Abstract:
An apparatus for controlling the temperature of an electronic device. The apparatus comprises a refrigeration system including a compressor and a multi-pass heat exchanger. The refrigeration system is operative to circulate a refrigerant fluid through a fluid flow loop such that the refrigerant fluid will change between gaseous and liquid states to alternately absorb and release thermal energy. The refrigerant fluid is pre-cooled in the heat exchanger by a pre-cooling refrigerant stream. A thermal head is connected into the fluid flow loop and has a temperature controlled surface.

Description:
PRIORITY CLAIM 
   This application claims priority to Provisional Application Serial No. 60/507,732, filed Oct. 1, 2003, the entire disclosure of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates to temperature control systems for maintaining the temperature of an electronic device at a predetermined temperature such as while the device is being tested. 
   Electronic devices, such as integrated circuits, are often tested at temperatures below ambient temperature. This requires a supply of a coolant below the test temperature, which can be used both to bring the device to the setpoint temperature and to act as the conduit for heat rejection when power is applied to the device. In one such implementation, liquid refrigerant at ambient temperatures is expanded in an isenthalpic process to provide cooling at temperatures below ambient at a thermal head. See, for example, pending application Ser. No. 09/871,526, filed on May 31, 2001, which is hereby incorporated by reference. 
   The temperatures that can be achieved at the thermal head with this process depend on the refrigerant(s) used as well as the temperature and pressure of the refrigerant at the entrance of the isenthalpic expansion device. Generally, as pressure increases or as temperature decreases for the unexpanded refrigerant, more cooling and/or lower temperatures can be achieved at the thermal head. However, there is a limit to the increase in pressure allowable by the unexpanded refrigerant due to the gas physical properties as well as due to practical structural considerations for supply tubing. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention provides an apparatus for controlling the temperature of an electronic device. The apparatus comprises a refrigeration system including a compressor and a multi-pass heat exchanger. The refrigeration system is operative to circulate a refrigerant fluid through a fluid flow loop such that the refrigerant fluid will change between gaseous and liquid states to alternately absorb and release thermal energy. The refrigerant fluid is pre-cooled in the heat exchanger by a pre-cooling refrigerant stream. A thermal head is connected into the fluid flow loop and has a temperature controlled surface. 
   In some exemplary embodiments, the apparatus contains a bypass flow to recirculate refrigerant through the heat exchanger without passing the refrigerant through the thermal head. 
   In another aspect, the present invention provides an apparatus for independently controlling the temperature of multiple electronic devices. The apparatus comprises a refrigeration system including a single compressor connected in parallel to multiple heat exchangers at their warm end. A thermal head with a temperature controlled surface is connected into the fluid flow loop of each heat exchanger. A flow balance heater control is connected to each heat exchanger to provide an additional thermal load and thereby prevent a temperature imbalance between the heat exchangers. 
   Other objects, features and aspects of the present invention are discussed in greater detail below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings, in which: 
       FIGS. 1–10  are diagrammatic representations of various apparatus constructed in accordance with different aspects of the present invention for controlling the temperature of an electronic device under test; and 
       FIG. 11  is a flow diagram of an embodiment for controlling the flow balance heater controls. 
   

   Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions. 
     FIG. 1  illustrates an apparatus for controlling the temperature of an electronic device under test  10 , such as an integrated circuit. Electronic device  10  is mounted in a suitable test fixture (not shown) so that various read/write commands can be performed to evaluate the performance of device  10 . 
   A thermal head  12  has a temperature controlled surface  14  which can be positioned into thermal contact with device  10 . A thermocouple (not shown) or other suitable sensor is provided at temperature controlled surface  14  or alternatively within thermal head  12  to detect the temperature of device  10 . Alternatively, the sensor can be mounted on or within device  10 , or within a block or device (not shown) interposed between thermal head  12  and device  10 . This information is fed to a processor for controlling the operation of thermal head  12  so as to maintain a predetermined temperature. For example, the temperature to be maintained could be entered by a user at a temperature selector. One skilled in the art will recognize that the functions of a processor and temperature selector may be performed by a single digital computer or the like. 
