Solid-state high voltage linear regulator circuit

A regulator circuit (10, 50) for connection to a high voltage generator (16, 52). The regulator circuit may be coupled to the generator in either a series or a shunt configuration. In the shunt configuration, the regulator circuit (10) varies the amount of current through a shunt resistor (R1) to change the output voltage provided to a load. The amount of current that is shunted by the regulator circuit is controlled by a feedback circuit consisting of a voltage divider (20) and an error amplifier (22). In the series configuration, the voltage across the regulator circuit (50) is added to the output from the high voltage generator. The current conducted through the regulator circuit therefore varies the summed output provided to the load.

FIELD OF THE INVENTION 
The present invention relates generally to high voltage regulators, and 
more particularly to solid-state circuits for high voltage regulation. 
BACKGROUND OF THE INVENTION 
Many applications demand a regulated high voltage that is free from 
variations in voltage level. Designing an inexpensive and reliable circuit 
that provides a regulated high voltage, however, has proved to be 
problematic. While it has been recognized that it would be advantageous to 
use solid-state devices in a regulator circuit because of their low cost 
and small size, it has been difficult to design such a circuit. For 
example, although bipolar junction transistors (BJTs) have been used in 
the design of high voltage regulator circuits, the regulator circuits have 
failed to achieve the necessary performance for practical use. In certain 
circumstances, the current necessary to drive the bipolar junction 
transistors can exceed the actual load current being regulated. Moreover, 
bipolar junction transistors cannot tolerate overvoltages for an extended 
period. Based on the perceived shortcomings of bipolar junction 
transistors in specific, and solid-state devices in general, current 
regulators have therefore typically been constructed using different 
technologies. 
SUMMARY OF THE INVENTION 
The present invention provides a solid-state regulator circuit for 
regulating a high voltage in a controlled manner. The regulator circuit 
consists of multiple MOSFET transistor stages connected in cascade. In the 
preferred embodiment, a blocking diode is connected in parallel with each 
stage. Each stage in the regulator circuit can be biased on or off. When 
biased on, the stage provides a conductive path. When biased off, the 
stage acts as an open circuit up to the breakdown value of the blocking 
diode across each stage. The first stage in the regulator circuit is a 
current regulation stage that includes a current sense resistor in the 
conductive path of the regulator circuit. The stages coupled to the 
current regulation stage do not contain a sense resistor, and will 
hereinafter be referred to as the component stages. 
In order to control the current flow through the regulator circuit, the 
current regulation stage is connected to a feedback circuit. The feedback 
circuit generates a signal that changes the bias point of a transistor in 
the current regulation stage. Changing the bias point of the transistor 
adjusts the amount of current that is flowing through the regulator 
circuit. 
In accordance with one aspect of the invention, the regulator circuit may 
be connected to a high voltage generator in a shunt configuration. In the 
shunt configuration, the high voltage generator is connected to a load 
through a shunt resistor. The last component stage and the feedback 
circuit are connected at a point between the shunt resistor and the load. 
The current regulation stage is connected to ground. If the output from 
the high voltage generator exceeds a desired level, the feedback circuit 
adjusts the bias point of the current regulation stage to shunt additional 
current through the shunt resistor connected to the high voltage 
generator. The additional current causes a greater voltage drop through 
the resistor, charging the output voltage applied to the load. In this 
manner, the voltage applied to the load is regulated by charging the 
current through the shunt resistor. 
In accordance with another aspect of the invention, the regulator circuit 
may be connected to a high voltage generator in a series configuration. In 
the series configuration, the component stages and the current regulation 
stage are connected in series with one of the output terminals from the 
high voltage generator. For example, the regulator circuit may be 
connected between ground and a first terminal of the high voltage 
generator that is floating with respect to ground. The feedback circuit is 
connected between a second terminal of the high voltage generator and the 
current regulation stage. Based on the monitored output voltage from the 
high voltage generator, the feedback circuit adjusts the amount of current 
flowing through the current regulation stage. In this manner, the output 
from the high voltage generator is maintained at a desired level. 
In accordance with still another aspect of the invention, the series of 
discrete blocking diodes across the regulator circuit will avalanche at a 
known voltage rating. The blocking diodes provide a measure of overvoltage 
protection by entering into avalanche if a voltage across the regulator 
circuit exceeds the sum total of the avalanche ratings of the blocking 
diodes. 
In accordance with still another aspect of the invention, the number of 
component stages can be varied to change the voltage that is regulated. 
