# Patent

## Title
Stacked half-bridge package

## Abstract

According to an exemplary embodiment, a stacked half-bridge package includes a control transistor having a control drain for connection to a high voltage input, a control source coupled to an output terminal, and a control gate for being driven by a driver IC. The stacked half-bridge package further includes a sync transistor having a sync drain for connection to the output terminal, a sync source coupled to a low voltage input, and a sync gate for being driven by the driver IC. The control and sync transistors are stacked on opposite sides of a common conductive leadframe with the common conductive leadframe electrically and mechanically coupling the control source with the sync drain. The common conductive leadframe thereby serves as the output terminal.

## Background
BACKGROUND OF THE INVENTION
This is a continuation of application Ser. No. 13/866,744 filed on Apr. 19, 2013, which is itself a continuation of application Ser. No. 13/278,695 filed Oct. 21, 2011, now U.S. Pat. No. 8,426,952. Application Ser. No. 13/278,695 in turn claims the benefit of and priority to a provisional application titled “Stacked Packaging Architecture with Reduced Form Factor and Increased Current Density with Application to Power Transistor Packaging,” Ser. No. 61/461,110 filed on Jan. 14, 2011. The disclosures in the above-identified patent applications are hereby incorporated fully by reference into the present application.


Moreover, application Ser. No. 11/986,848, filed on Nov. 27, 2007, titled “DC/DC Converter Including a Depletion Mode Power Switch,” now U.S. Pat. No. 7,902,809; and pending applications Ser. No. 12/928,102, filed on Dec. 3, 2010, titled “DC/DC Converter with Depletion-Mode III-Nitride Switches,” and Ser. No. 12/927,341, filed on Nov. 12, 2010, titled “Semiconductor Package with Metal Straps,” are hereby incorporated fully by reference into the present application.
FIELD OF THE INVENTION
The present invention relates generally to semiconductor devices. More particularly, the present invention relates to packaging of semiconductor devices.
BACKGROUND ART
To allow for high efficiency power conversion, power converters, such as buck converters, commonly employ power switching circuits in which a high side power transistor and a low side power transistor are connected to form a half-bridge. One such power converter that is frequently employed is a synchronous buck converter, where the high side transistor is a control transistor and the low side transistor is a synchronous transistor. The control transistor and the synchronous transistor are typically formed on their respective separate dies, i.e. a control transistor die and a synchronous transistor die, that are connected in a package (i.e. co-packaged) to form the half-bridge.
One approach to connecting the control transistor and the synchronous transistor in a package would be to arrange the control transistor and the synchronous transistor side by side on a substrate, such as a printed circuit board (PCB). However, this arrangement would result in the package having a large footprint, as the package must be large enough to accommodate footprints of the control transistor and the synchronous transistor. Furthermore, conductive traces on the PCB could be used to connect the control transistor and the synchronous transistor so as to form the half-bridge package. However, the conductive traces would form long routing paths on the PCB, causing high parasitic inductance and resistance. Thus, this approach to packaging the control transistor and the synchronous transistor would result in a package having a large form factor where the package significantly degrades performance of the half-bridge.
What is needed is an approach to packaging control and synchronous transistors that is capable of achieving packages having a small form factor where the packages do not significantly degrade performance of the half-bridge.
SUMMARY OF THE INVENTION
A stacked half-bridge package with a common conductive leadframe, substantially as shown in and/or described in connection with at least one of the figures, and as set forth more completely in the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an exemplary circuit schematic of a synchronous buck converter, according to one embodiment of the invention.
FIG. 2A illustrates a selective top view of a stacked half-bridge package, according to one embodiment of the invention.
FIG. 2B illustrates a selective top view of a stacked half-bridge package, according to one embodiment of the invention.
FIG. 2C illustrates a selective top view of a stacked half-bridge package, according to one embodiment of the invention.
FIG. 2D illustrates a cross sectional view of a portion of a stacked half-bridge package, according to one embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
The present application is directed to a stacked half-bridge package with a common conductive leadframe. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order not to obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skill in the art.
The drawings in the present application and their accompanying detailed description are directed to merely exemplary embodiments of the invention. To maintain brevity, other embodiments of the invention, which use the principles of the present invention, are not specifically described in the present application and are not specifically illustrated by the present drawings.
FIG. 1 shows an exemplary circuit schematic of synchronous buck converter 100 including half-bridge 102, which is formed in a stacked half-bridge package according to an embodiment of the invention, and is thus also referred to as “stacked half-bridge package” 102 or “half-bridge package” 102 in the present application. Synchronous buck converter 100 also includes, driver integrated circuit (IC) 104, output inductor 106, and output capacitor 108.
