Abstract:
An improved cooling mechanism for a power electronics device is provided. More specifically, a cooling mechanism is provided that includes an air passageway configured to allow cooling air to bypass a portion of a heatsink adjacent to the rectifier circuitry and direct cooling air into an area of the heatsink that is nearer to the inverter circuitry. Another embodiment employs an air passageway with an air directing structure configured to provide an air flow that impinges on a lateral surface of the heatsink. In another embodiment, the air directing structure is chosen to provide a turbulent air flow in the heat dissipating structure within the vicinity of the inverter circuitry.

Description:
BACKGROUND 
       [0001]    The invention relates generally to the field of power electronic devices such as those used in power conversion or for applying power to motors and other loads. More particularly, the invention relates to a power electronic module with an improved cooling arrangement which provides enhanced air flow characteristics and enhanced heat dissipation. 
         [0002]    In the field of power electronic devices, a wide range of circuitry is known and currently available for converting, producing and applying power to loads. Depending upon the application, such circuitry may convert incoming power from one form to another as needed by the load. In a typical arrangement, for example, constant (or varying) frequency alternating current power (such as from a utility grid or generator) is converted to controlled frequency alternating current power to drive motors, and other loads. In this type of application, the frequency of the output power can be regulated to control the speed of the motor or other device. Many other applications exist, however, for power electronic circuits which can convert alternating current power to direct current power, or vice versa, or that otherwise manipulate, filter, or modify electric signals for powering a load. Circuits of this type generally include rectifiers (converters), inverters, and similar switched circuitry. For example, a motor drive will typically include a rectifier that converts AC power to DC. Often, power conditioning circuits, such as capacitors and/or inductors, are employed to remove unwanted voltage ripple on the internal DC bus. Inverter circuitry can then convert the DC signal into an AC signal of a particular frequency desired for driving a motor at a particular speed. The inverter circuitry typically includes several high power switches, such as insulated-gate bipolar transistors (IGBTs), controlled by drive circuitry. 
         [0003]    The motor drive circuitry detailed above will typically generate substantial amounts of heat, which must be dissipated to avoid damaging heat sensitive electronics. Typically, therefore, some form of cooling mechanism is usually employed to enhance heat extraction and dissipation. Often, the motor drive circuitry is packaged together as a unit with a built-in cooling channel that carries cool air to several components. Because the air within the channel is heated as it travels through the channel, components near the exhaust end of the air channel will usually experience a diminished cooling effect. Therefore, as packaged control units become more compact, the need for efficient heat dissipation becomes more critical. 
         [0004]    Additionally, as the workload or motor speed changes, the temperature of the inverter circuitry (e.g., the IGBTs) generally increases, causing higher failure rates and reduced reliability. The power output of the unit is often, therefore, limited by the maximum temperature that the inverter circuitry can handle without substantially increasing the risk of failure. A more effective cooling mechanism that provides additional cooling for the inverter circuitry would, therefore, allow the motor drive to operate at higher motor speeds. 
         [0005]    Therefore, it may be advantageous to provide a motor drive with an improved cooling mechanism. In particular, it may be advantageous to provide a cooling mechanism that provides increased cooling for the inverter circuitry of a power electronic module such as a motor drive. 
       BRIEF DESCRIPTION 
       [0006]    The present invention relates generally to a cooling configuration designed to address such needs. One embodiment employs an air passageway configured to allow cooling air to bypass a portion of a heatsink adjacent to the rectifier circuitry and direct cooling air into an area of the heatsink that is nearer to the inverter circuitry. Another embodiment employs an air passageway with an air directing structure configured to provide an air flow that impinges on a lateral surface of the heatsink. In another embodiment, the air directing structure is chosen to provide a turbulent air flow in the heat dissipating structure within the vicinity of the inverter circuitry. 
     
    
     
       DRAWINGS 
         [0007]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0008]      FIG. 1  is a diagrammatical representation of an exemplary motor drive circuit in accordance with one embodiment of the present invention; 
           [0009]      FIG. 2  is a perspective view of an exemplary motor drive unit in accordance with one embodiment of the present invention; and 
           [0010]      FIGS. 3-5  are cross sectional views of the motor drive unit shown in  FIG. 2 , illustrating exemplary air passageways with exemplary air directing structures. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a diagrammatical representation of an exemplary motor drive circuit  10  employing an air passageway with an air directing structure for providing enhanced cooling of the motor drive circuitry. The motor drive circuit  10  includes a three phase power source electrically coupled to a set of input terminals  12 ,  14  and  16  that provides three phase AC power of constant frequency to a rectifier circuitry  18 . In the rectifier circuitry  18 , a set of six silicon-controlled rectifiers (SCRs)  32  provide full wave rectification of the three phase voltage waveform. Each input terminal entering the rectifier circuitry  18  is coupled between two SCRs  32  arranged in series, anode to cathode, which span from the low side  38  of the DC bus  34  to the high side  36  of the DC bus  34 . Inductors  42  are coupled to both the high and low sides of the DC bus  34  and act as a choke for smoothing the rectified DC voltage waveform. Capacitors  40  link the high side  36  of the DC bus  34  with the low side  38  of the DC bus  34  and are also configured to smooth the rectified DC voltage waveform. Together, the inductors and capacitors serve to remove most of the AC ripple presented by the rectifier circuitry  18  so that the DC bus  34  carries a waveform closely approximating a true DC voltage. It should be noted that the three-phase implementation described herein is not intended to be limiting, and the invention may be employed on single-phase circuitry, as well as on circuitry designed for applications other than motor drives. 
