Patent Publication Number: US-6713991-B1

Title: Bipolar shunt regulator

Description:
BACKGROUND 
     In the field of high voltage power conversion, there are a number of methods that can be employed to design a single polarity solid state shunt regulator that is controlled from a low voltage signal. However, in some applications, a regulator is required to accept a high voltage input that has a changing polarity relative to ground and regulate the output to a given magnitude of the same polarity as the input. Typically, circuitry designed around vacuum tube technology has been used to satisfy this requirement. 
     An example of a single polarity shunt regulator is provided in U.S. Pat. No. 6,222,350 to Mosley (“the Mosley reference”). The high voltage shunt regulator including a series of Zener diodes connected in series with a thermal compensation circuit that includes a plurality of MOSFET switches and resistive voltage dividers. When a voltage in excess of the Zener threshold voltage is applied to the series of Zener diodes from a high voltage source, the Zener diodes conduct substantially all of the current applied at the input terminal to the output terminal. However, because the threshold voltage of a Zener diode decreases as the temperature of the Zener diode increases (which happens as the Zener diode conducts current), the MOSFETs of the thermal compensation circuit must be engaged to provide a compensatory voltage drop so that the diversion of current takes place at the appropriate voltage level. U.S. Pat. No. 5,949,122 to Sccaccionoce discloses a similar system using bipolar junction transistors instead of MOSFETs. Although these references discuss the use of shunt regulators in high voltage applications, the disclosed regulators are unsuitable for use in dual polarity applications. Accordingly, neither discusses the need for quick switching to accommodate changes in the polarity of the high voltage source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit schematic diagram depicting a solid state high voltage switch according to an embodiment of the present invention; 
     FIG. 2 is a circuit schematic depicting a solid state high voltage bipolar shunt regulator according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are directed to a high voltage solid state bipolar input and output shunt regulator. According to the present invention, the output of the shunt regulator automatically tracks the polarity of the input from a high voltage source, without the need of any polarity-indicating input. Current may be shunted in either a positive or negative manner in order to maintain regulation of the output voltage. The shunt regulator of the present invention may combine a first conduction path to act as a shunt when the input from the high voltage source has a positive polarity and a second conductive path to act as a shunt when the input from the high voltage source has a negative polarity. 
     In an embodiment of the invention, each conduction path may include a solid-state switch coupled between the high voltage input terminal and ground. FIG. 1 depicts an isolated high voltage MOSFET transistor stack that may be used as a solid-state switch according to embodiments of the present invention. The solid-state switch of FIG. 1, as shown, may be used as a switch element in which a control signal causes all of the transistors to either be fully enhanced (or closed) or fully “off” (or open). As discussed in greater detail with respect to FIG. 2, the solid-state switch shown in FIG. 1 may be slightly modified for advantageous use in a shunt regulator application. In the particular embodiment shown, the transistor stack consists of nine stages, each including a transistor Q 1  to Q 9 . While a MOSFET transistor is preferred for a switching element, other types of FETs or other transistors may be substituted. The number of transistor stages may be increased or decreased depending on the maximum input voltage the solid-state switch will receive. The high voltage conduction path of the switch is from a high voltage input terminal E 2  to an output terminal E 6  when the switch is in the closed mode. In the open mode, the switch will block transmission of a positive (as measured from terminal E 2  to E 6 ) voltage from the high voltage input terminal E 2  to the output terminal E 6 . An isolated bias voltage may be connected between bias terminal E 1  and bias terminal E 5 . Bias terminal E 5  may be coupled to the output terminal E 6 . Voltage regulators VR 1 , VR 3 , VR 5 , VR 7 , VR 9 , VR 11 , VR 13 , VR 15  and VR 17  (shown as zener diodes) may be provided to prevent damage to a transistor from the application of an excessive voltage across its drain and source terminals. 
     The switch may be operated by supplying a biasing voltage at a bias terminal E 1  relative to bias terminal E 5  of a magnitude great enough to fully enhance the gate of transistor Q 9  such that current is allowed to flow from the drain terminal to the source terminal of the transistor. In alternative embodiments in which bipolar junction transistors (BJTS) are used, a current is supplied to the base terminal of the BJT to induce current flow between a collector terminal and an emitter terminal. Current regulating elements CR 1  to CR 9  (shown as diodes in FIG. 1) may be used to prevent current flowing into the biasing stage when transistors Q 1  to Q 9  are open. 
