Inverter, Converter, and Power Electronics
The three boxes
The traction inverter converts DC bus voltage to three-phase AC using six switches (IGBTs or SiC MOSFETs) with freewheeling diodes, driven by gate drivers and controlled by PWM. The DC-DC converter bucks HV down to roughly 13–14 V for the 12 V system. The on-board charger rectifies AC, corrects power factor, and boosts to pack voltage. SiC devices switch faster with lower losses; IGBTs remain common.
Energy stored on the bus
The DC bus capacitor holds a dangerous charge after the contactors open. Inverters have an active discharge circuit, but that circuit can fail - which is exactly why you verify 0 V at the inverter terminals instead of trusting a timer. The precharge resistor limits inrush into that capacitor; if it fails or is bypassed, contactor pitting and welding follow.
Diagnosis
Typical faults: DC bus overvoltage (often regen into a full or disconnected pack), overcurrent, gate-drive/desaturation faults, inverter over-temperature, and coolant leaking into the housing. Phase-to-phase winding resistance is very low - often under 0.1 Ω - so a standard DVOM cannot resolve it. Use a four-wire milliohm meter and compare phases.
Traps
Probing phase outputs with a DVOM and expecting a steady AC reading, when PWM output is not a sine wave; condemning an inverter for a fault actually caused by a bad resolver or chafed cable; forgetting the DC-DC converter can drain the pack on a vehicle that sits unplugged.
Worked example: a 350 V bus shows 0.042, 0.043, and 0.041 Ω phase-to-phase - balanced. Any phase to the aluminum housing must read open (megohms); a few ohms means a shorted winding or a chafed cable.