Each component stage contributes to the regulation of a voltage roughly 
equivalent to the avalanche voltage rating of the blocking diode across 
the stage. The number of component stages may therefore be selected 
depending on the voltage that is to be regulated, allowing the regulator 
circuit to be simply and easily configured to operate in different 
environments. 
An advantage of the disclosed regulator circuit is that it allows high 
voltages to be regulated using MOSFET transistors. MOSFET transistors are 
readily available, relatively inexpensive, displace a very small volume, 
and are of minimal weight. Constructing the regulator circuit using MOSFET 
transistor stages coupled in cascade therefore creates a very economical 
and small high voltage regulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 depicts the preferred embodiment of a regulator circuit 10 in 
accordance with the present invention. Regulator circuit 10 consists of a 
number of component stages 12a, 12b, and 12c connected in cascade with a 
current regulation stage 14. As will be described in additional detail 
below, the regulator circuit may operate in one of two states. In an "off" 
state, the component stages 12a, 12b, and 12c and the current regulation 
stage 14 are initially biased off so that there is no conductive path 
provided through the regulator circuit. In an "on" state, the component 
stages and the current regulation stage are biased on so that a conductive 
path is provided through the regulator circuit. The amount of current that 
flows through the regulator circuit is controlled by the current 
regulation stage 14 in a manner that will be described below. 
The regulator circuit 10 is depicted in FIG. 1 in a shunt configuration. 
One end of the regulator circuit 10 is connected between the output of a 
high voltage generator 16 and a load. The other end of the regulator 
circuit is connected to ground 18. A feedback circuit comprised of a 
voltage divider 20 and an error amplifier 22 is connected between the load 
and the current regulation stage 14. The feedback circuit monitors the 
output voltage supplied to the load, and changes the amount of current 
that is shunted by the regulator circuit 10 in order to maintain the 
output voltage at a desired level, i.e., provide an essentially constant 
voltage to the load despite variations that otherwise would affect the 
output voltage at terminal V.sup.out. 
Examining the feedback circuit in closer detail, the output voltage from 
the high voltage generator 16 is connected in series with a shunt resistor 
R1. The current flowing through shunt resistor R1 determines the output 
voltage at the load. That is, the voltage drop across the resistor is 
subtracted from the output voltage generated by the high voltage generator 
to determine the voltage applied to the load. The regulator circuit 10 
therefore adjusts the current flowing through the shunt resistor in order 
to maintain a desired output voltage at the load. 
The voltage divider 20 consists of a resistive and capacitive network that 
steps down the output voltage at the load. The voltage divider consists of 
a resistor R2 in series with a resistor R3 connected between line 24 and 
ground. Resistor R3 is preferably much smaller than resistor R2 so that 
the output voltage produced by the high voltage generator is greatly 
stepped down for use in the feedback circuit. A line 26 is connected to 
the point where resistor R2 connects with resistor R3. Line 26 provides 
the stepped-down voltage from the voltage divider to the error amplifier 
22. The capacitive network includes capacitors C2, C3 and C4 connected in 
series between the output end of resistor R1 and ground, and an additional 
capacitor C1 connected between the junction of resistors R2 and R3 and the 
junction of capacitors C2 and C3. A resistor R4 is connected in parallel 
with capacitor C3. A resistor R5 and a Zener diode Z1 are connected in 
parallel with capacitor C4. The capacitive network provides instantaneous 
feedback information to the error amplifier. The capacitive coupling 
associated with the capacitive network increases bandwidth of the voltage 
divider. A provision which defeats the capacitive coupling allows 
capacitor C2 to charge upon initial circuit actuation is composed of 
components C3, Z1, C4, R4, R5. Zener Z1 performs the function of a switch 
providing a current shunt of smaller value capacitor C4 during the 
charging of C2. The Zener voltage is set for approximately five volts. 
In an actual embodiment of the voltage divider, the components of the 
voltage divider have the following values: 
______________________________________ 
Component Part Number or Rating 
______________________________________ 
Resistor R2 500 Meg 
Resistor R3 250K 
Resistor R4 47 Meg 
Resistor R5 47 Meg 
Capacitor C1 0.01 .mu.F 
Capacitor C2 1000 pF 
Capacitor C3 0.68 .mu.F 
Capacitor C4 0.10 .mu.F 
Zener Diode Z1 IN6489, 4.74 
______________________________________ 
The error amplifier 22 compares the stepped down output at the load with a 
reference voltage and produces an error signal that is proportional to the 
difference in the two voltage levels. The error amplifier consists of an 
operational amplifier U1 having the non-inverting input connected to line 
26 through a resistor R7. The inverting input of operational amplifier U1 
is coupled to a voltage reference (V.sub.ref) terminal 28 through a 
resistor R8. The inverting input of the operational amplifier U1 is also 
connected to the output of the amplifier by a capacitor C5, and by the 
series connection of a resistor R9 and a capacitor C6. The voltage 
reference terminal is maintained at a reference voltage level that 
corresponds to the desired output at the load. In the preferred 
embodiment, the reference voltage is a stable DC voltage that does not 
fluctuate like the high voltage generator. The reference voltage may be 
supplied by a number of circuits, such as from an LH0070-2 device. 
