CIRCUIT ASSEMBLY INCLUDING GALLIUM NITRIDE DEVICES

Abstract

A circuit assembly includes an insulated metal substrate (IMS), a switching device located on the IMS, and a printed circuit board (PCB) directly attached and electrically connected to the IMS with no gap or substantially no gap therebetween and including a cutout that receives the switching device.

Claims

1. A circuit assembly comprising: an insulated metal substrate (IMS); a switching device located on the IMS; and a printed circuit board (PCB) directly attached and electrically connected to the IMS with no gap or substantially no gap therebetween and including a cutout that receives the switching device.

2. The circuit assembly according to claim 1, wherein a surface of the PCB mates with a surface of the IMS.

3. The circuit assembly according to claim 1, wherein the PCB routes power and signals to the switching device.

4. The circuit assembly according to claim 1, wherein the PCB is electrically and mechanically connected to the IMS via solder pads.

5. The circuit assembly according to claim 1, wherein the PCB further includes negative-temperature-coefficient temperature sensing circuitry.

6. The circuit assembly according to claim 1, further comprising a heatsink attached to the IMS.

7. The circuit assembly according to claim 1, further comprising an L-shaped metal plate that is attached to the heatsink and that is in contact with a top surface of the switching device.

8. The circuit assembly according to claim 1, wherein the switching device is a gallium nitride switching device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 is a circuit diagram of a conventional totem-pole bridgeless PFC circuit using GaN devices.

[0025] FIG. 2 is a circuit diagram of a conventional three-phase, six-switch boost converter circuit using GaN devices.

[0026] FIG. 3 is a circuit diagram of a half-bridge LLC converter circuit using GaN devices.

[0027] FIG. 4 shows a conventional GaN circuit assembly.

[0028] FIGS. 5 and 6 show a conventional GaN circuit assembly using an Insulated Metal Substrate.

[0029] FIGS. 7A and 7B a circuit assembly including a PCB with a cutout and an IMS.

[0030] FIG. 8 shows solder pads on an IMS that provide electrical connection to a gate-driver circuit.

[0031] FIGS. 9 and 10 show a circuit assembly with separate switching-device PCB and gate-driver PCB.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] FIGS. 7A, 7B, and 8 show a circuit assembly with a PCB 10 with a cutout 11 and an IMS 20. FIG. 7A is a plan view of the circuit assembly, and FIG. 7B is an exploded view showing the IMS 20 and PCB 10 separated from each other. As shown in FIGS. 7A and 7B, the circuit assembly can include an IMS 20 with a PCB 10 attached to the IMS 20 with no gap or substantially no gap between the IMS 20 and the PCB 10 within manufacturing tolerances of the IMS 20 and the PCB 10. The opposing surfaces of IMS 20 and the PCB 10 can directly contact each other or substantially directly contact each within manufacturing tolerances of the IMS 20 and the PCB 10 so that there is no gap or substantially no gap within manufacturing tolerances of the IMS 20 and the PCB 10. FIG. 7A also shows locations to connect to the gates of switches S1 and S2 to provide gate driver signals GS1 and GS2 and to connect to the power connections +Vdc, Vdc, MID, where MID can be the connection between the top and bottom switches in each leg of a converter. These voltages and signals can be connected to the IMS 20 through the PCB 10. FIG. 7B shows that the IMS 20 can include switches S1 and S2 that can high-power switches such as GaN switches, and can include DC bus filter capacitors 21. FIG. 7B also shows that the PCB 10 includes a center portion that is cut out to define an opening or cutout 11 to fit around the switches S1 and S2, which can be, for example, GaN devices, and related circuitry on the IMS 21. The cutout 11 in the PCB 10 allows opposing surfaces of the PCB 10 and the IMS 20 to mate flush, or substantially flush within manufacturing tolerances of the PCB 10 and IMS 20, where there are no circuit components. Although one cutout 11 is shown in FIGS. 7A and 7B, it is possible to use more than one cutout.

[0033] For a high power density design, the IMS 20 can include copper because copper can provide better thermal performance with a smaller heatsink. It is also possible to use other materials for the IMS 20. The most used materials for the metal plate of the IMS 20 are aluminum and copper. An IMS 20 that includes aluminum can be more cost effective. However, the material characteristics of copper offer many advantages in terms of thermal and electrical behavior compared to aluminum. Furthermore, the thermal expansion coefficient of copper compared to aluminum is advantageous, especially in supporting highly reliable solder connections between the PCB 10 and power devices.

