Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRIC PULSE SHAPING CIRCUI~
Background of the Invention '
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This invention relates generally ~o pulse
amplifying and shaping circuits and more particularly
to an improved circuit of this kind for use as a gate ;
5 driver for a periodically triggered thyristor. ,
Many electronic circuits and apparatus employ
solid-state controllable switching devices known as '~,?~
thyristors or silicon controlled rectifiers ~SCRs).
A thyristor is typically a three-electrode device ;~;
10 having an anode, a cathode, and a control ox gate
terminal. When its anode and cathode are externally
connected in series with an electric power load and ,
a source of forward anode voltage (i.e., anode
potential is positive with respect to cathode)~ a
15 thyristor will ordinarily block appreciable load
current until a firing signal of appropriate amplitude
and duration is applied to the control terminal, ri;'~
whereupon it switches from its blocking or "off" state ~;i;
to a conducting or "on" state in which the ohmic value :;
20 o~ the anode-to-cathode resistance is very low. To ;.!
generate the firing signal required to trigger a high-
power thyristor, it is common practice to use a gate
driver that is activated by a command or gating signal
which in turn is supplied by associated control mea~sD
25 The criteria for designing a gate driver are well known
in the art - see, for example, the chapter entitled
"Gate Trigger Characteristics, Ratings, and Methods"
on pages 71-122 of the SCR Manual tFifth Ed., 1972)
published by the General Electric Company (Electronics
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Park~ Syracuse, NY). Generally speaking the waveform i,
of the firing signal should be characterized by a ~;',
sharply rising front, a high amplitude, and a sufficient ~.
length (duration) to ensure successful turn on of the "
5 thyristor when commanded by the c:ontrol means. ~ r
In many practical applications a thyristor
is triggered by a single pulse-like firing signal
having a very fast rise time, a ~igh amplitude, and .
a relatively short duration (of t:he order of 25
10 microseconds or less~. When only a single pulse is .
required, it is not difficult to design a gate driver fi
for such applications. For example, in a known gate
driver of this type a precharged capacitor is
discharged through the primary winding of a pulse
15 transformer so that th~ secondary current ~which is th~
firing signal) rises abruptly to a relatively high ~~
peak, and a series LC circuit is connected across the s~J,
capacitor so that its subsequent discharge will slightly
broaden this pulse of current. In other applications,
20 a thyristor requires a ~iring signal having a much
longer duration (e.g., longer than 1 millisecond). In
this case the initial rise time of the firing signal
is not critical and can be relatively slow, and gate
drivers using long pulse width transformers or known
25 filtering techniques to prolong or sustain the firing
signal can therefore be applied.
In some cases a combination of short single ;
pulse and long sustained firing signals is needed. An
example of this requirement is found in ~:
~30 Canadian patent application Serial Number 3 S ~, &~ ,
filed ~g~t ~,J~o for R. B. Bailey and T. D.
Stitt and assigned to the General Electric Company,
which application discloses a chopper type electric
propulsion system for d-c traction motors. For reasons "~!
35 that are explained in the re~erenced patent application,
in normal operation the main thyristor o~ the chopper
, is periodically fired in response to gating signals of
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relatively short duration (approximately 10
microseconds) that recur at either a constant frequency ',.
(300 Hz) or a variable frequency that is lower than the ,~/
constant frequency, but to turn o,n the chopper at the .~;
beginning of an electrical braking mode of operation of
the propulsion system, the main thyristor is intially
triggered by an extended firing signal generated in
response to a 2-millisecond burst of high frequency
(e.g., 21.6 KHz) discrete pulses (10 to 20 microseconds
10 each). In this case the use of prior art
techniques in the gate driver to obtain the extended
firing signal is not practical because the resulting
retardation of rise time would be unsatisfactory in .
normal operation. Other known pulse shaping circuits
15 that can generate extended firing signals are not ideal r'i;
for dual-purpose gate drivers because of one or more of .;
the following shortcomings: electrical isolation between .
control and power circuits is insufficient, an a-c '
control power source is required, cost is too high, or -~
20 initial rise time is too slow.
Summary of the Invention
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Accordingly, it is a general objective of
the present invention to provide an improved pulse
amplifying and shaping circuit useful in a dual purpose ;
25 gate driver that can successfully provide either a
single fast rising short duration firing signal or an
extended firing signal of much longer duration.
