Patent Document

FIELD OF THE INVENTION 
     The present invention relates to a switching power supply (SPS), and more particularly to a packing structure and a packing method of a mini-size power supply. 
     BACKGROUND OF THE INVENTION 
     Presently, for the development of electronic device, the volume trends to be smaller and smaller, and the current and power requirements trend to be bigger and bigger, especially for a high density switching power supply (SPS). Therefore, it is an important issue to achieve excellent heat-dissipating effect and reduce the current loading in such tiny space. 
     FIG. 1A is a diagram illustrating a switch circuit used in the switching power supply according to the prior art. Referring to FIG. 1A, an induced current is generated and inputted to a secondary winding  101  via a transformer  10 . A set of rectifiers  102  and  103  then rectify the induced current to generate the output DC current to output through inductances  104  and  105  connected with a positive terminal (Vo) and a negative terminal (−Vo). The rectifiers  101  and  102  can either be a diode or a metal-oxide-semiconductor field effect transistor (MOSFET). FIG. 1B is a diagram illustrating an alternate circuit used in the switching power supply arranged slightly differently than that of FIG.  1 A. 
     FIG. 2 is an exploded three-dimensional view illustrating a packed circuit in FIG. 1A which is a general packed structure used in the industry. Cores  200 ,  201  and a windings  202  shown in FIG. 2 is corresponded to the secondary winding  101  in FIG.  1 A. Cores  203 ,  204  and a windings  207  shown in FIG. 2 are corresponded to the inductance  104  in FIG. 1A, and cores  205 ,  206  and a winding  208  shown in FIG. 2 are corresponded to the inductance  105  in FIG.  1 A. The MOSFETs  211  and  212  are corresponded to the rectifiers  103  and  102  in FIG. 1A, respectively. 
     Currently, most rectifiers applied in SPS are MOSFETs. FIG. 3 is a perspective view illustrating the typical structure of a packed MOSFET. As shown in FIG. 3, a chip  301  is soldered onto a copper plate  300  which is a drain of the MOSFET. A source and a gate are bonding to two pins  302  and  304  via metal lines  306  and  305 . After testing the electricity, the top of the chip  301  is packed by epoxy. Generally, the conductivity of copper is about 380 W/mk, while that of epoxy is smaller than 1 W/mk. For the general heat-dissipating mechanism, the MOSFET is connected onto a pad or a metal of a substrate by soldering or screwing, and a thermal pad is placed between the MOSFET and the substrate for heat-dissipating. Usually, the substrate is a FR4 printed circuit board and the metal is aluminum. Thus, the heat conduction pathway is to transfer the heat generated from the MOSFET to the pad of the substrate via the copper plate  300 , and then dissipate the heat to the air by natural convection or forced convection. 
     On the other hand, most electric devices are soldered on the surface of substrate by the surface mounting technology (SMT). For SPS, the surface mounted device (SMD) is generally used in SPS designation. FIG. 4A is an exploded diagram illustrating a standard packed MOSFET bound to a printed circuit board according to the prior art. Generally, MOSFET has three pins, i.e. a gate  401 , a source  402 , and a drain  400  which is a copper plate. The copper plate  400  is soldered on a pad  404  of a printed circuit board  403 , and the gate  401  and the source  402  are soldered on plates  405 ,  406  of the printed circuit board  403  respectively. After assembling, the structure is as shown in FIG.  4 B. As shown in FIG. 4C, the heat conduction pathway is from the drain  400  located at the back of MOSFET to the printed circuit board  403  via a soldering material  407 , i.e. the conductive materials such as tin or silver. Generally, since the material of printed circuit board is FR4 having conductivity of about 0.8 W/mk, the conduction effect is very small, i.e. the heat resistance is very large. Hence, most heat is directly transferred to the position just under the printed circuit board  403 , i.e. under the MOSFET, by conduction, and dissipated into the air by convection as shown in FIG.  4 C. Thus, if an electronic device which is not tolerance to heat such as capacitance is placed under the MOSFET, then the device lifetime will reduce because of high temperature generated by the MOSFET. However, since the device which could generate heat is soldered on the printed circuit board which is a FR4 material and is a bad conductor for heat, the generated heat is not easily taken away. According to the law of the conservation of energy, the temperature of the device which could generate heat will keep increasing because the generated heat cannot be dissipated, and further results in losing efficacy of the device because of the thermal run away effect. 
