# Electronic – Inductive kick-back voltage calculation

diodesinductivekickback

I am controlling a stepper motor (M42SP-5P) with BUZ11 MOSFETs. The winding resistances are \$22\Omega\$ and each winding is 18 mH as I measured. I am driving it with 24V, so let's say the drive voltage is about 1A. I know that the datasheet doesn't agree with me, however, this motor is M42SP-5P, not x-5K or x-5.

According to the MOSFET datasheet, turn off time is about 150 nS.

I want to pick a kick-back diode, however I am confused with the kick-back voltage calculation. The equation is:

\$v(t)=L*\frac{di(t)}{d(t)}\$

\$v(t)=18*10^{-3}*\frac{1}{150*10^{-9}}\$

That gives us 120 KV. This is insane and cannot be true, right? Where is my mistake in this scenario?

Also, which one of the following should one look for in the datasheet of a kick-back diode for this specific purpose?

\$I_{FSM}\$ : Non-repetitive peak forward current?

\$t_{rr}\$ : Maximum reverse recovery time ?

\$V_{RRM}\$: Maximum repetitive peak reverse voltage ?

Will 1N4148, BA157 be suitable for this application?