   Thermal head  12  may be configured having both cooling and heating capability to accurately maintain a wide range of possible temperatures. For example, thermal head may include heating elements  16  or the like to supply thermal energy if the temperature of device  10  drops below the selected temperature. Alternatively, thermal head  12  may be supplied with refrigerant that has been suitably heated to provide a heating action rather than a cooling action upon passing through the thermal head. 
   If the temperature of the unexpanded refrigerant can be made colder than ambient conditions, it can provide more cooling than a typical refrigeration system providing unexpanded refrigerant at ambient conditions. Toward this end, an auto-cascade system can be provided where a single compressor is used to compress a gas mixture and liquid-gas phase separation and multi-pass heat exchanger is used to achieve low temperatures of the unexpanded refrigerant. General principles of such an arrangement can be discerned with reference to U.S. Pat. No. 5,724,832 to Little et al. and U.S. Pat. No. 3,768,273 to Missimer, both incorporated herein by reference. 
   In the configuration shown in  FIG. 1 , a high pressure supply stream  18 , a high pressure pre-cooling stream  20 , and a low pressure return stream  22  are employed and exchange heat in a heat exchanger  24 . Pre-cooling stream  20  enters heat exchanger  24  and is cooled by return stream  22 . Pre-cooling stream  20  is then expanded to reduce temperature further and exhausted to return stream  22  part-way within the heat exchanger  24 . Supply stream  18  is pre-cooled by the return stream  22  to reach low temperatures. 
   Upon exiting heat exchanger  24 , the flow of supply stream  18  entering thermal head  12  is controlled by a control valve  26 . Control valve  26  can provide flow control by pulse-width-modulation of flow through a low impedance valve together with subsequent flow through a fixed restriction, as shown, or can be proportionally controlled by a mechanically or electrically variable restriction within the control valve  26 . The amount of restriction within the control valve  26 . The amount of restriction of the flow control valve  26  is selected according to the desired test temperatures, cooling capacities, and/or control algorithms required at the thermal head. Return stream  22  exits thermal head  12  as a vapor or 2-phase mixture of liquid refrigerant plus vapor and cools supply stream  18  in heat exchanger  24 . Pre-cooling stream  20  and return stream  22  thus provide a bootstrapping operation that allows low temperatures to be reached. 
   With the system shown in  FIG. 1 , it is practical to physically locate the multi-pass heat exchanger in close proximity of the thermal head to limit the exposure of any cold tubing and components to parasitic heat leaks from the ambient to the cold refrigerant both before and after expansion and before and after passage of the refrigerant through the thermal head. 
   This configuration works well when there is a steady flow of refrigerant through heat exchanger  24  and thermal head  12 , since steady temperatures are maintained throughout the system and cold liquid refrigerant is always available at control valve  26 . In a test environment, however, there are often relatively long periods when control valve  26  is closed, and no return stream  22  flows through heat exchanger  24 . After some time, heat exchanger  24  warms, and cold, high pressure refrigerant is no longer available upstream of control valve  26  until after an additional cool down period. This may be undesirable in a test environment, where it is desired to have continuous availability of the cold refrigerant at a consistent initial temperature at the control valve  26  and thermal head  12 . 
     FIG. 2  shows a temperature control system containing a bypass restriction that allows a continuous bypass stream  30  of refrigerant to flow from supply to return regardless of the status of control valve  26 . The restriction is sized to balance the constraints of parasitic heat load to heat exchanger  24 , minimization of the compressor work to flow the bypass stream, and maximization of thermal stability and capacity of the thermal head. 