     In addition, for the power supply design having high current and high power characteristics, many MOSFETs are generally parallel connection for enhancing the efficiency, so the printed circuit board requires more thick copper line for loading larger current. Therefore, the space on the printed circuit board is occupied. 
     Summarily, the problems of the heat-dissipating effect, loading larger current and the space-consumption are still required to be solved in current industry. Therefore, the purpose of the present invention is to develop a method to deal with the above situations encountered in the prior art. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to propose a packing structure of a switching power supply for enhancing heat-dissipating effect. 
     It is therefore another object of the present invention to propose a packing structure of a switching power supply for loading and outputting more current. 
     It is therefore an additional object of the present invention to propose a packing structure of a switching power supply having the smaller packaged volume. 
     According to one aspect of the present invention, there is provided a packing structure of a switching power supply for enhancing heat-dissipating effect. The packing structure includes a printed circuit board, a transformer, an inductor having an inductive winding, a converter placed on a pad of the printed circuit board, wherein the pad is electrically connected to a secondary winding of the transformer and the inductive winding, and a metal cover directly covered on the converter. 
     Certainly, the metal cover can be made of copper. 
     Certainly, the converter can be a metal-oxide-semiconductor field effect transistor (MOSFET) having a drain directly connected to the metal cover and a source and a gate directly connected to the pad of the printed circuit board. 
     Preferably, the packing structure further includes a heatsink placed on the metal cover for enhancing heat-dissipating, or/and a thermal pad placed between the metal cover and the heatsink for conducting heat. 
     Preferably, the packing structure further includes a metal strip electrically connected to the metal cover, the inductive winding and the secondary winding of the transformer. The metal strip and the inductive winding can be integrally formed. The metal strip can be made of copper. 
     Certainly, the converter can be a diode having an anode electrically connected to the inductor and a cathode directly connected to a pad of the printed circuit board. 
     Certainly, the printed circuit board can be made of a material selected from FR4 and thermal clad. 
     According to another aspect of the present invention, there is provides a packing structure of a switching power supply for enhancing heat-dissipating effect. The packing structure includes a printed circuit board, a transformer, an inductor having an inductive winding, a metal strip electrically connected to the inductive winding, and a converter electrically connected to the metal strip and covered by the metal strip. 
     According to an additional aspect of the present invention, there is provides a method for packing a switching power supply to enhance heat-dissipating effect. The method includes steps of placing a converter on a pad of a printed circuit board, placing an inductor and a transformer on the printed circuit board, and electrically connecting a metal cover to the pad, an inductive winding of the inductor and a secondary winding of the transformer for enhancing heat-dissipating effect. 
