Patent Publication Number: US-6222350-B1

Title: High temperature voltage regulator circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates to generally to the field of discrete component shunt voltage regulator circuits. More specifically, it relates to a shunt voltage regulator circuit suitable for high voltage, low current applications, especially in high temperature environments. 
     Shunt voltage regulators are components or circuits that are usually connected in parallel with a particular electronic device, or across the input or output terminals of a circuit, to limit the voltage that can be applied across the device or between the terminals. The shunt regulator performs this function by conducting very little current until a preset voltage is reached, at which point the regulator becomes a very low resistance device that conducts a high current. 
     A well-known type of shunt voltage regulator is the zener diode. A zener diode exhibits a very high resistance, and thus allows the passage of very small currents, until a predefined reverse threshold voltage (or “zener” voltage) is applied across it. When the zener voltage is reached or exceeded, the zener diode becomes conductive with a variable current at the zener voltage. Zener diodes are commonly available with zener voltages of about 2 volts to about 400 volts. A problem with zener diodes is that those with zener voltages above about 5 or 6 volts exhibit large positive temperature coefficients (expressed in V/° C.), as shown in the graph of FIG.  1 . Thus, high voltage zener diodes are not suitable in many applications in which high ambient temperatures may be experienced. Of course, a large number of low voltage zener diodes may be connected in series to provide a high voltage regulator that is relative temperature-stable, but this is usually impractical in terms of cost and space considerations. 
     U.S. Pat. No. 5,949,122—Scaccianoce discloses an integrated circuit that provides thermal compensation for a series string of zener diodes, in which several bipolar transistors are connected as V BE  multipliers. While this circuit provides temperature-stable high voltage regulation, it may not work well at very low collector currents. This is because the bipolar transistors are connected in a common emitter configuration, in which the collector current (I C ) in each transistor is equal to the base current (I B ) multiplied by the common emitter gain (H FE ) of the transistor. The value of H FE  for a typical bipolar transistor is in the range of about 10 to about 200. Since the collector current in the Scaccianoce device is the shunt regulation current, the base current would be between 0.5% and 10% of the shunt regulation current. Thus, at low shunt regulation currents (i.e., about 25 μamps to about 500 μamps), the base current would be at or near the value of the collector cutoff current (the collector-to-base leakage current, or I CBO ) for typical bipolar transistors. There are bipolar transistors with values of I CBO  low enough to allow the Scaccianoce device to work at low shunt regulation currents, but the value of I CBO  exhibits a large positive temperature coefficient, especially at temperatures above about 100° C. Thus, as a practical matter, a device constructed in accordance with the Scaccianoce disclosure to operate at low shunt regulation currents would be limited to operation in temperatures below about 125° C. 
     The prior art also includes a gas discharge tube device that operates in the corona mode of discharge. This device operates as a high voltage equivalent of a zener diode, and it functions well with low shunt regulation currents and at high temperatures (100° C. to 200° C.). These devices are fragile, however, and expensive, and they require a radioactive component (a beta emitter), which may present a health concern in some contexts. 
     There is thus a need for a high voltage regulation device that can operate with low shunt regulation currents in high temperature environments. There is a further need for a device that meets these operational criteria, and that may also be realized in a space-efficient and shock-resistant package. 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention is a high voltage shunt regulator circuit comprising a high voltage device with a predetermined reverse-conduction threshold connected in series with a thermal compensation device comprising a plurality of gate threshold amplifiers connected in series with one another. The high voltage device comprises a plurality of zener diodes connected in series. Each of the gate threshold amplifiers comprises a resistive voltage divider and a voltage-controlled resistive device, preferably a MOSFET. Specifically, the voltage divider comprises first and second resistors connected in series between first and second terminals of the gate threshold amplifier, with a MOSFET having its drain connected to the first terminal, its source connected to the second terminal, and its gate connected to an intermediate tap of the voltage divider. 