Half-bridge package 102 includes control transistor Q1 and synchronous transistor Q2 (also referred to as “sync transistor”), high voltage input terminal VH, low voltage input terminal VL, output terminal Vout, control gate terminal VG1, and sync gate terminal VG2.
In half-bridge package 102, high voltage input terminal VH receives high voltage input VHI, which can be, for example, a direct current (DC) voltage. Low voltage input terminal VL receives low voltage input VLI, which can be, for example, ground. Also in half-bridge package 102, control gate terminal VG1 receives a control gate signal from driver IC 104 for driving control transistor Q1. Similarly, sync gate terminal VG2 receives a sync gate signal from driver IC 104 for driving sync transistor Q2. Driver IC 104 may drive control transistor Q1 and sync transistor Q2 utilizing any suitable method. As a specific example, in one embodiment, driver IC 104 can vary a duty cycle of the sync and control gate signals responsive to a measurement of output voltage VO of synchronous buck converter 100 to maintain a desired output voltage VO, regardless of changes in high voltage input VHI. It will be appreciated that in other embodiments, output voltage VO is not measured in synchronous buck converter 100.
Control transistor Q1 includes control source S1, control drain D1, and control gate G1. Sync transistor Q2 includes sync source S2, sync drain D2, and sync gate G2. In various embodiments of the present invention, control transistor Q1 and sync transistor Q2 can be any combination of an enhancement more transistor and a depletion mode transistor. For example, in one embodiment, control transistor Q1 and sync transistor Q2 are both depletion mode transistors. Control transistor Q1 and sync transistor Q2 can be P-channel or N-channel transistors. Also, control transistor Q1 and sync transistor Q2 can be field-effect transistors (FETs). In one embodiment, at least one of control transistor Q1 and sync transistor Q2 is a silicon FET. However, at least one of control transistor Q1 and sync transistor Q2 may also comprise a non-silicon FET or any other FET in general. In one embodiment, at least one of control transistor Q1 and sync transistor Q2 is a metal-oxide-semiconductor field-effect transistor (MOSFET). At least one of control transistor Q1 and sync transistor Q2 can also be, as an example, a high electron mobility transistor (HEMT), such as a GaN HEMT.
Control transistor Q1 has control drain D1 for connection to high voltage input VHI through high voltage input terminal VH of half-bridge package 102. Control transistor Q1 also has control source S1 coupled to output terminal VOUT of half-bridge package 102. Also, control transistor Q1 has control gate G1 coupled to control gate terminal VG1 of half-bridge package 102 for being driven by driver IC 104.
Sync transistor Q2 has sync drain D2 for connection to output terminal VOUT of half-bridge package 102. Sync transistor Q2 also has sync source S2 coupled to low voltage input VLI through low voltage input terminal VL of half-bridge package 102. Also, sync transistor Q2 has sync gate G2 coupled to sync gate terminal VG2 of half-bridge package 102 for being driven by driver IC 104.
In some embodiments, at least one of control transistor Q1 and sync transistor Q2 can be connected to a diode. For example, FIG. 1 shows optional diodes 110 and 112. In the embodiment shown in FIG. 1, diode 110 is coupled to control source S1 and control drain D1, such that diode 110 is in parallel with control transistor Q1. Similarly, diode 112 is coupled to sync source S2 and sync drain D2, such that diode 112 is in parallel with control transistor Q2. In some embodiments, at least one of diodes 110 and 112 can be connected with a reverse polarity to that shown in FIG. 1. In various embodiments, diodes 110 and 112 can be internal to or external to control transistor Q1 and sync transistor Q2. As an example, in one embodiment, control transistor Q1 and sync transistor Q2 are MOSFETs and diodes 110 and 112 are respective body diodes of control transistor Q1 and sync transistor Q2.
Control transistor Q1 and sync transistor Q2 are commonly included on respective dies (i.e. a control transistor die and a synchronous transistor die). The respective dies may include other components, for example, diode 110 may be on the control transistor die and diode 112 may be on the synchronous transistor die. These other components may also be provided external to the respective dies and can be, for example, on a different die.
Synchronous buck converter 100 illustrates one power converter, which can benefit from including stacked half-bridge package 102, in accordance with embodiments of the present invention. However, half-bridge package 102 is not limited to being included in a synchronous buck converter and can be included in other types of power converters.
In accordance with embodiments of the present invention, control transistor Q1 and sync transistor Q2 can be connected in a stacked half-bridge package, which includes stacked half-bridge package 102, while achieving a small footprint with low parasitic inductance and resistance. Thus, the stacked half-bridge package can have a small form factor where the stacked half-bridge package does not significantly degrade performance of half-bridge 102.
FIGS. 2A-2C illustrate selective top views of stacked half-bridge package 202, corresponding to stacked half-bridge package 102 in FIG. 1, in accordance with one embodiment of the present invention. Various features of stacked half-bridge package 202 are not shown in FIGS. 2A-2C for clarity of description. FIG. 2D illustrates a cross sectional view of a portion of stacked half-bridge package 202, according to one embodiment of the invention. The cross sectional view shown in FIG. 2D corresponds to a cross section along line 2D-2D in FIGS. 2A-2C.