[0034] Because of the limited layout density of an IMS 20, a PCB 20 can be used to provide more copper layers to route signals including the gate driver signals GS1 and GS2 and power connections +Vdc, Vdc, MID to the main board to which the circuit assembly is attached (not shown). The connections to the main board can be provided by fingers or connectors on the PCB 10 that also provide mechanical support of the circuit assembly.

[0035] The layout design should reduce or minimize inductance of the high frequency AC current loop caused by the fast switching of the switching devices. Therefore, the cutout 11 in the PCB 10 is arranged so that the PCB 10 can be directly attached to the IMS 20 to eliminate the gap between the PCB 10 and the IMS 20. The electrical connections between the PCB 10 and IMS 20 can be provided by solder pads so that the PCB 10 can effectively become a surface mounted device. However, any other suitable method can be used to provide electrical connection between the PCB 10 and the IMS 20. FIG. 8 shows an example of the solder pads on the IMS 20. The solder pads used with the gates of switches S1 and S2, the power connections +Vdc, Vdc, and MID provide solder connections to corresponding solder pads on the rear of the PCB 10. The negative-temperature-coefficient temperature sensing circuitry NTC can also be included to monitor temperature and to provide over-temperature protection via signals NTCS. Portions of the power supply gate drive circuit can also be integrated with the PCB 10 if there is enough board space. Alternatively, the power supply gate drive circuit can be located on a second gate driver PCB (not shown) (with the PCB 10 shown in FIGS. 7A, 7B, and 8 as the first PCB).

[0036] A heatsink can be directly attached to the metal plate of the IMS 20 without electrical insulation between the metal plate because the metal plate has been electrically isolated from the gate driver circuit by thermal insulating layers. A thermal interface material (TIM) such as a grease or a phase-change thermal material with very high thermal conductivity can be used to reduce or minimize any air voids between the metal plate and the heatsink.

[0037] The cooling of the switching-device PCB 10 improves the overall thermal performance of the circuit assembly. Therefore, the thermal resistance of the switching-device PCB 10 needs to be reduced as small as possible to have the greatest effect on cooling. Copper-filled vias can be used in the PCB layout design that can significantly reduce the thermal resistance of the switching-device PCB 10. Reducing or minimizing the thickness of the PCB 10 can also help reduce the thermal resistance. A thickness of about 1 mm has been found to provide an acceptable balance between the thermal resistance and rigidness of the PCB 10. In this preferred embodiment of the present invention, the gate drive circuit of the circuit assembly is also integrated in the PCB 10 to reduce or minimize any looping of the gate driver signals GS1 and GS2.

[0038] Double-sided cooling can be applied to an IMS-based circuit assembly described with respect to FIGS. 7A, 7B, and 8. For example, a heatsink can be attached to the IMS 20, and a copper plate can contact the tops of the switches S1 and S2 through the cutout 11 in the PCB 10.

[0039] FIGS. 9 and 10 show another preferred embodiment of the present invention in which gate driver circuitry can be integrated into a gate driver PCB 30 and separated from the switching-device PCB 40 that includes the switching devices. As shown in FIGS. 9 and 10, the switching-device PCB 40 is attached to the heatsink 45. A TIM layer 44 can be used between the switching-device PCB 40 and the heatsink 45 to provide electrical isolation. If needed, a thermal grease or phase-change thermal material with very high thermal conductivity can be used to remove the air voids between the switching-device PCB 40 and the heatsink 45.

[0040] As shown in FIG. 10, surface-mount connectors 41 can be used to transfer electrical signals between the switching-device PCB 40 and the gate driver PCB 30, although other connection mechanisms are possible. As shown in FIGS. 9 and 10, an L-shaped plate 35 can be added to provide additional top side cooling to further improve the thermal performance. Although the L-shaped plate 35 can include copper, the L-shaped plate 35 can include aluminum, an alloy, or any other suitable material. As shown, the L-shaped plate 35 can include two cutouts 36 through which the surface-mount connectors 41 extend from the switching-device PCB 40 to the gate driver PCB 30 to connect to the gates of the switching devices. FIG. 10 shows that TIM layers 44 can be used between the heatsink 45 and the switching-device PCB 40 and between the switching-device PCB 40 and the L-shaped plate 35 to provide electrical isolation. A thermal pad (not shown) can be placed on the outer surface of the L-shaped plate 35 at an interface between the L-shaped plate 35 and a chassis of the power supply circuitry such that heat can transfer to the chassis. As such, thermal management can be more effective with the combination of different cooling methods, including force convection, conduction, and radiation. The thermal pad can extend along the L-shaped plate 35 at an interface between the L-shaped plate 35 and the gate driver PCB 30.

[0041] It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variances that fall within the scope of the appended claims.