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A Boost for Automotive Start-Stop: Dual Use of EMC Components

Automobile manufacturers are increasingly turning to start-stop systems to glean an extra 3% to 12%* of fuel economy out of conventional vehicles. With this approach, the Internal Combustion Engine (ICE) is turned off during idle periods, such as at traffic lights or in stop-and-go traffic, and then automatically restarted when the driver depresses the accelerator pedal. To implement this feature, relatively minor changes are needed for the existing powertrain, such as a more rugged starter motor and battery combination to accommodate the more frequent starts.

But beyond the powertrain, key electronics modules may require modification to remain functional during engine restart when the battery voltage drops well below the nominal level. While drivers are accustomed to the radio and other electronics turning off momentarily when they manually start the vehicle, this will not be tolerated if it happens at every traffic light!

ON Semiconductor has created a new boost converter controller that is specifically designed for start-stop systems. The NCV8876 integrates many functional and protection features to support this application, for an efficient, low-cost/small-PCB-footprint implementation. This may also include reuse of the module’s existing EMC components.

The figure above is a reference design that is available in SystemVision® Cloud, a free on-line schematic capture and simulation platform. It shows the reuse of a typical module's EMC components (i.e., the “pi” filter, highlighted in green), which can serve a dual purpose as part of the boost converter. The NCV8876 is designed specifically for this application, requiring a minimum of new external components (highlighted in blue). These include the NMOS switch, the Schottky rectifier diode, and the 0.011 Ohm current sense resistor. They also include a few peripheral passives, such as the simple current-mode compensation network and the frequency programming resistor r9. This is used to adjust the switching frequency to accommodate the available EMC component values.

Simulation results show the output to the 5W load during battery drop-out and recovery. Note that in this application, the output voltage (red waveform) is maintained above 6.0V during the drop-out transient and regulates at 6.8V as the input voltage at the PCB drops to a minimum value of 3V (light blue waveform).

The second figure is an alternate reference design that is also available for simulation and design reuse in SystemVision Cloud. It is a more complex but higher efficiency implementation, with additional components highlighted in blue. It is intended for use with higher power loads.

This circuit includes a PMOS bypass switch, such that the rectifier diode voltage drop is not present under normal battery voltage conditions. A conventional diode can be used; no need for a Schottky in this version, because the load current flows through the bypass PMOS during normal operation. When the battery voltage falls below the “wake-up” level, the logic transition of the NCV8876 status pin provides an early indication that the boost switching is about to begin. This signal is inverted and level-shifted by the BJT circuit. This deactivates the bypass PMOS, forcing the boosted load current through the rectifier diode. When the battery voltage recovers and the output voltage rises, the status signal re-activates the PMOS bypass and normal operation resumes.

Simulation results show the output to a 20W load during battery drop-out and recovery. Note that in this application, the output voltage (red waveform) drops slightly below 6V momentarily during the fast drop-out transient, but then regulates at 6.8V as the input voltage at the PCB again drops to the 3V minimum value (light blue waveform). The bypass transition is also observed, with the PMOS gate voltage (purple waveform) and the Ids current (dark green waveform) shown in the lower waveform viewer. Note that no bypass current flows during boost switching.

Further insight into this design can be gained from the online version. The user can observe any other signal in the circuit, simply by moving one of the waveform probes to the desired net or component. He can also make a copy of the design, make any parameter or component changes desired, and then run a new simulation to see the effect of those changes.

ON Semiconductor provides a physical Evaluation Board that potential customers can buy and then test the device in various applications and operating scenarios. A free and equivalent “Virtual Evaluation Board” is available in SystemVision Cloud, where the user can test similar scenarios and get an early understanding of the part’s performance, as well as safely assess its protection features.

The author would like to thank Robert Davis, Automotive Power Supply System Architect at ON Semiconductor, for his contributions to both creating the high-fidelity model of the NCV8876, as well as providing key design insight for automotive start-stop applications.

*Reference

Do Start-Stop Systems Really Save Fuel?

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