30 The Future of Automotive
paralleling SMT devices is a viable solution. If using two SMT
devices in parallel to replace one SMT device, each of the two
paralleling SMT devices should have twice the on-resistance
compared to using only one SMT device. In this case, the
current in each is halved, but on-resistance is doubled
for each, so dissipation is half of a single part. The total
dissipation of two paralleled SMT devices would be slightly
lower compared to using just one SMT device with half
the on-resistance. Thermally, each device would be much
cooler because for the same thermal management (thermal
resistance from junction to ambient or coolant), each
paralleling device only dissipates half the loss of a single
SMT device. So theoretically, each paralleling SMT device's
temperature rise from ambient- or coolant-to-junction should
also be half of a single SMT device. Apart from this, the lower
package inductance of the D2PAK-7L might allow faster
switching edge-rates for even lower dynamic losses.
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A surface-mount alternative would seem attractive, but at
the 22kW level? Actually, yes, it can be viable with UnitedSiC
(now Qorvo) D2PAK-7L devices with little or no effect on
performance, depending on the power conversion stage
considered. Looking at the headline differences between
the package styles in Table 1 above, the D2PAK-7L wins
except for die pad size, which results in an overall junction
to cooling fluid thermal resistance of around 1.3°C/W for an
18-milliohm device bonded to an insulated metal substrate,
about 30 percent more than the TO-247-4L package.
The practical effect of a higher thermal resistance is a higher
junction temperature for given power dissipated, all things
being equal, but because of the substantial assembly savings
with an SMT device, perhaps lower resistance parts can
be used, reducing temperature. However, if using only one
SMT device reaches thermal limits—Tj becomes too high—
Table 1: Comparison between D2PAK-7L and TO-247-4L. (Source: Qorvo)