     In embodiments of the invention, the control switch U 1  may be an optocoupler that includes a photodiode and a phototransistor, as shown in FIG.  1 . While an optocoupler is advantageous because it provides for isolation of the control signal from the phototransistor and the remainder of the circuit, other types of switches may be used. The biasing voltage may be applied at the bias terminal E 1  relative to bias terminal E 5  and a control signal drives current into control terminal E 3  and through the photodiode to return through control terminal E 4 . As a result, the photodiode may transmit light to the phototransistor, maintaining the control switch U 1  in a saturated state, i.e., the control switch is in the closed or “ON” state. When the current driven through the photodiode by the control signal is removed, the phototransistor will turn to the open or “OFF” state. In alternative embodiments of the invention, the control switch U 1  may be configured so that the open or “OFF” state is achieved when a “high” control signal is applied and the closed or “ON” state is achieved when no or a “low” control signal is applied. Alternatively, in embodiments of the invention, control switch U 1  may be removed and the state of the solid-state switch may be controlled by directly turning on or off the biasing voltage connected between bias terminal E 1  and bias terminal E 5 . 
     In alternative embodiments of the invention, other mechanisms may be used as the control switch U 1 , such as electrical coupling elements (e.g., a diode and transistor), switches, rectifiers, relays, resonant circuit elements (e.g., transformers), or the like. However, it may be advantageous to use an optocoupler as the control switch U 1  in applications where isolation of the control signal from the controlled load and fast response to changes in the control signal are desirable. It should be further understood that while use of an optocoupler including a phototransistor is shown in FIG. 1, different types of optocouplers, such as those including phototriacs, photodarlingtons or light-activated rectifiers, may also be used. 
     In the embodiment shown in FIG. 1, with the control switch U 1  in saturation (i.e., in the closed or “ON” state), the gate-source voltage of transistor Q 9  will be insufficient to bring the transistor Q 9  into enhancement mode. Current from the biasing terminal E 1  will be drained off to bias terminal E 5  (which may be electrically coupled to the output terminal E 6 ) rather than passing through the drain path resistor R 10 . Therefore, the solid-state switch will remain in the “open” state. The solid-state switch may then be “closed” by removing the control signal from (or applying a “low” control signal to) the control switch U 1 , thereby opening the control switch U 1 . Opening the control switch allows the gate of transistor Q 9  to become enhanced by the biasing voltage and thereby reduces the drain source impedance of Q 9  to effectively a short. Where transistor Q 9  is a FET, the transistor Q 9  may include an inherent capacitance between the gate and source terminals, and enhancement may occur when the biasing voltage is applied to the gate terminal. In alternative embodiments, such as those in which a BJT is used for transistor Q 9 , a biasing element may be connected between terminals of the BJT to forward bias the base-emitter junction. The impedance of Q 9  transitions from high to low, allowing current to flow from the bias terminal E 1 , through drain path resistor R 9  and transistor Q 9  to the output terminal E 6 . As a result, the biasing voltage may be applied to the gate terminal of transistor Q 8  and transistor Q 8  may also become enhanced. This cascading process continues up the stack until all of the transistor elements Q 1  to Q 9  are fully enhanced, at which time the solid-state switch is considered closed. Voltage-regulating Zener diodes VR 2 , VR 4 , VR 6 , VRS, VR 10 , VR 12 , VRl 4 , VRI 6  and VR 18  may be chosen so that their “breakdown” voltage is less than the maximum voltage difference permissible between the gate and source terminals at which transistors Q 1  through Q 9  can operate. 
     Where transistors Q 1  to Q 9  are FETs, drain path resistors R 2  to R 10  may be coupled across the gate and source terminals of the transistors Q 1  to Q 9  as shown in FIG.  1 . When the solid-state switch transitions from the closed state to the open state, charge built up at the gate terminal of each FET may be drained to prevent the stage from closing again until the biasing voltage is reapplied. Thus, during this transition, charge from the gate terminal of each transistor Q 1  to Q 9  may be drained away as current passing through the drain path resistors R 2  to R 10  to the output terminal E 6 , since current may be prevented from draining through the current-regulating diodes CR 1  to CR 9 . As previously discussed, in alternative embodiments in which the transistors Q 1  to Q 9  are BJTs, resistors may be coupled across the terminals of the transistors Q 1  to Q 9  to act as biasing elements. Furthermore, in alternative embodiments of the invention, the transistors Q 1  to Q 9  and control switch U 1  may be configured such that the transistors Q 1  to Q 9  transition to the closed state when the control switch U 1  closes. 
