Back to the future – forward from the past?

  • Jun 18, 2020
  • UnitedSiC

Abstract

Electric vehicles seem to be the realization of decades of futuristic ambitions, but actually, in 1900 they outnumbered gas-powered vehicles by nearly two to one. Today, EVs are finally taking their place again as serious contenders in the vehicle market, enabled by advances in motors, batteries, chargers and the electronics that interconnect them. Power semiconductor switches are critical in the application and new advances in wide bandgap silicon carbide devices are enabling yet higher performance.

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An eye to the past

‘Flux capacitors’ have become a standing joke to represent futuristic technology but the writers of ‘Back to the Future’ in 1985 perhaps really were before their time? Magnetic flux in motors, and ‘capacitors’ in the form of batteries are enabling technology today’s EVs. Fortunately, they don’t need the De Lorean’s 1.21 gigawatts from a plutonium-powered nuclear reactor and 88 mph to operate.

It’s all about efficiency

Doc Brown would have appreciated that energy efficiency and consequent range of EVs have been their weakness from the middle of the 19th century when they first appeared as road transport. Even so, developments in battery and motor technology led to EVs outselling combustion engine cars in 1900 in the US by a factor of nearly two to one. With 38% of the road vehicle market, electric power was just outdone by steam at 40% but Henry Ford’s mass production of cheap gas-powered cars quickly turned the ICE into the 20th century’s dominant power plant.

Today, the desirability of the EV is without question; it is far more environmentally friendly and can have supercar performance with running costs a fraction of those of a gas-powered vehicle. The remaining barriers to wholesale adoption are initial cost, charge times and a nagging ‘range anxiety’. While purchase costs are slowly falling with economies of scale, improvement to battery charge time and distance you can travel with that charge are the focus of power electronics engineers’ efforts.

Chargers split into on-board and home/roadside types. Any inefficiency in the OBC translates directly to longer charge time, and power wasted must be dissipated in a larger, heavier cooling arrangement, affecting range. The roadside or fast home chargers allow much quicker re-charge but electrical inefficiencies here result in higher energy bills and bigger and costlier products, making them less economical to make widely available. Improvements to efficiency are therefore key to progress.

Wide band-gap semiconductors enable higher efficiency

The semiconductor switches in EVs until recently have been IGBTs, a technology that dates back to the 1960s. Devices have of course improved over the years, but they still suffer from high losses if switched at high frequencies. For this reason, EV motor drives operate around 10kHz which is hard to filter. This results in ripple on inverter drive outputs causing increased motor wear and lowered efficiency due to iron losses. EV drives also need to work bi-directionally to charge the battery when coasting or braking, requiring parallel diodes across the IGBTs which do not conduct in the reverse direction.

If the IGBTs are replaced with wide band-gap semiconductors, specifically silicon carbide FETs, frequencies can be pushed much higher, with better efficiency than IGBTs. Filtering is then easier, so motors operate more efficiently, and faster switching enables better control of the motor to help eliminate effects such as torque ripple which produces audible noise and motor wear. The SiC FETs also have an integral low-loss diode effect allowing reverse conduction, eliminating the need for a separate parallel diode. As a package, a bi-directional EV boost converter and inverter can be more efficient overall, smaller and lighter, all of which help to boost range. A major benefit is that SiC FETs are available in packages that can drop-in as replacements to many existing IGBTs for an instant performance boost. Devices are now available from UnitedSiC with on-resistances below 10 milliohms for 1200V-class devices and below 7 milliohms for 650V types, suitable for 800V and 400V battery systems respectively.

EV traction inverter using a 2-level voltage source converter architecture CHANGING IMAGE

The on-board charger might already use Si-MOSFETs switching at high frequency but SiC FETs for all stages give efficiency benefits, from the front end ‘totem pole’ PFC stage to the main bridge converter switches and even to output rectifiers which can be synchronously driven SiC FETs. Home chargers are smaller and more efficient with SiC FETs, saving energy. Fast roadside chargers can utilize SiC FETs in ‘Vienna rectifier’ three-phase front ends and SiC FETs can be paralleled with ease in the main conversion stage for powers up to hundreds of kW. The efficiency gains are again substantial resulting in energy savings and less cooling hardware, reducing costs and encouraging roll-out of more charging stations with consequent further economies of scale.

Electric vehicles are set to be the transport of the future with wide band-gap semiconductors such as SiC FETS the power semiconductor switch of choice. Doc Brown and Marty won’t have to travel very far into the future to see the past repeating itself, with EVs winning over gas-power with reservations about charge time and range a thing of the past.