In what situation does Toyota use a Doubler on the Main Pack?
My understanding is that the doubling is integrated in the Power Control Unit with the inverter. I've seen references to this starting in model year 2010. Here's some text from Toyota Global > Technology File > Hybrid >
Power Control Unit
The power control unit converts AC/DC power and appropriately adjusts the electrical voltage. Hybrid vehicles powered by motors are equipped with a power control unit that consists of an inverter, a boost converter, and a DC/DC converter.
The boost converter controls the voltages of the motor and generator. It steplessly increases the normal roughly 200 V DC supply voltage to a maximum of 650 V DC as required. This means more power can be generated from a small current to bring out high performance from the high output motors, enhancing overall power control unit efficiency.
There are references in this page to changes for 2015, but the voltage 'doubling' (the boost converter) is not flagged as new. A non-authoritative set of
Prius Specifications for 2004-2009 show a battery nominal voltage of 201.6 V and a motor voltage of "500V maximum". Authoritative
specs from Toyota (but for an unknown year, 2010 or later) show similar voltage levels.
There is little design difference between Toyota hybrid models of the same generation (other than the obvious transverse or longitudinal powertrain arrangement, and presence or not of an electric drive unit for the rear axle of AWD transverse models), so if a Prius is doing it, it is likely that everything else is.