FIG. 2A shows control drain leadframe 220, control gate leadframe 222, common conductive leadframe 224, and sync transistor Q2 of stacked half-bridge package 202. Sync transistor Q2 in FIGS. 2A-2D corresponds to sync transistor Q2 in FIG. 1. Common conductive leadframe 224 is over sync transistor Q2. In FIG. 2A, dashed lines indicate portions of synch transistor Q2 that would not otherwise be visible, as they are covered by common conductive leadframe 224. For example, an outline of synch transistor Q2, which can be a semiconductor die, is depicted using a dashed line. Furthermore, sync transistor Q2 includes sync source S2 and sync gate G2, which are depicted using dashed lines. Sync transistor Q2 also includes sync drain D2, a portion of which is shown in FIG. 2A, as it is not fully covered by common conductive leadframe 224 in the present embodiment.
Sync source S2 in stacked half-bridge package 202 corresponds to sync source S2 in FIG. 1, sync gate G2 in stacked half-bridge package 202 corresponds to sync gate G2 in FIG. 1, and sync drain D2 in stacked half-bridge package 202 corresponds to sync drain D2 in FIG. 1. In the present embodiment, sync transistor Q2 has top and bottom surfaces, which are opposite one another. Sync drain D2 is on the top surface and sync source S2 and sync gate G2 are on the bottom surface.
Also in the present embodiment, sync drain D2 is provided with a sync drain pad that substantially covers an entirety of the top surface of sync transistor Q2. Furthermore, sync source S2 is provided with a plurality of sync source pads 228a, 228b, 228c, 228d, and 228e (also referred to collectively as “sync source pads 228”). Sync source S2 is exposed for electrical connection on a bottom surface of sync transistor Q2 via sync source pads 228 (See FIG. 2D) and corresponds to low voltage input terminal VL in FIG. 1. Sync source pads 228 of sync source S2 can include solderable front metal (SFM) that can be used for electrical connection to sync source S2.
In the present embodiment, sync gate G2 is also exposed for electrical connection on a bottom surface of sync transistor Q2 via a gate pad (See FIG. 2D) and corresponds to sync gate terminal VG2 in FIG. 1. Sync gate G2 can also include SFM that can be used for electrical connection to sync source G2.
It is noted that in some embodiments, sync drain D2, sync source S2, and sync gate G2 are not provided as shown in stacked half-bridge package 202. For example, sync drain D2, sync source S2, and sync gate G2 can be provided on different surfaces of sync transistor Q2 and can be provided using one or more pads arranged in any suitable manner. As one example, in some embodiments, a single sync source pad can replace sync source pads 228. It is further noted that in some embodiments, sync source S2 and sync gate G2 are not exposed for electrical connection as described above, and other layers or features can be connected to sync source S2 and sync gate G2.
Control drain leadframe 220, control gate leadframe 222, and common conductive leadframe 224 are electrically conductive and can comprise, for example, an easily solderable metal such as aluminum, and other solderable materials such as a metal alloy or a tri-metal. Common conductive leadframe 224 is over sync transistor Q2 and is electrically and mechanically connected to sync drain D2.
FIG. 2B is similar to FIG. 2A, with an addition of showing control transistor Q1 of stacked half-bridge package 202 over common conductive leadframe 224 and sync transistor Q2. Control transistor Q1 in stacked half-bridge package 202 corresponds to control transistor Q1 in FIG. 1. In FIG. 2B, dashed lines indicate portions of control transistor Q1 that would not otherwise be visible. More particularly, control transistor Q1 includes control source S1 and control gate G1, which are depicted using dashed lines. Control transistor Q1 also includes control drain D1, which is visible in FIG. 2B.
Control source S1 in stacked half-bridge package 202 corresponds to control source S1 in FIG. 1, control gate G1 in stacked half-bridge package 202 corresponds to control gate G1 in FIG. 1, and control drain D1 in stacked half-bridge package 202 corresponds to control drain D1 in FIG. 1. In the present embodiment, control transistor Q1 has top and bottom surfaces, which are opposite one another. Control drain D1 is on the top surface and control source S1 and control gate G1 are on the bottom surface.
In the present embodiment, control drain D1 is provided with a control drain pad that substantially covers an entirety of the top surface of control transistor Q1. Also in the present embodiment, control source S1 is provided with a plurality of control source pads 234a and 234b (also referred to as control source pads 234). It is noted that in some embodiments, control drain D1, control source S1, and control gate G1 are not provided as shown in stacked half-bridge package 202. Rather, control drain D1, control source S1, and control gate G1 can be provided on different surfaces of control transistor Q1 and can be provided using one or more pads arranged in any suitable manner. For example, in some embodiments, a single control source pad can replace control source pads 234.