         [0012]    An inverter  22  is coupled to the DC bus  34  and generates a three phase output waveform at a desired frequency for driving a motor  30  connected to the output terminals  24 ,  26  and  28 . Within the inverter  22 , two switches  44  are coupled in series, collector to emitter, between the high side  36  and low side  38  of the DC bus  34 . Three of these switch pairs are then coupled in parallel to the DC bus  34 , for a total of six switches  44 . Each switch  44  is paired with a flyback diode  46  such that the collector is coupled to the anode and the emitter is coupled to the cathode. Each of the output terminals  24 ,  26  and  28  is coupled to one of the switch outputs between one of the pairs of switches  44 . The driver circuitry  48  signals the switches  44  to rapidly close and open, resulting in a three phase waveform output across output terminals  24 ,  26  and  28 . The driver circuitry  48  is controlled by the control circuitry  50 , which responds to the remote control and monitoring circuitry  52  through the network  54 . 
         [0013]    As discussed above, many of the circuit components depicted in  FIG. 1  will generate significant amounts of heat, which can lead to component failure due to overheating. Therefore, to increase the heat dissipating properties of motor control circuit  10 , the motor control circuit  10  will usually be packaged within a unit that includes a cooling channel and a heatsink, as shown in  FIG. 2   
         [0014]    Turning now to  FIG. 2 , a perspective view of an exemplary motor drive unit in accordance with one embodiment of the present invention is shown. The motor drive unit  56  includes a cooling channel  58  enclosed by side plates. The motor drive unit  56  also includes a set of fans  60  to provide a flow of cooling air through the cooling channel  58 . The SCRs  32 , IGBTs  44 , driver circuitry  48 , and the control circuitry  50  are situated above and adjacent to the cooling channel  58  so that the flow of cool air draws heat from the circuitry. To make efficient use of the space within the motor drive unit  56 , the SCRs  32  will generally be grouped together with the control circuitry  50  near the input of the cooling channel  58 , and the IGBTs  44  will generally be grouped together with the driver circuitry  48  further downstream, i.e. toward the exhaust end of the cooling channel  58 . It will be appreciated that, given a typical cooling channel arrangement, the downstream circuitry, such as the IGBTs  44 , will experience diminished cooling compared to the upstream components. Embodiments of the present invention, however, provide improved cooling techniques that allow the cooling effects of the cooling air to be shifted downstream, toward the IGBT circuitry, as will be explained below in respect to  FIGS. 3-5 . 
         [0015]      FIG. 3  is a cross-sectional view of the control unit  56 , and provides a better view of cooling channel  58 . As can be seen in  FIG. 3 , the cooling channel  58  includes a heatsink  62  mounted below a lower plate  64  adjacent to the SCRs  32 , the IGBTs  44 , the driver circuitry  48 , and the control circuitry  50 . The heatsink  62  may include a series of parallel fins oriented toward the fans  60  to allow cooling air from the fans  60  to pass between the fins. Cooling air may also pass between the fins at a lateral face  66  of the heatsink  62 . Also inside the cooling channel  58  is an open passageway  68  located adjacent to the heatsink  62  and extending some portion of the length of the heatsink  62 . The open passageway  68  allows some portion of the air entering the cooling channel  58  to bypass the heatsink  62  for a certain distance. The cooling channel  58  also includes an air directing structure  70  positioned below the heatsink  62  and configured to direct air into a lateral face  66  of the heatsink  62 . The air directing structure  70  may be formed by the bottom plate  72  of the cooling channel  58  as shown in  FIG. 3 , or alternatively, the air directing structure  70  may be a separate baffle located inside the cooling channel  58 . As explained below, the cooling channel  58  provides improved cooling properties over the prior art by shifting a portion of the cooling air to a downstream location and by imparting an angular direction to the air flow. 
         [0016]    In embodiments of the present invention, cooling air is forced by the fans  60  into the cooling channel  58 , at which point, some of the air enters the leading edge of the heatsink  62 , as illustrated by arrow  74 , while some portion of the air enters the open passageway  68 . The air entering the leading edge of the heatsink  62  will be warmed by the control circuitry  50  and the SCRs  32 . However, because the open passageway  68  is not significantly thermally coupled to the heatsink  62 , the air passing through the open passageway  68  will be relatively cool. Air entering the open passageway  68  is later forced up into the heatsink  62  by the air directing structure  70  at a location downstream from the control circuitry  50  and the SCRs  32 , as illustrated by arrow  76 . By directing cooler air into the heatsink  62  at the downstream location, rather than guiding all of the cooling air into the leading edge of the heatsink, the combined temperature of the cooling air adjacent to the driver circuitry  48  and the IGBTs  42  may be reduced, making those components relatively cooler. At the same time, however, the flow of cooling air adjacent to the control circuitry  50  and the SCRs  32  will be reduced, making those components relatively warmer. It can be seen, therefore, that using the techniques described above, the cooling influence of the air flow in the channel  58  may be shifted from an upstream location to a downstream location. In this way, the cooling air may be directed to circuitry that may have a greater need for cooling, such as the IGBTs  42 , for example. 
