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.