Abstract:
An electronic device includes a cooling apparatus having: a primary cooling unit which is disposed in close proximity with an electronic device so as to face a surface thereof; an auxiliary cooling unit which is disposed in close proximity with the electronic device so as to face a surface thereof; and a controller which drives at least one of the primary cooling unit and the auxiliary cooling unit so as to cool the electronic device.

Description:
TECHNICAL FIELD 
   The present invention relates to a technology for cooling an electronic device and, more particularly, to a technology for cooling the surface of an electronic device by coordinating multiple cooling units with different characteristics. 
   BACKGROUND TECHNOLOGY 
   Electronic devices such as central processing units (CPUs) and digital signal processors (DSPs) are comprised of various electronic components including active elements such as transistors and passive elements such as capacitors. Part of electric energy for driving the electronic components is converted into thermal energy and dissipated. Since the performance of electronic components normally depends on temperature, dissipated heat affects the electronic components and, in turn, the performance of the electronic device. Thus, technology for cooling an electronic device is extremely important to control an electronic device properly. 
   An example of technology for cooling an electronic device is air-cooling using an electric fan. In this method, an electric fan is provided to face the surface of an electronic device. Air taken in from an air inlet is blown to the surface of the electronic device by the electric fan. Air heated as a result of absorbing heat generated from the surface of the electronic device is discharged from an air outlet. Thus, the electronic device is cooled by removing the heat generated from the surface of the electronic device by using the electronic fan. 
   A device called a heat sink for dissipating heat is also used commonly. A heat sink is usually designed to have a large heat dissipation surface in order to transfer heat from a heat source such as an electronic device to a cool fluid or air that surrounds the electronic device. Heat generated from an electronic device is conducted to a heat sink and dissipated through an extensive heat dissipation surface of the heat sink. 
   A cooling method using an electric fan and a heat sink in combination is generally used to cool a semiconductor chip. 
   Time response of an electric fan or a heat sink to temperature variation is generally slow compared to variation in the amount of heat generated in an electronic device. In other words, the related-art cooling method cannot efficiently cool an electronic device in the face of minute-by-minute variation in the amount of heat generated in an electronic device. This necessitates holding extra cooling capacity in reserve in order to maintain the reliability of an electronic device. One drawback of this approach consists in large power consumption and noise of an electric fan. Another drawback is that the volume  6 f an electric fan has to be larger than is necessary. 
   SUMMARY OF THE INVENTION 
   A general purpose of the present invention in this background is to provide a technology for cooling an electronic device efficiently. 
   An electronic device cooling apparatus according to at least one embodiment comprises: a primary cooling unit which is disposed in close proximity with an electronic device so as to face a surface thereof; an auxiliary cooling unit which is disposed in close proximity with the electronic device so as to face a surface thereof; and a controller which drives at least one of the primary cooling unit and the auxiliary cooling unit so as to cool the electronic device. The primary cooling unit maybe based on a cooling mechanism different from that of the auxiliary cooling unit. For example, with regard to cooling time response, which is time response to temperature variation in a target of cooling, an auxiliary cooling unit based on a cooling mechanism with faster cooling time response than the primary cooling unit may be employed. Alternatively, the cooling capacity of the auxiliary cooling unit per unit time may be higher than that of the primary cooling unit. The primary cooling unit and the auxiliary cooling unit may be disposed in close proximity with different areas on the surface of an electronic device cooled. 
   Cooling time response may be defined as a time required for a cooling unit to lower the temperature of a target of cooling to an original temperature, subsequent to a stepwise increase in the temperature. A primary cooling unit is represented by a cooling means, such as an electric fan, which is provided with large cooling capacity but is slow in responding to time variation of heat. In contrast, an auxiliary cooling unit is represented by a cooling means, such as a cooling nozzle for emitting a coolant to a target of cooling, which may be low in cooling capacity but is fast in responding to time variation of heat. Heat generated on a constant basis in an electronic device is dealt with by the primary cooling unit and instantaneous heat generation is dealt with by the auxiliary cooling unit. This approach uses two types of cooling units that differ in properties, depending on whether heat from an electronic device is generated on a constant basis or generated instantaneously, thereby enabling efficient cooling. An electronic device may be a device produced by packaging a semiconductor circuit. 
