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Claims  |
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We claim:
1. A high voltage power supply providing a plurality of concurrent output
signals, comprising:
input terminal means (P5) for providing energization potential from a
unidirectional source;
output terminal means (P1) for providing a common reference for each of the
plurality of concurrent output signals;
a first voltage generating circuit (12) comprising:
first oscillator circuit means (22) having a first tuned network including
first transformer means (46), for developing a first sinusoidal signal at
the resonant frequency of said first tuned network,
first regulator circuit means (20) for developing pulses at a particular
repetition rate and duty cycle, for controlling the amplitude of said
sinusoidal signal by interrupting the flow of energization potential to
said oscillator circuit means from said input terminal means,
second transformer means (24) for significantly increasing the amplitude of
said sinusoidal signal, and
voltage multiplier and rectifier circuit means (76, 76') including a second
output terminal (P2), coupled to said second transformer means (24) for
developing a unidirectional output voltage of several tens of kilovolts
amplitude across said common output terminal means (P1) and said second
output terminal (P2);
a second voltage generating circuit (14) comprising:
second oscillator circuit means (30) having a second tuned network
including a third output terminal (P3) and third transformer means (32)
for developing a second sinusoidal signal at the resonant frequency of
said tuned network floating at said unidirectional output voltage across
said common output means (P1) and said third output terminal (P3),
second regulator circuit means (28) for developing pulses at a particular
repetition rate and duty cycle for controlling the amplitude of said
second sinusoidal signal by interrupting the flow of energizing potential
to said second oscillator circuit means from said input terminal means;
and a third voltage generating circuit (16) comprising:
third oscillator circuit means (38) having a third tuned network including
fourth transformer means (40), for developing a third sinusoidal signal at
the resonant frequency of said third tuned network,
third regulator circuit means (36) for controlling the amplitude of said
third sinusoidal signal by varying the magnitude of the flow of energizing
potential from said input terminal means (P5) to said third oscillator
circuit means, and rectifier circuit means (D.C.) for converting said
third sinusoidal signal into a unidirectional output signal of an
amplitude varying slightly from that of said unidirectional output voltage
across said first output terminal means (P1) and a fourth output terminal
(P4).
2. The power supply of claim 1 wherein the pulse repetition rate of each of
said first and second regulator circuit means (20, 28) is at least twice
the resonant frequency of each of said tuned networks of their respective
voltage generating circuits.
3. The power supply of claim 2 wherein each of said first and second
oscillator circuit means comprises a push-pull constant current
oscillator.
4. The power supply of claim 2 wherein each of said first and second
regulator circuit means (20, 28) includes means (R1, R2) for individually
establishing the particular duty cycle of the pulses developed by each of
said first and second regulator circuit means.
5. The power supply of claim 2 wherein each of said first and second
regulator circuit means (20, 28) comprises a switching type regulator.
6. The power supply of claim 2 wherein said third regulator circuit means
(36) comprises a linear impedance type regulator.
7. The power supply of claim 2 wherein each of said first, third and fourth
transformer means (46, 32, 40) includes a single secondary winding (W9,
W4, W6) and at least one primary winding coupled across a capacitive
reactance (C3, C22, C28) to form respective ones of said tuned networks.
8. The power supply of claim 7 wherein at least said third and fourth
transformer means (40, 32) are constructed to exhibit a significantly high
electrical insulation between their primary and secondary windings.
9. The power supply of claim 2 wherein said voltage multiplier and
rectifier circuit means (76, 76) comprise complementary pairs of multiple
units of diodes and capacitors and said complementary pairs are parallel
coupled across said common output terminal means (P1) and said second
output terminal (P2).
10. The power supply of claim 2 wherein each of said first, second and
third voltage generating circuits (12, 14, 16) further comprises circuit
means which include a sense winding (W10, W13, W14) on each of said
second, third and fourth transformer means for providing a feedback signal
indicative of a variation in the amplitude of the output voltage across
the output terminals of each of said voltage generating circuits to adjust
the operation of each of said first, second and third regulator circuit
means thereby to vary the amplitude of the sinusoidal signal developed by
each of said oscillator circuit means (22, 30, 38).
