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 higher current. Many types of shunt voltage regulators are in widespread use today. U.S. Pat. No. 5,023,543--Tse, U.S. Pat. No. 5,029,295-- Bennett et al., U.S. Pat. No. 5,519,313--Wong et al., and U.S. Pat. No. 5,621,307-- Beggs, all disclose a version of a temperature compensated voltage regulator. These and similar patents refer to basic low voltage regulator methods to which we will not presume to add, modify or improve.
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, called the "zener" voltage, is applied across it. When the zener voltage is reached and exceeded, current conduction across the zener diode junction interface increases rapidly. Zener diodes are commonly commercially available with zener voltages of about 2 volts to about 200 volts. A problem with zener diodes is that those with zener voltages above about 6 volts exhibit large positive temperature coefficients, and an increase in noise and negative resistance characteristics at low currents; these traits worsen as higher voltage zener diode devices are selected. Lower voltage zener diodes exhibit a negative temperature coefficient, but above about 6 volts true zener breakdown does not occur, with avalanche mode breakdown taking over and imparting a positive tempe rature coefficient. Thus, high voltage zener diodes are not generally suitable in very low current applications, especially at elevated temperatures.
A number of temperature coefficient correcting schemes involving multiple diodes are possible; included are combinations of higher and lower voltage zener diodes or combinations of lower voltage zener diodes and conventional silicon diodes. While these schemes may compensate the temperature coefficient, all are less than ideal due to space considerations, likely noise problems at very low currents, and the fact that any such temperature coefficient correction can be optimized for only a narrow current range.
A variation is the use of a thermistor as a temperature compensating element for a string of zener diodes. While this scheme can result in a more compact assembly due to the use of just a few high voltage zener diodes, the temperature coefficient optimization can only be made over a very narrow current range, and there remains the likelihood of noise problems at very low currents.
One manufacturer (Comprobe) of oil well logging sondes utilized conventional silicon diodes as high voltage regulator devices. The diodes had to be selected for appropriate reverse leakage characteristics, a time consuming process resulting in a low yield of usable devices. The leaky diode high voltage regulator was unreliable and extremely temperature unstable, and its use was long ago discontinued.
The prior art also includes a gas discharge diode tube operating 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 (characterized to 150°C and usable to 200°C). However, these devices are fragile, expensive, and they require a radioactive component (a beta emitter). They are no longer manufactured, at least in part due to misplaced concerns about their radioactivity. One would have to literally eat such a device in order to sustain any real chance of damage from the radiation, but even then the more likely injury would be from the broken glass of the tube envelope.
Scaccianoce discloses an integrated circuit that provides thermal compensation for a series string of zener diodes, in which several bipolar transistors are connected as VBE 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 (IC) in each transistor is equal to the base current (IB) multiplied by the common emitter gain (HFE) of the transistor. The value of HFE 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 uA to about 500 uA), 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.
Mosley discloses a high voltage shunt regulator
circuit comprising high voltage zener diodes connected in series with a thermal compensation
device. This Mosley apparatus includes the mistake of using a string of noisy avalanche mode
zener diodes at currents that virtually guarantee negative resistance and the specter of
oscillation or other noise. Furthermore, the design requires costly adjustment of each and
every thermal compensation device used in the unit. Note that operation above 150° C is
questionable, since very few components are specified above 125° C, and none above
175° C. Further, note that the FET temperature compensation units are
physically distanced from the zener units, resulting in poor compensation when temperature
gradients are present. Large temperature gradients are also encouraged by the teaching due
to the virtual impossibility of placing each zener diode in thermal contact with a
corresponding FET gate-source voltage multiplier.
Temperature testing of such a unit made by Titan, done by a division of Halliburton,
shows voltage excursions across the shunt regulator exceeds the voltage excursions of the
power supply; that is to say, this configuration makes things worse during and after
temperature changes. The data indicates that it can take about an hour for the Mosley/Titan
regulator configuration to stabilize.
Improvements were suggested.
One object and advantage was to provide an improved stable high voltage regulation method and apparatus that can operate with low shunt regulation currents over a wide temperature range, and especially at elevated temperatures.
Another object and advantage is the avoidance of the use of avalanche mode zener diodes.
The invention is a high voltage shunt regulator circuit comprising a voltage- controlled resistive device to regulate voltage, a low voltage device having a predetermined reverse conduction threshold voltage in series with said voltage controlled resistive device, and a thermal compensation device comprising a voltage sensing resistive divider in combination with said thermal compensation device. The voltage-controlled resistive device is preferably a Mosfet, with its source connected to ground via an appropriate stable voltage reference. The voltage divider is formed by a first resistor connected from the voltage to be regulated and the gate of the Mosfet and continues to the thermal compensation device, and then to a second resistor connected to ground. The thermal compensation device is selected and its operating bias is also selected so as to substantially track the temperature coefficient of the Mosfet. This allows efficient operation at temperatures at least as high as 125° C, and usable operation above 150° C is observed. Stable and low noise operation is exhibited with low shunt regulation currents of about 25 microamps up to about 5 milliamps at room temperature. However, the present invention is preferably realized by optimization in the 25 uA to about 500 uA range.
The zero temperature coefficient point of current regulation can be adjusted over a wide range, thereby allowing the use in a wide variety of applications.
Again, large temperature gradients are also encouraged by the teaching due to the virtual impossibility of placing each zener diode in thermal contact with the corresponding Mosfets, and of placing the Mosfets in thermal contact.
The Provisional Patent Application "High Voltage Shunt Regulator Circuit" covering this device was filed Jan 16, 2003; Application # 6/440,903.
Our Codatron® high temperature high voltage shunt regulator uses a zener and a temperature compensating diode mounted on the same metallic substrate, which guarantees that they see the same temperature within a few milliseconds in the worst case. That means there is no thermal histeresis, making it superior to other implementations.