I don't really understand this as the actual Toshiba TB6560 chips have an absolute max VmA and VmB voltage rating of 40 volts and the operating rage is up to 34 V. Since it was the chip that blew and not something else on the board then I can see it was a pure over voltage problem providing your PSU was putting out less than 40V but preferrably less than 34 V. The Chinese data sheet for the whole circuit board states a voltage range of 16 to 24V for NEMA 23 spec steppers and 24 to 36V for NEMA 34 steppers. I don't really understand why there is a difference in voltage stated for NEMA 23 and 34 providing the current limit DIP switches are set so as not to exceed the particular Stepper motor current per phase for your wiring configuration (series or parallel). The power supply voltage selected just defines how quick the current limit value is reached for each pulse as it is driving an inductive load.

The when the current pulse is turned off the inductance of the winding tries to maintain current flow so generates a large reverse voltage. The protection diodes are there to clip this reverse voltage and absorb the current from the motor in discharging the winding inductance.

The larger your winding inductance the slower it will be before the current limit is reached when applying a pulse causing reduced speed and torque and also causes a bigger turn off reverse current as the inductance stores energy.

A good fast stepper will have low inductance and series resistance.

What is you stepper motor specifications Resistance, Current rating and Inductance in the wiring configuration you are using?