Control transistor Q1 is over common conductive leadframe 224, which is electrically and mechanically connected to control source S1. Thus, control and sync transistors Q1 and Q2 are stacked on opposite sides of common conductive leadframe 224, with common conductive leadframe 224 electrically and mechanically coupling control source S1 with sync drain D2. Common conductive leadframe 224 is thereby serving as output terminal VOUT of stacked half-bridge package 202, corresponding to output terminal VOUT in FIG. 1. Advantageously, a short current path is formed between control source S1 and sync drain D2 that has low parasitic inductance and resistance (See FIG. 2D). As such, stacked half-bridge package 202 does not significantly degrade performance of half-bridge 102 in FIG. 1. Furthermore, stacked half-bridge package 202 has a small footprint that does not incorporate footprints of control transistor Q1 and sync transistor Q2 individually, thereby allowing for a small form factor.
Control transistor Q1 is also over control gate leadframe 222, which is electrically and mechanically connected to control gate G2. In the present embodiment, control gate leadframe 222 corresponds to control gate terminal VG1 in FIG. 1. Control gate leadframe 222 provides electrical and mechanical connection for control gate G1. FIG. 2B shows a majority of control transistor Q1 being over common conductive leadframe 224, with a portion of control transistor Q1 overhanging common conductive leadframe 224 so as to accommodate connection between control gate G1 and control gate leadframe 222.
FIG. 2C is similar to FIG. 2B, with an addition of showing conductive clip 230 of stacked half-bridge package 202 over control transistor Q1, common conductive leadframe 224 and sync transistor Q2. Conductive clip 230 comprises conductive material, such as a metal or metal alloy. In one embodiment conductive clip 230 is a copper clip. Conductive clip 230 includes web portion 230a and leg portion 230b and provides connection between control drain D1 and control drain leadframe 220. In FIG. 2C, a dashed line indicates a boundary of web portion 230a and leg portion 230b. In the present embodiment, control drain leadframe 220 corresponds to high voltage input terminal VH in FIG. 1 with leg portion 230b being over and electrically and mechanically connected to control drain leadframe 220 and web portion 230a being over and electrically and mechanically connected to control drain D1 of control transistor Q1.
As noted above, FIG. 2D illustrates a cross sectional view of a portion of stacked half-bridge package 202 corresponding to a cross section along line 2D-2D in FIGS. 2A-2C. FIG. 2D shows hermetic sealant 236, which is optional as indicated by dashed lines. In one embodiment, hermetic sealant 236 is a molding compound, which encapsulates stacked half-bridge package 202. FIG. 2D shows hermetic sealant 236 leaves common conductive leadframe 224, sync gate G2, sync source S2, control gate leadframe 222, and control drain leadframe 220 exposed on bottomside 240b of stacked half-bridge package 202 and thus, available for electrical connection.
FIG. 2D shows that in the present embodiment, common conductive leadframe 224 has web portion 224a and leg portion 224b. Leg portion 224b is of thickness 242, which is greater than that of sync transistor Q2 and web portion 224a. Furthermore, control gate leadframe 222 is of thickness 242, which is greater than that of web portion 224a of common conductive leadframe 224 and is greater than that of sync transistor Q1. Similarly, control drain leadframe 220 is of thickness 244, which is greater than that of web portion 224a of common conductive leadframe 224 and is greater than that of sync transistor Q1. In the present embodiment, respective bottom surfaces of sync transistor Q1, leg portion 224a of common conductive leadframe 224, conductive gate leadframe 222 and conductive drain leadframe 220 can thereby be substantially flush with one another, as shown in FIG. 2D. As such, stacked half-bridge package 202 can easily be mounted on and electrically connected to a substrate (not shown in FIGS. 2A-2D).
FIG. 2D also shows that conductive clip 230 is connected to control drain D2 at topside 240a of stacked half-bridge package 202. Leg portion 230b of conductive clip 230 is of thickness 244 that is greater than that of control transistor Q1 and web portion 230a. In the present embodiment, respective top surfaces of control gate leadframe 222, control drain leadframe 220, and common conductive leadframe 224 can thereby be substantially flush with one another, as shown in FIG. 2D. Thus, in some embodiments, control gate leadframe 222, control drain leadframe 220, and common conductive leadframe 224 can be formed from a shared leadframe. In one embodiment, common conductive leadframe 224 is an etched or half-etched leadframe that can be formed from the shared leadframe. Etching or half-etching common conductive leadframe 224 can be used to form leg portion 224b of common conductive leadframe 224. FIG. 2D also illustrates that common conductive leadframe 224 electrically and mechanically couples the control source (source of control transistor Q1) with the sync drain (drain of sync transistor Q2) to form the output terminal of stacked half-bridge package 202.
Thus, as described above with respect to FIGS. 1 and 2A-2D, embodiments of the present invention can provide for a stacked half-bridge package including a control transistor and a sync transistor. The control transistor and the sync transistor can advantageously be connected in a half-bridge package with low parasitic inductance and resistance. Furthermore, the control transistor and the sync transistor can be connected with the stacked half-bridge package having a small footprint. As such, among other advantages not specifically described herein, the stacked half-bridge package can have a small form factor, where the stacked half-bridge package does not significantly degrade performance of the half-bridge.
From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skills in the art would recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. As such, the described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein, but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention.

