Drill Teardown

So my latest project is a teardown of a Milwaukee 2604 Cordless drill.  I picked up a used drill on eBay and proceeded to disassemble.

The drill contains 3 PCB’s: power, logic, and motor sense.

The power stage is shown below.  The ribbon cable carries the gate and current sense signals, while the battery and motor phase wires are soldered directly to the board.  The FET's are back-to-back, using a single insulated heatsink.

A single phase-leg of the inverter is shown below.  The high-side uses a zener pair in series to function as a double-sided voltage clamp.  It's easy to see how the FET's gate voltage could exceed 15V w/o the clamp by imagining turning on the gate and floating the source or vice versa.

The gate resistors are 100Ω and 510Ω for the low and high-side, so with a 15V gate driver the currents are 150mA and 30mA respectively.  I'm not completely sure why the variation, my guess would be to limit the amount current the zeners might see.  The logic board is shown below, with a good amount of urethane potting to keep out dust and moisture.

After a healthy soaking in Strypeeze it softened up and was removed.  This drill uses TI's drv91670 chip, a MSP430 core w/ integrated 3-phase driver, what?!? 2 chips in one, how did I not know about this one?  Well according to TI it’s ‘not a broad market distribution item and information regarding this part is not made publicly available.’

The motor sense board is pretty straightforward, 3 hall sensors and a temp resistor.  The individual motor coils (2 for each phase) are soldered directly to the PCB, as are the motor phase wires.

More down here..