     FIG. 2 depicts a high voltage shunt regulator according to an embodiment of the present invention. As shown, the shunt regulator may regulate the output voltage provided at the output terminal J 2 . A high voltage source may be connected at the input terminal J 1 . For a positive input voltage, current flows from the input terminal J 1  to the output terminal J 2  through a resistor R 25 . Resistors R 23  and R 24  may be selected such that their combined resistance is much greater than the equivalent load impedance connected to J 2 . Under this condition, it may be assumed that the current shunted from the input terminal J 1  through resistors R 23  and R 24  is negligible. As a result, the maximum output voltage at J 2  may be approximately equal to the input voltage received at input terminal J 1  minus the product of the output current (I out ) and the resistance of resistor R 25 . Furthermore, in embodiments of the invention, resistor R 23  may be chosen to have a resistance significantly greater than that of resistor R 24  so as to produce a relatively low voltage signal at the input terminal to control circuit  103  that is still proportional to the output voltage. 
     In order to regulate the output voltage (V out ) to a desired output voltage less than this maximum output voltage when the high voltage source providing the input power has a positive polarity, current may be shunted to ground through a first conduction path including current regulator CR 1  (shown as a diode) and its corresponding solid-state shunt  101 . Shunting current through the first conduction path may increase the voltage drop across resistor R 25  such that the output voltage V out =Vin−R 25  * (I out +I shunt ), where R 25  represents the resistance of resistor R 25 , since the sum of the output current and the shunted current will generally be larger than the output current when no current is shunted through the first conduction path. Conversely, in the negative polarity condition, current may be shunted from ground to the input terminal J 1  through a second conduction path including current regulator CR 7  (also shown as a diode) and its corresponding solid-state shunt  102 . 
     The solid-state shunt  101  (as well as the solid-state shunt  102  in the alternate conduction path) may be a transistor stack type solid-state switch similar to the one shown in FIG.  1 . However, as shown in FIG. 2, the solid-state shunts  101  and  102  may differ from those shown in FIG. 1 insofar as the control switch U 1  used in the device of FIG. 1 may be incorporated into the control circuit  103 , such that the control circuit  103  directly produces or removes the biasing voltage to cause transistors in the transistor stack to open and close in response to detecting that the output voltage exceeds the desired voltage. Furthermore, as shown in FIG. 2, terminals E 5  and E 6  of the solid-state switches  101  and  102  (as identified in FIG. 1) may be coupled to ground. The transistors Q 1  to Q 5  and Q 6  to Q 10  in the transistor stacks themselves may be BJTs instead of FETs (as shown in FIG.  1 ). Accordingly, base resistors R 1  to R 6  and R 12  to R 17  may be included with each stage, such that when a biasing voltage is applied by the control circuit, a portion of the current flowing through the base resistor R 1  to R 6  and R 12  to R 17  for a particular stage will be diverted through the current regulators CR 2  to CR 6  and CR 8  to CR 12  toward the base terminals of the transistor Q 1  to Q 10  for that stage and the remainder of the current will be passed through the base resistor R 1  to R 6  and R 12  to R 17  for the next stage. Moreover, resistors R 7  to R 11  and R 18  to R 22  may be used as biasing elements as well as drain and/or noise filtering elements as described in connection with the FET embodiment shown in FIG.  1 . Accordingly, a portion of the current passing through current regulators CR 2  to CR 6  and CR 8  to CR 12  may be passed through these resistors R 7  to R 11  and R 18  to R 22  to ensure that the a sufficient potential difference exists (or does not exist, depending on the amount of current sent through the biasing resistor) at the base-emitter junction of a particular transistor Q 1  to Q 10  of a transistor stack. Moreover, the solid-state shunts  101  and  102  may be biased such that at least one transistor element of the stack is operated in its linear region. 
