High voltage power supply having multiple high voltage generators

A high voltage power supply (40) having a plurality of high voltage generators (50a, 50b, . . . 50j) that produce a controlled current in a load (24) having a capacitive component. Each high voltage generator consists of a pulsewidth-modulated (PWM) inverter, a high voltage transformer, and a high voltage full bridge rectifier. The high voltage generators are divided into three groups: Group I, Group II, and Group III. During a driving mode of power supply operation, the Group I, Group II, and Group III high voltage generators produce an output voltage (V.sub.gen) from the power supply which exceeds a voltage (V.sub.c) across the capacitive component of the load, causing the current in the load to increase. When the current in the load reaches a desired value, the power supply enters a tracking mode of operation. During the tracking mode, the Group I and Group II high voltage generators produce an output voltage (V.sub.gen) from the power supply which equals the voltage (V.sub.c) across the capacitive component of the load. The current through the load remains constant while the power supply tracks the voltage across the capacitive component of the load. The driving mode and the tracking mode are repeatedly performed, as the high voltage generators are driven by a square wave voltage (V.sub.sw) produced by a timing generator (66). A control circuit (56) is also provided in the power supply to enable or disable additional Group II high voltage generators in response to fluctuations in the voltage across the load.

FIELD OF THE INVENTION 
The present invention relates generally to high voltage power supplies and, 
in particular, to a high voltage power supply for generating a controlled 
current in a load having varying inductive and capacitive components. 
BACKGROUND OF THE INVENTION 
High voltage power supplies often are used in applications where a load 
connected to the power supply includes a significant inductive or 
capacitive component. In many applications, inductive components comprise 
a major component of the load. For example, electrostatic precipitators 
contain electrode arrays connected to a high voltage power supply via high 
voltage cables and buses. The array, the cables, and the buses all 
contribute to the overall inductive value of the system. Even in 
applications that do not contain large inductive components, elements of 
the system may have non-negligible inductive values. For example, a load 
connected to a high voltage power supply may be connected to the power 
supply by a high voltage cable having an inherent inductance. As a general 
rule, therefore, all high-power electrical circuits can be modeled with 
resistive, inductive, and capacitive components. 
The presence of capacitive and inductive load components often makes it 
difficult to maintain a desired current in the load because the components 
prevent the current in a load from quickly changing. This is a 
disadvantage because in many applications, it is beneficial to maintain 
the current in a high power circuit at a desired level. A constant current 
allows the transfer of a maximum amount of electrical energy in a minimum 
time. A limited amplitude also increases the safety of those exposed to 
the circuit. Finally, a limited amplitude is ideal for power supplies 
constructed of power transistors. Since power transistors are rated to 
handle a maximum current, transistors used in power supplies that generate 
current waveforms that may periodically exceed an average current level 
must be selected to handle the periodic spikes to a much higher current 
level. If the amplitude of a load current is controlled to remain around 
an average level, typically less expensive power transistors having a 
lower current carrying capability may be selected for use in the power 
supply. 
Several different approaches exist for designing a power supply that 
produces a direct current (DC) high voltage. For example, a common 
approach is to convert a DC voltage to a high frequency alternating 
current (AC) square wave voltage with an inverter, step-up the square wave 
voltage using a high voltage transformer, and rectify the stepped-up 
output of the transformer to produce an approximately DC output voltage. A 
drawback to this generation method is that it does not allow simple 
control of the magnitude and waveform of the current generated in the 
transformer and in the load. The magnitude of the current in the load is 
affected by two changing voltages. First, fluctuations in the primary DC 
voltage source can cause the amplitude of the AC square wave voltage from 
the inverter to vary. Variance in the output from the inverter will have a 
direct effect on the output voltage from the power supply. Second, changes 
in the voltage drop across the inductive or capacitive components in the 
load will affect the current through the load if the output voltage from 
the power supply remains constant. For example, in precipitator 
applications, the voltage drop across the electrode array in the 
precipitator will fluctuate in response to varying amounts of pollutants 
passing through the precipitator. The two changing voltages, i.e., the 
magnitude of the output voltage generated by the high voltage power supply 
(hereinafter V.sub.gen) and the voltage across the capacitive components 
in the load (hereinafter V.sub.c), often change independently of each 
other. The variation of both V.sub.gen and V.sub.c effects the rate of 
change of current in the load. Because prior art power supplies were not 
successful in rapidly controlling the difference between the output 
voltage from the power supply and the voltage in the load, the magnitude 
and waveform of the current in the supply and load often went 
uncontrolled. 
Generating a controlled current in an electrical circuit that includes 
inductive and capacitive components is therefore a difficult problem. 
Several solutions have been suggested, all of which limit the switching; 
speed of the power supply. For example, one solution is to convert the 
unstable DC input voltage to a stable current by adding a current 
regulator or limiter to the output of the power supply. While an added 
regulator will limit the output current from the high voltage power 
supply, a current regulator also contains reactive elements which store 
energy. When the power supply is turned off, the stored energy must 
dissipate through other components, preventing the output of the supply 
from rapidly decreasing to zero. Those techniques which allow the 
production of a regulated current from a high voltage power supply 
therefore have a tendency to prevent the rapid shut down of the power 
supply. It is an object of the present invention to create a power supply 
that produces a regulated current in a load and allows rapid power down. 
SUMMARY OF THE INVENTION 
A high voltage power supply is disclosed having a plurality of high voltage 
generators that may be divided into three groups, hereinafter referred to 
as Groups I, II, and III. Each of the high voltage generators comprises a 
pulsewidth-modulated (PWM) inverter, a high voltage transformer, and a 
high voltage full bridge rectifier. In operation, the PWM inverter 
generates a high frequency AC voltage that is stepped-up by the 
transformer and rectified by the rectifier. The output voltage 
(hereinafter V.sub.gen) generated by the power supply is equivalent to the 
sum of the output voltages from each of the three groups of high voltage 
generators. The output voltage may be applied to a load having varying 
inductive, capacitive, and resistive elements. A voltage (hereinafter 
V.sub.c) is produced in the load across the capacitive components. 
