Improving engine stop-start system design

Automatic engine stop-start is effective in helping reduce CO2 emissions from private cars. When integrated in an otherwise conventional combustion-engine power train, engine stop-start technology can deliver fuel savings of 5 to 15% at a relatively low incremental cost of about 300 USD. Offering worthwhile reductions in fuel consumption and emissions, at a price accessible to a significant proportion of car buyers, the stop-start vehicle is an important staging point in the transition to so-called mild hybrids or full hybrids and, ultimately, plug-in electric vehicles. Market analyst Yole Développement predicts strong demand for stop-start vehicles, rising from 5 million vehicles in 2012 to some 45 million in 2020. Electrical design challenges Automatic engine stop-start challenges several areas of vehicle electrical design. One is to protect systems such as the radio, climate control, GPS, and interior or exterior lights against supply-voltage fluctuations during engine cranking. The battery voltage can fall to as low as 6 V during cranking, whereas the electrical systems require a stable supply, or board-net voltage, of 13 V nom to ensure correct operation. Hence an additional subsystem is needed, containing a power switch and associated control circuitry, to disconnect the battery when the engine is cranking, allowing an auxiliary battery or DC-DC converter to supply the loads temporarily. When the vehicle is operating normally, the power switch contained in this subsystem must supply all electrical loads in the car. Consequently, low conduction losses are imperative because the switch is on at all times except when cranking. This calls for a power MOSFET rated for continuous drain current on the order of 240 A and having low on-state resistance of about 1 m?. Connecting several devices in parallel can achieve a further reduction in on-state resistance. Because the battery must remain connected when the car is off, the power switches and the controller, combined, must have a low quiescent current to minimise battery drain. A quiescent current of around 50μA is acceptable. Controlling the power switch As soon as the vehicle's auto-start function activates engine cranking, the main battery voltage begins to fall. The power switch must turn off quickly to allow the auxiliary supply to maintain the board-net voltage at 13 V. This is important, because the low resistance of the power switch can allow a high current to flow to the main battery side with a voltage drop of only a few mV. The turn-on time is less critical because the current will flow in the power switch's body diode. A controller, or gate driver, is necessary to turn on and turn off the power switch. A suitable controller must be capable of providing a gate voltage of 12 V to 15 V when operating from the low main battery voltage. Additional circuitry performs voltage and current monitoring, on- and off-time control, fault diagnostics, and thermal protection. Designing a driver having low quiescent current and meeting all these requirements, using discrete components, is challenging. Some integrated gate-driver ICs are available, which not only simplify design but can also help increase reliability. However, many are primarily for such applications as mobile phones or PDAs. IR's AUIR3240S is an example of gate driver ICs for automotive engine stop-start applications. It contains a voltage converter capable of operating from an input voltage in the range 4 V to 36 V and provides a MOSFET-gate drive voltage of 12.5 V. The company optimized the device architecture for intermittent operation, reducing current consumption to less than 50 μA. The gate driver requires only a few external passive components to complete the design: an inductor and a capacitor for the boost converter and resistors for the IC's diagnostic and measurement circuitry (Figure 1).

 

Key functions include the main DC-DC switch, K1, and freewheeling diode, D, and two comparators to control K1's state (Figure 2). The uppermost comparator monitors the gate voltage compared to VCC, to turn K1 on. The lower comparator monitors the voltage across the shunt resistor connected to the RS pin, which controls the turn off

 

 

A monostable timer with a typical interval of 7.5 μs guarantees a minimum boost converter off time. It activates when the inductor current reaches the peak current fixed by the shunt resistor (Figure 3). The driver operates at variable frequency. K1's on time depends on the values of supply voltage, shunt resistor, and inductor. The output current fixes the off time. When the output voltage is below 12.5 V, the driver turns on the power MOSFETs and the monostable timer's off time, Toff, fixes the frequency. In this mode, the driver can provide a relatively large currentâ€"on the order of tens of mAâ€"to charge the MOSFET gates quickly and turn on the power switch. The stop-start system can achieve maximum output current by optimising the values of the inductor and the shunt resistor. Once the gates of the power MOSFETs charge above 12.5 V, the driver enters a low-current-consumption mode. Then the driver activates only when the gate voltage falls below 12.5 V. With this architecture, the driver can hold the power switch on, with very low current consumption. Gate discharge is mainly due to the driver's leakage current.

Safety and temp monitoring Since the power switch supplies all the electrical loads in the car, it is a safety relevant function and must be able to detect failures. The AUIR3240S provides two diagnostic mechanisms for monitoring both correct output current and excessive system temperature. By adding an RC filter to resistor RS, the system can monitor the average output current (Figure 1). The values of current flowing in the output and at the RS pin link by

 

 

 

In low-current-consumption mode, with current of 50μA and RS equal to 10 ?, the voltage reading will be 0.5 mV. If a short circuit is present at the output, the driver will try to regulate the output resulting in a voltage of several hundred mV depending of the values of L and RS. Additionally, when the system operates to turn on the power MOSFET, a peak current will charge the gate. By monitoring the current during the turn-on, the system can detect continuity between the MOSFET and the output. The driver provides an NTC interface allowing the system to monitor the MOSFET die temperature, using one standard resistor and one NTC device located close to the MOSFET's source. The circuitry operates as a current mirror between the NTC_EN and NTC pins. The ratio is typically 2:1 so 500 μA must flow in the NTC_EN to achieve a current of 1 mA in the NTC pin. R_Ntc and V_Dg_In determine the NTC current. For example, 7 k? and 5 V will result in an NTC current of 1 mA. International Rectifier