Abstract:
A method and apparatus for use with a transistor array for enhancing the array&#39;s performance and/or reliability and/or longevity by sensing the temperature of each transistor in the array and initiating corrective action to maintain its temperature within a desired operating range. Each array transistor has a temperature sensor closely thermally coupled thereto preferably mounted within the transistor package, in close physical proximity to the transistor junction.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to electronic systems, e.g., communications, radar, etc., which employ a plurality of radio frequency (RF) power transistors and more particularly to a method and apparatus for enhancing system performance and reliability by controlling the operating temperature of each transistor. 
     BACKGROUND OF THE INVENTION 
     It is well known that the overall performance (including longevity and/or reliability) of electronic systems can be adversely affected if component operating temperatures are permitted to rise above certain rated values. Thus, systems utilizing large transistor arrays frequently incorporate cooling subsystems which monitor array operating temperatures and act to cool the array by, for example, increasing coolant flow rate. Although such cooling subsystems typically enhance system performance, they generally fail to rapidly respond to localized temperature excursions of individual transistors which can produce “hot spots” and lead to thermal runaway with potentially catastrophic failure. Thus, the functionality of an entire transistor array can be significantly compromised as a consequence of a single transistor experiencing an excessive junction temperature excursion. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for use with a transistor array for enhancing the array&#39;s performance and/or reliability and/or longevity by sensing the temperature of each transistor in the array and initiating corrective action to maintain its temperature within a desired operating range. 
     In accordance with the invention, each of a plurality of array transistors has a temperature sensor closely thermally coupled thereto, preferably mounted within the transistor package, in close physical proximity to the transistor junction. 
     In a preferred embodiment, each temperature sensor is configured to supply digital output signals representing the junction temperature of a single transistor. The output signals are supplied to a digital controller which is connected in a feedback loop to control one or more parameters affecting the temperature producing behavior of each transistor. More particularly, a controllable temperature modifying device is coupled to each transistor and is controlled by the digital controller to maintain junction temperature within a specified range. Each such temperature modifying device can control one or more parameters; e.g., the magnitude of the drive current and/or supply voltage applied to the transistor and/or the flow rate of coolant provided to the transistor. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic diagram of an exemplary prior art electronic system including an array of RF power transistors; 
     FIG. 2 is a schematic diagram of an exemplary system incorporating a temperature control subsystem in accordance with the invention for eliminating hot spots attributable to temperature excursions of individual transistors; and 
     FIG. 3 is a schematic block diagram showing a portion of a temperature control subsystem in accordance with the invention including a temperature sensor mounted to sense the junction temperature of a transistor and connected to a controller in a feedback loop for controlling the junction temperature of the transistor. 
    
    
     DETAILED DESCRIPTION 
     Attention is initially directed to FIG. 1 which schematically illustrates a portion of an exemplary prior art RF system  10  used, for example, for communications or radar surveillance. The system  10  typically includes an array  12  of RF transmit/receive (T/R) elements  14 . Each element  14  is shown coupled to a dedicated circuit module  16  which typically includes multiple components such a variable impedance  18 , a variable phase shifter  20 , and a variable gain amplifier  22 . The variable components  18 ,  20 ,  22  are typically controlled by a controller  24  which can be manually and/or algorithmically controlled to affect, for example, beam steering, RF signal optimization, etc. Each module  16  is shown coupled to a common signal processor  25  Each amplifier  22  is typically comprised of one or more transistors which can be of various types, e.g., Bipolar Junction (silicon), Gallium Arsenide, Silicon Carbide, Gallium Nitride, etc. 
     Systems of the type represented in FIG. 1 (e.g., radar systems) can employ upwards of a thousand modules, any one of which could experience a potentially damaging transistor temperature excursion. A consequence of such an excursion is a reduction in the useful life of the array inasmuch as individual transistors experiencing large temperature excursions are at increased risk of failure. Moreover, such temperature excursions can significantly degrade system performance. For example, in an active radar system, patches of non-functioning or poorly functioning modules are likely to lead to degraded sidelobes and reduced gain. Typically, failed modules tend to raise overall sidelobe levels of an active radar array and degrade angular beam pointing accuracy. In order to enhance array reliability and longevity, prior art systems typically utilize some type of sensor  26  for measuring the array temperature and a controller  28  responsive to the measured temperature for controlling a heating/cooling mechanism  30  to raise or lower the temperature of the entire array. 