Each of the groups of high voltage generators contributes a different 
component to the output voltage produced by the power supply. The power 
supply cycles through two modes of operation to maintain a desired current 
in the load. During a driving mode, Groups I, II, and III of the high 
voltage generators collectively provide an output voltage that exceeds the 
voltage across the capacitive components in the load. To accomplish this 
objective, Group I generates a tracking voltage (V.sub.1) that decreases 
as the current rises in the load, Group II generates a base voltage 
(V.sub.2) approximating the voltage across the load, and Group II 
generates an accelerator voltage (V.sub.3) at a level that ensures that 
the sum of the three voltages exceeds the voltage across the load (i.e., 
V.sub.1 +V.sub.2 +V.sub.3 &gt;V.sub.c). During the driving mode the rate of 
the change of the current in the load and in the high voltage generator 
transformers is at a maximum value. 
Operation in the tracking mode begins when the current in the load reaches 
a desired level. During the tracking mode, Groups I and II of the high 
voltage generators operate to collectively provide an output voltage that 
is equal to the voltage across the capacitive components in the load. The 
Group II component of the output voltage is a substantially constant base 
voltage (V.sub.2), which is equal to or less than the voltage across the 
load. The Group I high voltage generator produces a tracking voltage 
(V.sub.1) equal to the difference between the voltage provided by the 
Group II high voltage generators and the voltage appearing across the 
capacitive components (i.e., V.sub.1 +V.sub.2 =V.sub.c). Because the 
output voltage from the power supply tracks the voltage across the 
capacitive component in the load, the current in the load and in the high 
voltage generator transformers remains constant during the tracking mode. 
To expand the range of the power supply, a control circuit is provided to 
enable or disable additional high voltage generators contained in Group 
II. In order to track a voltage change across the capacitive components in 
the load, the high voltage produced by the Group II generators must be 
capable of producing a broad range of output voltages. To produce these 
voltages, additional high voltage generators may be turned on-or-off by 
the control circuit to adjust the output of the power supply. The 
generators are dynamically enabled or removed in response to changes in 
the voltage across the load. 
It will be appreciated that the power supply disclosed herein offers 
several advantages over other high voltage power supplies disclosed in the 
prior art. In particular, the present power supply generates a high 
voltage while controlling the magnitude and waveform of the current in the 
load and the transformers. The current may be controlled despite 
fluctuations in the difference between the primary voltage and the voltage 
across the capacitive components in the load. 
Additionally, the number of high voltage generators that are included in 
each power supply may be selected based on the desired operating range of 
the power supply. More or less Group II generators may be included to 
alter the base voltage level applied by the power supply in the driving 
mode and the tracking mode. The use of a modular design therefore 
increases the applications and flexibility of the power supply. 
A still further advantage is that very few energy storage components are 
included in the power supply design. The minimal use of inductors or 
capacitors allows the high voltage power supply of the present invention 
to be quickly turned on-or-off. The rapid response of the power supply 
allows it to be used in situations where it would be desirable to quickly 
turn off the power during the occurrence of a failure condition, such as a 
short in the load. The rapid switching time of the power supply disclosed 
herein therefore makes it a especially suited for applications such as 
electrostatic precipitators, x-ray equipment, pulse and continuous lasers, 
and radar systems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A high voltage power supply design that is known in the art is represented 
in the block diagram of FIG. 1. The power supply consists of a primary DC 
voltage source 20 that provides an unregulated DC voltage to a high 
voltage generator 22. The high voltage generator is connected to a load 
24, and contains an inverter 26, a transformer 28, and a rectifier 30 to 
produce a high voltage for application to the load. Inverter 26 converts 
the unregulated DC voltage from the primary voltage source into a high 
frequency square wave. The high frequency square wave is stepped-up by 
high voltage transformer 28 and rectified by high voltage rectifier 30. 
The resulting output voltage applied to the load is a fully rectified 
square wave having a nearly constant DC amplitude. 
The current flowing in the load depends on the voltage generated and 
applied to the load by high voltage generator 22. Load 24 may be modeled 
with three components: an inductive component 32, a capacitive component 
34, and a resistive component 36. The current produced in the inductive 
component when a voltage is applied by the high voltage generator may be 
represented by the following equation: 
##EQU1## 
Where: 
V.sub.gen =voltage generated by high voltage generator 22; 
V.sub.c =voltage across capacitive component 34; 
i=current through inductive component 32; 
L=equivalent value of inductive component 32; and 
di/dt=the rate of change of current in the inductive component. 
From equation (1) it is apparent that the rate of change of the current in 
the load depends on the difference between V.sub.gen and V.sub.c. Several 
factors can cause these voltages to fluctuate. With respect to V.sub.gen, 
the output voltage from the high voltage generator is proportional to the 
magnitude of the voltage produced by primary voltage source 20. 
Fluctuations in the primary voltage supply due to instability or voltage 
ripples directly cause variations in the output voltage V.sub.gen. In 
contrast, V.sub.c can change its magnitude due to a change in the voltage 
drop across the inductive component in the load. The voltage drop across 
the inductive component may change due to the discharge of energy, for 
example, during precipitator operation. As V.sub.gen and V.sub.c change, 
the difference between the voltages typically changes, causing a 
proportional current fluctuation through the inductive component of the 
load. In general, large fluctuations in load currents are to be avoided, 
as they have the potential to overheat or destroy electric components in 
the load. 
The object of the present invention is to maintain the magnitude of the 
current in the load at a desired level independently of any fluctuations 
in the primary voltage and changes in voltage across the capacitive 
components. 
1. General Power Supply Construction and Operation 
A block diagram of a high voltage power supply 40 constructed according to 
the present invention is shown in FIG. 2. High voltage power supply 40 
consists of three sets of high voltage generators 50 identified as Groups 
I, II, and III. In a preferred embodiment of the power supply, Group I 
consists of a single high voltage generator 50a, Group II is made up of 
eight high voltage generators 50b, 50c, . . . 50i, and Group III is formed 
by a single high voltage generator 50j. Upon understanding the invention, 
it will be appreciated that greater or fewer high voltage generators may 
be included in each group so as to configure the invention for operation 
at a desired voltage and current level. 