11. The power supply of claim 10 wherein said first voltage generating
circuit (12) further comprises circuit means (D2, D8, Q4, R-20-23) to
which the feedback signal is applied for terminating the development of a
sinusoidal signal by said first oscillator circuit means (22) in response
to a substantial load curent drain across the output terminals (P1, P2) of
said first voltage generating circuit.
12. A high voltage power supply providing a plurality of concurrent high
voltage output signals, comprising:
a first circuit (12) having a pair of input ports connectable across a
source (18) of electrical energy, comprising:
first regulating means (20) coupled to said input ports and having a first
intermediate terminal (P7), for inverting said electrical energy into a
first train of pulses characterized by a first pulse frequency and a first
average amplitude, said first regulating means including first means (R1)
for varying said first average amplitude;
first oscillator means (22) including a first resonant circuit exhibiting a
first resonant frequency connected to said first intermediate terminal,
for transforming said first train of pulses into a first sinusoidal signal
having a peak-to-peak amplitude exceeding said first average amplitude;
first transformer means (24) having a primary winding (W1) coupled to
receive said first sinusoidal signal, and a secondary winding (W2), for
amplifying said first sinusoidal signal; and
first and second rectifying means (76, 76') having opposite polarities and
collectively providing a common terminal (P1) and a reference terminal
(P2), additively coupled in parallel across said secondary winding of said
first transformer means for rectifying said amplified first sinusoidal
signal and for multiplying the amplitude of said second sinusoidal signal
to provide a substantially constant high voltage signal across said first
pair of output terminals;
a second circuit (14) having a pair of input ports connectable across said
source, comprising:
second regulating means (28) coupled to said input ports and having a
second intermediate terminal (P22), for inverting said electrical energy
into a second train of pulses characterized by a second pulse frequency
and a second average amplitude, said second regulating means including
second means (R2) for varying said second average amplitude;
second oscillator means including: second transformer means (32) having a
secondary winding (W4) providing at one end a first output terminal (P3)
and coupled at its other end to said common terminal and a center-tapped
primary winding (W3), for inducing a second sinusoidal signal across said
secondary winding (W4); first reactive means (C22) coupled across said
primary winding (W3) to form a second resonant circuit with said second
transformer means; second reactive means (L2) coupled between said second
intermediate terminal and said center tap; and first switching means (30)
having a pair of alternately conducting switching devices (Q6, Q7)
connected between said second intermediate terminal and different ends of
said primary winding (W3) for cyclically providing paths of current flow
from alternate of said different ends of said primary winding (W3); a
third circuit having a pair of input ports connectable
across said source, comprising:
third regulating means (36) interposed between said input ports and an
intermediate terminal (P26) for providing an intermediate potential
difference between said intermediate terminal and said reference terminal,
said third regulating means including third means (R3) for varying the
amplitude of said intermediate potential difference;
second oscillator means including: third transformer means (40) having a
second winding (W6) and a primary winding (W5) for inducing an alternating
signal across said secondary winding; third reactive means (C28) coupled
across said primary winding (W5) to form a third resonant circuit with
said third transformer means; and switching means (Q9) coupled to said
intermediate terminal for cyclically coupling said intermediate terminal
to one end of said primary winding (W5); and
means (D1, C1) providing at one end a second output terminal (P4) and
coupled at its other end to said common terminal, for converting said
alternating signal into an output signal having an amplitude varying from
the amplitude of said high voltage signal between said reference terminal
and said second output terminal.
13. The power supply of claim 12 wherein said second and third transformer
means comprise:
core means (T2, T3) each including pairs of legs, for concentrating lines
of magnetic flux in ferromagnetic paths within said core means;
a plurality of electrically insulating means (A2, B2, A3, B3) each
encircling different ones of said legs;
a first and equal plurality of coatings (S1) of an electrically conducting
material exhibiting a first electrical conductivity completely covering
the surface areas of different ones of said insulating means adjacent to
said core means; and
means (X7, X8) for interconnecting said core means and said first plurality
of coatings to one side of said secondary winding of said first
transformer means.
14. The power supply of claim 13 wherein said primary (W3, W5) and said
secondary (W4, W6) windings of said second and third transformer means
have a second and greater electrical conductivity and are wound around
different ones of said insulating means to generate a magnetic flux in
corresponding ones of said core means, further comprising a second
plurality of coatings (S3, S4, S5, S6) of said electrically conducting
material for separating and completely surrounding different ones of said
primary and secondary windings and lining the surface areas of
corresponding ones of said insulating means adjacent to said windings.