     The zener diodes provide high voltage regulation (up to at least about 1600V), while the thermal compensation device exhibits a negative temperature coefficient that substantially offsets the positive temperature coefficient of the zener diodes. This allows efficient operation at temperatures at least as high as about 200° C. The gate threshold amplifiers, each including a voltage-controlled resistive device, allow operation at low shunt regulation currents, i.e., on the order of about 25 μamps to about 500 μamps. 
     The present invention is preferably realized with discrete components, thereby minimizing costs. Because only a few components are needed, even for the regulation of high voltages in relatively high temperature environments, efficient use of space is achieved. Furthermore, the use of solid state components provides a high degree of resistance to mechanical shocks and vibrations. These and other advantages of the present invention will be better appreciated from the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a representative graph of temperature coefficient versus reverse breakdown voltage for a typical zener diode; 
     FIG. 2 is a representative graph of gate threshold voltage versus temperature for a typical MOSFET; 
     FIG. 3 is a circuit diagram of a MOSFET gate threshold amplifier, of the type used in a preferred embodiment of the present invention; and 
     FIG. 4 is a circuit diagram of a 1600 volt regulator constructed in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention, in its preferred embodiment, exploits an advantageous characteristic of MOSFET devices that is illustrated in FIG.  2 . The MOSFET device has a gate threshold voltage (V GS(TH) ), defined as the lowest voltage from the source to the gate at which a specified (low) value of drain current begins to flow. As shown in FIG. 2, the value of V GS(TH)  (normalized in the graph to a value of 1 at 25° C.) decreases substantially linearly as a function of temperature between 0° C. and 200° C. Thus, it can be seen that as temperature increases, the value of V G(TH)  for a MOSFET decreases, while the value of the zener voltage for a zener diode increases. By appropriately combining a MOSFET gate threshold amplifier, having suitably selected component values, with a zener diode chain selected for a specified total zener voltage, the offsetting temperature coefficients of the MOSFETs and the zener diodes can be used to maintain the total zener voltage sufficiently close to the specified total zener voltage for practical utility at elevated temperatures. 
     A MOSFET gate threshold amplifier  10 , suitable for use in the present invention to achieve the above-mentioned goal, is shown in FIG.  3 . The gate threshold amplifier  10  comprises a MOSFET  12  having a drain D, a source S, and a gate G. The drain D is connected to a first terminal  14 , and the source S is connected to a second terminal  16 . The MOSFET  12  shown in FIG. 3 is an n-channel MOSFET. It will be understood that a p-channel MOSFET can be used instead, with circuit modifications that will readily suggest themselves to those skilled in the pertinent arts. 
     A voltage divider is connected between the first and second terminals. The voltage divider comprises a first resistor R 1  connected between the drain D and the gate G of the MOSFET  12 , and a second resistor R 2  connected between the gate G and the source S of the MOSFET  12 . Thus, the gate G of the MOSFET  12  is connected to an intermediate tap  18  between the resistors R 1  and R 2 . 
     The MOSFET  12  will not conduct until the gate-source voltage (V GS ) is at least equal to the gate threshold voltage (V GS(TH) ). If the drain-source voltage (V DS ) is sufficient to result in a gate-source voltage that is greater than V GS(TH) , then the MOSFET will conduct until the gate-source voltage (created by the voltage divider R 1 +R 2 ) decreases to the value of V GS(TH) . A state of equilibrium is then reached at the condition of V GS =V GS(TH) . 
     The gate-source voltage may be expressed as: 
     