     The present invention recognizes that even if the array operating temperature is properly maintained, individual transistors within the array can malfunction leading to elevated junction temperatures which can reduce system reliability and longevity. More particularly, if a transistor&#39;s junction temperature is permitted to rise considerably above its optimum operating value (which shall hereinafter be assumed to be about 150° C.), the transistor can develop a thermal runaway condition (e.g., to a junction temperature approaching 350° C.), thus damaging the transistor and reducing the performance of the entire array  12 . Thus, it is important to recognize that even if the overall temperature of the array  12  is cooled to a proper level, local hot spots can degrade the performance and operating longevity of the entire system  10 . The present invention is primarily directed to means for reducing the occurrence of transistor hot spots in order to enhance overall system performance and operational longevity. 
     Attention is now directed to FIG. 2 which depicts the system of FIG. 1 modified in accordance with the invention to enhance system operational reliability and longevity. In accordance with the invention, the temperature of each transistor in the array  40  is separately monitored and utilized by a feedback loop to maintain the transistor&#39;s junction temperature within prescribed limits, e.g., 150° C.±10°. 
     For simplicity in illustration, FIG. 2 depicts an array  40  of amplifiers  42  where each amplifier  42  will be assumed to comprise an RF power transistor contained in a transistor package  44 . In accordance with a preferred embodiment, each transistor package  44  also contains a separate temperature sensor  46  mounted so as to be closely thermally coupled to the junction of the transistor therein. An exemplary temperature sensor  46  suitable for mounting in the transistor package  44  comprises a small die (e.g., 50 by 60 mil.) which is available as an Analog Devices Serial Digital Output Thermometer TMP03/TMP04. To avoid calibration errors, spacing between each sensor and its associated transistor junction should be minimized. Temperature calibration is preferably performed using a reference infrared sensor (not shown) to read junction temperature. In operation, the sensed junction temperature is employed in a feedback loop (FIG. 3) to control a temperature modifying device  50  to maintain the junction temperature within a prescribed window, e.g., 150° C.±10°. A temperature modifying device in accordance with the invention functions to modify the temperature of a transistor in one or more of multiple ways, e.g., 1) by varying the magnitude of electrical energy delivered to the transistor, and/or 2) by active cooling using either liquid coolant circulating through a coldplate or forced-air directed by high speed blower. 
     Note in FIG. 2 that each of a plurality of transistors  42  in the array  40  is thermally coupled to a different one of a plurality of temperature sensors  46 , each of the sensors preferably being mounted in the package  44  of the transistor. Each sensor preferably provides a digital output signal representing the temperature of the junction of the transistor thermally coupled thereto. The digital output signals from all of the sensors  46  are applied to the input of a digital temperature controller  48 . The controller  48  is preferably a microprocessor based device which functions to determine whether the temperature of each transistor is within prescribed limits and based thereon generates control signals for controlling a plurality of temperature modifying devices  50 . Each device  50  is configured to adjust the temperature of a different one of the plurality of transistors. Thus, the temperature of each transistor is effectively controlled by a dedicated feedback loop comprised of a related temperature sensor  46 , the temperature controller  48 , and an associated temperature modifying device  50 . That is, each temperature sensor  46  and associated modifying device  50  defines a functionally distinct feedback loop including the temperature controller  48  which is preferably common to and time shared amongst the plurality of loops. The temperature controller  48  can be algorithmically or manually programmed to introduce a desired transfer function for responding to the output signals from sensors  46  for controlling the respective modifying devices  50 . 
     FIG. 3 depicts, as an example, the feedback loop coupling a single exemplary temperature sensor  46 X to a temperature modifying device  50 X Inputs to the feedback loop controller  48  include, in addition to the temperature sensor output  60 , the present coolant flow rate  62 , the drive signal impedance level  64  and the supply voltage level  66 . 
     Each feedback loop operates such that when a transistor is too hot, its related temperature modifying device  50  operates to modify the transistor&#39;s temperature generating parameters. Thus the modifying device  50  can operate to reduce the drive signal and/or supply voltage applied to the transistor to reduce the electrical energy dissipated therein and/or increase the flow rate of coolant to the transistor. On the other hand, when the transistor is too cool, the supplied electrical energy can be increased and/or the coolant flow rate decreased. 
     In a typical active radar systems, a temperature control subsystem in accordance with the invention can be employed to assure: 
     1. Correct two-dimensional aperture tapering for a low sidelobe distribution. 
     2. Accurate two-dimensional antenna pointing angles. 
     3. Optimized operation in situations of transistor failure which are removed from service. This involves algorithms to optimize gain, sidelobe and pointing performance for an array containing both operating and failed T/R modules. 
     From the foregoing, it should now be appreciated that a temperature control subsystem has been disclosed herein for controlling temperature excursions of individual transistors in an array of RF power transistors to enhance system performance and reliability. Although the invention has been described with reference to a specific preferred embodiment, it is recognized that various alternative implementations and modifications will readily occur to those skilled in the art which fall within the spirit of the invention and the intended scope of the appended claims.