Abstract:
A high-temperature, regulated power supply uses wide band gap transistors to drive a Royer circuit. Pulses output from the Royer circuit are gated through a pulse width modulator to control the duty cycle of wide band gap transistors that drive an output transformer. The output of the transformer is rectified and filtered to provide the regulated D.C. output voltage. Regulation is accomplished by sampling the output voltage, comparing it to a reference voltage and using the difference between the output voltage and the reference voltage to control the pulse width modulator. High temperature operability is provided by using wide band gap transistors and iron or steel core transformers. This technique also provides a radiation hard assembly.

Description:
FIELD OF THE INVENTION 
   This invention relates to power supplies. More particularly, this invention relates to a lightweight power supply, capable of operating in ambient temperatures as high as 300° C. 
   BACKGROUND OF THE INVENTION 
   Existing power supplies use well-known silicon semiconductors, which work well at temperatures up to approximately 125° C. but silicon semiconductors are ill suited for applications where ambient temperatures are above 125° C. At temperatures over approximately 125° C., charge carriers in silicon leak across P-N junction. 
   Even at temperatures below 125° C., silicon semiconductors that require high-power dissipation require a heat sink to dissipate heat in order to protect the devices from being damaged. Heat sinks take up space and add weight. Accordingly, there exists a need for a power converter also known as a regulated power supply that is usable in high temperature environments but which is also operable with minimally-sized heat sinks to minimize the volume of the power converter as well as its weight. 
   SUMMARY OF THE INVENTION 
   A high-temperature, regulated power supply is provided in part by a Royer inverter circuit driven by wide-band gap transistors. Square waves output from the Royer circuit drive a “magnetic amplifier,” which operates as pulse width modulator. Output pulses from the pulse-width modulator drive wide-band gap transistors that drive current through an output transformer, the secondary of which is rectified and filtered. 
   Output voltage from the supply can be varied by adjusting the pulse width or duty cycle of pulses output from the pulse width modulator. The pulse width can be adjusted automatically using a feedback loop that drives the pulse width modulator to maintain an output voltage that is equivalent or proportional to a reference potential. 
   It is well known that heat transfer from a body to its surrounding environment by thermal radiation is proportional to T 4  where “T” is the body&#39;s temperature. Therefore, raising the temperature of the active devices in a power supply increases heat transfer significantly. Prior art silicon semiconductor devices leak current across their junctions at temperatures over 125° C., making them ill suited for applications where the ambient temperature is over approximately 125° C. By using “wide-band gap” semiconductors however, junction leakage current at high ambient temperatures is much less, making it possible to operate a semiconductor power supply in ambient temperatures over 300° C. using relatively small heat sinks. 
   The term “band gap” used herein refers to the energy difference between a material&#39;s non-conductive state and its conductive state. There is virtually no “band gap” in most metals, but a very large one in an insulator (dielectric). Technically, the “band gap” is the energy it takes to move electrons from the valence band to the conduction band. In most semiconductors, the “band gap” is relatively small. Silicon semiconductors have a band gap of approximately 1.12 eV. As used herein, a “wide band gap transistor” is a semiconductor made from materials that have an energy difference between the non-conductive state and conductive state that is greater than the band gap of silicon-based semiconductors. Silicon carbide is considered a “wide band gap” semiconductor. Its band gap is approximately 3 eV; it is also radiation hard. Crystalline silicone carbide can be doped to be either P-type or N-type semiconductor. A P-N junction made from silicon carbide transistor has a much higher “turn on” voltage than silicon (3 volts for silicon carbide vs. 0.7 volts for silicon) but will also have a much smaller leakage current at high temperatures because they have a much wider energy band gap. Although silicon carbide is a preferred semiconductor material, other wide band gap semiconductors that maintain functionality at temperatures above 125° C. and that are radiation hard are considered equivalent embodiments of a wide band gap semiconductor. 
   In addition to using wide band gap semiconductors, the transformers and coils used in the high temperature power supply are made using iron cores instead of composite materials. Iron and steel core transformers are operable at higher temperatures than are composite core transformers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an advantageous embodiment of a high temperature power supply. 
