Methods for high current DC motor emergency cutoff

Question

I need to add a secondary means of cutting off power to a motor for safety reasons if a state change is detected on a 3.3V logic level signal while it is operating. The requirement I have is that it must be external to the existing system, fitting inline with the motor connector.

Currently, the brushed DC motor is driven bidirectionally with 12V (SLA battery, so realistically 11.5-13.5V) PWM using an H-Bridge onboard like the image below. This protection cutoff would essentially need to fit between the bridge and the motor, in the area of the red box

Right now I've considered a few solutions, but have the following concerns:

• High Motor Current: Nominally about 5A, but if stalled could reach ~50A for a short period before FW detects the condition or our 20A slow fuse goes. I'm seeking parts rated to ~75A to provide some margin.
• Bidirectional Drive: Whereas a high-side PFET may have been an easy solution if driven in one direction, bidirectional drive with the H-bridge makes a discrete solutions with FETs more complicated.
• Flyback voltage: Since it is driven bidirectionally with the H-bridge, a simple flyback diode can't be placed across the terminals. I'm unsure with how to mitigate flyback voltage from this cutoff without interfering with normal bidirectional operation. I'm also concerned that flyback will damage my cutoff circuitry, particularly if I use a semiconductor device

Here are some solutions I'm considering so far:

• MOSFET Discrete "E-Fuse"
• Integrated E-Fuse, or Load switch, possibly with external FET(s)
• Relays, Electromechanical or solid state
• Some sort of SPDT switch, which could be used to simultaneously cutoff power and short the battery terminals to circulate flyback current (though I haven't found any sort of high power component for this...)

Any feedback on my considered solutions or suggestions would be great. My main difficulty is that finding parts that work bidirectionally and handle adequate current has not been easy, so I'm wondering if I'm looking at the wrong types of solutions.

Answer 1

I see 3 solutions :

1. If you are allowed to, simply cut the power supply of the H bridge : it is still independent from your main system, and it's far easier (a mosfet or a relay is enough, and you don't have to worry about voltage spikes or energy dissipation, as this should already be taken care of by the H bridge itself). EDIT : I have just seen the comment you added that you don't have access to anything except ground, the signal and the 2 wires going to the motor. If it's really so, then it won't work. But if for example you have access to the battery, then you can cut the current right at the output of the battery

2. If you are not allowed solution 1), then you might consider using a relay. To avoid voltage spikes, you might put 2 zener diodes (with nominal voltage above the normal voltage of the supply) in serie (one in each direction) between the 2 sides of the motor: they will clamp the over-voltage. You can find zener diodes with very high current ratings. Just be carefull about power : if you have a lot of inertia, then the zener diodes are likely to overheat. If that's the case, you might use a relay with 2 position, and when you disconnect the motor from the H-bridge, you connect a power resistor between the 2 sides of the motor : the zener diodes will then only conduct for the short time until the relay finished to change position

3. How unlikely is that emergency stopping. If it is only theoretical, then a simple relay on one of the motors wire is enough : you will get voltage spikes, it's not unlikely that you brake something, but the motor is no longer powered. If you expect the likelyhood of using this emergency higher than 1%, then forget about this solution

Answer 2

If your 3.3 V line is able to provide at least about 2 mA of current, do the following:

Run the line through a photovoltaic isolator providing ~9V isolated voltage. Get two fat NMOSFETs back to back, sources tied together. Gates tied together. Turn them on with 9 V from the isolator.

When the 3V signal goes low, the MOSFETs switch off slowly and current cannot pass through the motor.

Advantage over a relay is that it is easier to drive and that the transistors turn off slowly. Depending on the size of the motor, due to the slow turn off, you won't need TVS in parallel. For a large motor you will though.

Answer 3

You could add a resistor (small value less than an ohm) to monitor the current of the bridge (between ground and the bridge). You will have to monitor this with an ADC or have a dedicated circuit to stop the drive to the H-bridge transistors in case of a high current.

Answer 4

Why don't you connect a fuse in series with the motor? In order for the fuse to open the circuit at 3.3V divide that by the resistance of the motor Rm and buy a fuse of the same rating with the current through the motor at 3.3V.