   An electronic device cooling apparatus according to at least one embodiment comprises: a primary cooling unit which is disposed in close proximity with an electronic device so as to face a predetermined surface thereof; an auxiliary cooling unit which delivers a jet of coolant to the electronic device via a through hole provided in a substrate that faces a surface of the electronic device different from the predetermined surface; and a controller which drives at least one of the primary cooling unit and the auxiliary cooling unit so as to cool the electronic device. For example, the primary cooling unit may be disposed in close proximity with the front surface of the electronic device, and the substrate may be provided to face the back surface, so that the auxiliary cooling unit may deliver a jet of coolant through a hole provided in the substrate. 
   An electronic device cooling apparatus according to at least one embodiment comprises: a heat dissipating mechanism which is disposed in close proximity with an electronic device so as to face a predetermined surface thereof and which dissipates heat generated from the predetermined surface; an auxiliary cooling-unit which delivers a jet of coolant to the electronic device via a through hole provided in a substrate that faces a surface of the electronic device different from the predetermined surface; and a controller which drives the auxiliary cooling unit so as to cool the electronic device. For example, the heat dissipating mechanism may be provided in close proximity with the front surface of the electronic device, and the substrate is provided to face the back surface, so that the auxiliary cooling unit may deliver a jet of coolant via a through hole provided in the substrate. 
   Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, systems, recording mediums and computer programs may also be practiced as additional modes of the present invention. 
   According to the present invention, an electronic device can be effectively cooled. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing the mechanism of an electronic device cooling apparatus; 
       FIG. 2  shows a first example of the mechanism whereby a jet cooling apparatus delivers a jet of coolant; 
       FIG. 3  shows a second example of the mechanism whereby the jet cooling apparatus delivers a jet of coolant; 
       FIG. 4  shows a third example of the mechanism whereby the jet cooling apparatus delivers a jet of coolant; 
       FIG. 5  is a functional block diagram of the electronic device cooling apparatus; 
       FIG. 6  is a flowchart showing a process whereby the electronic device cooling apparatus cools the electronic device; and 
       FIG. 7  is a graph showing a relation occurring in the electronic device cooling apparatus between time variation of the temperature and the control of the electronic device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Heat generated from the surface of an electronic device such as a CPU or a DSP represents thermal energy as converted from electric energy supplied to electronic components and wires connecting the electronic components and dissipated therefrom. The heat is not necessarily generated from the surface of the electronic device on a constant basis. The amount of heat generated may instantaneously increase depending on the process performed by the electronic device. Even if the amount of heat generated from the surface of the electronic device is close to its maximum, instantaneous heat generation may occur concurrently. One cooling method adapted to instantaneous heat generation is based on a jet cooling mechanism. Jet cooling cools a heat generating object such as an electronic device by spraying the object with a jet of coolant. 
   Jet cooling is known as a method capable of ensuring high local heat transfer efficiency. The method is effective to deal with a large amount of heat generated locally in, for example, a machining operation. It cools a heat generating object by spraying the object with a jet of coolant from a cooling nozzle. The term “coolant” refers to fluid such as air or liquid such as water for absorbing and dissipating heat generated from the surface of an electronic device. Heat transfer on a plane perpendicular to the flow of coolant delivered occurs concentrically around a jet axis point. The term “jet axis point” refers to a point on the surface of a cooled object intersecting with an axis of jet from the cooling nozzle. 
   Heat transfer coefficient h 0 [W/m 2 K], occurring when the impinging radius is denoted by r 0 [m] and the thermal conductivity of a coolant is denoted by λ f [W/mk], is given by
 
 h   0   =λ   f   ·Nu   0   /r   0  
 
where Nu 0  denotes an average Nusselt number at the impinging radius of r 0  [m], which is given by
 
 Nu   0 =1.25· Pr   0.45   ·Re   0.45  
 
where Pr denotes a constant called Prandtl number, and Re denotes the Reynolds number. Re is given by the following expression.
