Demand for higher power density drives innovative solutions for power tools

“The configuration of DC motors in power tools has changed significantly from brushed DC to more reliable and efficient brushless DC (BLDC) solutions. Typical brushed DC topologies such as chopper configurations usually implement one or two power metal oxide semiconductor field effect transistors (MOSFETs) depending on whether the bidirectional switch is used or not. On the other hand, a three-phase BLDC configuration requires three half-bridges or at least six field effect transistors (FETs), so the shift from brush current to brushless current means that the global power tool FET total regional market has increased by 3 to 6 times (see figure 1).

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Author: Texas Instruments Brett Barr

The configuration of DC motors in power tools has changed significantly from brushed DC to more reliable and efficient brushless DC (BLDC) solutions. Typical brushed DC topologies such as chopper configurations usually implement one or two power metal oxide semiconductor field effect transistors (MOSFETs) depending on whether the bidirectional switch is used or not. On the other hand, a three-phase BLDC configuration requires three half-bridges or at least six field effect transistors (FETs), so the shift from brush current to brushless current means that the global power tool FET total regional market has increased by 3 to 6 times (see figure 1).

Figure 1: Switching from a brushed topology to a brushless topology means a 6-fold increase in the number of FETs

But BLDC design puts forward new technical requirements on these FETs. For example, if a 6-fold increase in the number of FETs on a circuit board also means a 6-fold increase in the printed circuit board (PCB) area required to drive the motor, then the BLDC design is unlikely to still be suitable for manufacturers of electric tools and gardening tools. Power electronics are usually located in the handles of these tools; therefore, to accommodate the smallest hand sizes, applications are often greatly affected by space constraints (see Figure 2). The market needs solutions with higher power density, in other words, FETs that need to handle more current in a narrow space.

Figure 2: In most power tools, the Electronic equipment is located in the handle

Traditionally, FETs suitable for driving high-power motors have large and heavy packages, such as TO-220, DPAK and D2PAK. However, the latest quad flat no-lead (QFN) packages such as TI’s small leadless package (SON) 5mm×6mm FET can provide a smaller package resistance between the silicon chip and the source pin. Smaller resistance per unit area means less conduction loss per unit area, and it also means higher current capability and power density. Therefore, as the resistance per unit area (RSP) of FET silicon continues to decrease (roughly equivalent to half of each generation of products in the past), it is not surprising that QFN solutions in the power tools, garden tools, and household appliances industries have seen rapid growth. These smaller FETs are now generally capable of driving DC motor currents of up to 30A or higher; even for higher power designs, using multiple QFNs in parallel is sometimes more suitable for larger packages. After all, two 5mm×6mm devices are at 60mm²The total PCB area of ​​D2PAK is still only equivalent to a small part of the size of a D2PAK. The total PCB space of D2PAK is about 10mm×15mm, which is 150mm.²(See Figure 3).

Figure 3: PCB space required for the half-bridge (not drawn to scale)

TI recently attributed this trend to a logical conclusion by vertically integrating two FETs into a single package, providing the entire half-bridge in a SON 5mm×6mm power module. The 40V CSD88584Q5DC and 60V CSD88599Q5DC use the same stacked die technology as Texas Instruments’ low-voltage power modules for high-frequency power applications, while using optimized silicon to reduce conduction losses in high-current motor drive applications. In addition to reducing the parasitic inductance caused by arranging two FETs on the PCB, the vertical integration of two FETs can accommodate more silicon in the same package, thereby achieving higher power density than discrete QFN devices.

These devices also have a thermally enhanced DualCool™ package with an exposed metal top. Therefore, although there are still some circumstances, power tool manufacturers may prefer to use TO-220 FETs for surface mount FETs, because these FETs can be mounted on large heat sinks to remove heat from the PCB, but these power modules use The QFN package provides the same benefits. For example, even in the most ideal thermal environment, it is generally discouraged to dissipate more than 3W of power in a typical 5mm×6mm QFN. But through the proper use of heat sinks, these DualCool devices can cope with power consumption of 6W or more, the power density has doubled, and the PCB footprint has been reduced by half.

Nowadays, power density becomes the most important factor when introducing the more popular BLDC motors in power tools, garden tools, and battery-powered household appliances. TI’s new power module solutions can achieve this goal on an unprecedented level.