse generator circuit under
consideration is that a commutation circuit is not required
to turn off the switching element. The scr is turned off when
the magnitude of the anode current is reduced to a level
below the holding current, ZH. This is facilitated by choosing
RS
I
V
max = (e)c.
(4)
Therefore, the above value of the rate of rise of anode voltage
must be less than the specified critical rate of rise of the
anode voltage to assure reliable firing of the ser.
82
The average electrical power, PAvE, delivered to the output
circuit (including the damping resistor) is given by:
1 cv,”
= - --
P*"E
2
T
= ;- CV,2f
where T is the period of the pulse repetition
the firing circuit.
frequency
of
If the turn-on time of the scr is very small ( and usually
attempts are made to achieve this in any practical design),
it is reasonable to assume that the output voltage exhibits
a step edge followed by an exponential decay depicted by:
Vomax e-*‘ICR
1
1
where
(7)
tr ZZ t - t,,
Att=t,,,v,=
(1) as
Vomx,
V Omax
Transforming
V,(jw)
-
=
6
which can now be calculated from
cR(1
_
e
*on/CR)
t on
(7) into the frequency
=
domain yields
v, maX L1 +jwr
where
7 = CR.
P = Kvo(jw)v;(jo)
To prevent the scr from re-firing when the storage capacitor
is being charged, it is necessary that the rate of rise of the
anode voltage, dV,/dt, be less than the critical rate of rise
of the anode voltage, dV/dt,,,,
of the ser. The maximum
rate of rise of anode voltage can be calculated to be
dt
pulse current of the ser.
The power of the pulse can then be expressed as
I,.
An scr may be triggered on by a rapidly rising anode voltage
in the absence or presence of a gate signal. A fast changing
anode voltage produces a pulse of current which flows
through the relatively small reactances of the junction
capacitances of the scr and is large enough to promote an
avalanche effect and thus turn on the scr.6
s
is the maximum
where I,,,
ve = -
Therefore, the rate specified by (2) should be less than the
guaranteed critical value of the ser.
3 <
In pulse echo systems, it is desirable that the damping
resistance be small such that the pulse energy transmitted
to the transducer is dissipated quickly to allow the transducer to receive echoes from close flaws or interfaces. The
lowest value of damping resistance depends essentially on
the maximum pulse current that the scr can deliver and the
supply voltage magnitude as
= Iw;,,,
L
1 t cIJ272
where K is a constant.
At the angular frequency, CJ* = l/7, the 3dB point occurs.
Presuming that the piezoelectric crystal is modelled as a
pure resistor, it can be seen from (8) that 50% of the pulse
energy is delivered to the output circuit and contained
within the band from zero to 0,. It should be noted that
since the pulse power is spread over the whole frequency
spectrum, a low Q transducer, like that usually used in
pulse echo applications, is excited by more energy than a
high Q transducer. Thus, if the central frequency of a low
Q transducer circuit is f,, then from (9) 50% of the
electrical power will be coupled to the output circuit if
f,
=
_-1
(10)
27rRC
ULTRASONICS.
MARCH
1979
Rate generator
Pulse amplifier
Pulse shor&ng
cwut
f High
I
1 “5
voltage pulse generotor
56 k
47nF
25 V
t
PI
611 B
Hammond
I
I
56
IN4006
620 k
2 2M
DI
’
2
t:
47k
16 nF
Fig. 2
0033==
oc
Circuit
diagram
of the scr pulse generator
If RC < 1/(27rf,), there is more energy m frequency bands
higher thanf, and if RC > 1/(27rfc), there would not be
enough energy to excite a higher frequency transducer but
more energy would be concentrated at lower frequency
bands. Equation (10) can therefore give an optimum value
of the time constant RC for a particular transducer. If the
transducer is modelled as a parallel RC equivalent circuit,
the bandwidth calculations given above will vary depending
on the effective bandwidth change caused by the crystal
capacitance.
Choice of the components
The choice of the time constant, RC, involves some conflicting requirements. The larger RC is, the less charge is
lost by the capacitor C while the scr is turning on, as
evidenced by (1). On the other hand, the smaller the time
constant RC, the more energy there is to excite high
frequency transducers as shown by (10). Although the
damping resistance is required to be small to dissipate the
pulse energy quickly, it cannot be made smaller than the
value that exceeds the scr’s capabilities (5). The value of the
capacitor, C, is required to be small not only for its small
size but also to keep its self-inductance to a minimum
value. A high self-inductance would reduce the rate of rise
of the scr anode current to an unacceptable value.
The scr should have a fast turn-on time, high switching
current capability and be able to withstand the required
MARCH
supply voltage. The reason for the fast turn-on time is
evident from (1). A high switching current is needed to
allow the use of a small-valued damping resistance (5). It
should be noted that this requirement becomes more
stringent as the supply voltage is increased.