15. The power supply of claim 14 wherein each of said second plurality of
coatings are separately coupled to corresponding ones of said primary and
secondary windings.
16. A high voltage power supply providing a plurality of concurrent output
signals, comprising:
first (20) and second (28) regulator stages each having a pair of input
ports connectable across a source of electrical energy, each providing a
separate intermediate terminal (P7/P22), each inverting said electrical
energy into separate trains of pulses characterized by average amplitudes
and respective first and second operational frequencies, and each of said
regulator stages including separate impedance means (R1/R2) for
independently varying the duty cycle of a corresponding one of said
regulator stages and thereby changing said average amplitude of the
corresponding one of said trains of pulses;
first and second oscillator stages (22/30+32) each including respective
ones of a first and second reactive impedance (L1/L2) separately coupled
to corresponding ones of said intermediate terminals, first and second
transformer means (46/32) each having a secondary winding (W9/W4) and a
primary winding (W7/W3) having a center tap (P6/P21) connected to said
corresponding ones of said intermediate terminals via one of said first
and second reactive impedances for inducing first and second sinesoidal
signals across corresponding ones of said secondary windings, third and
fourth reactive impedances (C3/C22) forming first and second resonant
circuits exhibiting respective first and second resonant frequencies with
respective ones of said transformer means, said first and second
operational frequencies being greater in value than corresponding of said
first and second resonant frequencies, and first and second switching
means (22/30) having a pair of alternately conducting switching devices
(Q2, Q3/Q6, Q7) connected between said intermediate terminal and different
ends of a corresponding one of said primary windings (W7/W3);
third transformer means (24) having a secondary winding (W2) and a primary
winding (W1), coupled to said first transformer means (46) and providing a
step-up relation to said secondary winding (W2), for transforming said
first sinusoidal signal into a third sinusoidal signal;
first (76) and second (76') complementary voltage multiplier stages
additively coupled in parallel across said secondary winding of said third
transformer means, having a common terminal (P1) and a reference terminal
(P2) forming a first pair of output terminals, and providing an output
voltage having an amplitude on the order of several tens of kilovolts at
said common terminal, said common terminal being coupled to one side of
said secondary winding of said second transformer means;
a third regulator stage (36) having a pair of input ports connectable
across said source of electrical energy and providing an intermediate
potential difference, said third regulator stage including means (R3) for
varying the amplitude of said intermediate potential difference;
third oscillator stage (38+40) including fourth transformer means (40)
having a primary (W5) and a secondary (W6) winding, a fifth reactive
impedance (C28) forming a third resonant circuit with said fourth
transformer means, and third switching means (Q9) connected to said third
resonant circuit coupled between said third regulator stage and said third
resonant circuit for cyclically applying said intermediate potential
difference to said third resonant circuit; one side of said secondary
winding (W6) of said fourth transformer means being connected to said
common terminal;
said second and fourth transformer means having cores of magnetic material
electrically coupled to one side of said secondary winding of said third
transformer means; and
means (D1, C1) coupled to said secondary winding of said fourth transformer
means for converting a signal occurring across said secondary winding into
an output signal having an amplitude differing from the amplitude of said
output voltage.
17. The power supply of claim 16 wherein said first and second operational
frequencies are greater than respective ones of said first and second
resonant frequencies by at least a factor of two.
18. The power supply of claim 17, further comprising circuit means which
include a sense winding (W10, W13, W14) on each of said first, second and
forth transformer means for providing a feedback signal indicative of a
variation in the amplitude of the output voltage across said secondary
windings of each of said first, second and fourth transformer means to
adjust the operation of each of said first, second and third regulator
stages and thereby vary the amplitude of the sinusoidal signal developed
by each of said oscillator stages. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention relates to electrical power supplies and, more particularly,
to a power supply for providing a plurality of high voltage output
signals.
BACKGROUND ART
High voltage power supplies have typically been low frequency networks
characterized principally by their mass, bulk and relative inefficiency.
Recent advances in such fields as diagnostic x-ray devices (e.g., the Low
Intensity X-ray and Gamma-ray Imaging Device disclosed in U.S. Pat. No.