A single-phase AC power source 84 provides power to the high voltage power 
supply. In a preferred embodiment of the high voltage power supply, AC 
power source 84 provides a 120 volt AC voltage at 60 Hz. The AC power 
source supplies operating power to an unregulated AC-to-DC rectifier 52 
which is connected via lines 100 and 102 to each of the high voltage 
generators 50b, 50c, . . . 50j. Connected between lines 100 and 102 is a 
filter capacitor 88. AC power source 84 also provides operating power to a 
current control circuit 64 that is coupled to high voltage generator 50a. 
In the currently preferred embodiment, rectifier 52 is a full-bridge diode 
network that produces an unregulated DC voltage. In a realization of such 
an embodiment, the DC voltage produced by the rectifier is approximately 
170 volts. 
Each high voltage generator 50a, 50b, 50j is comprised of a high voltage 
inverter 70a, 70b, . . . 70j, a high voltage transformer 72a, 72b, . . . 
72j, and a high voltage rectifier 78a, 78b, . . . 78j. As shown in FIG. 2, 
the output of each inverter 70a, 70b, . . . 70j is connected to supply a 
signal to a corresponding primary winding 74a, 74b, . . . 74j of 
transformer 72a, 72b, . . . 72j. An associated high voltage rectifier 78a, 
78b, . . . 78j is connected to the secondary winding 76a, 76b, . . . 76j 
of each transformer. The high voltage generators are connected in a 
cascode configuration so that the output voltage V.sub.gen from the power 
supply is the sum of the output voltages produced by each of the high 
voltage generators. That is, each group in the high voltage power supply 
produces an output voltage that is summed to form V.sub.gen. 
The high voltage power supply is connected by a line 80 to a load 24. Load 
24 may be modeled by an inductive component 32, a capacitive component 34, 
and a resistive component 36. When current flows through inductive 
component 32, a voltage V.sub.c is produced across the capacitive 
component of the load. As discussed above, voltage V.sub.c will vary as a 
result of changes in the voltage drop across inductive component 32. A 
change in V.sub.c caused by, for example, energy discharge, would 
typically result in a change in load current. As will be demonstrated 
below, however, the power supply disclosed herein generates a variable 
output voltage that maintains a nearly constant load current of desired 
amplitude through the inductive component of the load. 
As noted earlier, each of the generator groups is responsible for 
generating a different component of the output voltage from the power 
supply. Representative timing diagrams of the output voltages produced by 
Groups I, II, and III of the power supply are provided in FIG. 3, and will 
be discussed generally below. It will be appreciated that the graphs 
showing the output voltage from each high voltage generator group are 
idealized in FIG. 3. For example, the voltage transitions of the inverters 
will be dampened somewhat by the load of the high voltage transformer and 
high voltage rectifier. The waveforms shown in FIG. 3 are thus to be 
considered as representative of the shape of the high voltage generator 
output voltages that produce a desired current (I.sub.d) in the load. The 
specific construction of each high voltage generator group will be 
subsequently discussed with respect to FIGS. 6, 7, 8, and 9. 
In FIG. 2, each of the high voltage generators is connected to a timing 
generator 66. The timing generator is provided to synchronously drive each 
of the inverters in the high voltage generators with a high frequency 
timing signal. In a preferred embodiment of the invention, the timing 
signal produced by the timing generator is a square wave voltage having an 
amplitude of 15 volts and a frequency within the range of 10 to 20 kHz. A 
representative square wave timing signal generated by the timing generator 
is shown by graph V.sub.sw of FIG. 3. As shown in FIG. 3, each of the 
generator groups generates a characteristic output voltage beginning with 
the leading or falling edge of the square wave. 
The Group III high voltage generator 50j generates a periodic rectangular 
pulse as shown in graph V.sub.3 of FIG. 3. The rectangular pulse begins 
with the leading edge of the timing signal, and has an amplitude that 
corresponds to the magnitude of the DC voltage provided by AC-to-DC 
rectifier 52 and the turns ratio of transformer 72.sub.j. A current 
monitor 58, connected between the output of inverter 70j and the primary 
winding 74j of transformer 72.sub.j, senses the current being delivered by 
the inverter. When the current measured by current monitor 58 corresponds 
to a desired load current (I.sub.d) through inductive component 32, the 
inverter of the Group III high voltage generator is switched off. When 
switched off, the output voltage generated by the Group III generator 
falls to zero. 
The Group II high voltage generators 50b, 50c, . . . 50i of FIG. 2 supply 
an approximately DC output voltage as shown in graph V.sub.2 of FIG. 3. 
That is, each Group II inverter 70b, 70c, . . . 70i produces a square wave 
output that is synchronous with the timing signal produced by timing 
generator 66. The square wave voltage produced by each inverter is 
stepped-up by the associated transformer and is full wave rectified by the 
corresponding high voltage rectifier. In the arrangement of FIG. 2, the 
output voltage collectively produced by the Group II generators is 
determined by a control circuit 56 that selectively enables high voltage 
generators 50b, 50c, . . . 50i. Control circuit 56 receives from a voltage 
divider network 82 on a line 54 a voltage representative of the voltage 
V.sub.c across the capacitive component of the load. The control circuit 
monitors the voltage across the load, and enables or disables the high 
voltage generators so that the total voltage generated by the Group II 
generators is less than or equal to the voltage appearing across the 
capacitive component of the load. The output voltage from the Group II 
generators may therefore change in incremental steps that correspond to 
the output voltage produced by each high voltage generator in the group. 
The Group I high voltage generator 50a produces an output voltage V.sub.1 
that can range between zero and a value limited by the maximum input 
voltage being supplied to current control circuit 64. The Group I high 
voltage generator is connected to a current control circuit 64. Unlike the 
Group II and Group III high voltage generators, the Group I high voltage 
generator is a current fed inverter. Connected between the current control 
circuit 64 and the inverter 70a is a current monitor 60. Current monitor 
60 monitors the mount of current flowing in the inverter. The monitored 
current is a scaled amount of the current flowing through the secondary 
winding of the transformer contained in the high voltage generator, and 
therefore proportional to the current flowing through the power supply and 
to the load. If the current measured by the current monitor 60 indicates a 
current level in the load below the desired current I.sub.d, the current 
control circuit 64 provides additional current to the inverter 70a to 
increase the voltage generated by the high voltage generator. Conversely, 
if the current measured by the current monitor 60 indicates a current in 
the load greater than the desired current I.sub.d, the output voltage from 
high voltage generator 50a is reduced. In this manner, a variable voltage 
is generated by the Group I high voltage generator in order to track the 
change in the voltage across the capacitive component and maintain the 
current in the load at a desired level. A representative output voltage 
generated by the Group I generator is shown in graph V.sub.1 of FIG. 3. 
