Powermanagement of a minicomputer in an automotive vehicle by engine start/stop and voltage threshold detection

Question

Revison 1.3 (New circuit, more background information added, PCB made and tested, it WORKS)

Background: I would like to use a BeagleBone Black (=Minicomputer) for data acquisition in an automotive vehicle. It will be plugged into the OBD interface and will therefore be supplied by the 12.0 V - 14.0 V battery power / alternator.

The following specifications were defined:

1. The minicomputer has to power on when the engine starts.
2. The minicomputer has to stay alive when the engine goes out (e.g. new start/stop system in new vehicles at traffic jams)
3. A clean shutdown (i.e. not cutting the power) should be made when no CAN or OBD messages have been received for 2 minutes.
4. After the BeagleBone Black shutdown the power has to be cut to the automotive battery. (i.e. no further power consumption)

Actual progress/solution (Thanks to Olin and Dave!)

Two separate circuits. The trigger voltage indicates if the main switch is open or closed.

1. Voltage sensing by TL431: When a voltage > 13.25 V was detected, the main switch will be opened by a P-Mosfet.
2. A NP-MOSFET switches the power supply for the BeagleBone Black. When the battery voltage is above a specific threshold (e.g. 13.25 V) it closes the P-Channel (engine started, vehicle battery ~ 14.0V, the BeagleBone starts up). When the BeagleBone Black starts up, the internal 3.3 V closes also the N-channel MOSFET (This will keep the BeagleBone on, also when the car engine is out)
3. When the engine is out, and the BeagleBone receives the poweroff system call, the BeagleBone powers down. The connection to the vehicle battery is cutted (No trigger voltage and no 3.3 V of the BeagleBone).

The circuit is shown below. Of course there is a protection circuit before and a step down converter after that part.

• MOSFET used: VISHAY SI4564DY-T1-GE3
• +12.0V = Vehicle Battery
• TRIGGER = When vehicle battery is > 13.25 V, this voltage will be pulled down
• VDD = Supply voltage for the BeagelBone Black (Goes into a step down converter)

Answer 1

What you are attempting sounds backwards. You want to detect higher than idle voltage, not lower. When the engine is running, the alternator raises the battery voltage to the float charge level. This is usually around 13.6 V, but can vary quite a bit. A threshold of around 13 V or a little higher is probably right. Measure your battery voltage with everything off and with the engine running, then pick a voltage in between.

The rest comes down to detecting this threshold voltage using very little current. Something like a TL431 and a couple precision resistors may be good enough. Carefully compute the error band to make sure it is acceptable. Then turning on power to something else when the TL431 triggers is trivial.

Answer 2

Assuming you really want the computer to start up on cranking rather than running, I would propose a circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

The zener diode D2 determines how low the vehicle power must "droop" before switching on the computer. The computer itself determines when to shut down — by pulsing the base of Q3 high.

This circuit draws no quiescent current. If you replace D2 with something like a TL431, there will be some current through its bias network.

The general idea is that the computer will fire up on any cranking event (or other transient load), and then it can evaluate conditions and decide whether or not to keep running.

Answer 3

I strongly suggest that you do a little bit of searching around automotive power supply design. There's a lot more going on than just a slightly (12-14V) power supply. A good starting point is Little Fuses's AN9312 app note which gives a good overview of what's going on.

For example a figure of 14V is mentioned in the original post but the alternator on my TDV8 L322 is regularly outputting 14.5V because there is so much demand on the battery from all the electrical systems. Similarly cranking a 3.6L V8 draws quite a bit of current, particularly over the winter, and it will easily drop to 6V, and these are not abnormal figures.

The main thing to be aware of is the rating on various devices. For example Q1 in the schematic above is connected "directly" to the battery and noisy circuitry. Care must be taken to chose a FET that is appropriately rated for the environment.

NB: there are devices (Linear produce some for example) that are specifically designed as automotive protection circuits and power supplies.