4,142,101) have established a demand for portable, battery driven power
supplies able to deliver a plurality of regulated, static output voltages
on the order of tens of kilowatts for periods of several hours. The
operational characteristics of such devices usually require that each
potential furnished by a power supply be separately adjustable.
Currently available battery operated power supplies typically have a
push-pull inverter stage coupled across a center tapped primary winding of
a saturable core step-up transformer. When the core saturates, a potential
is induced in a second winding which causes the inverter stage to reverse
its mode of conduction. The frequency of operation of the inverter stage
is principally controlled by the saturation time of the transformer. Often
in such configurations, a feed-back potential is derived from a secondary
winding and applied to control the amplitude of the voltage applied by the
inverter stage to the transformer. A voltage multiplier stage driven by
another secondary winding is used in such configurations to provide an
increased output voltage while a voltage divider stage converts the output
from the multiplier stage into a plurality of different output signals.
The use of voltage induced in the secondary winding, as practiced by these
prior art power supplies for switching the conduction mode of the inverter
stage causes the output current provided by the transformer's secondary
winding to be generated as a series of square waves pulsing at the same
frequency at which the inverter is being switched. In effect, such power
supplies rely upon the transformer to establish the switching frequency of
the inverter stage; harmonics of the switching frequency are, therefore,
included in any signal appearing across the secondary windings of the
transformer. The presence of such harmonics introduces substantial
undesired ripple into the output signals provided by the power supply.
Moreover, the use of a divider stage to provide multiple output signals
prevents independent adjustment of the output potentials. Furthermore, the
presence of high current spikes occurring during changes in core
saturation and the sudden change in the amplitude of current which occurs
at each transition between pulses causes a reflected ripple current which,
in turn, causes electromagnetic noise that detrimentally interferes with
the operation of any neighboring electrical equipment. Also, the saturable
core of the transformer is a significant source of energy loss, a factor
which renders these configurations unsuitable for use in battery powered,
high potential supply sources. The presence of such ripple renders this
type of power supply unsuitable for use in applications where both a
constant high voltage and a well regulated but much lower amplitude
voltage floating at the high voltage are required as output signals
because the ripple from the high voltage stage destroys the regulation of
the low voltage stage.
Attempts to improve the regulation of output potentials have included
efforts to compensate for variations in output voltages due to causes such
as changes in loading. Such efforts typically rely upon a pulse width
modulator to regulate a chopping transistor driving the center tap of the
primary winding. In these configurations a feedback loop, such as a
current sensing stage is often used to provide an analog signal for
controlling the duty cycle of the modulator in proportion to changes in
the loading of the transformer's secondary winding. This type of power
supply is not suitable for providing high voltages, however, because of a
lack of electrical insulation between the input and output sides of the
circuit. Moreover, such power supplies require synchronization between the
pulse width modulator and the transformer, a feature which restricts the
range over which the duty cycle of the modulator may be varied to
compensate for changes in loading of the power supply.
Other power supplies have attempted to obtain well regulated output signals
by using a separate control circuit having ancillary oscillator and base
drive stages to regulate switching of transistors driving a transformer in
a power converting stage. The ancillary circuits themselves require a
power supply. The presence of such ancillary circuits and their individual
power supply undesirably adds to the complexity and physical bulk of the
overall design. Moreover, the control circuit in such power supplies is
driven by a feedback signal obtained directly from the output terminals of
the power supply, a feature which prevents insulation of the control
circuit from the output voltages and, therefore, renders these power
supplies unsuitable for generation of high output voltages.
Recent efforts to enable a power supply to provide high voltages suitable
for operation of x-ray tubes have included a variety of capacitive
discharge circuits. One power supply, for example, included a motor
driven, rotating commutator providing sequential discharge of individual
capacitors through the primary winding of a step-up transformer. Although
capacitive discharge type power supplies are adequate for providing high
voltage impulses of short duration, without extensive, power consuming
filtering, the transient phenomenon accompanying discharge renders such
power supplies unsuitable for providing well regulated output voltages.
Additionally, the presence of motor driven, rotating communators makes
such power supplies less than ideal for use in small, portable devices.
STATEMENT OF INVENTION
Accordingly, it is an object of the present invention to provide an
improved high voltage power supply.
It is another object to provide a power supply able to continuously furnish
a well regulated output potential on the order of tens of kilovolts.