     The control circuit  103  may monitor the difference between the desired output voltage and the actual output voltage, and adjust the drive signal (e.g., the biasing voltage in the embodiment shown in FIG. 2) for the solid-state shunts  101  and/or  102  accordingly. In the embodiment shown, that magnitude of the biasing voltage, as well as the amount of load present may determine how many of the transistors Q 1  to Q 10  in a solid-state shunt  101  and  102  will be closed, while the polarity of the biasing voltage may determine whether transistors Q 1  to Q 10  in the solid-state shunt  101  and  102  of the positive or negative conduction path are closed. For example, the control circuit  103  may produce a positive biasing voltage that is capable of developing a base current that is sufficient to saturate transistors Q 3 , Q 4  and Q 5  of solid-state shunt  101  and will not cause any of the transistors Q 6  to Q 10  in solid-state shunt  102  in the negative conduction path to close. In this scenario, current may be shunted away from output terminal J 2  to ground through the closed transistors Q 3 , Q 4  and Q 5  and voltage regulators VR 1  and VR 2 . As is evident from this example, the solid-state shunts  101  and  102  as shown in FIG. 2 may provide the added benefit of acting as inherent output voltage clamps, because according to this example, the output voltage at output terminal J 2  may be clamped to a voltage approximately equal to the sum of the voltage drops across the voltage regulators of stages of the solid-state shunt in which the transistors are open (i.e., the sum of the zener voltages of voltage regulators VR 4  and VR 1  in the example given). 
     In embodiments of the invention in which the control circuit  103  provides a biasing voltage to the solid-state shunts  101  and  102  as a drive signal, such as that shown in FIG. 2, the magnitude of the biasing voltage may depend upon the difference between the output voltage and a desired output voltage level. In particular embodiments of the invention, this may be accomplished via the use of an error amplifier (not shown). As shown in FIG. 2, the output voltage may be divided down by resistors R 23  and R 24  to a level low enough to be inputted into a terminal of an error amplifier. A voltage representative of the desired output voltage level (e.g., the desired output voltage level divided by the same ratio by which the output voltage is divided for input to the error amplifier) may be input to the other terminal of the error amplifier. If the magnitude of the output voltage increases beyond the desired output voltage level, the control circuitry may raise the magnitude of the biasing voltage accordingly. The increase in biasing voltage may cause a solid-state shunt  101  or  102  to shunt more current (by closing additional transistors Q 1  to Q 10  within the solid-state shunt  101  or  102 ) and thereby decrease the magnitude of the output voltage. Conversely, if the magnitude of the output voltage drops below the desired output voltage level, the control circuitry may reduce the amount of biasing voltage applied to the solid-state shunt  101  and  102  on the stack, thereby opening one or more of transistors Q 1  to Q 10  in solid-state shunt  101  or  102  to cause the solid-state shunt  101  or  102  to shunt less current. This allows the magnitude of the output voltage to increase. The biasing voltage may be applied to both solid-state switches  101  and  102  even though only one conduction path may shunt current. In embodiments of the invention, the control circuit  103  may include two separate error amplifiers, one for each polarity condition. Furthermore, in embodiments of the invention, the desired output voltage level may be provided by an internal or external source and may be fixed or adjustable. In embodiments of the invention, each conduction path may have an independent control circuit for supplying the biasing voltage. 
     As shown in FIG. 2, when the magnitude of the output voltage is equal to the sum of the zener voltages of voltage regulators VR 1  to VR 5  in the positive polarity condition or VR 6  to VR 10  in the negative polarity condition, a conduction path will shunt current even though none of the transistors Q 1  to Q 5  or Q 6  to Q 10  may be closed. Accordingly, the number of stages in a solid-state switch  101  or  102  or the characteristics of the voltage regulators (e.g., zener voltages) in each stage may be chosen based on the desired level for output voltage regulation. Furthermore, the characteristics of the voltage regulator used in a stage may depend in part on the characteristic of the transistor used in a stage (e.g., a zener voltage may be selected to be less than the level of collector-emitter junction voltage at which a corresponding BJT may be damaged). 
     In some embodiments of the invention, the magnitude of the biasing voltage applied to the solid-state switch  101  in the positive polarity conduction path may be greater or less than the magnitude of the biasing voltage provided to the solid-state switch  102  in the negative polarity conduction path. Furthermore, the number of stages in each of the solid-state switches need not be the same. The number of stages in each solid-state switch may depend on the symmetry of the high voltage source, as well as the symmetry or relevant characteristics of the transistor elements used in each solid-state switch. 