As each group of high voltage generators produces the above described 
output voltage, the power supply cycles through two modes of operation, 
hereinafter referred to as a driving mode and a tracking mode. During a 
driving mode, represented by the time interval from t.sub.1 to t.sub.2 in 
FIG. 3, the sum of the Group I, Group II, and Group III generators exceeds 
the voltage V.sub.c measured across the capacitive component of the load. 
The Group I generator produces a high voltage that will dynamically vary 
during the driving mode. The Group II generators produce a substantially 
constant output voltage comprising a rectified square wave voltage and the 
Group III generator produces a positive rectangular pulse. The amplitude 
of V.sub.2 and V.sub.3 are selected so that the sum of V.sub.2 and V.sub.3 
is a constant voltage that exceeds V.sub.c (i.e., V.sub.2 +V.sub.3 
&gt;V.sub.c). Since V.sub.gen is greater than V.sub.c during the driving 
mode, according to equation (1) the rate of change of current is positive 
during the period from t.sub.1 to t.sub.2. The current through the load 
therefore increases during the driving mode until it reaches a desired 
level I.sub.d. When the current reaches a desired level, current monitor 
58 senses the desired current and switches off the Group III inverter 70j. 
At this point, the power supply enters a tracking mode of operation. 
When the desired current level is reached, the power supply enters a 
tracking mode of operation represented by the time interval from t.sub.2 
to t.sub.3 in FIG. 3. During the tracking mode of operation, the voltage 
V.sub.gen generated by the power supply substantially tracks the voltage 
V.sub.c across the capacitive component of the load. In particular, the 
output from the high voltage generator in Group III is switched off by the 
feedback provided by current monitor 58. The voltage V.sub.gen therefore 
drops to the level of the sum of the output from the Group I and Group II 
high voltage generators. The Group II generators collectively generate a 
sum DC voltage less than or equal to the voltage across the load. 
Simultaneously, the high voltage generator in Group I is controlled so 
that the sum of the voltages V.sub.1 and V.sub.2 is substantially equal to 
the voltage V.sub.c across the capacitive component of the load (i.e., 
V.sub.1 +V.sub.2 =V.sub.c). As previously pointed out, by monitoring the 
current being provided to the load, current monitor 60 causes the Group I 
generator to increase or decrease its output voltage to track voltage 
changes across the capacitive component in the load. Voltage V.sub.c may 
change for a variety of reasons, for example, due to any charging or 
discharging processes in the load. Absent the invention, any such change 
in V.sub.c would cause a change in load current. However, during the 
tracking mode, the power supply disclosed herein maintains voltage 
V.sub.gen at a level substantially equal to V.sub.c. Thus, in accordance 
with equation (1), the current in the load does not appreciably change its 
magnitude during the tracking mode because the voltage drop across the 
inductive component of the load is in the ideal sense equal to zero. By 
controlling the output voltage during the driving mode and the tracking 
mode it is possible to generate a nearly constant current having minimal 
ripple in the load. Additionally, a nearly square wave current is 
maintained in the transformers, which is beneficial from the energy 
transfer standpoint. 
The two-mode voltage generation process repeats during the second half of 
the period of square wave voltage V.sub.sw produced by timing generator 
66. At time t.sub.3, the polarity of the square wave voltage reverses, and 
a negative voltage drives the high voltage generators. The change in 
polarity of the voltage driving the high voltage generators causes a 
momentary drop in the output voltage V.sub.gen of the high voltage power 
supply. This causes V.sub.c to exceed V.sub.gen, and generates a falling 
current in the load since, in accordance with equation (1), the change in 
the current through the inductor is negative. 
During the driving mode from t.sub.3 to t.sub.4, the application of the 
rectified sum of V.sub.1, V.sub.2, and V.sub.3 causes V.sub.gen to exceed 
V.sub.c, and the current to increase through the inductive component in 
the load. At time t.sub.4, the current through the inductor reaches the 
desired value. When the desired current is reached, the power supply of 
FIG. 2 begins operation in the tracking mode. The output from the high 
voltage generator in Group III is switched off, and the voltage V.sub.gen 
drops to the level of V.sub.2, the output from the Group II high voltage 
generators. At the same time, the high voltage generator in Group I 
changes its output so that the sum of V.sub.2 and V.sub.1 are equivalent 
to the voltage V.sub.c across the capacitive load. Driving mode operation 
begins again at t.sub.5 in FIG. 3, i.e., at the end of the negative 
polarity portion of the square wave timing signal. 
FIGS. 4 and 5 are timing diagrams showing the effect of a change in the 
voltage across the capacitive components V.sub.c that will result in 
enabling or disabling a Group II high voltage generator. Recall from FIG. 
2 that the Group II output voltage V.sub.2 is produced by a plurality of 
generators 50b, 50c, . . . 52i. The number of Group II generators that are 
producing a voltage is controlled by control circuit 56, which can 
selectively enable or disable the output from each of the Group II 
generators. FIG. 4 represents a case when the voltage V.sub.c across the 
capacitive component in the load is increasing. As the voltage increases, 
the output voltage V.sub.1 produced by the Group I high voltage generator 
continues to increase in magnitude to insure that V.sub.gen tracks 
V.sub.c. At a time t.sub.1, the voltage produced by the Group I high 
voltage generator reaches a maximum dictated by the input voltage to the 
current control circuit. From time t.sub.1 to time t.sub.2, the output 
voltage V.sub.1 remains at a maximum. Because V.sub.c continues to 
increase during this period, however, V.sub.c becomes greater than 
V.sub.gen, causing the current I.sub.L through the load to drop. 
Recognizing that the high voltage power supply output voltage can no 
longer track V.sub.c, the control circuit in the power supply enables an 
additional generator in the Group II high voltage generators. The 
additional generator in Group II increases the output voltage V.sub.2 
produced by the Group II generators, thus increasing the output voltage 
V.sub.gen. By time t.sub.3, the output voltage produced by Group I and 
Group II generators is sufficient to again track V.sub.c without causing 
V.sub.1 to saturate. Current I.sub.L through the load is therefore 
returned to the desired amplitude and waveform after time t.sub.3. 