It is a further object to provide a battery powered power supply able to
continuously furnish a well regulated output potential on the order of
tens of kilovolts.
It is yet another object to provide a power supply providing a plurality of
independently adjustable, high voltage output potentials.
It is also an object to provide a power supply providing a plurality of
well regulated, high voltage outputs.
It is a still further object to provide a portable, battery powered power
supply able to continuously furnish a well regulated output potential on
the order of tens of kilovolts.
These and other objects are achieved with a high voltage power supply
having a plurality of circuits coupled in parallel between an
unidirectional energy source and a common output terminal, and providing a
plurality of concurrent output signals. Two of the circuits include
modulated regulator stages driving corresponding oscillator stages with
pulse trains having particular repetition rates. A third one of the
circuits has a regulator stage controlling the flow of energy between the
energy source and a third oscillator stage. Each of the oscillator stages
includes a transformer forming part of a tuned network and providing a
sinusoidal signal at the frequency of the tuned network across secondary
windings coupled on one side to the common output terminal. One of the
first two circuits also includes a step-up transformer supplying an
amplified corresponding one of the sinusoidal signals to a parallel pair
of complementary poled rectifying, voltage multiplier stages which, in
turn, provide a unidirectional output voltage having an amplitude on the
order of several tens of kilovolts between the common output terminal and
a network reference. The sinusoidal signal provided by the second of the
two circuits floats at the amplitude of the unidirectional output voltage
while the third circuit includes a rectifier stage converting the
corresponding sinusoidal signal into a unidirectional signal having an
amplitude varying slightly from that of the unidirectional output voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of this invention and many of the attendant
advantages thereof will be readily apparent as the same becomes better
understood by reference to the following detailed description in which
like numbers indicate the same or similar components, and wherein:
FIG. 1 is a block diagram showing an embodiment of the present invention
arranged to provide power to a typical load.
FIG. 2 is an electrical schematic diagram of a high voltage circuit
included in the embodiment of FIG. 1.
FIGS. 3A through 3B illustrate waveforms of signals occurring at several
points throughout the high voltage circuit shown in FIG. 2.
FIG. 4 is an electrical schematic diagram of a second circuit included in
the embodiment of FIG. 1.
FIG. 5 is an electrical schematic diagram of a third circuit included in
the embodiment of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Refer now to the drawings and, in particular, to FIG. 1 which illustrates
the interconnections between the various stages of a power supply for
providing several independently adjustable output potentials to such
devices as a triode type x-ray tube 10. The power supply is formed by
three multi-stage circuits 12, 14, 16 coupled in parallel between a direct
current source such as a battery 18, and a common output terminal P1.
Circuit 12 includes a switching regulator stage 20 having its input coupled
across battery 18 for driving a sine wave oscillator 22. An adjustable
resistance, such as a rheostat, R1 is included in regulator stage 20 to
adjust its duty cycle. The output of oscillator 22 is applied across a
primary winding of a step-up isolation transformer 24. A secondary winding
of transformer 24 is coupled across a voltage multiplier 26 which
furnishes a negative potential difference on the order of several tens of
kilovolts between output terminals P1, P2. Output terminal P1 is common to
all three circuits while terminal P2 is connected to a reference potential
such as a network ground.
Circuit 14 has a switching regulator stage 28 coupled across battery 18 and
a rheostat R2 providing adjustment of its duty cycle. Regulator stage 28
provides a power to a push-pull drive stage 30 which, in turn, has its
output applied across a primary winding of an isolation transformer 32
which, in turn, develops an alternating voltage on its secondary winding.
One side of the secondary winding of transformer 32 is coupled to common
output terminal P1 while the other side thereof forms output terminal P3.
Circuit 16 includes a linear regulator stage 36 coupled across battery 18
for providing power to oscillator drive stage 38. Regulator stage 36
operates as a linear resistance which consumes power from oscillator 38 to
control the amplitude of generated signals in response to variations in
output voltage sensed by the regulator. As adjustable resistance R3 is
provided for the regulator. The output of drive stage 38 is applied across
a primary winding of an isolation transformer 40. A secondary winding of
transformer 40 has one leg coupled to common output terminal P1 and its
other leg coupled to terminal P4. This winding develops an output signal
which is rectified by a diode D1 and filtered by a capacitance C1 to
provide a low amplitude direct voltage between terminals P1 and P4.