       FIG. 2  is a schematic diagram of an advantageous embodiment of preferred embodiment of a high temperature power supply. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  is a block diagram of functional elements of the high temperature power supply  10 . The square wave generator  12 , which is implemented by a Royer circuit and described more fully below, generates a square wave output pulse stream depicted by the waveform  13  shown in  FIG. 1 . The pulse width modulator  14  allows only a portion of each pulse from the Royer circuit to turn on a current driver  16 - 1 , the output of which drives current through the primary winding of an output transformer  16 - 2 . The output winding of the transformer  16 - 2  is rectified and filtered  18  to produce a D.C. output voltage  20 . 
   The output of the pulse with modulator  14  is a stream of pulses,  15  that are output from the Royer circuit  12 . Inasmuch as the pulses output from the pulse width modulator  14  will vary according to the load on the power supply and its desired output voltage, the pulse width modulator  14  output signal can be considered a “variable pulse width output” signal. The time duration of each pulse relative to the pulses duration from the Royer circuit is a function of a control signal  26  to the pulse width modulator from the error detector circuit  22  and will determine the power supply&#39;s D.C. output voltage. 
   A control signal  26  input to the pulse width modulator  14 , increases and decreases the width (also known as time duration) of each pulse from the Royer circuit  12  in order to keep the voltage output  20  equivalent to a reference potential  24  that is input to the error detector  22 . The error detector  22  compares the power supply D.C. output voltage  20  to a reference potential voltage  24  and generates an output signal  26  that controls the pulse width modulator  14 . 
   As set forth above, the semiconductors are implemented using wide band gap, silicon carbide, which is well-known to be operable in ambient temperatures up to and above 300° C. and which is also known to be radiation hardened, i.e., relatively able to withstand intense, nuclear radiation. Indeed, all semiconductors are wide band gap semiconductors making the power supply operable at temperatures as high as 300° C. In a preferred embodiment, the transformer cores used in the power supply are steel or iron and therefore operable at high temperatures. 
     FIG. 2  is a schematic diagram of a preferred embodiment of a high-temperature regulated power supply  10 . 
   As set forth above, the square wave generator  12  is a Royer circuit comprised of an iron-core transformer  12 - 2 , which in the preferred embodiment is a non-saturating core  12 - 4  having at least three windings. One of the windings on the non-saturating core  12 - 4  is a first primary winding  12 - 2  that has two input terminals identified by reference numerals  12 - 6  and  12 - 8 . This primary winding also has a center tap terminal identified by reference numeral  12 - 9 . 
   This first transformer  12 - 2  has two secondary windings that are identified by reference numerals  12 - 12  and  12 - 30 . The first secondary winding  12 - 12  has a center tap, which is identified by reference numeral  12 - 15 . This first secondary winding  12 - 12 , also has first and second output terminals,  12 - 14  and  12 - 16 , respectively, which are directly coupled to first and second magnetic amplifiers  14 - 10  and  14 - 30 , the operation of which is explained below. 
   The non-saturating core transformer  12 - 2  has a second, secondary winding identified by reference numeral  12 - 30 . The second, secondary winding  12 - 30  terminals are identified by reference numerals  12 - 32  and  12 - 34  and are directly connected to the terminals  12 - 56  and  12 - 58  respectively of the primary winding  12 - 54  of a second transformer that is a “saturating core transformer” identified in  FIG. 2  by reference numeral  12 - 50 . 
   For purposes of claim construction, a “saturating” core is considered a transformer core that becomes fully saturated with magnetic flux. When a transformer&#39;s core is fully saturated, the transformer cannot inductively couple voltage signals between its primary windings and secondary windings. When a transformer&#39;s core is saturated, the windings also lose their inductive character. A saturation current provided by one winding can therefore act to control or gate voltage through the other winding as a pulse width modulator. 