Similarly, FIG. 5 represents a case when the voltage across the capacitive 
component in the load is decreasing. As the voltage V.sub.c decreases, the 
output voltage V.sub.1 produced by the Group I high voltage generator 
continues to decrease in magnitude to insure that V.sub.gen tracks 
V.sub.c. At a time t.sub.1, the voltage produced by the Group I high 
voltage generator reaches a minimum value (substantially equal to zero) 
wherein it is not producing any component of V.sub.gen. From time t.sub.1 
to time t.sub.2, the output voltage V.sub.1 remains at zero. Because 
V.sub.c continues to decrease during this period, however, V.sub.c becomes 
less than V.sub.gen, causing the current I.sub.L through the load to 
increase. Recognizing that the high voltage power supply output voltage 
can no longer track V.sub.c, the control circuit in the power supply 
disables a generator in the Group II high voltage generators. Eliminating 
a generator in Group II decreases the output voltage V.sub.2 produced by 
the Group II generators, thus decreasing the output voltage V.sub.gen. At 
time t.sub.3, the output voltage produced by the Group I and Group II 
generators is sufficient to again track V.sub.c without forcing V.sub.1 to 
zero. Current I.sub.L through the load is therefore returned to the 
desired amplitude and waveform after time t.sub.3. 
Having described the general operation of the disclosed high voltage power 
supply, more detailed circuit diagrams of the components in the preferred 
embodiment of the power supply will be described relative to FIGS. 6, 7, 
8, and 9. 
2. Group III High Voltage Generator 
The Group III high voltage generator is designed to produce a periodic 
variable-width rectangular pulse that is used as an accelerator voltage 
during the driving mode of the power supply. In the arrangement of FIG. 2, 
the Group III generator includes a single high voltage generator 50j. The 
unregulated DC voltage from the AC-to-DC rectifier 52 is directly coupled 
via lines 100 and 102 to high voltage generator 50j. FIG. 6 is a circuit 
diagram of a pulsewidth-modulated (PWM) inverter suitable for use in high 
voltage generator 50j. The PWM inverter 70j is shown schematically as 
comprising the following primary elements: transformer 148, four power 
metal-oxide-semiconductor field-effect transistors (MOSFETs) 157, 158, 
159, and 160, current transformer 165, resistor 166, and triacs 146 and 
147. Transistors 157, 158, 159, and 160 are connected in a full-bridge 
configuration. When one transistor pair, for example transistors 157 and 
160, are biased ON (i.e., conducting), the other transistor :pair 
comprising transistors 158 and 159 are switched OFF, allowing current to 
flow from line 100 through transistor 157 and transformer 165 to terminal 
168. Terminals 168 and 170 are connected to the primary winding 74j of the 
high voltage transformer 72j. The current flows from terminal 168, through 
the primary winding of the high voltage transformer, and through terminal 
170 and transistor 160 to ground. When transistors 157 and 160 are 
switched OFF, and transistors 158 and 159 are biased ON, the current flows 
through the primary winding 74j in the opposite direction. That is, 
current flows from line 100 through transistor 159, terminal 170, primary 
winding 74j, terminal 168, and transistor 158 to ground. Alternately 
biasing or switching each transistor pair ON or OFF therefore causes 
current to flow in the primary winding of high voltage transformer 72j in 
one direction or the other. 
The switching of the MOSFET pairs is controlled by the timing signal 
generated by timing generator 66 and connected to the inverter at 
terminals 105 and 106. In a preferred mode of operation, the square wave 
timing signal generated by the timing generator has a frequency of 10 to 
20 kHz and a magnitude of 15 volts. Terminals 105 and 106 are connected to 
the primary winding of transformer 148. Applying a square wave voltage to 
transformer 148 biases the transistor pairs ON and OFF. When terminal 105 
is of a positive polarity with respect to terminal 106, a positive voltage 
is applied to the gates of transistors 157 and 160, biasing the 
transistors ON. At the same time, a negative voltage is applied to the 
gates of the transistors 158 and 159 due to the reverse polarity of the 
secondary winding of transformer 148 connected to those transistors. While 
transistors 157 and 160 are biased ON, transistors 158 and 159 will 
therefore be switched OFF. Similarly, when voltage across the terminals 
106 and 105 reverses its polarity so that terminal 106 is positive with 
respect to terminal 105, transistors 157 and 160 are switched OFF and 
transistors 158 and 159 are biased ON. 
Additional components are present in the inverter to optimize the inverter 
operation. Diodes 161, 162, 163, and 164 are coupled between terminals 168 
and 170 and lines 100 and 102 to allow current to flow from high voltage 
transformer 72j to lines 100 and 102 when all four transistors are 
switched OFF. The transistors are switched OFF during shut down, or when 
no voltage is generated by the high voltage generator. Resistors 149, 151, 
153, and 155 are connected between the secondary winding of transformer 
148 and the gates of the transistors in order to limit the gate voltage 
applied of the transistors. Resistors 150, 152, 154 and 156 are connected 
between the gate and the source of transistors 157, 158, 159 and 160. The 
resistors create a path for current flowing from the gate-to-source 
capacitance of the transistors to allow the transistors to switch to an 
OFF position more quickly. 
Each inverter may be enabled or disabled by an appropriate control signal 
applied to a terminal 144. Terminal 144 is connected to the gate of a 
triac 146, which is in series with the primary winding of transformer 148. 
When triac 146 is biased ON by applying a voltage at terminal-144, the 
connection between terminal 106 and transformer 148 is completed, allowing 
the square wave voltage from the timing generator to be applied to the 
gates of transistors 157, 158, 159, and 160. When the triac 146 is 
switched OFF, however, the connection between terminal 106 and transformer 
148 is broken and transistors 157, 158, 159, and 160 remain switched OFF. 
When triac 146 is switched OFF, no output voltage is therefore generated 
by the PWM inverter. In a preferred embodiment of the invention, triac 146 
remains biased ON in inverter 70j to ensure that the Group III high 
voltage generator is always enabled. 