 
 Re=u   0   ·d   0 /ν
 
where u 0  [m/s] denotes a representative speed derived from dividing the volume of jet flow by the cross section of the outlet of the cooling nozzle. d 0  [m] denotes the diameter of the outlet, and ν[s/m 2 ] denotes the viscosity of the fluid.
 
   Jet cooling achieves improved cooling effect in the neighborhood of a jet axis point. By disposing multiple cooling nozzles in close proximity with one another to cover a target of cooling, even more advanced cooling effect is achieved. 
     FIG. 1  is a schematic diagram showing the mechanism of an electronic device cooling apparatus  100  according to an embodiment of the present invention. An electronic device  200  is a heat generating object and a target of cooling. A spreader  250  is fitted to the upper surface of the electronic device  200 . Normally, the spreader  250  is formed of copper. A heat sink  252  is mounted on the spreader  250 . Heat generated from the upper surface of the electronic device  200  is conducted to the heat sink  252  via the spreader  250 . The heat sink  250  is provided with an extensive heat dissipation surface exposed to the environment. By blowing air from an electric fan (not shown) to the heat sink  252 , heated air in the neighborhood of the heat dissipation surface of the heat sink  252  is efficiently removed. The electronic device  200  may be in direct contact with the heat sink  252 . 
   The heat sink  252  may be exposed to cooling water. A waterproof case covers the surface of the electronic device  200 . Cooling water is led to the heat sink  252 . The cooling water is heated by absorbing heat generated by the electronic device  200  and conducted to the heat sink  252 . The cooling water is then discharged through a cooling water outlet. 
   The electronic device  200  is mounted on a package substrate  254 . The package substrate  254  serves as an intermediate plate for mounting the electronic device  200  on a mounting substrate  256 . The mounting substrate  256  is attached to the package substrate  254  by solder. Normally, the distance between the package substrate  254  and the mounting substrate  256  is about 300-500 μm. Normally, several capacitors  258  are attached to the surface of the package substrate  254  facing the mounting substrate  256 . In case the capacitance of the capacitors  258  is greater than a certain level, the capacitors  258  may be too large to fit in the clearance of about 300-500 μm. For this reason, the mounting substrate  256  is usually provided with an opening as shown immediately below the electronic device  200 , in order to secure a height to provide the capacitors  258 . 
   A jet cooling apparatus  300  sprays the package substrate  254  with a jet of coolant via the opening in an upward direction. The jet cooling apparatus  300  is provided with multiple coolant jet outlets. The delivering mechanism of the jet cooling apparatus  300  will be described later in detail. The jet of coolant delivered from the jet cooling apparatus  300  is collected by a coolant collecting hole (not shown). In case the coolant is a liquid, collection of the coolant warmed by the heat generated from the electronic device  200  may be achieved by capillary action as known in the art. Alternatively, the coolant may be collected actively by using a pump. The coolant collected is cooled by ambient air. The coolant is supplied again to the jet cooling apparatus  300 . In case the coolant is air, the air may be wasted after delivering the jet. 
   The diameter of each of the coolant jet outlets is configured to be approximately ⅓ the distance between the upper surface of the jet cooling apparatus  300  and the package substrate  254 . For example, if the distance is about 6 mm, the diameter of each of the coolant jet outlets is configured to be about 2 mm. The coolant jet outlets are arranged in a distributed manner, disposing the outlets both at the center and toward the edge of the jet cooling apparatus  300 . If a location in the electronic device  200  which generates a relatively large amount of heat is identified beforehand, the diameter of each of the coolant jet outlets corresponding to the location may be configured to be larger than that of the other outlets. As demonstrated by the expression for jet cooling, the larger the diameter of the coolant jet outlet, the greater the heat transfer coefficient. 
   The jet cooling apparatus  300  may cool the surface of the package substrate  254  by directly spraying it with a coolant. Alternatively, the jet cooling apparatus  300  may indirectly cool the surface by spraying a case covering the surface of the package substrate  254 . The effects intended by the present invention are achieved as long as the jet cooling apparatus  300  is configured to ensure the heat generated from the electronic device  200  is dissipated by jet cooling. For example, a mechanism similar to the jet cooling apparatus  300  may be provided inside the heat sink  252 . More specifically, heat generated from the upper surface of the electronic device  200  on a constant basis may be removed by the heat sink  252  and an electric fan. Heat generated instantaneously may be removed by allowing the jet cooling apparatus  300  provided in the heat sink to spray the upper surface of the electronic device  200  with a coolant. 