In applications where the voltage requirement exceeds the
voltage rating of available high current, fast switching scr’s
two or more scr’s of the same type may be connected in
series. The supply voltage is made to divide across them
with a resistive divider, so that the breakdown voltage of
the individual scr’s is not exceeded.
of the basic circuit
For most industrial flaw detection applications where the
pulse repetition frequency ranges from 50 to 2500 Hz, the
time constant CR, must be kept small so that the storage
capacitor will be charged to the supply voltage at the end
of the pulse period. The resistance, R,, should be chosen
such that the current flowing through it is less than the
holding current specified by (3).
ULTRASONICS.
12k
1979
Practical circuit
Fig. 2 shows the scr pulse generator circuit. The rate
generator is realized by operating a 5.55 timer in an astable
mode. The output of the timer is connected both to the
input of a pulse amplifier and to the time base sync of a
scope.
The inverter (CD 4049) and the transistor Tl drive the dual
secondary transformer to make up the pulse amplifier. The
pulse transformer, PI, is used to couple the pulse amplifier
to the pulse sharpening circuits. The two secondary windings
provide isolation between the gates of the two scr’s and also
allow simultaneous triggering of the two series-connected
scr’s.
In order to obtain minimum turn-on time of the seriesconnected scr’s, it is desirable not only to trigger the gates
with a strong drive but also to produce a triggering pulse
with a fast rise-time.’ To accomplish this, the slowly rising
pulse produced at the secondary of the pulse transformer
is shaped to give a fast rise-time using a pulse sharpening
circuit first introduced by General Electric.’ The ensuing
pulse delivered to the scr has a rise time of about 50 ns.
83
for damping transducers having central frequencies in the
1 MHz range. For damping resistors of 33, 56 and 120 R.
the pulse widths of the output pulse are 0.61, 1.3 and
2 ps respectively. Fig. 3 shows the output waveform of the
scr pulse generator.
Conclusion
Design considerations for a high voltage scr pulse generator
have been given along with a practical circuit realization. The
treatment has been general in approach and is applicable to
other switching elements and voltage requirements. The
pulse generator circuit in Fig. 2 has been useful in ultrasonic
defectoscopy of wood with shock excited transducers
spanning the range from 0.25 to 2.2 MHz.~
Fig. 3 Output voltage wavetorm. Vertical scale 200 V div-’ ,
horizontal scale 1 us div-‘. R = 120 a. vs = 800 V
References
1
Scr 2N4203 was used because of its high peak forward
blocking voltage (700 V), high switching current capability
(100 A) and reasonably fast turn-on time (100 ns). High
valued resistors have been connected across each of the scr’s
to ensure that the supply voltage divides equally between
the scr’s. With the holding current, IH, of the scr being about
20 mA, the resistance R, was found using (3).
2
3
4
The storage capacitor, C, was chosen to be 4.7 nF. This
value allows the time constant CR, LO be small enough to
enable the capacitor to charge to the supply voltage V,
when the rate generator is operated at a pulse repetition
frequency of 1000 Hz. At the same time, the chosen value
of the storage capacitor (refer to (10)) enables the values of
the damping resistance to be tens or a few hundred ohms
5
6
7
8
Connolly, C.C. An Ultrasonic Generator
Giving Widely
Variable Parameters, Bio-Medical Engg, 3 (1968) 72-75
Cheney, S.P., Lees, S., Gerhard, Jr., Kranx, P.R. Step
Excitation
Source for Ultrasonic Pulse Transducers,
Ultrasonics, 11 (1973) 111-113
Myers, G.H., Thumin, A., Feldman, S., Santis, G., Lupo, F.J.
A Miniature Pulser-preamplifier
for Ultrasonic Transducers‘
Ultrasonics, 10 (1972) 87-89
Krautkramer,
J. Ultrasonic Testing of Materials, SpringerVerlag, New York (1969)
Wells, P.N.T. Physical Principles of Ultrasonic Diagnosis,
Academic Press, London (1969)
Dewan, S.B., Straughen,
A. Power Semiconductor
Circuits,
John Wiley and Sons, New York (1975)
‘SCR Manual’, 5th Ed, Semiconductor
Products Department,
General Electric, Syracuse, New York (1972)
Okyere, J.G., Cousin, A.J. OnFldw Detection in Live Wood,
IEEE Tram Sonics and Ultrasonics, in press
TheEvaluationandCalibration
of Ultrasonic Transducers
conference organized by the Materials and Testing Group
of the lnstitu te of Physics
The Geological Society, London, 11-12 May 1977
The proceeding of a two-day meeting concerned with
various aspects of the standardization of ultrasonic
transducers. The meeting attracted over a hundred
deiegates, from the UK, Austria, Belgium, Denmark,
France, West Germany, Italy, The Netherlands, Norway
and Switzerland, bringing together workers from
universities, medical establishments and from nondestructive testing laboratories.
The meeting was organized by the Materials and Testing
Group of the Institute of Physics in collaboration with
the Hospital Physicists Association, the Institute of
Acoustics, and the Institute of Quality Assurance.
It represents an invaluable survey of current transducer
technology
240 x 170mm/200
pages /illustrations
November 19781lSBN 0 902852 84 1
Price f 16.OOB41.60
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l
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84
theoretical considerations
practical evaluation
development and design features
absolute calibration
information
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ULTRASONICS.
MARCH
1979