A circuit comprising a current transformer 165, a resistor 166, and a triac 
147 serves as overcurrent protection for the PWM inverter. The primary 
winding of current transformer 165 is connected in series with the output 
line connected to terminal 168 and the primary winding of high voltage 
transformer 74j. A resistor 166 is connected in parallel with the 
secondary winding of current transformer 165. The secondary winding is 
also coupled between the primary winding of transformer 148, and the gate 
of triac 147. Triac 147 is connected between terminals 105 and 106. Since 
resistor 166 is connected across the secondary winding of current 
transformer 165, a voltage drop is generated across resistor 166 that is 
proportional to the output current of the inverter. A value is selected 
for resistor 166 so that when the output current magnitude reaches a 
preselected limit, the voltage drop across resistor 166 is sufficient to 
bias triac 147 in the ON position. When triac 147 is biased ON, terminal 
106 is shorted across a resistor 145 to terminal 105, preventing the 
square wave voltage generated by the timing generator from being applied 
across transformer 148. When the current in the inverter passes the 
overcurrent limit, the inverter is therefore disabled by the triac 
shorting terminal 105 to terminal 106. 
For the Group III high voltage generator, the circuit comprising current 
transformer 165, resistor 166, and triac 167 also serves as the current 
monitor 58 to control the width of the rectangular voltage pulse produced 
by high voltage generator 50j. Resistor 166 in the Group III inverter 70j 
is selected to have a value that is greater than the value of resistor 166 
used in PWM inverters 70a, 70b, . . . 70i. The value of resistor 166 is 
selected so that when the output current from the inverter exceeds a 
target current, a sufficient voltage drop is generated across resistor 166 
to bias triac 147 ON. The target current is the current through the 
inverter that indicates that the current in the load is equal to or 
greater than the desired current I.sub.d. Biasing triac 147 ON will cause 
inverter 70j to be switched off, turning off the output voltage from the 
Group III generator. The Group III inverter 70j therefore produces 
rectangular voltage pulses rather than the square wave voltage produced by 
inverters 70a, 70b, . . . 70i. 
With reference to FIG. 2, the rectangular voltage pulses produced by 
inverter 70j are applied to high voltage transformer 72j. The high voltage 
transformer steps-up the input voltage to a desired output level. In a 
preferred embodiment of the high voltage power supply, the high voltage 
transformer has a ferrite core, with a primary winding 74j having 90 turns 
and a secondary winding 76j having 550 turns. The alternating square wave 
voltage driving inverter 70j has a frequency of 10 to 20 kHz. After being 
stepped-up by the high voltage transformer, the Group III high voltage 
generator therefore produces an output voltage having the same frequency 
of 10 to 20 kHz and a magnitude of about 1,000 volts. 
After the inverter output voltage is stepped-up by transformer 72j, it is 
applied across high voltage rectifier 78j. FIG. 7 is a circuit diagram of 
an exemplary high voltage rectifier 78j suitable for use in high voltage 
generator 50j. The high voltage rectifier comprises four high voltage 
diodes 180, 182, 184, and 186 that are connected in a full-bridge 
configuration. Input lines 188 and 190 of the bridge are connected to the 
secondary winding 76j of high voltage transformer 72j. The output lines 
from the bridge are connected across a ;parallel combination of a 
capacitor 192 and a resistor 194. In operation, a high voltage square 
pulse generated by inverter 70j and stepped-up by transformer 72j is 
applied across the input to the diode bridge. The full-wave rectified 
output from the bridge is then filtered and loaded by capacitor 192 and 
resistor 194. The rectified output from the Group III high voltage 
generator corresponds to the V.sub.3 graph as shown in FIGS. 4 and 5. 
3. Group II High Voltage Generators 
With reference to FIG. 2, in a preferred embodiment the eight Group II high 
voltage generators 50b, 50c, . . . 50i are constructed using a design 
nearly identical to the Group III high voltage generator 50j. Inverters 
70b, 70c, . . . 70i are constructed substantially as shown in FIG. 6. A 
slight variation is that the value of resistor 166 is selected so that the 
Group II inverters will not be disabled when the desired current is 
indicated in the load. The value of resistor 166 is instead selected to 
provide overcurrent protection for each high voltage generator. As a 
result, in normal operation the Group II inverters 70b, 70c, . . . 70i 
produce an output voltage that is substantially identical to the square 
wave voltage produced by the timing generator. That is, the inverters are 
driven at the 10 kHz to 20 kHz frequency of the timing square wave, and 
produce a square wave output voltage at this frequency. The remaining 
components in each high voltage generator 50b, 50c, . . . 50i are 
identical to the components in high voltage generator 50j. High voltage 
transformers 72b, 72c, . . . 72i are identical to transformer 72j, and 
rectifiers 78b, 78c, . . . 78i correspond to the construction of rectifier 
78j as shown in FIG. 7. The square wave generated by the inverter is 
stepped-up by the transformer and full wave rectified by the rectifier. 
Each Group II high voltage generator therefore produces an approximately 
DC output voltage having an amplitude of 1,000 volts. 
Control circuit 56 is connected to each of the Group II high voltage 
generators 50b, 50c, . . . 50i. The control circuit is designed to monitor 
the voltage V.sub.c across the capacitive component of the load and to 
enable or disable the Group II high voltage generators so that the output 
voltage V.sub.2 from the Group II generators is less than or equal to 
V.sub.c. FIG. 8 is a block diagram of a control circuit suitable for 
monitoring the voltage drop across the capacitive, component of the load 
and enabling or disabling additional Group II high voltage generators in 
the high voltage power supply. 
As shown in FIG. 8, high voltage divider network 82 consists of resistors 
200a, 200b, . . . 200n connected in series between V.sub.c and ground. The 
resistors are selected such that the voltage drop across resistors 200b, 
200c, . . . 200n reduces the voltage drop across resistor 200a to a 
desired level. The voltage across resistor 200a is provided on line 54 to 
control circuit 56. 