   The second transformer  12 - 50  has a center tapped secondary winding  12 - 52 , the outputs of which are identified by reference numerals  12 - 60  and  12 - 62  and are coupled through resistances to the bases  12 - 64  and  12 - 66  of two Darlington-pair transistors  12 - 18  and  12 - 20 . As is known in the art, Darlington pair transistors have a higher input impedance than does a single transistor. They also have a much higher current gain than does a single transistor. The center tap of the transformer is coupled to a power source so as to provide a bias current for the Royer&#39;s Darlington-pair drive transistors. Relatively small input signals on a bases  12 - 64  and  12 - 66  will cause the corresponding transistor to turn “on” thereby pulling current through the primary winding of the first transformer  12 - 2 . 
   The operation of a Royer circuit  12  is well known. Implementing a Royer circuit using iron or steel core transformers and driving the Royer circuit with wide band gap transistors is heretofore new however because wide band gap semiconductors require higher operating voltages. The behavior of a Royer circuit is known but briefly described here nonetheless for purposes of clarity hereafter. 
   When a voltage is applied to power supply input terminal  13 , a voltage is applied to the center tap  12 - 70  of the second, saturating transformer  12 - 50  causing a voltage to appear at both output terminals  12 - 60  and  12 - 62  of the same winding  12 - 52 . Because each of these terminals  12 - 60  and  12 - 62  is directly coupled to a corresponding base  12 - 64  and  12 - 66  of one of the two, wide band gap transistors  12 - 18  and  12 - 20 . Although the bases of both transistors are driven high at ostensibly the same time, only one of the transistors  12 - 18  and  12 - 20  will turn “on” because of component differences between the base drive circuits of the two transistors  12 - 18  and  12 - 20 . Therefore, upon the application of a voltage at the supply terminal  13 , only one of the two Royer circuit drive transistors  12 - 18  and  12 - 20  will turn on before the other. 
   Regardless of which transistor turns on first, the one that does turn on and draws current through the primary winding  12 - 6  and will thereby induce a voltage on the first secondary winding  12 - 12  of the first transformer  12 - 2 . A voltage will also be induced on the second secondary  12 - 30 . As the connections to the windings are shown in  FIG. 2 , a voltage is induced on the second secondary winding  12 - 30  by the current flowing through the primary  12 - 6 . The voltage induced on the secondary winding  12 - 30  is coupled to the primary winding  12 -  54  of the second transformer  12 - 50 , this induces a voltage on the secondary of  12 - 50  and forces one of the two Royer circuit drive transistor devices  12 - 18  and  12 - 20  on. Transformer  12 - 50  will maintain the drive to one of the base circuits until the core of  12 - 50  saturates. Once the core of the transformer  12 - 50  saturates, the phase of the output voltage from the transformer  12 - 50  will flip and turn off the previously “on” transistor and turn on the other transistor. As the input voltage  13  increases the transformer  12 - 2  has outputs  12 - 14  and  12 - 16  that are connected to rectifiers  17  which are connected as a control voltage to transistor  12 - 72 . Transistor  12 - 72  disables the startup voltage by clamping the center tap terminal  12 - 70  to ground. 
   As is well-known, the output  13  of the Royer circuit  12  is a square wave or pulse train of relatively fixed-with pulses, the frequency of which is determined by the saturation constant or volt-seconds required to saturate the core  12 - 51  of the second transformer  12 - 50 . Pulse trains of 10 kilohertz to 100 kilohertz or even 1 megahertz are readily possible using the Royer circuit. By using wide band gap transistors for the transistors  12 - 18 ,  12 - 20  and  12 - 72 , and by using saturating core transformers, the Royer circuit can be made to operate at temperatures as high as 300° C. 
   Inasmuch as a desired objective of the invention is to provide regulated output power at high temperatures, the preferred embodiment of the invention uses silicon carbide transistors which although they require higher power supply voltages, they have low leakage currents at elevated temperatures (as compared to silicon devices). Such transistors can be implemented as bipolar junction transistors or field-effect transistors. As set forth above, wide band gap materials other than silicon carbide can also be used so long as such materials exhibit low current leakage at high-temperatures as does silicon carbide. Although  FIG. 2  shows Darlington pairs driving the Royer circuit, single transistors could used as well. 