Another objective of the invention is the
provision of a relatively simple and low cost pulse
30 shaping circuit that enables a gate driver to generate -
a continuous firing signal havin~ a fast rise time
in response to a burst of high frequency gating signalsO
A more specific objective is to provide a
pulse shaping circuit well suited for practical
35 application in the gate driver for the main thyristor
of the electric power chopper that is used in the
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propulsion system disclosed in the above- ,~
cited Canadian Patent Application Serial
Numher 3~7,~g~ . :
In carrying out the invention in one form,
there is provided a capacitor in series with the primary
winding of a pulse transformer, al~d the capacitor is ,'
also connected through an inductor to an a-c powsr ;~
supply from which i~ accumulates a charge. Normally
open switching means is connected in parallel with the
lO primary winding and the capacitor, and means for
periodically closing the switching means is provided.
When the switching means is c~osed, it conducts `
discharge current from the precharged capacitor ,~
through the primary winding of the pulse transformer,
15 and it also conducts current from the d-c power supply '.
through the inductor. Subsequently, when the switching :,~
means is next opened, current in the inductor will '~
flow through the primary winding and assist the /,
recharging of the capacitor. A econdary winding of
20 the pulse transformer i5 connected through full wave
rectifier means to a pair of d-c output terminals adapted
to be connected to the gate and cathode of a thyristor. ,
The rectifier means includes a first diode connected
between the secondary winding and one of the output
25 terminals for conducting gate-to-cathode current when ,;
the precharged capacitor is discharging through the
primary winding, and a second diode connected between
the secondary winding and the same output terminal for
conducting gate current when capacitor recharging
30 current is flowing in the primary winding. A third
diode is connected in s ries with the second diode,
and a second capacitor is connected between the `
junction of the second and third diodes and the other ~ ~,
output terminal, whereby the second capacitor is
35 charged during th~ recharging intervals of the -;
capacitor on the primary side of the pulse transformer.
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20-TR-1292
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Consequently the second capacitor does not impede "
the rise time of the gate-to-cathode current when the
switching means is initially closed. If the switching
means is repetitively closed and opened at a suffi-
5 ciently high frequency, the charge on the second '
capacitor will supply gate current during the short i'
intervals when the transformer primary current is low
or crossing zero, thereby ensuring continuous gate
current after the first half cyclle of high frequency '~
10 operation.
The invention will be better understood and
its various objects and advantages will be more fully
appreciated from the following description taken in
conjunction with the accompanying drawing.
Brief Description of the Drawing
Fig. 1 is a schematic circuit diagram of a ;
pulse shaping circuit embodying the present invention;
and
Fig. 2 is a diagram of the waveform of the
20 output current that is generated by the Fig. 1 circuit
during the initial few cycles of operation in a burst ;:
firing mode. ;.
D~scription of the Preferred Embodiment
The pulse shaping circuit comprises a pulse ~;
25 transformer T1 having a primary winding 11 and at least
one secondary winding 12. Preferably the primary
winding 11 has twice the number of turns as the
secondary winding 12. The dot end of the primary 11
is connected to an input terminal 13 at ground
30 potential. A capacitor Cl is connected to the other
end of the primary, and the series combina~ion of the
primary winding 11 and the capacitor Cl is connected
via an inductor Ll to a relatively positive input
terminal 15 that is adapted to be connected to a
35 source of d-c control power from which the capacitor
Cl accumulates charge. The d-c control power source :
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includes a battery or the like (not shown) whose ;,
negative terminal i5 connected to ground and whose ~'
positive terminal is connected to a terminal 16, and it
should have a relatively high voltage (e.g., 100 volts). -
s In Fig. 1 a filter capacitor 17 of relatively larcJe ,
capacitance value (e.g., 50 microfarads) and a series
resistor 18 are shown connected between the positive
control power terminal 16 and the grounded terminal 13,
and the input terminal 15 is the junction of this ~;
10 capacitor and resistor. ,;~
A voltage clipping circuit comprising back-to-
back diodes 19 and 20 is connected across the inductor
Ll. As is shown symbolically in Fig. 1, the diode 19
is preferably a ~ener diode.
The relatively positive plate of the capacitor
Cl is connected through a current limiting resis~or 22
of small ohmic value (e.g., 20 ohms) and an input line 23
~o the collector of an NPN transistor 25 whose emitter ;,-
is grounded. Thus the transistor 25 is connected in
20 parallel with the capacitor Cl and the primary winding
11. This transistor is normally biased off by associated
bias means 26, and it serves as a normally open switching
means in the pulse shaping circuit of Fig. 1. The ~`
transistor 25 can be physically located remotely from ,
25 the other components of the illustrated circuit, in :;
which case the line 23 has measurable inductance which ,,!,
is shown symbolically at 27. Whenever the bias means
26 is commanded to forward bias the base-emitter
junction of the transistor 25, this devic switches
30 to a turned on or closed state, and while it remains
closed ~he collector current in line ~3 comprises a ~,
gating signal for activating the pulse shaping circuit.
The bi.as means 26 is suitably controlled so
as periodically to turn on the transistor 25.