   The opening provided in the mounting substrate  256  may not necessarily be provided at the center of the mounting substrate  256  as shown. For example, the opening may be provided to address the locality of heat generation in the electronic device  200 . Alternatively, multiple openings may be provided. Further, multiple holes may be drilled through the mounting substrate  256  so that multiple locations of the package substrate  254  are sprayed with a coolant through the holes. 
   Multiple heat detection sensors are provided inside the electronic device  200  to detect the amount of heat it generates. The heat detection sensors may be provided on the electronic device  200  or the package substrate  254 . The heat detection sensor may be a sensor, such as an infrared sensor, which detects the amount of generated heat by detecting infrared radiation from the surface of the electronic device  200 . The heat detection sensor may be a thermometer for measuring the temperature inside the electronic device  200 . 
   Normally, heat generated from the electronic device  200  is conducted to the heat sink  252  via the spreader  250  and released from the dissipation surface. If the amount of heat dissipated from the heat sink  252  is large, heat can be more efficiently removed by blowing air to the heat sink  252  using an electric fan. As a result, the electronic device  200  can be cooled more intensively. By controlling the rotational speed of the electric fan to be increased in accordance with the amount of heat generated from the electronic device  200 , cooling adapted to the amount of generated heat is achieved. In the example described below, the heat sink  252  and the electric fan are used as cooling units of one type for cooling the electronic device  200 . 
   Meanwhile, the electronic device  200  may generate the heat instantaneously. In the event of instantaneous heat generation, i.e., if the amount of heat generated rises abruptly per unit time, the jet cooling apparatus  300  is driven for cooling. The coolant is emitted by the jet cooling apparatus  300  toward the package substrate  254 . Part of the heat generated from the electronic device  200  is conducted to the package substrate  254 . By allowing the jet cooling apparatus  300  to deliver a jet of coolant, the package substrate  254  is cooled. As a result, heat conducted to the package substrate  254  is dissipated via the opening of the mounting substrate  256 . If a location in the electronic device  200  which is heated to a relatively high temperature is known beforehand, the jet cooling apparatus  300  may be designed such that the coolant jet outlets of the jet cooling apparatus  300  are concentrated to cover the location. Jet cooling using a cooling nozzle allows heat, particularly the heat generated in the neighborhood of the jet axis, to be removed effectively. The larger the number of cooling nozzles and the higher the coolant delivering capability of the cooling nozzles, the higher the cooling effect. 
     FIGS. 2 through 4  show mechanisms whereby the jet cooling apparatus  300  delivers a jet of coolant. 
     FIG. 2  shows a first example of the mechanism whereby the jet cooling apparatus  300  delivers a jet of coolant. The jet cooling apparatus  300  includes a coolant passage  310  and a chamber  320 . The coolant introduced into the coolant passage  310  is transferred to the chamber  320 . The chamber  320  temporarily stores the coolant. Multiple coolant jet outlets are provided on the upper surface of the chamber  320 . The coolant jet outlets function as cooling nozzles. A fan driver  302  comprises, for example, a piezoelectric element to drive a fan  304 . When a predetermined voltage is applied to the fan driver  302 , the fan driver  302  is deformed in accordance with the voltage value. The deformation is transferred to the fan  304  so that the fan  304  functions as a paper fan, so to speak, and delivers the coolant introduced into the chamber  320  through the coolant jet outlets on the upper surface of the chamber  320 . 