The voltage applied to the load is compared by the error amplifier 22 with 
the desired voltage as represented by the reference voltage on the 
V.sub.ref terminal. The error amplifier produces an error signal that is 
proportional to the difference between the desired voltage and the output 
voltage at the load. The error signal is provided to the current 
regulation stage 14 on a line 30. The slew rate of the error amplifier is 
slowed by the network consisting of capacitors C5, C6 and resistor R9, 
which filter any high frequency variations in the error signal. In an 
actual embodiment of the error amplifier, the components of the error 
amplifier have the following values: 
______________________________________ 
Component Part Number or Rating 
______________________________________ 
Resistor R7 10K 
Resistor R8 10K 
Resistor R9 100K 
Capacitor C5 10 pF 
Capacitor C6 0.01 .mu.F 
Operational Amplifier U1 
TL064, LM124 
Resistor R10 100 .OMEGA. 
______________________________________ 
The output from the error amplifier 22 is connected to the current 
regulation stage 14 of the regulator circuit 10 through a resistor R10. 
The current regulation stage is constructed around a pair of transistors 
TRA and TRB, preferably both MOSFETs. A sense impedance, preferably a 
sense resistor RS, is connected between the source of transistor TRA and 
ground 18. The sense resistor RS is selected to have a peak power 
capability sufficient to conduct the desired current when the regulator 
circuit is turned on. A diode DD and a capacitor CD are connected between 
the source of transistor TRA and the drain of transistor TRB. A capacitor 
CF is also connected in parallel with the sense resistor RS. 
Transistors TRA and TRB are both biased by the error signal produced by the 
error amplifier. A resistor RG and a Zener diode ZG are connected in 
parallel between the gate and source of transistor TRB. Resistor RG and 
Zener diode ZG are selected to prevent the transistor from conducting due 
to leakage current during biased-off operation, to protect the transistor 
from gate-to-source stress during biased-on operation, and to allow the 
desired gate-to-source voltage to turn the transistor on when a conductive 
path is generated through the regulator circuit. The gate of transistor 
TRA is connected in series with a diode DB and a resistor RB. Diode DB is 
selected to ensure that reverse current will not flow from the current 
regulation stage. Resistor RB is sized to limit the current flow into the 
transistor when the regulator circuit is turned on. In an actual 
embodiment of the regulator circuit, which is designed to regulate an 
approximate 10,000 volts output, the circuit elements for the current 
regulation stage are as follows: 
______________________________________ 
Component Part Number or Rating 
______________________________________ 
Diode DD BYD37M 
Capacitor CD 10 pF 
Transistors TRA 1RFR020, MTD IN80E 
Zener diode ZG BZX84015, 15V 
Resistor RG 10K ohm 
Diode DB BYD37M 
Resistor RB 1K ohm 
Resistor RS 1K ohm 
Capacitor CF 0.01 .mu.F 
Resistor RZ 4.99K ohm 
______________________________________ 
The drain of transistor TRB is connected to the first component stage 12a. 
It is noted that each component stage 12a, 12b and 12c is constructed with 
the same circuit elements. For purposes of this description, a generic 
component stage 12a will therefore be discussed as representative of all 
the component stages. Component stage 12a is constructed around a pair of 
transistors TR, which in the preferred embodiment of this circuit are a 
pair of MOSFETs connected in cascade. Component stage 12a is similar to 
the current regulation stage, in that both stages are constructed around a 
pair of transistors. The component stages do not, however, contain a sense 
resistor in the conductive path. A diode DD and a capacitor CD are 
connected across the transistors TR. Diode DD and capacitor CD serve the 
same functions as the corresponding components in the current regulation 
stage, that is, they are selected to provide overvoltage protection for 
the circuit. A Zener diode ZG and a resistor RG are also connected across 
the gate and source of each transistor. The Zener diode ZG and the 
resistor RG also serve the same roles as they do in the current regulation 
stage. 
