Automotive
Straight up – a better approach to cooling semiconductors
By Carlos Ramirez Ramos, product line director, automotive power discretes onsemi W
hile it may not be immediately apparent due to the relatively large size of vehicles, the space available for
technology solutions in vehicles is generally quite small and cramped. The primary reason for this is that most of the available space is given to the passenger cabin and electronic systems are tucked away in out- of-the-way spaces.
While this makes sense, it does make cooling these solutions challenging – especially as power levels are high in many automotive applications. Therefore, the industry continues to seek ways in which cooling may be improved, delivering a number of benefits to automakers and vehicle owners. In this technical article, onsemi will look at how innovations at the semiconductor package level are making huge strides in improving thermal management within modern automotive applications. As vehicles move to electric propulsion and many systems that were previously mechanical or hydraulic are replaced with electrical actuators, the amount of high-power conversion on modern vehicles is rising significantly. Huge efforts and significant budgets are being expended to increase the overall efficiency of these new electrical systems, primarily to increase the range of the vehicle.
However, greater efficiency has another benefit for the system designer, the generation of waste heat is significantly reduced. From a thermal management perspective this means that cooling techniques such as heatsinks can be reduced or removed entirely, thereby reducing the size, weight and cost of the solution. In fact, as any power engineer will know, the best way to remove heat is to not generate it in the first place. The second best thing is to ensure that any waste energy has as direct a path as possible to ambient air.
While wide-bandgap technologies such as silicon carbide (SiC) have made huge leaps in efficiency improvement, there is not (and
16 February 2023 Components in Electronics
www.cieonline.co.uk.uk
probably never will be) a power device that does not incur some energy loss.
Conventional approaches to cooling semiconductors
In power applications, metal–oxide– semiconductor field-effect transistors (MOSFETs) tend to be surface mount devices (SMD) such as SO8FL, u8FL, and LFPAK package types. SMD is the preferred technology as it gives good power capability and the convenience of automatic placement and soldering as well as making for a compact solution. However, the heat dissipation with SMD devices is not ideal because the heat propagation path usually travels through the printed circuit board (PCB).
In conventional components, the lead frame, including an exposed drain pad, is soldered directly onto a copper footprint on the PCB which provides an electrical connection and a thermal path from the die to the PCB. This is the only direct galvanic thermal connection to the PCB as the rest of the device is enclosed in a mold compound and cooled only through
Figure 2: Top Cool devices place the heatsink above, improving layout and thermal performance
convection to surrounding air.
With this approach, the efficiency of heat transfer away from the device is heavily dependent on PCB properties such as the copper (Cu) plane size, Cu layers, Cu weight, and Cu layout. This holds true whether the board is attached to a heatsink, or not. As a result of the restricted thermal path, the maximum power capability of the device is restricted as the low thermal conductivity of the PCB impedes heat dissipation.
The Top Cool concept
To address this issue, onsemi has developed a new MOSFET package that exposes the lead frame (drain) on the top side of the package. This approach brings benefits in both application layout / space, and in thermal transfer.
While the traditional approach to cooling power MOSFETs can deliver reasonably compact solutions, the underside of the PCB must remain unpopulated to allow for a heatsink to be applied. In this approach, a larger PCB is generally required to
accommodate all necessary components. As the thermal path is upwards with Top Cool devices, the heatsink is placed above the MOSFETs, allowing placement of components such as power devices, gate drivers and other components on the underside, thereby permitting a smaller PCB to be used. This more compact layout also allows for shorter gate drive traces which can be a benefit in high frequency operation.
Additionally, as there is no longer a requirement for the heat to pass through the PCB, the PCB itself will remain cooler and components surrounding the MOSFETs will operate at lower temperatures which will improve their reliability.
Alongside the layout benefits of Top Cool devices, there are also significant thermal benefits as the package allows for direct heatsinking to the lead frame of the device. The most used heatsinks are aluminium due to the high thermal conductivity (typically between 100 − 210 W/mk). Aluminium, or similar metals, greatly reduce the thermal resistance
Figure 1: Conventional cooling requires heat to travel via the PCB to a heatsink
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