## Claims

1. A method for manufacturing a stacked half-bridge package, said method comprising:
providing a control transistor having a control drain, a control source, and a control gate;
providing a sync transistor having a sync drain, a sync source, and a sync gate;
stacking said control and sync transistors on opposite sides of a common leadframe, said common leadframe serving as an output terminal by coupling said control source with said sync drain.

2. The method of claim 1, wherein said common leadframe comprises a web portion and a leg portion.

3. The method of claim 1, wherein respective bottom surfaces of said sync transistor and a leg portion of said common leadframe are substantially flush with one another.

4. The method of claim 1, further comprising connecting said control drain to a control drain leadframe by a conductive clip.

5. The method of claim 1, wherein said stacked half-bridge package further comprises a conductive clip including a web portion that is coupled to said control drain and including a leg portion that is coupled to a control drain leadframe.

6. The method of claim 1, further comprising coupling a conductive clip to said control drain at a topside of said stacked half-bridge package.

7. The method of claim 1, wherein said sync source comprises a solderable front metal (SFM).

8. The method of claim 1, wherein said sync source is exposed on a bottom surface of said sync transistor.

9. The method of claim 1, wherein said sync gate is exposed on a bottom surface of said sync transistor.

10. The method of claim 1, wherein said control transistor has top and bottom surfaces, said control drain being on said top surface and said control source and said control gate being on said bottom surface.

11. The method of claim 1, wherein said sync transistor has top and bottom surfaces, said sync drain being on said top surface and said sync source and said sync gate being on said bottom surface.

12. A method for manufacturing a stacked half-bridge package, said method comprising:
providing a control GaN HEMT having a control drain, a control source, and a control gate;
providing a sync GaN HEMT having a sync drain, a sync source, and a sync gate;
stacking said control and sync GaN HEMTs on opposite sides of a common leadframe, said common leadframe serving as an output terminal by coupling said control source with said sync drain.

13. The method of claim 12, wherein said common leadframe comprises a web portion and a leg portion.

14. The method of claim 12, wherein respective bottom surfaces of said sync GaN HEMT and a leg portion of said common leadframe are substantially flush with one another.

15. The method of claim 12, further comprising connecting said control drain to a control drain leadframe by a conductive clip.

16. The method of claim 12, wherein said stacked half-bridge package further comprises a conductive clip including a web portion that is coupled to said control drain and including a leg portion that is coupled to a control drain leadframe.

17. The method of claim 12, further comprising coupling a conductive clip to said control drain at a topside of said stacked half-bridge package.

18. The method of claim 12, wherein said sync source comprises a solderable front metal (SFM).

19. The method of claim 12, wherein said sync source is exposed on a bottom surface of said sync GaN HEMT.

20. The method of claim 12, wherein said sync gate is exposed on a bottom surface of said sync GaN HEMT.