SMD package for PowerMOSFETs in automotive applications -- developments and trends

Ever since the first SMD PowerMOSFETs in SOP-8 packages started their career in electronics some 20 years ago, the demands made on PowerMOSFETs and their packages have increased dramatically with respect to performance and reliability. This is especially the case in the automotive segment. In present-day applications such as valve control in fuel injection systems or in new applications such as start-stop systems large currents have to be switched reliably. Moreover, even in the case of extreme ambient conditions, such as in the engine compartment, the device may under no circumstances be allowed to fail, because this might have catastrophic consequences and system failure.

Electrical and thermal properties

For applications with high power requirements, devices in SOP-8 packages will often not be suitable because of their lower current carrying capacity and their low thermal conductivity. In such applications, DPAK (TO-252) and D2PAK (TO-263) packages, with their superior electrical and thermal properties, have become a de facto industry standard. In order to meet the steadily increasing power requirements, these packages are being improved all the time. Fig. 1 tracks these improvements for DPAK and D2PAK variants developed by NEC Electronics.

Fig. 1: Increase in maximum current carrying capacity of DPAK and D2PAK variants

Inherent package optimizations such as improved leadframes and new bond technologies increase the current carrying capacity of DPAKs from 60 A to 90 A.The result has been that, in conjunction with NEC Electronics' 30 V UMOS-4 trench technology an on-resistance RDS(on)max of 3.2 mΩ can be realized in the TO-252ZP, making it possible to use DPAK devices in applications that previously had been the preserve of D2PAK devices. Moreover, by implementing comparable package optimizations as for DPAK and by increasing the number of external pins to 7, the TO-263-7 package can switch currents up to 180 A and reach RDS(on)max values of 1.5 mΩ for a breakdown voltage of 40 V.

SMD devices are cooled using the PCB itself; this requires detailed thermal management for the overall PowerMOSFET/PCB system. The channel temperature must be kept as low as possible, at any rate below the specified maximum value, to minimize power loss and increase reliability. The role of the package is to provide an efficient thermal path from the channel via the drain path to the PCB to abduct heat from the device. It may be necessary to modify the PCB to improve heat dissipation. For instance, the effective cooling area could be enlarged by introducing copper sections below the PowerMOSFET or by providing thermal vias to the PCB's underside.