   As shown in  FIG. 1 , the output  13  of the Royer circuit is coupled into a pulse with modulator  14 . Pulse width modulators can be implemented in many ways, including semiconductor devices. As shown in  FIG. 2  and in order to provide a pulse width modulator that will operate at high temperatures, in a preferred embodiment the pulse width modulator  14  is implemented using two so-called magnetic amplifiers  14 - 10  and  14 - 30 , also referred to herein as “MAG AMPS.” As can be seen in  FIG. 2 , these magnetic amplifiers  14 - 10  and  14 - 30  are implemented using saturating core transformers. 
   With respect to the first one of these two magnetic amplifiers  14 - 10 , it is comprised of two windings on a saturating core made up of either iron or steel, i.e., preferably not of any composite material. The “primary” winding has a first input terminal  14 - 16  coupled to the power supply potential  13 . The other end of this winding is identified by reference numeral  14 - 20  and is coupled to the first input terminal of the “primary” of the second magnetic amplifier  14 - 30 . The first terminal  14 - 16  of the primary of the first MAG AMP  14 - 10  is considered a “control current input terminal” of the primary winding of the first magnetic amplifier  14 - 10 . The second terminal  14 - 20  of the primary is considered a “control current output terminal.” 
   As shown in  FIG. 2 , the first MAG AMP  14 - 10  also has a secondary winding, one terminal of which is considered a first input voltage terminal. This first input voltage terminal is identified by reference numeral  14 - 12 . The other terminal of the secondary winding is considered an output voltage terminal. This output voltage terminal is identified by reference numeral  14 - 18 . 
   The second magnetic amplifier  14 - 30  is also comprised of two windings on a saturating core that is also made up of either iron or steel. The “primary” winding of the second MAG AMP  14 - 30  has a first control current input terminal identified by reference numeral  14 - 34 . This saturation input terminal for the second MAG AMP is coupled to the “bottom” terminal  14 - 20  of the primary of the first MAG AMP  14 - 10  so that control current flowing through the first MAG AMP  14 - 10  primary winding must also flow through the second MAG AMP  14 - 30  primary winding. 
   The “bottom” terminal of the primary winding of the second MAG AMP is identified by reference numeral  14 - 38  and considered the “control current output terminal” for the second MAG AMP. This control current output terminal  14 - 38  is coupled to the output of the error amplifier  22 , which is described more fully below. Current flowing out into the error amplifier  22  flows through the primary windings of both MAG AMPS  14 - 10  and  14 - 30  insuring that both cores of these MAG AMPS are equally controlled to the desired volt-second value. 
   As shown in  FIG. 2 , the second MAG AMP  14 - 30  also has a secondary winding, one terminal of which is considered a first input voltage terminal for the second MAG AMP and identified by reference numeral  14 - 32 . The other terminal of the secondary winding is considered an output voltage terminal for the second MAG AMP  14 - 30 . This output voltage terminal is identified by reference numeral  14 - 36 . 
   The MAG AMPs are effective as pulse width modulators by controlling the saturation of the transformer cores. As the control current  27  increases, thereby increasing the current through the primary windings and thereby increasing the flux through the core, a voltage pulse input to either of the cores at its first input voltage terminal will propagate through the secondary, if the core is fully saturated with flux induced by current flowing through the MAG AMPS other windings. The width of the pulse appearing on the output voltage terminals  14 - 18  and  14 - 36  of the two MAG AMPS will therefore be function of the current  27  through the MAG AMPs and the signal input to the input voltage terminals. By increasing and decreasing the control current  27 , the portion or duration of output pulses with respect to input pulses can be increased and decreased. 
   The pulses output from the pulse width modulator  14  are coupled to the bases  16 - 8  and  16 - 10  of current driver transistors  16 - 4  and  16 - 6  respectively. Like the other transistors of the circuit shown in  FIG. 2 , the driver transistors are preferably embodied as wide band gap transistors, which are capable of operating at much higher temperatures but which require higher operating voltages. As shown in  FIG. 2 , the current driver transistors  16 - 4  and  16 - 6  are actually paired transistors to increase the current drawn through the primary  17 - 2  of output transformer  17 . Alternate embodiments of the invention would include using single driver transistors. A center tap on the transformer primary  17 - 2  allows the current driver transistors  16 - 4  and  16 - 6  to induce a “bi-polar” voltage across the primary winding  17 - 2 , which yields a truly bi-polar output voltage across the secondary winding  17 - 4 . 