     FIG. 3  shows a second example of the mechanism whereby the jet cooling apparatus  300  delivers a jet of coolant. In the illustrated example, too, the jet cooling apparatus  300  includes the coolant passage  310  and the chamber  320 . The coolant introduced into the coolant passage  310  is transferred to the chamber  320 . The chamber  320  temporarily stores the coolant. Multiple coolant jet outlets are provided on the upper surface of the chamber  320 . The coolant jet outlets function as cooling nozzles. A driver (not shown) drives a movable film  306  by a force derived from, for example, electrostatic repulsion, piezoelectric effect or magnetism, in accordance with a control signal from an external source. The movable film  306  delivers a jet of coolant by pumping the coolant contained in the chamber  320  through the coolant jet outlets on the upper surface of the chamber  320 . 
     FIG. 4  shows a third example of the mechanism whereby the jet cooling apparatus  300  delivers a jet of coolant. The jet cooling apparatus  300  includes the coolant passage  310  and the chamber  320 . The coolant introduced into the coolant passage  310  is transferred to the chamber  320 . The chamber  320  temporarily stores the coolant. Multiple coolant jet outlets are provided on the upper surface of the chamber  320 . The coolant jet outlets function as cooling nozzles. The coolant passage  310  includes a pump  308 . The pump  308  transfers the coolant introduced into the coolant passage  310  to the chamber  320 . In this process, as the pump  308  pumps the coolant introduced into the coolant passage  310  into the chamber  320  at a high pressure, the coolant introduced into the chamber  320  is delivered through the coolant jet outlets on the upper surface of the chamber  320 . 
     FIG. 5  is a functional block diagram of the electronic device cooling apparatus  100 . The blocks as shown may be implemented hardware wise by elements such as a CPU of a computer or by a mechanism, and software wise by a computer program or the like.  FIG. 5  depicts functional blocks implemented by cooperation of hardware and software. Therefore, it will be obvious to those skilled in the art that the functional blocks may be implemented in a variety of manners by a combination of hardware and software. 
   A controller  110  exercises overall control of the cooling mechanism of the electronic device cooling apparatus  100 . A nozzle unit  102  is a mechanism for delivering a jet of coolant to the package substrate  254 . The jet cooling mechanism  300  represents an example of the nozzle unit  102 . An electric fan unit  106  is a mechanism for blowing air to the heat sink  252 . The electric fan described with reference to  FIG. 1  corresponds to the electric fan unit  106 . A temperature measuring unit  108  measures the temperature of the electronic device  200  by referring to the heat value detected by a thermal sensor provided in the electronic device  200 . The temperature measuring unit  108  also measures time variation of the temperature in the electronic device  200 . The temperature measuring unit  108  may directly measure the temperature of the electronic device  200  by providing a thermometer inside the electronic device  200 . 
   The controller  110  includes a nozzle controller  120 , an electric fan controller  130  and a selector  134 . The nozzle controller  120  controls the nozzle unit  102  by sending a control signal thereto. If, for example, the jet cooling apparatus  300  is of a structure described with reference to  FIG. 2 , the nozzle controller  120  controls the nozzle unit  102  by applying a predetermined voltage to the fan driver  302 . The electric fan controller  130  controls the electric fan unit  106  by sending a control signal thereto. The nozzle controller  120  includes an emission time calculator  122 , and the electric fan controller  130  includes a rotational speed calculator  132 . The selector  134  directs one or both of the nozzle controller  120  and the electric fan controller  130  to exercise control, in accordance with the temperature characteristic of the electronic device  200  measured by the temperature measuring unit  108 . 
   If the temperature of the electronic device  200  measured by the temperature measuring unit  108  exceeds a predetermined threshold value, the selector  134  detects it accordingly and sends a selection signal to the electric fan controller  130 . The electric fan controller  130  sends a control signal to the electric fan unit  106  so as to drive the electric fan. The rotational speed calculator  132  calculates the number of revolutions of the electric fan in accordance with the temperature of the electronic device  200  measured by the temperature measuring unit  108 . The number of revolutions may be the number of revolutions of the electric fan per unit time. Alternatively, the number of revolutions may be the total number of revolutions, i.e., drive time, of the electric fan. If the surface temperature of the electronic device  200  is high, the electric fan controller  130  directs the electric fan unit  106  to drive the electric fan at a high speed. Alternatively, the electric fan controller  130  directs the electric fan unit  106  to drive the electric fan for a longer period of time. 