    
    
     The present invention may best be understood through the following description with reference to the accompanying drawings, in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1B are diagrams illustrating two alternative switch circuits used in the switching power supply according to the prior art; 
     FIG. 2 is an exploded view illustrating a packed circuit in FIG. 1A according to the prior art; 
     FIG. 3 is a perspective view illustrating a packed structure of MOSFET according to the prior art; 
     FIG. 4A is an exploded diagram illustrating a standard packed MOSFET bound to a printed circuit board according to the prior art; 
     FIG. 4B is diagram illustrating an packed structure of MOSFET bound to a printed circuit board in FIG. 4A; 
     FIG. 4C is a lateral diagram illustrating a MOSFET bound to a printed circuit board in FIG. 4B; 
     FIG. 4D is a diagram illustrating a thermal resistance distribution in FIG. 4C; 
     FIG. 5A is an exploded diagram illustrating a standard packed MOSFET bound to a printed circuit board according to one preferred embodiment of the present invention; 
     FIG. 5B is diagram illustrating an packed structure of MOSFET bound to a printed circuit board in FIG. 5A; 
     FIG. 5C is a diagram illustrating a thermal dissipation distribution in FIG. 5B; 
     FIG. 6A is an exploded diagram illustrating a standard packed MOSFET bound to a printed circuit board according to another preferred embodiment of the present invention; 
     FIG. 6B is diagram illustrating an packed structure of MOSFET bound to a printed circuit board in FIG. 6A; 
     FIG. 6C is a diagram illustrating a thermal dissipation distribution in FIG. 6B; 
     FIGS. 7A-7B are exploded diagrams illustrating two alternative switch circuits used in the switching power supply according to one preferred embodiment of the present invention; and 
     FIGS. 8A-8B are exploded diagrams illustrating two alternative switch circuits used in the switching power supply according to another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 5A-5C, a standard packaged MOSFET and a copper cover used for heat-dissipating are soldered to a printed circuit board  503  simultaneously in order to increase heat-dissipating effect and prevent too much current from transmitting to the printed circuit board  503 . As shown in FIG. 5A, a drain  500  of the MOSFET  50  is soldered on a pad  504  of the printed circuit board  503 . A gate  501  and a source  502  are soldered on a pad  505  and a pad  506  of the printed circuit board  503 , respectively. Then, a copper cover  507  is soldered on the pad  504  of the printed circuit board  503 . Because the copper cover  507  is directly connected to the pad  504 , the copper cover  507  is also a drain of the MOSFET  50 . FIG. 5B is an assembly structure of FIG.  5 A. Because the thermal conduction coefficients of the copper and the soldering materials such as tin and silver are much greater than that of the substrate, the major thermal flow is transferred from the drain  500  of the MOSFET to a soldering material  508 , then transferred to the copper cover  507 , and is dissipated into air by convection, as shown in FIG.  5 C. In addition, partial heat will be conducted to the bottom of the printed circuit board  503 , and is also dissipated into the air by convection. Hence, the electronic devices directly under the MOSFET  50  are still affected by the heat occurred by the MOSFET. Thus, if the heat-dissipating area of copper cover is increased, e.g. adhering a thermal pad on the copper cover as a medium for conducting heat and adding a heatsink thereon, then the temperature of the MOSFET will decrease. Further, the temperature of the electronic device located under the printed circuit board  503  will decrease, too. 
     Referring to FIG. 6, the MOSFET is pinged into the printed circuit board instead of being placed on the printed circuit board by SMT technology. Three pins of MOSFET  60 , i.e. a drain  602 , a source  601  and a gate  603 , are soldered into three holes  605 ,  604 ,  606  of the printed circuit board  61 , respectively. A heat-dissipating copper plate  607  is soldered on a copper plate  600  of the MOSFET  60  directly, so the heat-dissipating copper plate  607  is also a drain of the MOSFET  60 . A thermal pad  608  is adhered on the heat-dissipating copper plate  607  as a medium for conducting. Finally, a heatsink  609  is added on the thermal pad  608 . FIG. 6B is an assembly structure of FIG.  6 A. Because the major thermal flow is directly transferred from the drain  600  of the MOSFET to the heat-dissipating copper plate  607 , to the thermal pad  608 , then to the heatsink  609 , and is dissipated into air by convection, as shown in FIG.  6 C. In addition, the printed circuit board  61  is slightly affected by the heat occurred from the MOSFET  60  because there are only three pins  602 ,  601 ,  603  of the MOSFET  60  to connect to the printed circuit board. Therefore, the thermal effect on the electronic devices under the printed circuit board by the heat transferred from the MOSFET also reduces. 
     In order to explain the relation of the heat-dissipating effect and the heat-dissipating area, FIGS. 4C and 4D are as examples. As shown in FIG. 4C, the thermal flow is transferred from the MOSFET to the printed circuit board by conduction, then is dissipated into air by convection. 
     During conduction, the conduction resistance (R conduction ) and the convection resistance (R convection ) are occurred as shown in FIG.  4 D. 
