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Power supplies for car start/stop systems

28 Oct 2013  | Mark Scholten

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In order to minimise fuel consumption, many automobile makers are incorporating the “Start/Stop” function into their next generation of vehicles, many of which are already on the road. These systems turn off the engine when the vehicle comes to a stop and then automatically turn it back on when the foot is moved from the brake pedal to the accelerator pedal – or, in the case of a manual transmission, when the clutch pedal is depressed to re-engage gear. This helps reduce fuel consumption in city driving and stop-and-go rush hour traffic.

Such systems introduce some unique engineering challenges to the vehicle's electronics however, since the battery voltage can drop to 6.0 V or lower when the engine re-starts. In addition, typical electronic modules have a reverse polarity diode included to protect the electronics in the event the car is jump-started (bopsted) with the jumper cables accidentally reversed. This diode causes another 0.7 V drop in the battery voltage, leaving 5.3 V or less for down-stream circuitry. With many modules still requiring a 5 V supply, power sources are simple running out of headroom to operate properly.

Boost supply
One approach is a boost supply. This takes a lower input voltage and creates a higher voltage on the output. Many suppliers are currently employing some type of boost supply at the front end of electronic modules in order for them to operate properly through dropout conditions caused by the Start/Stop system.

As with most engineering problems, there are a number of ways to solve an issue. If the battery voltage only drops down to 6 V at the input, then the first and simplest solution is to find an extremely low drop-out linear regulator that requires less than 0.3 V of headroom. This can work for modules with lower current requirements, but one soon runs out of options for modules with higher current needs.

An alternative approach is to replace the standard P-N junction diode used for reverse battery protection on the front end with either a Schottky diode or a P-channel MOSFET. A Schottky diode has about half of the forward voltage drop of a standard rectifier, so it adds a few more tenths of a volt to the headroom. Changing to a Schottky diode is straight-forward enough, since it typically fits right onto the same PCB pads as the standard diode, eliminating the need for layout changes.

Figure 1: Reverse battery with P-FET.

The P-channel MOSFET however requires a PCB change, along with some extra circuitry.

Figure 1 illustrates the three components that are required; a P-FET, a Zener diode, and a resistor. The P-FET needs to be sized so that it can handle the voltages applied to the input of the module, along with the load currents required. In addition, it is important to consider the thermal requirements of the system, since the power dissipation in the FET is the current squared times the on-resistance of the FET. The Zener diode protects the MOSFET's gate oxide from damage due to over-voltage conditions. Most P-FETs can handle between 15-20 V from their gate to source connections, so the Zener has to be chosen to clamp before this point. The resistor pulls the gate down to ground to turn on the P-FET, but it also must be sized appropriately.

It cannot have too little resistance, because then too much current will be allowed to flow through the Zener – thereby creating power dissipation issues for it. However, if it gets too large, then the P-FET may not turn on as hard as preferred, and the whole idea for this scheme is to reduce the voltage drop across the drain to source connection.

Voltage dips to under 5 V
It is likely that one, or a combination, of the above schemes, will work for a given application. But what happens if the input voltage actually drops below 5 V? Some manufacturers are looking at 4.5 V during "cold cranking" conditions. If this is the case, then a switching power supply needs to be used in order to boost the input voltage. The three most common switchers are the boost supply, the buck/boost supply, and the SEPIC power supply.

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