   The temperature measuring unit  108  temporarily registers time-dependent change of the temperature measured. If a rise in the temperature of the electronic device  200  per unit time measured by the temperature measuring unit  108  exceeds a predetermined threshold value, the selector  134  reads the data and sends a selection signal to the nozzle controller  120 . The nozzle controller  120  sends a control signal to the nozzle unit  102  so as to drive the jet cooling apparatus  300 . The emission time calculator  122  calculates a period of time during which a jet of coolant should be delivered, in accordance with a rise in the temperature of the electronic device  200  per unit time. The emission time calculator  122  may calculate the speed of a jet of coolant or the quantity of coolant delivered, in accordance with a rise of the temperature of the electronic device  200  per unit time measured by the temperature measuring unit  108 . To control the nozzle unit  102  to repeatedly deliver a jet of coolant, the emission time calculator  122  may calculate a ratio between time in which the jet cooling apparatus  300  delivers a jet of coolant and time in which it does not deliver a jet of coolant. The nozzle controller  120  causes the nozzle unit  102  to deliver a jet of coolant by sending a control signal thereto in accordance with the calculation by the emission time calculator  122 . 
   The temperature measuring unit  108  may detect heat distribution in the electronic device  200  instead of the temperature of the electronic device  200  as a whole. The nozzle controller  120  may exercise control such that the coolant is delivered from those coolant jet outlets corresponding to the location in the electronic device  200  generating a large amount of heat. This can achieve effective cooling even in the event of localized heat generation in the electronic device  200  as well as instantaneous heat generation in the electronic device  200 . 
     FIG. 6  is a flowchart showing a process whereby the electronic device cooling apparatus  100  cools the electronic device  200 . The temperature measuring unit  108  measures the temperature of the electronic device  200  by referring to the heat value detected by a thermal sensor embedded in the electronic device  200  (S 10 ). The selector  134  determines whether the temperature of the electronic device  200  thus measured exceeds a threshold value (S 12 ). The predetermined threshold value is arbitrarily set by a designer in accordance with the performance of the electronic device  200  or the environment in which it is used. Hereinafter, the threshold value will be referred to as a first threshold value. If the temperature measured is lower than the first threshold value (N in S 12 ), the nozzle unit  102  and the electric fan  106  are not driven, whereupon the process is terminated. If the temperature measured is equal to or higher than the first threshold value (Y in S 12 ), the selector  134  sends a selection signal to the electric fan controller  130 . The electric fan controller  130  drives the electric fan unit  106  so as to drive the electric fan into rotation (S 14 ). 
   The selector  134  determines whether a rise in the temperature of the electronic device  200  per unit time measured by the temperature measuring unit  108  exceeds a predetermined threshold value (S 16 ). The predetermined threshold value is also a value set by a designer in accordance with the performance of the electronic device  200  or the environment in which it is used. Hereinafter, the threshold value will be referred to as a second threshold value. If the temperature rise per unit time measured is smaller than the second threshold value (N in S 16 ), the process is terminated. If the temperature rise per unit time measured is equal to or greater than the second threshold value (Y in S 16 ), the selector sends a selection signal to the nozzle controller  120 . The nozzle controller  120  drives the nozzle unit  102  so as to deliver a jet of coolant (S 18 ). The temperature measuring unit  108  detects the temperature on a regular basis so that steps S 10  through S 18  are repeated. 
   The nozzle unit  102  and the electric fan unit  106  may be driven concurrently. Even when the temperature of the electronic device  200  is lower than the first threshold value, the nozzle unit  102  may be driven if the rise in the temperature of the electronic device  200  per unit time is equal to or greater than the second threshold value. The nozzle unit  102  or the electric fan  106  may be controlled in accordance with the amount of heat generated in the electronic device  200  instead of the temperature. 
     FIG. 7  is a graph showing a relation between time variation of the temperature of the electronic device  200  and the control of the electronic device cooling apparatus  100 . In the graph, the horizontal axis represents elapsed time. It will be assumed that the electronic device  200  begins its process at time t 0 . The vertical axis represents the temperature of the surface of the electronic device  200 . The temperature is measured by the temperature measuring unit  108  using the thermal sensor in the electronic device  200 . T 1  denotes the first threshold value mentioned above. A chronological description will be now given of the process of the electronic device cooling apparatus  100 . 