       
           V   GS   =V   DS   R   2 /( R   1   +R   2 ),  (1)  
       
     
     so that when V GS =V GS(TM) , 
     
       
           V   GS(TH)   =V   DS   R   2 /( R   1   +R   2 ).  (2)  
       
     
     Solving for V DS  yields: 
     
       
           V   DS   =V   GS(TH) ×( R   1   +R   2 )/ R   2 .  (3)  
       
     
     As can be seen from FIG. 2, discussed above, V GS(TH)  decreases substantially linearly with temperature. The change in V DS  as a function of temperature could thus be expressed as: 
     
       
         Δ V   DS ( T )=Δ V   GS(TH) ( T )×( R   1   +R   2 )/R 2 .  (4)  
       
     
     Equation (4) means that the temperature-dependent change in drain-source voltage (ΔV DS ) is equal to the temperature-dependent change in the gate threshold voltage (ΔV GS(TH) ), amplified by the resistance ratio (R 1 +R 2 )/R 2 . 
     FIG. 4 shows a specific example of a voltage shunt regulator circuit  20 , constructed in accordance with a preferred embodiment of the present invention, using MOSFET gate threshold amplifiers, of the type described above and illustrated in FIG.  3 . The regulator circuit  20  is designed to provide a regulated voltage of 1600V at temperatures up to about 200° C., and with shunt regulation currents as low as about 25 μamps. 
     The circuit  20  includes a high voltage device  22  comprising a string of six zener diodes  24 , each having a zener voltage of 200V at 25° C. Thus, at 25° C., the high voltage device  22  has a total zener voltage of 1200V. The high voltage device  22  is connected in series with a thermal compensation device  26 . Thus, to achieve a total regulated voltage of 1600V, the thermal compensation device  26  must produce a voltage drop of 400V. 
     The thermal compensation device  26  preferably comprises at least one of the above-described MOSFET gate threshold amplifiers  10 . Because high voltage MOSFETS tend to be quite large in physical size, high voltage applications in which a small size for the voltage regulator circuit is desired will typically require a string of at least two gate threshold amplifiers  10  connected in series with each other, each having a medium voltage MOSFET  12 . In the illustrated embodiment, two gate threshold amplifiers  10   a  and  10   b  are employed, including MOSFETs  12   a  and  12   b , respectively. To achieve a voltage drop of 400V across the thermal compensation device  26 , there must be a drain-source voltage drop V DS  of 200V across each of the two gate threshold amplifiers  10   a  and  10   b.    
     Each of the gate threshold amplifiers  10   a ,  10   b  includes a first terminal  14   a ,  14   b , respectively, and a second terminal  16   a ,  16   b , respectively. Connected between the first and second terminals of each of the gate threshold amplifiers  10   a ,  10   b , is a resistive voltage divider comprising a first resistor R 1  and a second resistor R 2 , with an intermediate tap  18  therebetween, as described above in connection with FIG.  3 . The first terminal  14   a  of the first gate threshold amplifier  10   a  is connected to the high voltage device  22  and to the drain of the first MOSFET  12   a . The second terminal  16   a  of the first gate threshold amplifier is connected to the source of the first MOSFET  12   a  and to the drain of the second MOSFET  12   b  through the first terminal  14   b  of the second gate threshold amplifier  10   b . The second terminal  16   a  of the first gate threshold amplifier  10   a  and the first terminal  14   b  of the second gate threshold amplifier  10   b  are also commonly connected to the second resistor R 2  of the first gate threshold amplifier  10   a  and the first resistor R 1  of the second gate threshold amplifier  10   b . Each of the gates of the MOSFETs  12   a , and  12   b  is connected to the intermediate tap  18  of the voltage divider of the gate threshold amplifier in which that MOSFET is included. 
     For each of the gate threshold amplifiers  10   a  and  10   b , a type of MOSFET having a drain-source breakdown voltage well in excess of of 200V was selected. Within this type, the range of gate threshold voltages at 25° C. was about 2.0V to 4.0V. Specimens were selected that exhibited a test value of V GS(TH)  of 2.75V at 25° C. One can find the ratio of the resistances R 1  and R 2  using Equation (3) above, with the value of the gate threshold voltage at 25° C. (2.75V) and the desired drain-source voltage drop of 200V across each of the gate threshold amplifiers  10 : 
     
       
         200=2.75( R   1   +R   2 )/R 2   (5)  
       
     
     