     One or more of the individual transistors Q 1  to Q 10  of the solid-state shunts  101  and  102  may be operated in the linear region. As the desired output voltage level is reduced in magnitude, the transistor(s) Q 1  to Q 10  in operation shifts up the stack. As an example, in a positive polarity condition, the desired output voltage may range from 1 Volt (V) to 1000 V. Accordingly, voltage regulators VR 1  through VR 5  may be 200 V zener diodes. The output may be regulated by comparison of the divider voltage that is input to the control circuit (representative of the output voltage) against a variable reference voltage representative of a variable desired output voltage. Initially, the desired output voltage may be set at 1000 V and the transistors Q 1  to Q 5  may not conduct any current in order to maintain the output voltage at the desired level. As the reference voltage is adjusted down to correspond to a desired voltage limit within the 800 V to 1000 V range, transistor Q 5  may begin operating in the linear region, because the voltage difference at the collector-emitter junction of transistor Q 5  will be high (i.e., the amount by which the output voltage exceeds  800  V in this example). The control circuit  103  may also provide a large biasing voltage (since the biasing voltage may be approximately proportional to the difference between the output voltage and the desired output voltage), causing a relatively large current to be shunted through transistor Q 5  and voltage regulators VR 1  to VR 4 . When the difference between the output voltage and the sum of the zener voltages of voltage regulators VR 1  to VR 4  is sufficiently small, the transistor Q 5  may reach its saturation level, at which time voltage regulator VR 5  may be effectively shorted by transistor Q 5 . During the linear operation of transistor Q 5 , transistors Q 1  to Q 4  may be off and the conduction path to transistor Q 5  is through series elements VR 1  to VR 4 . While transistor Q 5  is operating in the linear region, the voltage of its collector terminal, which is also the voltage of the emitter terminal of transistor Q 4  up the stack, may be relatively high, maintaining current regulator CR 5  in a reverse-biased condition and preventing the base-emitter junction of transistor Q 1  from becoming biased. But as transistor Q 5  approaches saturation the voltage of its collector terminal, and that of the emitter terminal of transistor Q 4 , may drop to a relatively low level. Over the next region, 800V to 600V, transistor Q 5  may remain saturated and transistor Q 4  may operate over its linear region, while transistors Q 1  through Q 3  remain off and current is conducted through voltage regulators VR 1  to VR 3 . As the desired output voltage continues to drop, this transition may continue up the stack until all elements are fully saturated. Thus, the voltage regulators VR 1  to VR 10  may protect the corresponding transistors Q 1  to Q 10  from overvoltage conditions and provide a shunt-current path for transistors that are in the off state. 
     As the power received at input terminal J 1  changes polarity (for example, from positive to negative), the conduction path shunting current will automatically switch from the positive shunt element (e.g., the first conduction path as illustrated in FIG. 2) to the negative shunt element (e.g., the second conduction path as illustrated in FIG.  2 ). When the polarity of the input power received at input terminal J 3  is stable and of a roughly constant magnitude, the output power transmitted at output terminal J 2  may also maintain an approximately constant magnitude. At this magnitude, the first conduction path may shunt excess current through solid-state switch  101  to maintain the output voltage at the desired value. As the input starts to transition from this stable positive magnitude towards zero, the solid-state switch  101  in the positive first conduction path may shunt less and less current in order to keep the output regulated. When the shunt current through current-regulator CR 1  effectively reaches zero, the output voltage may no longer remain regulated and may start to drop in magnitude with the input voltage. As the input voltage transitions to negative polarity, it may reach a magnitude large enough to once again support output regulation. At this point, current may again be shunted, but from ground via the negative second conduction path through the current regulator CR 7  and its corresponding solid-state switch  102 . 
     In the embodiments described above, the output voltage may be regulated to the same magnitude when either a positive input polarity or a negative input polarity is present without the need for any external polarity reversal circuitry. It will be readily understood by those in the art that the embodiments described above may easily be modified for other regulation schemes, e.g., regulation of current, power or some other output power characteristic rather than voltage regulation. This may be accomplished by, for example, modifying the control circuit  103  to monitor and regulate output current, output power or another output power characteristic rather than output voltage. Furthermore, in embodiments of the invention, polarity reversal may be accomplished in either a symmetrical or non-symmetrical fashion. Accordingly, the conduction path for shunting current in the negative polarity condition may be engaged when the magnitude of the output voltage exceeds a first desired value and the conduction path for shunting current in the positive polarity condition may be engaged when the magnitude of the output voltage exceeds a second, different desired value. Accordingly, the number of stages in each of the solid-state shunts  101  and  102  may not be identical and the characteristics of the voltage regulators VR 1  to VR 5  may be different from those of voltage regulators VR 6  to VR 10 . 
     While the description above refers to particular embodiments of the present invention, it should be readily apparent to those in the art that a number of modifications may be made without departing from the spirit thereof The accompanying claims are intended to cover such modifications as would fall within the true spirit and scope of the invention. The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description. All changes that come within the meaning of and range of equivalency of the claims are intended to be embraced therein.