35 practical example of means for determining the timing
of the turned on periods of this transistor is shown
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20-TR-1292
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and described in the above-cited Canadian Patent -
`~ Applicatinn Serial Number ~
- ~ The transistor that is herein identified ~y the reference
number 25 corresponds to transistor 255 in the referenced
application. In normal operation it is turned on for
a short period of approximately 10 microseconds at a maximum
repetitive rate or frequency of the order of 300 Hz, ,~
but occasionally, in response to a burst iring signal
of approximately 2-millisecond duration at the
10 beginning of electrical braking of the propulsion system
disclosed in the referenced application, the transistor
is turned on for the same short period at a much higher
frequency (e.g., 21.6 KHz). As will soon be apparent,
the pulse shaping circuit of the present invention will
15 respond equally well to either situation by generating i,
an appropriate firing signal for a thyristor 70.
The secondary winding 12 of the pulse
trans~ormer Tl is connected through full-wave rectifier
means 30 to a pair of d-c output terminals 14C and 14G.
20 These output terminals are coupled to a load circuit
comprising the cathode and gate terminals of a ,;
thyristor 70 that is connected in an electric power
circuit such as a chopper (not shown). A resistor 31
connected across the output terminals provides noIse
25 immunity and improves the dv/dt capability of the
thyristor 70. Between a conductor 32 on the relatively
positive side of the rectifier means 30 and the output
terminal 14G there is a series resistor 33 that
promotes current sharing between the secondary circuit
that is illustrated in Fig. 1 and additional secondary
circuits that can be coupled to the same transformer '
primary winding 11.
As is shown in Fig. 1, the rectifier means 30
comprises a fire;t divde Dl connected between the dot
end of the transformer secondary winding 12 and the
conductor 32 ancl poled to cQnduct current from the dot
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end to the conductor 32, second and thixcl serially ,~
interconnected diodes D2 and D3 connected between the ,~
opposite end of the secondary winding and the ~'
conductor 32 and poled to conduct current from the ,,'
5 opposite end to the conductor 32, a fourth diode D4 '
connected between the dot end of the secondary and the
output terminal 14C and poled to conduct current fxom
this terminal to the dot end of t'he secondary, and a fifth '
diode DS connected between the op]posite end of the
10 secondary winding 12 and the same output terminal 14C ~'
and poled similarly to diode D4. A capacitor C2 is
connected between the output terminal 14C and th~ r
junction of the second and third diodes D2 and D3.
The charging path of this capacitor includes diodes ~,
15 D2 and D~, and the discharging path includes the diode ,.~
D3,. the resistor 33, and the gate-cathode junction of ''
the thyristor 70. '
The operation of the pulse shaping circuit will .,
now be described. The transistor 25 is normally in a :
20 turned off or open state, and usually the capacitor Cl
is precharged to the steady-state voltage level of the ~',
d-c control power source (100 volts) with the polarity
shown in Fig. 1. When the transistor is turned on or , '
closed, it provides a low-resistance path for discharging
25 the capacitor Cl through the primary winding 11 of th~ '!'
pulse transformer Tl, and at the same ~ime it also, .:-
conducts current from the positive input terminal 15
through the inductor Ll. ..
Nhen the fully precharged capacitor Cl is '!"
30 thus swi~ched across the transformer primary 11 (in
-series with the line 23), its discharge current rises
steeply to a relatively high peak in the primary
w'inding. The resulting "positive" half cycle of~ '
secondary current is conducted through the diodes Dl
35 and DS and the gate-cathode junction of the thyristor , ' ;;
70, and this current is therefore'the desired fast
.. rising single-pu:Lse firing signal iG fox triggering , ,r
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the thyristor 70. Its r:ise time is not impeded by the
capacitor C2 because the aiode D3 prevents this
capacitor from charging during the positive hal~ cycles
o~ secondary current. The capacitance value of the
capacitor Cl (e.g., 1 microarad) is selected so that
during the conducting period (e.g., approximately 10 ,
microseconds) of the transistor 25 this capacitor will
not fully di~charge and its voltage will not fall below
a predetermined level (e.g., 20 volts) at which the
10 magnitude of current iG in the transformer secondary
would fall below the minimum ga~e trigger current r;
(e.g., 0.3 ampere) of the thyristor 70. This ensures
tnat the magnitude of the initial pulse of secondaxy
current is higher than the minimum gate trigger current `
throughout the conducting period o~ the transistor 25.
