Thursday, May 3, 2012

48 V, what is so interesting this time?

By Jochen Langheim, STMicroelectronics

Most of us remember the discussion on the 42V in the nineties. There was a lot of excitement around the introduction of a second voltage level for on-board power supply besides 12V in passenger cars. With an ever increasing need for electric power, the fear was to reach too soon the limit of the on board generator to supply power to the devices such as air conditioning, heating or electric steering.
However, 42V didn't succeed, because engineers found ways to limit power consumption, implement intelligent power control, install buffers and Increase the capacity of on board generators to levels sufficient for a long time to supply energy.

What is different today?

There are needs for more power far beyond the present on-board needs of around 5 kW. Start-stop systems already push 12V technology to its limits with some kW. But they are only the beginning of further electrification towards mild hybrids. There, it will need something more powerfull in the range of 10 to 15 kW.

This would mean some 1000 A of current or an equivalent of cables with diameters of 10 to 15 mm. Heavy, costly, difficult to manufacture and install. In addition, each little resistance causes heavy losses according to P = R I²

And this leads to the second aspect, CO2.
Worldwide CO2 reduction and energy efficiency is a growing concern. The European Comission has adopted a very ambitious rule to reduce CO2 emissions. Limits are getting lower and lower and passing these limits will cost penalties, real money.
As an example: 3 g of CO2 above the limit of 120 g cost 45 € per vehicle. Avoiding this penalty can fund a nice efficiency improvement in an electric module leading to power savings of about 200W (or 3 g of CO2). Introduction of all sorts of innovations to reduce CO2 emissions become thus economically viable. Avoiding penalties can finance sophisticated start-stop systems, high efficiency generators, low loss air conditioning, drive train hybridization or... electric vehicles.
Finally, the definition of the 48V system specifies the maximum voltage below 60 V, which is the limit for LOW VOLTAGE, which means non-dangerous operation without specific security measures.

I want to emphasize at this point that the 12V system will not be replaced, but that there will be 2 on-board systems in parallel for 12 and 48 V linked by a DC/DC converter.

With the introduction of the 48V standard, every actor in the market can focus on the optimization of his products. Semiconductor makers can optimise their MOSFET and IGBTs, special connectors, capacitors, passives, etc. They will be able to better take into account the needs for load dump, EMC and automotive quality. Volume effects will lead to productivities that will further decrease the costs for innovation in these areas. Reuse of experience is increased.

What might be the consequence for electric mobility?

Imagine a drive system for a scooter with 48V and 5 to 10 kW. I would expect a nice acceleration for such a two-wheeler. Do you know the number of electric bikes and scooters in China? Tens or even hundreds of millions with 24, 36 or ...48V.
An ICE car with front wheel (conventional) motorisation can easily be equipped with an electric drive train on the rear wheels. Let's say 2 times 15 kW; makes 30 kW. As electric motors show more torque at low speeds, the drive feeling at low speeds can be compared to a car with 100 HP. A relatively easy and economic way to make a hybrid vehicle. Remember the cost advantage in combination with two-wheelers. Consequently one can also realise an electric vehicle with 4 units totalling 60 kW. Such cars are today studied all over the world.
 As a side effect, such a four-wheel drive system in a car can offer electric stability and anti-skid control without extra hardware. Furthermore the use of individual and small motors allows more flexibility in the desiign, which is and will be THE differentiating factor for car sales.

In conclusion, 48 V opens the door to electric driving with plenty of innovation at an affordable price.
48 V can initiate a real revolution!