   Additionally, bypass stream  30  allows operation of thermal head  12  at temperatures above the unexpanded refrigerant supply temperature by ensuring that the combined return stream  22  composed of bypass stream  30  and thermal head exhaust stream  32  is maintained sufficiently cold to ensure a supply of cold refrigerant is still available from heat exchanger  24 . At high temperature setpoints for thermal head  12 , without bypass stream  30 , the temperature of supply stream  18  would gradually climb with the return stream  22  inlet temperature, and control stability at thermal head  12  would be negatively impacted, or even lost entirely. 
   Another example bypass configuration is shown in  FIG. 3 . A switched bypass flow valve  33  can be turned on or off via an external control signal in coordination with control valve  26 , thereby allowing or preventing flow through bypass  34 . Several modes of operation for the switched bypass are possible. For example, bypass flow valve  33  can be maintained in an open position during testing, which operates flow through bypass  34  in the same manner as the fixed restriction. Bypass flow valve  33  can be closed during idle periods or left open as operating practices warrant. 
   In another mode, bypass flow valve  33  can be switched in a complementary fashion with respect to control valve  26 , so that flow from heat exchanger supply  18  and  20  flows either through thermal head  12  or through bypass, but not both at the same time. In this way, the compressor and heat exchanger  24  can see an almost constant flow of refrigerant, mimicking a pseudo-steady-state operation, so that from the point of view of heat exchanger  24  and the compressor, it is unknown whether at any given instant flow is through thermal head  12  or through bypass  34 . This provides temperature stability to heat exchanger  24  that improves control of thermal head  12 , and mechanical stability to the compressor that extends compressor life. 
   In another exemplary mode, bypass flow valve  33  can be activated only for thermal head temperature setpoints above the desired supply refrigerant temperature, so that the combined flow of thermal head exhaust stream  32  and bypass  34  maintains a temperature of return stream  22  which is colder than the desired temperature of supply stream  18 . 
   In another mode, the flow through bypass  34  could be controlled by varying the effective restriction of bypass  34 , such as by pulse width modulation. The variation in flow is useful to control the temperature of supply stream  18 , in particular as the thermal head setpoint temperature rises above the temperature of supply stream  18 . Also, the variation in flow can provide temperature stability when the flow through thermal head  12  varies considerably. Moreover, the variation in flow can provide a greater flow through heat exchanger  24  during cool down to speed attainment of the desired temperature setpoint during initial startup. The flow through bypass  34  can also be controlled using a proportional valve  36 , as shown in  FIG. 4 . 
   As shown in  FIG. 5 , temperature control of supply stream  18  can be further enhanced by the use of a stabilizing block  38  having its temperature actively controlled. Using a controlled bypass flow (such as proportional control as shown or switched control as previously described), through a block of material acting as a “thermal sink” or “thermal inertia,” the temperature oscillations that can occur during variations in thermal load at thermal head  12  can be damped. This provides a more consistent supply of refrigerant to control valve  26  that in turn improves refrigerant control to thermal head  12 , thereby improving temperature control of the thermal head  12  as desired for device testing. A heater control  40  can also be used to further enhance control of the temperature of stabilizing block  38 .  FIG. 5   a  shows another embodiment in which stabilizing block  38  is not thermally connected to the return stream  22 , but only to supply stream  18 . 
   Referring now to  FIG. 6 , when the temperature of thermal head exhaust stream  32  is below that of supply stream  18 , it is advantageous to return thermal head exhaust stream  32  through heat exchanger  24  to provide additional counterflow cooling to supply stream  18 , as described previously. When the temperature of thermal head exhaust stream  32  is above that of supply stream  18 , however, it can be advantageous to divert thermal head exhaust stream  32  past heat exchanger  24 . In this way, the relatively hot thermal head exhaust stream  32  does not tend to warm supply stream  18 . An example of such a system is shown in  FIG. 6 , in which a thermal head exhaust stream  32  is bypassed directly from thermal head  12  to the compressor. 