     Since the main heat-dissipating pathway is convection, the heat transfer is only focused on the convection. The thermal resistance of convection can be shown as the following equation:          R   convection     =       1   hA     =         T   MOSFET     -     T   AMBIENT       P                              
     Wherein h is a heat transfer coefficient, A is the heat-dissipating area, P is consumption power, T MOSFET  is the temperature of MOSFET, and T ambient  is the ambient temperature. Generally, for certain P, the smaller the thermal resistance of the convection is, the lower the temperature of the electronic device is under T AMBIENT . According to above equation, the convection resistance is inversely proportional to the average heat transfer coefficient and the heat-dissipating area. Under forced convection, the average heat transfer coefficient is changed along with the fluid flow speed, e.g. the ambient air flow speed. Hence, if the air flow speed and the ambient temperature are fixed, the convection resistance is only inversely proportional to the heat-dissipating area. 
     That is, the larger the heat-dissipating area is, the smaller the convection resistance is. Therefore, for enhancing the heat-dissipating effect of the electronic device, it is important to increase the heat-dissipating area. 
     FIGS. 7A-7B are exploded diagrams illustrating two packaged methods used in the switching power supply according to one preferred embodiment of the present invention. As shown in FIG. 7A, the packaged method in FIG. 7A is similar to that in FIG. 2 except that two copper covers  713  and  714  are added and soldered on the MOSFETs  711  and  712 , respectively. The copper covers  713  and  714  are also the drains of the MOSFETs  711  and  712 , respectively. The copper covers  713  and  714  are soldered to copper strips  709  and  710  respectively or a same part, then soldered to windings  702 ,  707  and  708 . Thus, the circuit structure such as FIG. 1A is formed. In addition, the heat-dissipating copper covers  713 ,  714 , the copper strips  709 ,  710 , and the windings  707 ,  708  can be formed integrally to a copper member as shown in FIG.  7 B. Referring to FIGS. 7A and 7B, the spaces under the copper strips  709  and  710  allow some electronic devices  715 , e.g. a resistance, to be placed, so the using-space of the printed circuit board could be increased according to the present invention. In addition, the heat-dissipating copper covers  713  and  714  can enhance heat-dissipating effect and prevent overloading current from conducting to the printed circuit board. A heatsink or spreader  718  can be placed on the top of the heat-dissipating copper covers  713  and  714  to further enhance heat-dissipating effect, and that have insulating thermal pads  716 ,  717  between heatsink  718  and copper cover  713 ,  714 , copper strips  709 ,  710 . 
     In addition, the two pins of MOSFETs  811 ,  812 , i.e. the gate and the source, can be formed at a right angle as shown in FIG.  8 A. The copper drains located at the back of the MOSFETs  811 ,  812  are directly soldered to copper strips  809  and  810  at the positions of  813  and  814 , respectively. Thus, the copper strips  809  and  810  are also the drains of the MOSFETs  811  and  812 . The copper strips  809  and  810  are soldered to windings  802 ,  807 , and  808  as shown in FIG. 8A, or are integrally formed therewith as shown in FIG.  8 B. Similarly, the spaces under the copper strips  809  and  810  also allow some electronic devices  815  to be placed in, so the using-space of the printed circuit board can be increased according to the present invention. In addition, the positions  813  and  814  can be copper covers. The copper covers  813  and  814  can enhance heat-dissipating effect and prevent overloading current from conducting to the printed circuit board. Furthermore, a heatsink or spreader  818  can be placed on the top of the heat-dissipating copper covers  813  and  814  to further enhance heat-dissipating effect, and that have insulating thermal pads  816 ,  817  between heatsink  818  and copper cover  813 ,  814 , copper strips  809 ,  810 . 
     In sum, the advantages of the present invention are as the following advantages of: 
     (1) increasing the heat-dissipating area for enhancing the heat-dissipating effect, 
     (2) soldering the copper plate to the drain of MOSFET for preventing too much current from transferring from the drain to the printed circuit board, and 
     (3) reducing the packaged volume for increasing using-space of the printed circuit board. 
     While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not to be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.

Technology Category: 5