Transformers 24, 32 and 40 are compact, high voltage isolation transformers
which are disclosed in a copending application entitled "High Voltage
Isolation Transformer" filed on June 21, 1983, and assigned Ser. No.
506,477; also identified as NASA Case No. GSC 12,817-1.
In the power supply of FIG. 1, circuit 12 is designed to provide a well
regulated voltage having an amplitude on the order of several tens of
kilovolts (e.g., -80 kV) between terminals P1, P2 while the other circuits
14, 16 are designed to provide output signals at voltages differing from
the voltage occurring between terminals P1, P2 by less than one kilovolt.
In the exemplary application shown, terminals P1, P2 are connected to
establish a potential difference between a filament 34 and an anode 44 of
x-ray tube 10. Output terminals P1 and P3 are coupled across filament 34
while terminal P4 is coupled to a grid 42 of x-ray tube 10. The
commonality of output terminal P1 to all three circuits assures that
output signals provided by circuits 14, 16 float at a high voltage near
the level of the constant potential difference between terminals P1, P2.
Consequently, the signal applied to filament 34 by circuit 14 has a direct
current potential of several tens of kilovolts (e.g., -80 kV) with respect
to terminal P2. The power supply also applies a direct voltage to grid 42
which has an amplitude varying between the sum and difference of the high
voltage appearing between terminals P1, P2 and the potential supplied by
circuit 16 between terminals P1, P4. The potential difference between
terminals P3, P4 is designed to be slightly more negative (e.g., 80 to 150
volts) than the potential applied across filament 34 to enable grid 42 to
repel and thereby focus electrons emitted by filament 34 in to a stream
directed toward anode 44, thus causing anode 44 to emit an x-ray beam 45.
Anode 44 is coupled to the network reference potential.
FIG. 2 illustrates in detail the several stages of high voltage circuit 12.
Regulator stage 20 is formed with a pulse width modulating regulator M1,
such as a commercially available sixteen pin SG 1524 integrated circuit
chip manufactured by Silicon General Company. Resistance R4 and
capacitance C2 are coupled between ports 6 and 7 of regulator M1 and a
reference potential, to establish the internal pulse repetition frequency,
f.sub.1, of regulator stage 20. Adjustable resistance R1, in conjunction
with fixed resistances R5, R6, form a voltage divider establishing an
adjustable reference voltage (across resistances R5 and R1) and a fixed
reference voltage (across resistance R6) to ports 2 and 16, respectively,
of regulator M1 for establishing the duty cycle of regulator M1. Power is
supplied directly to regulator stage 20 via input terminal P5 to port 15
of regulator M1 and to the remainder of circuit 12 via the emitter and
collector electrodes of a transistor Q1. The base of transistor Q1 is
biased by resistances R7, R8 and connected, via resistance R7, to output
ports 12, 13 of regulator M1. This enables regulator M1 to establish the
bias voltage applied to the base of transistor Q1 and thereby control
whether transistor Q1 is in a conducting or a non-conducting mode so that
while regulator M1 is in one state of its duty cycle, transistor Q1 is
held in a conducting mode, thereby allowing current to flow from terminal
P5 to the center taps P6, P8 of primary windings W7, W8, respectively, of
a step-up transformer 46 via the emitter and collector electrodes. While
regulator M1 is in the other state of its duty cycle, the bias voltage
applied by regulator M1 to the base of transistor Q1 holds the transistor
in a non-conducting mode, thereby interrupting the flow of current between
terminal P5 and transformer 46. Each output port of modulator M1 provides
regulation for fifty percent of a cycle. Coupling the base electrode of
transistor Q1 to both output ports enables regulator M1 to adjust the
modulation of the transistor between about five and ninety-five percent of
each cycle. Transistor Q1 is, in effect, a chopper which provides a
continuous train of direct current pulses to an inductance L1 and center
tap P6 of winding W7 of transformer 46. The width and the frequency of the
pulses are controlled by the duty cycle and frequency, f.sub.1,
respectively, of regulator M1. The waveform of the train of voltage pulses
appearing at node P7 is shown in FIG. 3A. With a battery 18 providing a
direct voltage of about fifteen volts to terminal P5, and regulator M1 set
to operate with a duty cycle of about fifty percent, the average voltage
appearing at node P7 will be approximately seven and one-half volts.