         [0017]    The degree of air flow shifting will depend on the setback  78  of the air directing structure  70 . For purposes of the present description, the setback  78  is defined at the distance from the leading edge of the heatsink  62  to the point at which the air directing structure  70  meets the heatsink  62 , as shown in  FIG. 3 . In some embodiments, the setback  78  of the air directing structure  70  may be selected to coincide with the leading edge of the IGBTs  42 , so as to favor increased cooling for the IGBTs  42 , as shown in  FIG. 3 . In other embodiments, the setback  78  may be increased or decreased to change the distribution of cooling air, and thereby favor certain components or spread the heat dissipation more evenly. In various embodiments, the setback  78  may range from ten percent to ninety percent of the length of the heatsink  62 , as shown in  FIGS. 4 and 5 . 
         [0018]    In addition to shifting the cooling air downstream, the air directing baffle  70  may also impart a directional component to the air that is perpendicular to the face of the heatsink, causing an angular airflow relative to the face of the heatsink. This angular, or impingent, air flow may tend to force cooler air deeper into the heatsink, closer to the heat source, while forcing warmed air out toward the exhaust of the heatsink. In this way, the rate of heat transfer from the heatsink  62  to the cooling air may be increased. 
         [0019]    The angularity of the air flow depends, at least in part, on the angle  80  of the air directing structure  70 . Furthermore, the angle selection may also affect the overall air flow resistance of the channel. In some embodiments, the angle  80  may be approximately forty-five degrees, as shown in  FIG. 3 . By orienting the air directing structure  70  to form an angle  80  of forty-five degrees a substantial level of angularity may be imparted to the air flow while, at the same time, maintaining a relatively low overall air flow resistance. In other embodiments, the angularity of the air flow and the air flow resistance may be increased or decreased by changing the angle  80  as shown in  FIGS. 4 and 5 , which are described below. In various embodiments, the angle  80  may range from 10 to 170 degrees. 
         [0020]    Turning now to  FIGS. 4 and 5 , additional embodiments of a motor drive unit with exemplary cooling channels are shown. Turning specifically to  FIG. 4 , an embodiment is shown in which the angle  80  of the air directing structure  70  is approximately ten degrees and the setback distance  78  is approximately ninety percent of the length of the heatsink  62 . In this embodiment, the air in open passageway  68  is guided into the heatsink  62  more gradually compared to the embodiment of  FIG. 3 . The relatively large setback may tend to shift cooling air further downstream, providing enhanced cooling to downstream components. The relatively small angle reduces the angularity of the cooling air flowing through the heatsink  62 . The large setback, in combination with the small angle tends to cause cooling air to be gradually guided into the heatsink  62  along a large portion of the lateral face  68  of the heatsink  62 , as indicated by the arrows  82 . In this way, the cooling effects of the cooling air may be more evenly distributed between the upstream and downstream components. Additionally, the small angle  80  may also decrease the overall airflow resistance of the cooling channel  58 . 
         [0021]    Turning to  FIG. 5  an embodiment is shown in which the angle  80  of the air directing structure  70  is approximately ninety degrees and the setback distance  78  is approximately ten percent of the length of the heatsink  62 . In this embodiment, the air in open passageway  68  is guided into the heatsink  62  more abruptly compared to the embodiment of  FIG. 3 . The ninety degree angle of the directing structure  70  increases the angularity of the cooling air flowing through the heatsink  62  in the vicinity of the upstream components, as indicated by the arrow  84 . Additionally, the relatively short setback distance will tend to allow very little downstream shifting of cooling air, providing additional cooling air to components further upstream. The small setback, in combination with the ninety degree angle, may tend to focus cooling effects of the cooling air on the upstream components, causing more heat to be extracted from the upstream components, such as the SCRs. However, the ninety degree angle may also tend to increase the overall airflow resistance of the cooling channel  58 . 
         [0022]    It will be appreciated that a wide range of angles and setback distances may be utilized in various embodiments besides those depicted above. For example, to achieve a higher degree of cooling in the vicinity of the IGBTs an embodiment may include a setback distance of approximately 60 percent and an angle of approximately ninety degrees, thereby creating an air flow under the IGBTs with a high degree of angularity. For another example, the angle  80  of the air directing structure may be up to 170 degrees, in which case the air directing structure may form a pocket of air that the cooling air travels past before being directed into the heatsink. 
         [0023]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.