The gate of each transistor TR in the component stage is connected to a 
biasing voltage through a resistor RB and a diode string DB. The diode 
string DB contains a different number of diodes for each transistor in the 
component stages. In order to ensure that only one component stage 
operates in a linear mode, the number of diodes within the diode string 
associated with a particular component stage increases by one for each 
transistor within the stage. Thus, in the representative regulator circuit 
depicted in FIG. 1, component stage 12a contains diode strings having two 
and three diodes, component stage 12b contains diode strings having four 
and five diodes, and component stage 12c contains diode strings having six 
and seven diodes. Before turning on, the voltage drop across the component 
stage must therefore exceed the voltage drop required to turn on the 
previous component stage by a value equal to the voltage drop across one 
diode DB. This method has the advantage of producing additional output 
stability due to the required voltage drop increase for conduction of an 
additional transistor. 
The drain of the transistor TR in the last component stage 12c is connected 
to the output voltage line 24 through the series connection of diode 
string DS and a resistor R6. Diode string DS is a string of Zener diodes 
that allow the output voltage at the load to exceed the voltage level that 
may be shunted by the component stages and current regulation stage alone. 
The diode string drops a fixed voltage providing a lower voltage at the 
component stages. The number of diodes within the diode string may 
therefore be changed rather than requiring the addition of component 
stages in certain applications. 
Before the regulator circuit is turned on, all the component stages are 
nonconducting. The biasing potential provided to each of the component 
stages is sufficient to raise the potential at the gates of the component 
stage transistors TR so that they will become biased on when the 
gate-to-source turn-on voltage for each transistor is exceeded by a 
voltage across resistor RG. That is, each transistor TR will become biased 
on when the current flow through the associated resistor RG causes a 
voltage drop across the resistor that exceeds the turn-on voltage of each 
transistor. When biased off, the resistance of each component stage 
exceeds one gigaohm. The regulator circuit therefore acts as an open 
circuit. 
The regulator circuit is turned on when the high voltage generator begins 
to generate an output voltage on line 24. The high voltage at the load is 
stepped down by the voltage divider 20 and compared by the error amplifier 
22 with the reference voltage level. The error signal generated by the 
error amplifier is applied to the current regulation stage 14, biasing 
transistor TRA so that it begins to conduct current through the sense 
resistor RS. After transistor TRA is biased on, a current path is provided 
through diode DB, resistor RB, and resistor RG of the directly adjacent 
transistor TRB, and through the current regulation stage transistor TRA 
and the sense resistor RS to ground. When the voltage across resistor RG 
rises sufficiently above the gate-to-source potential threshold of 
transistor TRB, the transistor is biased on. 
The turning-on process repeats for the transistors TR in the component 
stages. The transistors TR in each component stage remain biased off, and 
non-conducting, until the transistors in the component stage that is 
located nearer to the current regulation stages enter into conduction. The 
number of transistors TR that are biased on depends on the current through 
the current regulation stage 14. Depending on the current being shunted, 
some, but not necessarily all of the transistors in the component stages 
will be biased on. One transistor TR will operate in a linear mode. The 
transistors TR closer to the current regulation stage will operate in 
saturation. The transistors TR higher in the component stack will remain 
biased off, however the current will flow through the blocking diodes DD 
around the biased off transistors. The conductive path through the 
regulator circuit during operation therefore extends through the 
avalanching diodes DD, through the transistor TR operating in linear 
operation, through the transistors TR operating in saturation, and through 
the current regulation stage to ground 14. The transistor operating in a 
linear mode will change depending on the current being shunted. 
Ultimately, current is shunted through the regulator circuit 10 to 
maintain the output voltage at a desired level. When this occurs, current 
will be shunted through the regulator circuit 10 away from the load 
connected to output line 24. 
The amount of current that is shunted away from the load depends on the 
biasing point of the current regulation stage 14. The biasing point of the 
current regulation stage is adjusted by the changing voltage applied to 
the current regulation stage by the error amplifier 22. The reference 
voltage V.sub.ref is selected so that the output from the high voltage 
generator 16 is regulated at a desired level. In this manner, the amount 
of current through the current regulation stage is closely controlled. 