A constant current sinewave oscillator 22 is formed by transformer 46, a
pair of transistors Q2, Q3 coupled in a push-pull configuration between
the two center tapped windings W7, W8 of transformer 46, and inductance
L1. Inductance L1 is coupled between the collector electrode of transistor
Q1 and center tap P6 of primary winding W7. The energy stored in
inductance L1 maintains a continuous current flow through winding W7 while
transistor Q1 is held in a non-conducting mode. The waveform shown in FIG.
3B represents the amplitude of current flowing through inductance L1. The
base electrodes of transistors Q2, Q3 are coupled across winding W8. Bias
voltages are applied to the base electrodes by the two sections of winding
W8 through a potential applied to center tap P8 from current flowing from
node P7, through a series of resistances R10, R11 and a diode D3 which
restricts the current flow through center tap P8 to a single direction.
During each cycle of the oscillator, one of transistors Q2, Q3 is held in
a non-conducting mode by the amplitude of the potential applied to its
base electrode while the other transistor is held in a conducting mode
with current flowing through inductance L1 and across its collector and
emitter electrodes via center tap P6 and a corresponding section of
winding W7, and through the collector and emitter junction of either
transistor Q2 or Q3. Resistance R12 establishes a potential between the
coupled emitter electrodes of transistors Q2, Q3 and the network reference
potential for current sensing while a diode D4 assures a constant current
flow through inductance L1 when transistor Q1 is in a conducting mode.
A capacitance C3 is coupled across primary winding W7 while a capacitance
C4 is coupled across a secondary winding W9. Together, capacitances C3, C4
and transformer 46 form an equivalent tuned resonant circuit which
establishes a resonant frequency, f.sub.2, for oscillator stage 22. As
represented by FIGS. 3C and 3D, the amplitudes of voltages appearing
across the collector electrodes of transistors Q2, Q3, respectively, each
have half cycle sinusoidal waveforms with a frequency equal to f.sub.2.
The turns ratio between windings W7 and W9 is selected to provide a
step-up in the voltage across winding W9. A turns ratio of about 1:14.5,
for example, will provide a peak-to-peak voltage across winding W9 of
approximately one hundred and ten volts.
Switching between the conducting and non-conducting modes of transistors
Q2, Q3 occurs at twice the resonant frequency f.sub.2 (i.e., once every
one-half cycle) established by capacitors C3, C4 and transformer 46, and
is implemented by the bias voltages applied by winding W8 to the base
electrodes of each of the transistors. Voltages induced across winding W8
exhibit the same sinusoidal waveform (albeit with smaller amplitudes) as
voltages occurring across the collector electrodes. Windings W7 and W8 are
wound in a flux additive direction. Consequently, as the voltage on one
collector electrode of transistors Q2, Q3 falls to zero due to cycling of
the equivalent resonant circuit, a reversal of current occurs through
winding W8, thereby causing simultaneous shifts in the bias voltages
induced across winding W8 applied to the base electrodes. These shifts
cause the transistor which has a mininum collector voltage to be biased in
a conducting mode during a one-half cycle while the other transistor is
biased in a non-conducting mode. Assuming, for purposes of explanation,
that voltage on the collector of transistor Q3 reaches a minimum at time
t.sub.o, as is shown in FIG. 3D, then the simultaneous shift in the bias
voltages places transistor Q3 into its conduction mode and, for one-half
of a cycle, allows current to flow through inductance L1, one part of
winding W7, across the collector and emitter electrodes of transistor Q3,
and through resistance R12 to the network reference potential. The
amplitude of current flowing through the collector electrode of transistor
Q3 is represented by the waveform shown in FIG. 3E.
Capacitance C5 is coupled between resistances R10, R11 and the network
reference potential to provide filtering of transient currents caused by
the transition of transistor Q1 between its conducting and non-conducting
modes.
Transformer 24 exhibits a high degree of leakage current due to a large
number of closely spaced turns in its primary and secondary windings. A
capacitance C6 is serially connected between the network reference
potential and one end of primary winding W1 of transformer 24 to provide
series tuning with the leakage inductance and thereby assure that at the
oscillator frequency, the series combination of the leakage | | |