Control circuit 56 includes an analog-to-digital converter 204, a 
demultiplexer 206, and, in a preferred embodiment of the invention, eight 
optoisolators 208a, 208b, . . . 208h. The analog-to-digital converter 204 
is connected to the high voltage divider network by line 54. Converter 204 
measures the voltage across resistor 200a and generates a three-bit 
digital value that is representative of the voltage V.sub.c across the 
capacitive components of the load. It will be appreciated that resistors 
200a, 200b, . . . 200n should be selected such that the expected voltage 
variation across resistor 200a is within the input voltage range of 
analog-to-digital converter 204. 
The digitized value corresponding to voltage V.sub.c is input into 
demultiplexer 206. In a preferred embodiment of the invention, 
demultiplexer 206 is a 3.times.8 decoder that enables the output lines 
that correspond to the digital value received on the demultiplexer input 
lines. Each output line from the demultiplexer is coupled to an 
optoisolator 208a, 208b, . . . 208h. The optoisolators are then coupled 
via lines 210a, 210b , . . . 210h to terminal 144 and the gate of triac 
146 contained in each Group II inverter as shown in FIG. 6. Optoisolators 
208a, 208b, . . . 208h are provided between demultiplexer 206 and high 
voltage generators 50b, 50c, . . . 50i to ensure that the high voltage 
produced in the high voltage generators is isolated from the logic 
circuitry in control circuit 56. 
As the high voltage power supply generates and applies a high voltage to a 
load, a voltage across resistor 200a (and corresponding to the voltage 
V.sub.c across the capacitive component of the load) is provided to 
control circuit 56. Based on the voltage V.sub.c, the demultiplexer will 
enable or disable the inverters in the Group II high voltage generators in 
order to ensure that the sum voltage generated by the Group II generators 
is equal to or less than the voltage V.sub.c. If V.sub.c increases, the 
control circuit will enable additional Group II high voltage generators by 
generating additional signals on lines 210a, 210b, . . . 210h to bias ON 
triac 146 and the associated inverters 70b, 70c, . . . 70i. If V.sub.c 
decreases, the control circuit will disable additional Group II high 
voltage generators by switching OFF one or more of the triacs in the high 
voltage generator inverters. Ideally, the sum voltage produced by the 
Group II generators should not fail below the voltage V.sub.c by an amount 
greater than the maximum voltage that can be produced by the Group I 
generators (i.e., V.sub.c-V.sub.2 &lt;V.sub.1). This ensures that the sum of 
the Group I and II generators will allow the output voltage V.sub.gen of 
the high voltage power supply to accurately track V.sub.c. The sum of the 
output voltages from the Group II high voltage generators corresponds to 
the V.sub.2 graph shown in FIGS. 4 and 5. 
4. Group I High Voltage Generator 
With reference to FIG. 2, in a preferred embodiment the Group I high 
voltage generator 50a is constructed using a design nearly identical to 
the Group III high voltage generator 50j. Inverter 70a is constructed as 
shown in FIG. 6. The only variation in inverter construction is that the 
value of resistor 166 is selected to provide overcurrent protection, 
rather than to set a desired current in the load. High voltage transformer 
70a is constructed around a magnetic ferrite core, with a primary winding 
consisting of 90 turns and a secondary winding consisting of 640 turns. 
The additional turns in the secondary winding of high voltage transformer 
70a increases the output voltage produced by the Group I high voltage 
generator to about 1,200 volts. Rectifier 78a corresponds to the 
construction of rectifier 78j as shown in FIG. 7. 
The Group I high voltage generator is designed to produce an output voltage 
ranging between zero and a maximum value in order to track the difference 
in the voltage generated by the Group II high voltage generators and the 
voltage V.sub.c across the capacitive components in the load. During the 
driving mode, the Group I high voltage generator produces a 
dynamically-varying high voltage. During the tracking mode, the Group I 
high voltage generator produces a voltage such that V.sub.1 +V.sub.2 
=V.sub.c. 
To ensure that the high voltage generator 50a produces a desired output 
voltage, current control circuit 64 is provided to monitor and maintain a 
desired current through the high voltage generator. A schematic diagram of 
a current control circuit suitable for monitoring and maintaining a 
desired current through the Group I high voltage generator is provided in 
FIG. 9. The heart of the current control circuit is a PWM controller 220, 
which in a preferred embodiment of the invention is a Signetics NE5561. 
PWM controller 220 generates an internal sawtooth waveform voltage having 
a preferred frequency of 40 kHz for an inverter embodiment operating from 
a 20 kHz timing signal. The PWM controller frequency is determined by a 
resistor 248 and a capacitor 250 connected to the controller. 
The PWM controller 220 is connected to a switching network 222 that is used 
to connect and disconnect the DC voltage provided on line 100 with the 
inverter 70a through an inductor 224. Switching network 222 comprises a 
MOSFET transistor 226, having the drain electrode connected with line 100, 
and the base and source electrodes connected with inductor 224. The gate 
electrode of transistor 226 is connected to one lead of the secondary 
winding of a transformer 232 through a resistor 230. The other lead of the 
secondary winding is connected to the base and source electrodes of the 
transistor, and also to the gate electrode through a resistor 228. 
Resistor 230 limits the current applied to the gate of transistor 226, and 
resistor 228 creates a path for current flowing from the gate-to-source 
capacitance of the transistor to allow the transistor to switch to an OFF 
position more quickly. One lead of the primary winding of transformer 232 
is connected across a resistor 240 and capacitor 238 to ground, and 
through a resistor 236 to line 100. A resistor 234 is connected in 
parallel with the primary winding. The other lead of the primary winding 
is connected to the PWM controller 220. 
In operation, transistor 226 is switched by PWM controller 220 to achieve 
discontinuous current flow through inductor 224 and inverter 70a. When PWM 
controller is switched on, current is allowed to flow from line 100 
through resistor 236 and the primary winding of transformer 232 and to 
ground via the PWM controller on line 102. This generates a positive 
voltage across the gate of transistor 226, biasing the transistor ON and 
allowing current to flow through inductor 224 and over line 103 to 
inverter 70a. After flowing through inverter 70a, the current flows 
through a line 104, a shunt resistor 242, and on line 102 to ground. As 
the magnitude of the current increases through the shunt resistor, the 
voltage drop across the resistor increases. When the current reaches a 
desired level, the voltage drop is sufficient to turn the PWM controller 
off. On turning the PWM controller off, transistor 236 is switched OFF and 
switching network 222 no longer allows current to flow to the inverter. 
