Advanced power switches boost micro hybrid emissions gains

Start-stop operation of a car (shutting the engine when momentarily stopped) is a simple concept that can go a long way in improving fuel economy and cutting emissions.

Among around 80 million cars produced worldwide in 2011, the internal combustion engine (ICE) system remains as the predominant technology for vehicle propulsion. However, change is going to happen and is already on the way.

On one hand, the price of gasoline has been rocket high, which puts burden on every driver. On the other hand, legislation that governs emission standards is becoming more and more stringent in every country. In Europe, the CO2 emissions generated by vehicles are subject to a voluntary agreement between the EU and the car manufacturers, but the legislation has been pushed because the overall performance is still way off the voluntary goal. Moreover, the Euro 6 standard is gradually entering in to force that requires substantial reduction of oxides of nitrogen emissions. All these factors make the challenges facing the automotive industry much greater, with the bottom line for car manufactures to adhere to the standard.

Clearly reducing fuel consumption is one key to achieving these stringent requirements. Toward this, the hybrid electric vehicle (HEV) market will boom in the next ten years in all configurations: Micro, mild, full, and plug-in—not to mention the fully electric vehicle (EV). HEV/EV is expected to be a must in 2020 to meet CO2 emissions targets.

Figure 1, below, shows the EV/HEV annual demand forecast to 2020 in million units. Overall, the HEV/EV production will have a CAGR of +31% and will reach 50 million cars in 2020, which is about 50% of the cars produced—and most of this volume is projected by the analysts to come from micro hybrid vehicles.

Figure 1: EV/HEV annual demand is forecast to grow at 31% annually until 2020.

Hybrid types
The major difference between the micro hybrid and full/plug-in hybrid systems is whether or not it has the electric powertrain to propel the vehicle. The micro hybrid, also called start-stop system, shuts down and restarts the internal combustion engine to reduce the amount of time the engine spends idling, such as waiting at traffic lights or frequent stops in traffic jams. The mild hybrid has regenerative braking system in addition to start-stop feature. Fuel economy gains from these technologies are typically in the range of 5 to 10% compared to conventional vehicles.

HEV types by functions

Various types of start-stop systems are already available. The first type is the super starter, which uses a rugged DC starter plus a battery management system. It is estimated that this type represent two thirds of the start-stop market thanks to its low cost. With an average cost of $80, many car manufacturers, including BMW, are adopting this technology.

The second type of start-stop system is belt-driven alternator starter (BAS), which features a DC/AC inverter with the average power typically in 1.5 to 3kW range—and the system is quite silent. With the end user price estimated around $300, and the restart time as low as 400ms, BAS is found in many middle-class vehicles.

Finally, in cold conditions where the conventional start-stop types are not operational, a dual-battery solution or DC-DC boost solution could be used to maintain the bus voltage, the detailed operations of which will be discussed in the following sections.

Two batteries
Figure 2, below, shows the typical system diagram dual battery technology. When ICE is running, power switch Q1 stays on so that the load is fully supplied by main battery (Vbat) as well as an alternator. When a vehicle stops, the ICE will be turned off and only Vbat is the source of power supply to the load. At engine restart, the main battery Vbat also needs to supply a huge transient current as high as 1,000A to starter motor, which will result in a transient voltage drop at the Vbat terminal to as low as 6V. In order to prevent the power electronic circuit from shutdown due to the battery cranking transient event, a controller will send a turn off signal to Q1 to disconnect Vbat from the load. At this time the auxiliary battery (Vaux) will supplies power to the load and maintain the battery voltage.

Figure 2: Dual-battery switch technology in the micro-hybrid system uses an auxiliary battery to provide high starting currents for start-stop operations.

After engine restarts successfully and the alternator starts working again, Q1 is turned on and the system goes back to vehicle running mode. The power switch Q1 and the controller are also used as a part of reverse battery protection circuit. If the Vbat is connected in reverse polarity, Q1 stays off because of no signal coming from a controller—it protects a circuitry on the load by terminating a reverse current flowing path.

DC-DC boost
Another method, shown in Figure 3, below, is a similar configuration but uses a DC-DC boost converter instead an auxiliary battery. At engine restart, bypass switch Q1 disconnects main battery (Vbat) from the load and a DC-DC converter supplies a boosted voltage to the load during cranking period. DC-DC boost converter consists of one inductor, two power switches (Q2 and Q3), and one output capacitor.

Figure 3: DC-DC boost converter topology in micro hybrid system stores energy in the converter inductor.

All energy is stored at inductor when Q2 turns on (Q3 is off during that time), and then inductor transfers the energy to the load through Q3 when Q2 is off. Duty cycle of Q2 is determined by the voltage on main battery and the voltage on the load terminal. A PWM controller operates this type of synchronous DC-DC boost converter in continuous conduction mode to maintain Vload.