   Time t 1 : 
   The temperature of the electronic device  200  rises and exceeds T 1 , the first threshold value. The selector  134  sends a selection signal to the electric fan controller  130 . The electric fan controller  130  sends a control signal to the electric fan unit  106  so as to drive the electric fan. This mitigates the rise in the temperature in a period past time t 1 , causing the temperature to be lowered after a time. 
   Time t 2 : 
   The temperature of the electronic device  200  begins to rise instantaneously. At this stage, the temperature is still higher than T 1  so that the electric fan continues to be driven. However, since the electric fan&#39;s cooling time response is slower than that of jet cooling, the temperature is not lowered immediately. 
   Time t 3 : 
   The selector  134  detects that a rise in the temperature per unit time in a period between t 2  and t 3  is equal to or greater than the second threshold value. The selector  134  sends a selection signal to the nozzle controller  120 . The nozzle controller  120  sends a control signal to the nozzle unit  102  so as to drive the jet cooling apparatus  300 . This mitigates the instantaneous temperature rise that occurred at time t 2 . When a rise in the temperature of the electronic device  200  per unit time is smaller than the second threshold value, jet cooling is terminated. At this stage, the temperature exceeds T 1  so that the electric fan continues to be driven. 
   Time t 4 : 
   Since the temperature drops below T 1  due to the cooling by the electric fan, the electric fan controller  130  sends a signal directing the electric fan unit  106  to stop driving the electric fan. 
   According to the described embodiment, heat generated from the electronic device  200  on a constant basis is removed by air cooling using the heat sink  252  and the electric fan. Meanwhile, heat generated from the electronic device  200  temporarily or instantaneously is removed by jet cooling. The electronic device cooling apparatus  100  according to the embodiment is capable of promptly responding to variation in the amount of heat generated from the electronic device  200 , by driving a jet cooling mechanism as an auxiliary means. 
   Heat generated from the electronic device  200  on a constant basis is removed by the heat sink  252  and the electric fan and does not require driving the jet cooling apparatus  300 . Even if the amount of heat generated from the surface of the electronic device is close to its maximum, instantaneous heat generation may occur concurrently. By performing jet cooling when the temperature of the electronic device rises in this manner, the electronic device  200  can be cooled effectively depending on the type of heat generation. Since the opening in the mounting substrate  256  is used in this embodiment, there is no need to provide an excessively large apparatus for jet cooling. This results in a reduced overall size of the electronic device cooling apparatus  100 . 
   The technology used in the related-art air cooling and liquid cooling can be used unmodified in the system for supplying a coolant to the jet cooling apparatus  300  and the system for collecting the used coolant. The combination of an electric fan and jet cooling is described in the embodiment by way of example. The proposal of the present invention is to cool the electronic device  200  by driving cooling units based on cooling mechanisms that differ in respect of cooling time response. As such, the scope of the present invention is not limited to the cooling apparatus illustrated in the embodiment. For example, the heat sink  252  may be cooled by cooling water instead of an electric fan. Alternatively, the cooling mechanism facing the upper surface of the electronic device  200  may be implemented only by the heat sink  252 , i.e., only by the heat dissipating mechanism. Similarly, the cooling mechanism with fast cooling time response may not be limited to jet cooling by the jet cooling apparatus  300 . For example, heat may be removed from the package substrate  254  by water cooling using cooling water. Alternatively, a thermoelectric element may be used for cooling. It will be understood to persons skilled in the art that there are a variety of cooling methods based upon respective cooling mechanisms that differ in respect of cooling time response. 
   The description of the invention given above is based upon one illustrative embodiment. While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims. The described method cools an electronic device by coordinating two types of cooling units based upon different cooling mechanisms. Alternatively, more than two cooling units may be used. It will be understood to persons skilled in the art that a cooling unit may be designed in a variety of manners in accordance with the condition in which the electronic device is installed or the environment in which it is used.