When the transistor 25 changes to its normally lr~
- open state at the end of the above-described conducting
period, the current in the inductor Ll transfers from
this transistor to the capacitor Cl and the primary
20 winding 11, and it assists the subsequent recharging of
the capacitor Cl. The recharging curxent flows from
the positive terminal 15 and through inductor Ll, the
capacitor Cl, and the transformer primary 11 to the
grounded terminal 13. In the transformer primary its
direction is reverse to that o~ the discharging
current. A finite time, which can be referred to as ~he
commutation time, is required for the current in ^
inductor Ll to transfer from the transistor 25 to the
primary winding 11 when the transistor is turned off,
and during this time the transformer secondary current
decreases to zero and increases with opposite polarity.
While there is recharging current in the transformer
primary, a "negative" hal~ cycle o~ secondary current
is conducted through the diodes D2, D3, and D4 and the ;`,
thyristor gate. This negative ha~ cycle has a lower
amplitude but a longer duration than the preceeding
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positive half cycle of secondary current. During the
negative half cycle some of the current in diodes D2
and D4 is diverted into the capacitor C2 which is ?;
charged thereby. Later, during intervals when the
5 potential of line 32 is lower than the potential on
its positive plate, this capacitor will discharge into ;
the thyristor gate. ;~.
The inductance value of the inductor Ll (e.g., '
1 millihenry) is selected so that at the end of the
10 10-microsecond conducting period of the transistor 25
the curxent in Ll will have built up to at least a
predetermined minimum magnitude which is about equal to ;'
the magnitude of capacitor discharge current than flowing ;
in the trans~ormer primary winding 11 (e.g., approx-
15 imately 0.5 ampere) and so that during an ensuing non-
conducting interval of a predetermined duration the
recharging current of the capacitor Cl will not decay
below this predetermined minimum magnitude. This
ensures tha~ the magnitude of the second half cycle of
20 secondary current is above the minimum gate trigger
current of the thyristor 70 for at least as long as ``
said predetermined duration. Preferably the predeter-
mined duration of the non-conducting interval of the ;,
transistor 25 is of the order of 40 microsPconds or
25 less. Consequently, if the 10-microsecond conducting
periods of the transistor 25 are repeated at a
frequency of 20 KH~ or higher, the illustrated pulse
shaping circuit is operative, after its initial half
cycle, to supply a continuous gate current i~ to the
30 thyristor 7Q. The extended firing signal that the
pulse shaping circuit supplies to the thyristor during
the first thxee cycles of its operation in this burst
firing mode is illustrated in Fig. 2.
At time to in Fig. 2 the transistor 25 is
35 initially turnedL on, and the resulting discharge of
the precharged capacitor Cl produces a steep-rising
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high-amplitude pulse of gate current iG which is not
impeded by the capacitor C~. At the end of the first
conducting period of the transistor 25, current in
the transformer primary winding decreases to zero and
reverses direction as the capacitor Cl stops
discharging through the transistor and commences
recharging through the inductor Ll. The first zero
crossing of transformer primary current occurs at time
tl, and as is shown in Fig. 2 the gate current
lO momentarily decreases to zero at the same time. ~uring
the ensu-ng non-conducting interval, the capacitor Cl ~;
is being recharged and the negative half cycle of the
transformer secondary current serves as the gate signal
iG. The second capacitor C2 accumulates a charge
15 during this negative half cycle of secondary current.
When the transistor 25 is turned on for the
second time in this burst firing mode of operation,
current in the transformer primary winding will
again decrease to zero and reverse direction as the
20 capacitor Cl stops recharging through the inductor Ll
and commences discharging through the transistor, but
there is no corresponding decrease of iG because the ~
capacitor C2 is now charged and will supply gate current ,
during the short commutation time when the transformer
25 primary current is low or crossing zero. The gate
current is quickly driven to another high peak as the
capacitor Cl discharges through the transformer
primary and the transiskor 25 during the second conducting
period of the transistor. At the conclusion of the
30 secona conductin~ period, the capaci~or C2 is again
able to supply gate current while the transformer
primary current is crossing zero. The gate current '
waveform will continue generally as shown in Fig. 2
throughout the remainder of the burst firing mode of -;
35 operation, with some loss of amplitude due to the
partial discharge of the filter capacitor 17 during
the burst firing interval.
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In the illustrated embodiment of the ,.
invention, the capacitive value of the capacitor C2 ;~
(e.g., 2.2 microfarads) is selected so that the time
constant of the discharging path of this component
is longer than the commutation tilme of the transformer
primary winding 11. As previously mentioned, this
commutation time is the time required for the current
in the inductor Ll to transfer from the transistor 25
to the primary winding 11 when the transistor changes
10 from its closed state to its normally open state. In
practical embodiments of the invention, a time constant
of the order of 3 to 5 microseconds is contemplated.
While a preferred embodiment of the invention
has been shown and described by way of example, many
15 modifications will undoubtedly occur to persons skilled
in the art. The concluding claims are therefore
intended to cover all such modifications as fall within
the true spirit and scope of the invention.
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