Special Issue: Inside the Toyota Prius: Part 5 - Inverter/converter is Prius' power broker

The inverter/converter unit (ICU) is the ringmaster of all of the electrical conversion in the Prius. The hybrid's two motor generator units have distinct roles: MG1 recharges the high-voltage (approximately 200-V) nickel metal hydride battery pack located in the rear of the car and also applies direct power to drive the MG2 assembly. MG1 additionally serves as the electric motor used to start the gas engine portion of the power train. MG2 is the primary electric drive motor when energized and performs the reverse function to serve as the power generator during regenerative braking.

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Both MG1 and MG2 are permanent-magnet three-phase devices, providing torque when driven by ac power or providing ac output when rotated from outside sources (either the gas engine or the wheel rotation during braking).

To create the three-phase power for the motors, the dc battery source must first be stepped from 200 to 500 Vdc via a boost converter. A bank of insulated-gate bipolar transistors (IGBTs) with parallel diodes is mounted in bare-die form to a thermal plate, with connection to an ICU control board by way of feed connectors and ultraheavy-gauge aluminum wedge bonds for power interconnect.

With a boosted dc voltage available, the inverter is responsible for delivering the three-phase power needed in the MG1 and MG2 assemblies when they are used as motors. As in the boost converter, IGBTs are used for power modulation in the inverter. Again, a dedicated assembly plate supporting unpackaged transistor slices is used, with similar interconnect to the same controller board to which the boost converter is linked. The entire transistor/diode array assembly used by the boost converter and inverter is encapsulated in a gooey sea of protective silicone gel. Four of the six legs of the inverter have small current monitor assemblies to keep tabs on power delivery to and from MG1 and MG2.

This same power semiconductor plate used for the inverter supports a set of diodes, again mounted as bare chips, with one diode parallel to each of the IGBTs. The diodes are used during regenerative braking to rectify the ac output of the MG1 and MG2 assemblies so that after filtering and regulation (using the boost converter circuit in reverse), the recovered energy resupplies the high-voltage battery pack.

The Prius also uses an electric air-conditioning compressor motor so that cabin cooling is maintained even when running in electric mode only. A second dc/ac inverter, with circuits located on a second ICU controller circuit board ringed with TO-packaged IGBTs, is deployed to power the electric A/C compressor from the HV battery pack. The A/C inverter IGBT packages are bolted to one face of the substantial heat-sinking enclosure of the ICU. More on that shortly.

Since the Prius still needs a conventional electrical system to operate instrumentation, cabin lighting and the like, the ICU also supports step-down conversion from the 200-V NiMH battery to the 12-V subsystem, where a conventional lead-acid battery is used. Circuits for the dc/dc converter share space on the same circuit board that is believed to house the A/C compressor inverter, and TO-packaged devices are again bolted to the cooling plate of the overall ICU assembly.

The boost converter, MG1 and MG2 inverter, dc/dc converter and A/C compressor inverter all operate under the direction of the hybrid vehicle engine control unit.

With the large currents involved throughout the ICU, cooling of power semiconductor devices is paramount. The two sets of power components--both bare direct-mount slices and the TO-packaged parts--are mounted back to back on the metal case of the ICU. Heat transfer is achieved by way of a dedicated liquid cooling loop that runs through the ICU casing and is shared with the two MG assemblies.

Semiconductor content is critical in the ICU, but with the exception of a control 32-bit CPU from Renesas (the HD6437049), the IC content largely comprises switcher and inverter control components. An NEC µPC1099 switching regulator controller and NEC µPC494 inverter controller join with Toyota custom power control devices and a number of transistor drivers for implementing the active circuits. Toshiba GT30J324 and Renesas 2SK1517 N-Ch MOSFETs constitute the array of TO-packaged transistors, and Toyota custom IGBTs and diodes are used on the bare-chip power semiconductor plate referenced earlier.

Inductors, transformers, filter capacitors and a host of other passives complete the component set used in this impressive example of power engineering.

Much as the airflow, injectors, spark plugs and valves are the controlled aspects of the gas engine, the dc/ac, ac/dc and dc/dc converters are the managed elements of the electric drive train. Batteries and motors serve as rough parallels to the gas and mechanical pistons of the engine, and by teaming through the ECM and HVECU, the two systems create a fuel-efficient source of power to get you down the road.

David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)

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