Have you ever wondered why a car’s city fuel economy is always lower than its highway fuel economy? Most cars spend a lot of time stopping or traveling at low speeds, especially in a city. Start-stop technology helps with the former condition; if the vehicle is not in motion, why should the engine be on and waste gas? The latter condition of low-speed driving still presents an efficiency problem because your standard gasoline engine is not efficient at low speeds.
As Figure 1 shows, the engine produces lower torque during low-speed operation. More time spent at low speeds results in more inefficient driving and lower fuel economy. Even vehicles with start-stop technology can still have 30% lower miles per gallon when driving in the city vs. the highway.
Figure 1 An internal combustion engine’s general torque/speed curve shows how a vehicle produces much lower torque during low-speed operation. Source: Texas Instruments
Mild hybrid electric vehicles (MHEVs) provide that low-speed efficiency boost and here the starter-generator is the component at the heart of the MHEV design (Figure 2).
Figure 2 A starter-generator provides a low-speed efficiency boost in MHEVs. Source: Texas Instruments
Why starter generators are fundamental to MHEVs
Electric motors such as the starter-generator provide a huge benefit compared to internal combustion engines; they offer incredible torque at low speeds. An electric motor can actually output torque even while completely stalled, which is impossible for a traditional engine. Adding this electric motor in an MHEV will move the car from a stop up to low speeds—motor assistance mode—before starting the engine and allowing it to propel the vehicle.
The electric motor can also generate power—in generator mode—to store in the car battery when the vehicle is coasting or decelerating, and use that energy during the next start from a stop. The starter-generator combines the function of the engine starter and alternator into a single component, which reduces the vehicle weight.
How large is the starter-generator motor? First, picture what it needs to do: move a vehicle weighing between 3,000 and 7,000 lbs or 1,360 to 3,175 kg with a full complement of passengers and cargo, from a standstill up to a speed of around 5 mph (8 kph) within a few seconds. It’s no small feat, as anyone who has pushed a broken-down car will painfully tell you. Starter generators need to be able to output power from 5 kW to 30 kW, or even more.
Such a high-power requirement necessitates a 48-V battery, which makes the MHEV unique. A power of 30 kW coming from a standard 12-V battery amounts to an eye-watering 2,500 A (P = V × I). You would need about eight parallel wires of 0000 AWG—0.46-in or 11.7-mm diameter wire—to drive that current. If you use a 48-V battery instead of 12 V, 30 kW is possible with a “mere” 625 A, reducing the current and associated wiring thickness by a factor of four. The power requirement for such a large electric motor requires a new system power topology for MHEVs.
Figure 3 A starter-generator system comprises the motor, MOSFETs, gate driver, microcontroller, and other components. Source: Texas Instruments
Figure 3 shows the typical block diagram of such a system comprising the motor, MOSFETs, gate driver, microcontroller and some other components. As a design use case, an example of a motor spinning has been explained in a training video for the DRV3255-Q1 evaluation module.
Challenges in a high-power starter-generator system
A starter-generator motor functions in both motor assistance mode and generator mode, which results in some system challenges. Any electric motor can either convert electrical power (voltage and current) into mechanical power (torque and speed), or convert mechanical power back to electrical power. The microcontroller controls motor assistance mode and sends a command for the motor to apply a torque. Unfortunately, the generator process can occur under both controlled or uncontrolled circumstances.
If the vehicle is in motion and the starter generator is spinning without actively being controlled, the motor will generate electrical power and supply it into the 48-V battery. The MOSFET body diodes act as a rectifier circuit for the AC signal coming from the motor. It’s also known as “coasting” the motor. As mentioned earlier, this is a high-power motor spinning with the momentum of a heavy vehicle, so the amount of energy that can be generated is very large.
In the case of motor coasting event, the supply voltage will increase, given the current being generated by the starter generator. We call this phenomenon supply pumping since the motor’s back-EMF will supply current back to the battery. The problem that this poses to system designers is how to make sure that the supply does not increase to the point of system damage or failure. You can see the standard voltage levels for a 48-V system in Figure 4.
Figure 4 Voltage levels for a 48-V system are specified in ISO 21780. Source: Texas Instruments
Some of the most dangerous circumstances for these motor systems include the motor shutting down when it’s spinning at full power, as the system is handling a lot of energy and you cannot simply send it back to the battery. In the case of a fault condition, such as overcurrent, you can’t necessarily turn off all of the MOSFETs as you could in a lower-power system. While you do need to turn off MOSFETs that are experiencing faults, it’s important to control all of that motor energy such that it does not all go back into the battery.
How ASC can help prevent overvoltage events
The concept of active short circuit (ASC) is quite simple: it employs a “brake” mode to prevent supply pumping. In coast mode, the motor energy returns to the supply through the MOSFET body diodes, which causes the supply voltage to rise as the energy charges the bulk capacitors on the supply. In brake mode, the MOSFETs “short” the motor and prevent current from flowing into the supply. If you assert brake mode instead by turning either the high- or low-side MOSFETs on, the current will recirculate through the MOSFETs rather than returning to the supply.
It’s possible to implement ASC in a system with discrete components, as illustrated in Figure 5. A voltage monitor on the 48-V battery keeps tabs on the supply voltage. If the voltage goes too high, the circuit acts to turn on either the low- or high-side MOSFETs, contrary to the gate-driver commands. Once the high- or low-side MOSFETs are on, the generated energy is shunted away from the battery and prevents further supply pumping.
Figure 5 A discrete implementation of ASC is a viable design option. Source: Texas Instruments
For designers who have to tweak and tune the gate-drive setting for ASC in this discrete case, it involves trying out different series gate resistor values to see how to best protect the system. That includes soldering and unsoldering them onto the board after every test. It’s also important to know which MOSFETs to turn on into a brake state; if a system fault occurs and damages the high-side MOSFET, you cannot turn on the low-side MOSFET in that phase. It will lead to shoot-through. A high-side braking scheme is the best solution in case of damage to the high-side MOSFETs.
For an integrated design approach, take the example of the DRV3255-Q1 gate driver, which integrates ASC functionality, eliminating the need for a discrete implementation. The gate driver includes an adjustable voltage monitor to keep tabs on the supply voltage and automatically turns on either the high- or low-side MOSFETs to enter a brake state and prevent any further supply pumping. The ASC functionality is available not only while the device is active and powered, but also when it is in a standby state.
The gate driver also includes an integrated voltage monitor to watch the supply voltage and automatically turn on the high- or low-side MOSFETs to enter a braking state and prevent any further supply pumping, even without any input commands from the microcontroller. Dedicated nFAULT1 and nFAULT2 pins report failures on either the high- or low-side MOSFETs, which quickly provides the system microcontroller the information that it needs to configure ASC to apply brake on the high- or low-side MOSFETs.
The DRV3255-Q1 gate driver can even integrate the decision between high- and low-side braking on its own using the internal logic and voltage monitoring. This level of diagnostics and protection provides a huge benefit to system designers attempting to create new 48-V starter generator systems for three reasons: reduced design complexity, less firmware overhead and a smaller solution size.
Find out more about vehicle electrification, 48-V systems and MHEV motors in the below articles:
- Cars Get a Jolt from 48 Volt
- 5 Myths Shattered About 48-V Systems
- How Vehicle Electrification is Evolving Voltage Board Nets
- How to Optimize a Motor-Driver Design for 48-V Starter Generators
Adam Sidelsky, applications engineer at Texas Instruments, is author of Signal Chain Basics blog # 173 for Planet Analog.
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