Inverters fight for traction

When it comes to electric vehicles (EVs), battery technology makes most of the headlines. Automakers are on a well-chronicled quest to boost energy density and lighten up batteries that will power the next generation of EVs. But though it doesn't attract the fanfare, inverter technology is on the same do-more-with-less trajectory.

All EVs or hybrid electric vehicles (HEV) use solid-state inverters to convert the direct current from the batteries or generators to three-phase alternating currents that drives the propulsion motors. The critical power-handling semiconductor components in the inverter are insulated gate bipolar transistors (IGBTs), diodes, and a dc bus capacitor that suppresses voltage ripple and provides temporary energy storage.

When the vehicle accelerates, the inverter can get frying-pan hot. It typically must dissipate around 250 W of heat per power device at a heat flux reaching 300 W/cm² while keeping the chips below 125°C.

Heat removal technologies in this kind of environment are critical. But the first generation of thermal management hardware in EV inverters is relatively simple and unsophisticated. There is little attempt to save weight and space. The typical approach is to use passive heat spreaders and thermal grease between the power device and a heat sink. Problem is, this setup presents a high thermal resistance. As a result, current HEV electric power inverters circulate water-ethylene glycol coolant over the heat sinks at about 70°C. But this sort of dedicated cooling loop is complicated, heavy, bulky, and expensive.

That's important because inverters are in the “cannot fail, zero-tolerance” category of EV components. Another reason for concern about the cost of inverter electronics is that the inverter is lumped together with the EV battery when figuring the cost of the overall electric drive system. That may seem like a trivial point, but there is some thought in the industry that future government rebates and incentives could hinge on overall electric drive efficiency.

An indication of the trend is the latest Electrical and Electronics Technical Team Roadmap from the U.S. Dept. of Energy (DoE). It sets a high bar for manufacturers of EV components and vehicles. These requirements include getting the cost of the electric drive system to under $12/kW by 2015 and to $8/kW by 2020. Among other things, the roadmap also wants manufacturers to develop better coolants for handling higher temperatures, along with more efficient replacements for bulky electrolytic bus capacitors at the front end of the car's dc/dc converter and (dc/ac) inverter blocks.

Inverters are using advanced components and semiconductor technology to hit these goals. Some prototypes reportedly sport power modules fitted with silicon-carbide (SiC) and gallium-nitride (GaN) power semiconductor devices that have a high thermal conductance and thus can reduce system size to one-quarter of that today, but these aren't yet practical. The cost of the exotic power devices is still through the roof.

Silicon savvy

One way of reaching EV inverter roadmap targets is through novel topologies that reduce the power-handling demands on key components. An example is a Z-source inverter under investigation at Oak Ridge National Labs (ORNL). The “Z” moniker arises because the circuit uses what might be called an impedance-matching L-C network between the dc source and the switching devices. The L-C devices provide temporary energy storage. The components themselves can be physically smaller than the single capacitor storing energy in conventional V-source inverters. Elimination of this capacitor is important because it can account for over 20% of the cost and weight of an ordinary inverter, and about 30% of its volume, ORNL researchers say.

The Z-source inverter also is advantageous in that it can supply ac voltages exceeding the level of the dc source. The buck-boost action that makes this possible comes about through use of a “shoot-through” mode in the switching circuits, wherein both legs in one phase of the inverter bridge are on simultaneously. Conventional inverter designs guard against this condition because it can destroy the IGBTs in the bridge, and because it causes EMI. But the Z-source inverter manages the condition and uses it to realize the buck-boost operation.

The Z-source design at ORNL also eliminates any need for reverse-blocking diodes across the switching IGBTs, and it tolerates open circuits in the phase legs as well. Researchers say this could halve the footprint of the power modules used in the inverter.