   When the setpoint temperature of thermal head  12  can vary over a wide span, it may be advantageous to pass thermal head exhaust stream  32  through the heat exchanger  24  in some circumstances (primarily at cold temperature setpoints of the thermal head  12 ) while diverting it in other circumstances (primarily at hot temperature setpoints of the thermal head  12 ). This behavior can be achieved by the use of a three-way valve  44  on thermal head exhaust stream  32 , as shown in  FIG. 7 . The direction of the thermal head exhaust stream  32  can also be achieved by the use of two two-way valves, where the valves are operated in complementary fashion to direct the flow through only one or the other flow streams. 
   In some circumstances, thermal head exhaust stream  32  may be too warm to be introduced at the cold end of heat exchanger  24 , but may still be sufficiently cold to augment the cooling of the pre-cooling stream  20  if introduced at some intermediate point in heat exchanger  24 .  FIG. 8  shows an example arrangement of an array of hot head bypass valves  46  positioned at various points along heat exchanger  24  so that introduction of the thermal head exhaust stream  32  can be optimized to maximize the thermal effectiveness of heat exchanger  24  and maximize the available cooling capacity for thermal head  12 . In  FIGS. 5 ,  7  and  8 , flow is shown entering stabilization block  38  from thermal head  12 , where flow could instead be directed to heat exchanger  24 , bypassing stabilization block  38 . 
   To reduce equipment costs when multiple thermal heads are required in distinct locations remote from each other, it is sometimes advantageous to use a single compressor connected to several heat exchangers connected in parallel at their warm end. When configured this way, due to variations in parasitic loading as well as variations in manufacturing tolerances, it is possible for the multiple exchangers to experience a flow imbalance that leads to a thermal imbalance where the cold refrigerant supply temperatures do not match. When the temperature of one exchanger deviates from the other exchangers and gets colder, it can experience a thermal runaway effect where there is reduced pressure drop through the exchanger due to the changes in the heat exchange gas properties with temperature, which further increases the flow through the exchanger and further decreases its temperature. The increased flow to the runaway exchanger can prevent sufficient flow through the other exchangers, thereby causing them to warm. 
   The present invention provides methods to ensure that all heat exchangers have the same refrigerant supply temperature despite variations in loading and manufacturing tolerances. One such method of re-balancing the heat exchangers is by the introduction of a flow balance heater control  48 , as shown in  FIG. 9  for two thermal heads.  FIG. 9   a  shows another embodiment in which flow balance heater control  48  is positioned on supply line  18 , which serves both as the rebalancing component and as a pre-conditioning component to maintain the liquid supply temperature at the flow control valve  48  at a desired setpoint for stability of refrigerant and temperature control for the thermal head. 
   If a heat exchanger  24  starts getting colder than its desired setpoint by some tolerance, flow balance heater control  48  is turned on to provide additional thermal load to the heat exchanger  24 . This tends to warm the heat exchanger  24  back to the desired setpoint both by increasing the thermal load on the heat exchanger and by decreasing or inverting the temperature difference between supply stream  18  and return stream  22 . 
   Flow balance heater control  48  is preferentially placed at the return stream  22  inlet of heat exchanger, but could also be placed anywhere along heat exchanger  24  where supply stream  18  and return stream  22  exchange heat. Flow balance heater control  48  could also be placed on supply stream  18  inlet before heat exchanger  24 . 
   The bypass restriction shown in  FIG. 9  can be any of the previously described implementations, including the first shown implementation where there is no bypass. The heater control can be on/off, proportional, pulse width modulated (PWM), based on a proportional, integral, derivative (PID) or other control strategy. Multi-exchange balance control can also be achieved by operation of each flow legs&#39; thermal stabilization blocks controlled to equal set point temperatures. 
     FIG. 10  illustrates an example arrangement using two compressors for four thermal heads. 
     FIG. 11  illustrates an example flow diagram for controlling flow balance heater controls  48 . 
   While preferred embodiments of the invention have been shown and described, modifications and variations may by made thereto by those of skill in the art without departing from the spirit and scope of the present invention. It should also be understood that aspects of various embodiments may be interchangeable in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limitative of the invention described herein.