   Clamping diodes across the collectors and emitters of the driver transistors  16 - 4  and  16 - 6  become forward biased and allow current to flow through the primary winding as the field in the primary collapses thereby protecting the driver transistors  16 - 4  and  16 - 6  from damage. For example, the clamping diode across the collector and emitter of the first driver transistors  16 - 4  protects the driver transistors  16 - 4  when the second driver transistors  16 - 6  turn off. 
   A.C voltage induced at the secondary  17 - 4  winding of the output transformer  17  is full-wave rectified by wide band gap diodes  18 - 4  and  18 - 6 . In the preferred embodiment, these diodes  18 - 4  and  18 - 6  are paired to provide a higher current carrying capability in the power supply&#39;s output. Alternate embodiments would include using single diodes. The preferred embodiment contemplates a full-wave rectifier at the secondary of the output transformer, which could also be embodied as a bridge rectifier circuit. Alternate embodiments would also include using a half-wave rectifier at the output of the driver transformer. 
   A choke  18 - 7  at the output of the full-wave rectifier diodes  18 - 4  and  18 - 6  smoothes A.C. ripple from the full-wave rectifiers&#39; output providing a true D.C. output voltage  20 . This output voltage  20  is provided as an input to the error amplifier  22  (also referred to as a “difference amp”), which compares the output voltage  20  to a reference voltage  24  and provides an output current  26 . The output current  26  of the error amplifier  22  provides the control current through the aforementioned MAG AMPS  14 - 10  and  14 - 30 . Inasmuch as the error amplifier  22  provides an output current in response to an input voltage differential, the error amplifier  22  can be considered a voltage-to-current converter or, an adjustable current sink inasmuch as it sinks current through the MAG AMPS  14 - 10  and  14 - 30  that is sourced by the power source  13 . 
   While the present invention has been described in connection with the illustrated embodiments, it will be appreciated and understood that modifications can be made without departing from the true spirit and scope of the invention and that the scope of the invention should be determined by the following claims. For instance, those of skill in the art will recognize that the wide band gap transistors could be implemented with materials other than silicon carbide. Moreover, the transistors could be implemented bi-polar junction transistors or field-effect transistors. Single transistors could be used throughout the Royer circuit shown in  FIG. 2 , instead of using Darlington pairs as shown. Single driver transistors however having lower current gain and would require higher current drive than the Darlington pairs. 
   The Royer circuit might also be implemented with single driver transistor, on the primary of the first transformer  12 - 2 . The Royer circuit might also be implemented with only a single, saturating core transformer, i.e., without the second transformer  12 - 50 , which acts as to control the Royer circuit&#39;s oscillation. 
   The MAG AMPs are operable in very high temperature environments because they are not implemented using any active semiconductor devices. Depending on the particular application, the pulse-width modulator  14  could be implemented using well-known semiconductor circuits. If the pulse-width modulator  14  can be located out of the hostile environment, only the Royer circuit, and the driver transistors might need to be operable in high temperature environments. 
   The error amplifier  22  can be readily implemented using operational amplifiers and current drivers and is preferably operated outside of a hostile environment. Instead of using a “op amp” an alternate and equivalent embodiment would include using much more complex analog-to-digital converters, a processor to digitally computer input voltage differences and calculate a correction current produced by a digital-to-analog converter and high-power driver transistors. 
   Because the high-temperature power supply does not require the relatively massive heat sinks used to cool silicon devices, the claimed invention finds application in many hostile environments such as space vehicles, where radiation hardening and light weight is important. The high temperature power supply can also be used to control electric motors in high-temperature environments, such as in electrically powered automobiles. The high-temperature power supply is also useful in arcane applications, such as providing power to drilling equipment, such as drill point electronics and controllers.