Shunt resistor 242 therefore operates as current monitor 60 to control the 
output voltage of the Group I high voltage generator. 
When the switching network halts current flow on line 100, current flow in 
the secondary winding of high voltage transformer 72a and energy stored in 
the inductor 224 induces a current flow through shunt resistor 242 and a 
diode 244. If the current flow through the shunt resistor indicates a 
current level flowing through the load above the desired current level, 
then the voltage drop across the shunt resistor is sufficient to maintain 
the PWM controller 220 in an off state. When the current flow through the 
shunt resistor indicates a current level in the load below the desired 
current level, then the voltage across shunt resistor 242 falls below the 
necessary voltage to maintain the PWM control in the off position. 
Appropriate selection of the shunt resistor therefore selects the desired 
current I.sub.d that is to be maintained in the load. It will be 
appreciated that PWM controller adjusts the output voltage produced by 
high voltage generator 50a in order to maintain a desired current though 
the high voltage power supply and the load. The output voltage of the 
inverter is determined by the difference between V.sub.c and V.sub.2. 
A diode 246 is also provided in the current control circuit to protect the 
circuit from overvoltage. Those skilled in the art will recognize that 
other circuits or techniques may be used to maintain a desired current 
level from the output of high voltage generator 50a. The output from the 
Group I high voltage generator corresponds to the V.sub.1 graph as shown 
in FIG. 4 and 5. 
Having described the construction of the high voltage power supply, a brief 
example of the performance of the preferred embodiment of the power supply 
is provided below. For purposes of the example, a representative load is 
selected that has a resistive component of 20 kOhm, a capacitive component 
of 10 uF, and a inductive component of 6.times.10.sup.-3 H. Each high, 
voltage generator 50a, 50b, . . . 50j produces an output voltage of 
approximately 1,000 volts, thereby producing a potential maximum output 
voltage for the high voltage power supply of 10 kV. Timing generator 66 
produces a square wave tinting signal of approximately 15 kHz, having a 
period of 66.times.10.6 seconds. 
During the driving mode of the high voltage power supply, the sum of the 
Group I, II, and III generators produces a voltage that exceeds the 
voltage across the capacitive component of the load by approximately 1,000 
volts. From equation (1), the rate of change of the current in the load 
is: 
##EQU2## 
For the current in the load to initially drop during the driving mode and 
then reach a desired value of 0.5 amps therefore takes approximately 
6.times.10.sup.-6 seconds. Once the current in the load has reached the 
desired value, the high voltage power supply enters the tracking mode. The 
Group II high voltage generators continue to generate an output voltage 
that is less than or equal to the voltage across the capacitive component 
of the load. The Group I high voltage generator generates the difference 
between the Group II output voltage and the voltage across the capacitive 
component of the load. During the tracking mode, the current in the load 
remains constant since there is no voltage drop across the inductive 
component of the load. 
It will be appreciated that a high voltage power supply formed in 
accordance with the present invention offers many advantages over the 
prior art. Because few energy storage elements are used to maintain the 
output voltage level at a desired value, the high voltage power supply of 
the present invention may be quickly switched on or off. The lack of 
energy storage elements also provides for a fast response time to 
variations in load voltage. The high voltage power supply of the present 
invention is therefore suitable for use in dynamic applications wherein 
the load has quickly varying capacitive, resistive, and inductive 
components. 
A further advantage is that the use of multiple high voltage generators to 
generate a desired output voltage allows the high voltage power supply to 
be tailored for each particular application. High voltage generators may 
be added to the Group II generators to increase maximum generated voltage 
and produce a greater dynamic range of the power supply. Control circuit 
56 can similarly be expanded or reduced to control a greater or lesser 
number of high voltage generators. 
While the preferred embodiment of the invention has been illustrated and 
described, it will be appreciated that various changes can be made therein 
without departing from the spirit and scope of the invention. For example, 
while the preferred embodiment of the power supply contains only one high 
voltage generator in Groups I and III, those skilled in the art will 
recognize that in each group a number of high voltage generators could be 
connected in cascode fashion to create the desired output voltage. 
Similarly, although the preferred embodiment of the high voltage power 
supply has eight high voltage generators in Group II, the number of 
generators may be varied depending upon the expected load and load 
fluctuation. 
It will also be appreciated that the construction of the high voltage 
transformers contained within each high voltage generator may be varied. 
Instead of having a single secondary winding in each high voltage 
transformer, the transformers may be constructed with a plurality of 
secondary windings. Each of the plurality of secondary windings may be 
coupled to a high voltage rectifier, and the output voltage of each high 
voltage rectifier summed to produce the output voltage of the high voltage 
generator. 
Those skilled in the art will recognize that while current control circuit 
64 is used to ensure that the output voltage of Group I tracks the voltage 
across the capacitive component in the load, other control methods could 
be used to control the Group I output voltage. For example, a dedicated 
circuit could be used to monitor voltage V.sub.c and change the output 
voltage V.sub.1 of Group I to ensure the sum of V.sub.1 and V.sub.2 tracks 
V.sub.c. Those skilled in the art will recognize that this implementation 
would require additional circuitry to accurately monitor the voltage 
across the capacitive component of the load and to quickly modify V.sub.1 
in response to changes in V.sub.c. 
It will further be recognized that while the voltage V.sub.c across the 
capacitive component of the load was used as feedback in the preferred 
embodiment of the high voltage power supply, other measures of the voltage 
or current in the load could be used as feedback to control the number of 
Group II generators that are enabled or disabled. For example, a measure 
of the rate of charge or discharge of the capacitor in the load may be 
used as an indication of the current flowing in the load. Alternatively, a 
direct measurement may be made of the current magnitude or direction in 
the load. The actual parameter used to monitor the current in the load may 
depend on the availability and accessibility of the parameter in the load. 
While the preferred embodiment of the high voltage power supply is 
discussed above, other well-known parts, components, and their 
combinations may be employed in this circuit for generating a PWM square 
waveform voltage to maintain a desired output current. Several different 
designs of equivalent function and performance are available to replace 
the component blocks of the high voltage power supply in FIG. 2. 
Consequently, within the scope of the appended claims it will be 
appreciated that the invention can be practiced otherwise than as 
specifically described herein.