       
           R   1 =71.73 R   2   (6)  
       
     
     For proper operation of the gate threshold amplifiers  10   a  and  10   b , the drain-source current (I DS ) must be substantially greater than the current through the voltage divider. For example, one might design the gate threshold amplifier circuit so that I DS  is at least about ten times the value of the current through the voltage divider. Thus, if shunt regulation currents as low as about 25 μamps are desired, the resistances R 1  and R 2  may be selected so that the current through the divider is not more than about 2 μamps. Therefore, given the ratio set forth in Equation (6) above, if R 1  is selected to be 100 Megohms, then R 2  would be 1.39 Megohms. 
     At 200° C., as seen from FIG. 2, the value of V GS(TH)  is about 0.58 times the value of V GS(TH)  at 25° C. For the MOSFETs  12   a  and  12   b  selected as described above, the value of V GS(TH)  at 200° C. is therefore 0.58×2.75V=1.595V. The value of ΔV GS(TH)  (the difference between the values at 25° C. and 200° C.) is therefore 1.155V. Using Equation (4) above, with the values for R 1  and R 2  given above, the voltage drop across each of the gate threshold amplifiers  10   a  and  10   b  at 200° C. is: 
     
       
         Δ V   DS =1.155×( R   1   +R   2 )/R 2 =1.155×(100+1.39)/1.39=84.25 V.   (7)  
       
     
     The total change (decrease) in the voltage drop across the two gate threshold amplifiers  10   a  and  10   b  is thus twice the value of ΔV DS , or 168.5V. Thus, if the total voltage drop across the thermal compensation device  26  at 25° C. is 400V, the total voltage drop at 200° C. would be 231.5V. 
     From FIG. 1, it is seen that the temperature coefficient for each of the zener diodes  24  at 200° C. is about 0.16V/° C. Thus, the string of six zener diodes  24  will exhibit an increase in total zener voltage of: 
     
       
         6×0.16 V/° C.×ΔT= 0.96 V/° C.× (200−25)°  C.= 0.96 V/° C.× 175°  C.= 168 V,   (8)  
       
     
     where ΔT is the temperature differential between 25° C. and the expected ambient temperature at which the device is expected to operate, and for which the temperature coefficient is taken (in this case, 200° C.). 
     Therefore, the total zener voltage of the zener string will increase by 168V (from 1200V to 1368), which is substantially compensated by the 168.5V decrease in the voltage drop (from 400V to 231.5V) across the two gate threshold amplifiers  10 . Accordingly, the total voltage regulated by the regulator circuit  20  remains substantially the same at 200° C. as it is at 25° C. 
     In practice, the zener diodes  24  will have zener voltages that may vary from a nominal value by as much as about plus or minus 5 percent. Likewise, the MOSFETs  12   a ,  12   b  will have gate threshold voltages that may vary from the nominal value by a similar amount. These variations may be accommodated by using, for R 1 , a fixed precision resistor (e.g., 1% tolerance) of the same resistance in each gate threshold amplifier, and then selecting a value for R 2  that yields the desired results. The technique for doing this would be well-known to those of ordinary skill in the pertinent arts. 
     Although a specific example of a preferred embodiment of the invention has been described in detail above, the principles of the present invention will be readily employed in voltage regulator circuits having a wide range in the values of their operational parameters (e.g., total regulated voltage, shunt regulation current, ambient operating temperature). Thus, voltage regulator circuits in accordance with the present invention will be easily designed, with reference to the instant disclosure, by those skilled in the pertinent arts to accommodate a wide variety of needs and applications. 
     While a specific preferred embodiment has been described herein, it will be appreciated that a number of variations and modifications may suggest themselves to those skilled in the pertinent arts. For example, while the preferred embodiment described herein uses N-channel MOSFETs, P-channel MOSFETs may also be used, with circuit modifications that would be readily apparent to those skilled in the pertinent arts. These and other variations and modifications should be considered within the spirit and scope of the present invention, as defined in the claims that follow.