While three component stages 12a, 12b and 12c are depicted in FIG. 1, it 
will be appreciated that a greater or lesser number of component stages 
may be included within the regulator circuit. Each component stage 
contributes to regulating a voltage equal to the maximum avalanche voltage 
of the blocking diode for that stage. The diode ratings of each component 
stage and the current regulation stage are therefore used to determine the 
number of component stages necessary to regulate a particular voltage. For 
example, if the regulator circuit were to regulate 6,000 volts, and if 
blocking diodes DD rated at 1,000 volts were used in the regulator 
circuit, a total of five component stages would be required in the 
regulator circuit. The total avalanche voltage of the five blocking diodes 
in the component stages and the single blocking diode in the current 
regulation stage would add to a number approximating the required 
regulated voltage of 6,000 volts. It will be appreciated that a greater or 
lesser number of component stages could be used to select the regulated 
voltage of the regulator circuit. Moreover, diodes having different 
ratings may also be selected to change the regulated voltage capability. 
As noted above, the number of Zener diodes in the diode string DS may also 
be changed to reduce the number of required component stages. 
The regulator circuit 10 disclosed in FIG. 1 is advantageous in that it 
uses solid-state MOSFETs to regulate high voltages. Using MOSFETs reduces 
the cost of the regulator circuit, allows the regulator circuit to be 
incorporated into a very small package, and allows the regulator circuit 
to operate reliably in high voltage applications. 
FIG. 2 depicts an alternative embodiment of a regulator circuit 50 in a 
series configuration with a high voltage generator 52. The high voltage 
generator 52 is in a floating configuration, wherein the generator is not 
grounded. The construction and operation of the regulator circuit 50 is 
similar to the regulator circuit 10 depicted in FIG. 1. The operation of 
the regulator circuit will therefore be broadly described, with the reader 
directed to the corresponding text of FIG. 1 for additional details. 
The high voltage generator 52 is connector to a load by a line 54, and to 
the regulator circuit 50 by a line 53. Unlike the regulator circuit 10 
shown in FIG. 1 which contained multiple component stages, the regulator 
circuit 55 shown in FIG. 2 contains only a single current regulation stage 
55. The current regulation stage is constructed around a pair of 
transistors TRA and TRB, preferably both MOSFETs. A sense impedance, 
preferably a sense resistor RS, is connected between the source of 
transistor TRA and ground 66. The sense resistor RS is selected to have a 
peak power capability sufficient to conduct the desired current when the 
regulator circuit is turned on. A diode DD and a capacitor CD are 
connected between the source of transistor TRA and the drain of transistor 
TRB. 
The current regulation stage 55 operates in the same manner as does the 
current regulation stage in the regulator circuit 10 depicted in FIG. 1. 
The current regulator stage is connected to a feedback circuit consisting 
of an error amplifier 62 and a voltage divider 56. The voltage divider 56 
is coupled to the output line 54 that extends from the high voltage 
generator to the load. The voltage divider 56 generates a signal on a line 
58 that is proportional to the output voltage produced by the high voltage 
generator. The stepped-down signal is provided on line 58 to the error 
amplifier 62. 
The error amplifier 62 compares the stepped-down voltage signal with a 
reference voltage V.sub.ref. The error amplifier contains an operational 
amplifier U2 that acts as an inverting buffer. The output from operational 
amplifier U2 is provided to operational amplifier U3, which operates as a 
comparator to compare the measured voltage level on the output line 54 
with a voltage reference V.sub.ref. An error signal is generated that is 
proportional to the difference between the measured voltage on the output 
line 54 and the reference voltage V.sub.ref, and provided to the current 
regulation stage 55 on a line 64. 
The error signal changes the biasing point of transistor TRA, controlling 
the amount of current that is conducted through the current regulation 
stage. The impedance of the current regulation stage varies with the 
current flow through the stage. Since the current regulation stage 55 is 
coupled in series with the high voltage generator 52, the voltage drop 
across the current regulation stage will be summed with the voltage 
generated by the high voltage generator. By changing the amount of current 
that flows through the current regulation stage, the output voltage 
provided to the load is also changed. In this manner, the output voltage 
applied to the load is closely regulated. 
Those skilled in the art will appreciate that additional circuitry is 
present within the feedback circuit of the regulator circuit 50 to 
minimize noise, slow the response of the feedback circuit, and prevent 
oscillations in the output from the high voltage generator. Those skilled 
in the art will also appreciate that additional component stages may be 
added to the current regulation stage 55 if higher voltages are to be 
regulated. The use of the regulator circuit 50 in a series configuration 
allows the high voltage generator 52 to remain floating. 
While the preferred embodiment of the invention has been illustrated and 
described, it will be apparent that various changes can be made therein 
without departing from the spirit and scope of the invention. 
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.