Bi-Directional Converters are the Driving Force for MHEVs

Driving demand

Increased electrification of vehicles is also driving demand for 48V systems. A Lithium-ion battery is more efficient and can last up to 10 times longer than a lead-acid battery. They also significantly increase the driving range of a vehicle, with a Lithium-ion batteries typically having capacity of 150 to 250 Wh/kg, compared with 30 to 70 Wh/kg for a lead-acid battery. They are also able to handle more charge cycles than lead-acid batteries, allowing more cycles per unit and are capable of fast charging, up to 100% in 30 minutes. Lithium-ion batteries also weigh up to 75% less than their lead-acid counterparts and can be stored in any position, making integration more flexible.

The combination of the benefits of Lithium-ion batteries and SELV requirements found in bi-directional DC-DC converters has led to them being specified more frequently. They are now becoming mainstream in 48Vdc electrical systems which need larger energy storage and faster charging times.

Charge times of the Li-ion batteries are also much shorter than those of lead-acid batteries. As a result, adding 48V battery banks to an existing 12V system for energy storage is now feasible and cost-effective.

Mild hybrid electric vehicles

Mild hybrid electric vehicle (MHEV) architectures use the internal combustion engine (ICE) with a 48V electric system, or powertrain, to augment the engine and components in converting the engine’s power into movement. MHEVs use an electric motor to support the ICE during acceleration and when the vehicle is cruising.

The 48V battery serves the vehicle’s active chassis, electric steering, air conditioning system, engine cooling, heated windscreen, the positive temperature co-efficient (PTC) heater – which provides a heat source in the absence of engine exhaust heat – and turbo charging.

A bi-directional DC-DC converter, sits between the 12V and 48V batteries. It is a key component in the operation of mild hybrid systems. 48Vdc electrical systems require large energy storage capacity and fast charge times. The converter charges the 48V battery pack while the car is driving. The alternator on the ICE supplies 12Vdc to the converter, which in turn converts the 12V to 48V and charges the 48V battery pack.

Buck-boost converter operation

The number of sensors integrated into even basic models continues to rise, together with passenger comfort systems (seat positioning and electric windows), as well as transmission and engine control systems which require in-vehicle communication. A controlled area network (CAN) is a serial communication bus developed to reduce the vehicle’s cable wiring, which contributes to the weight and therefore the performance and fuel efficiency of vehicles. A vehicle’s CAN allows the separate electronic control units (ECUs) to communicate with only a single pair of wires.

Although vehicle electrical systems are moving from 12Vdc to 48Vdc, some auxiliary electrical loads will remain at 12Vdc. Loads such as headlights, driving lights, interior lights, radios, infotainment, and computer systems still require a 12Vdc input and will need to be powered by a DC-DC converter that steps down the 48V battery voltage.

A conventional buck-boost DC-DC converter can operate in step-up (boost) or step-down (buck) mode, but can only manage power flow in one direction. The converter’s mode of operation is determined by the input voltage, compared to the output voltage.

A bi-directional DC-DC converter, however, has the capability to manage the flow of power in both directions between two DC sources (batteries). The operational mode is determined by the required direction of current flow.

Mobile Energy Storage

Let us look at an example based on a recreational vehicle (RV). Using the two-way current flow capabilities of a bi-directional DC-DC converter, a 48V battery system can easily be added to a standard 12V system. While the RV is being driven, the 12V electrical system will charge the auxiliary 48V battery pack using the bi-directional converter in the boost (step-up) mode. While the RV is parked, the converter can be directed to change operation to a buck (step-down) converter and power the 12V accessories and appliances in the RV.

Another example is in recreational marine environments. Here too, the different modes of operation of a bi-directional DC-DC converter can be optimized to ensure the 48V battery can supply power to the 12V electrical system

Powering the 12V batteries from the 48V auxiliary battery extends the amount of time the boat’s auxiliary loads such as GPS, depth finders, trolling motors, radios, and refrigerators can operate.

As with the RV example, an auxiliary 48V battery system can be added to the existing 12V electrical system on these marine applications.

Regenerative Braking

Efficiency is extremely important in electric and hybrid architectures because it has a direct correlation to the vehicle’s range; the more efficiently the vehicle (or RV or boat) operates, the longer range it will have. One way to minimize, or lessen, losses and thus improve the efficiency of the vehicle is by recovering energy used to propel the vehicle.

Conventional traction or dynamic braking systems expend excess energy as heat. In contrast, regenerative braking recovers energy. It slows down a moving vehicle by converting its kinetic energy into energy that can be used immediately or stored until needed.

Battery Equalization

Another challenge for power system designers developing MHEV systems. When both 12V and 24V loads are connected, the lower battery will supply power to the 12V and 24V loads, and the top battery will only power the 24V loads. This will drain the lower battery faster than the top one. Balancing the two batteries extends the operating lifetime of each battery.

Battery equalizers actively transfer energy between two batteries to maintain each at the same voltage levels. It will balance (or equalize) the charge and discharge of the two batteries, but in order to balance the charge currents between both batteries, an equalizer must have the ability to change current directions. A battery equalizer is a version of a bi-directional DC-DC converter. It has specialized programming and circuitry to handle the requirements of balancing the charge and discharge currents of each 12V battery.