I'm struggling to understand how fast decay mode works on an h-bridge used for stepper motors . Particularly I've been looking at fast decay mode in this data sheet: http://www.ti.com/lit/an/slva321/slva321.pdf
What I'm struggling to understand is why when BH and AL are turned on, this doesn't cause damage due to the voltage that the motor coil would wind up putting out as the magnetic field collapsed. As far as I can understand it if we take V(motor-coil) = L * (change in current) / (change in time) if we have a stepper motor coil that is holding at 1A and then we start a step therefore switching which transistors are one, and say we turn off our transistors in 10ns (I just grabbed that from a random transistor datasheet) then wouldn't the voltage across the motor coil wind up being:
V(mc) = L * (0-1) / .000000001
V(mc) = L * -1000000000
So even if the inductance is something really small like 3 mH (taken from random NEMA 17 pololou data sheet) that would still cause a few thousand volt difference across the coil no?
If I look at the diagrams from that data sheet linked above, wouldn't that voltage be applied directly to the input voltage source and ground?
How does this not cause arcing or damage to components that may be up or down stream of this h bridge?
Best Answer
In 'slow decay' mode, the current flows through the two bottom FETs, which are on, and the motor voltage is near zero. The only voltage available to reduce current in the inductance is the voltage drop on the motor resistance and the FET resistance.
In 'fast decay' mode, the current flows through the FET body diodes to the rails, and from there into the power supply capacitors, briefly ramping the supply voltage up a little (called 'supply pumping'). The supply voltage is available to reduce the inductor current.
The body diodes are rated to the same current as the FETs themselves, so that they can be used for this purpose.
Supply pumping is a potential problem that has to be managed, to avoid over-volting the supply. This means at least one of using a much higher rated H-bridge than the supply volts, having sufficient reservoir capacitance, limiting the number of fast decay events, or having an active clamp across the rails to limit the peak voltage.