Protection capacitors for stepper driver IC ?

Hello


We are driving a bipolar stepper motor from a 36V rail via a chopper-type stepper motor driver.
We are worried about damaging the stepper driver IC when the stepper motor is accidentally disconnected from the stepper driver PCB whilst the stepper motor is actually running.


(we are actually driving two stepper motors but I’ll just speak of one so as to make it easier to follow.)


Stepper motor phase current is 320mA.
-we are doing full-stepping and half-stepping

The stepper driver IC is on a PCB, and its connected to the stepper windings via a
4- way plug in connector.

Obviously, its possible to disconnect the stepper motor whilst its actually running by simply pulling the connector out.

I am advised that this causes damage to the stepper driver IC, -but I cannot see how?




Anyway, the previous engineer fitted 4n7 ceramic capacitors to the switching nodes of the H bridge stepper drivers, and said that these caps are needed in order to prevent damage to the stepper driver IC when the stepper motor is suddenly disconnected by pulling apart the connector whilst the stepper motor is running.

Here are the 4n7 caps (C9, C10, C14, C21, C37, C42 , C43 ,C44)………..

CIRCUIT DIAGRAM (with MC34920 stepper driver IC):
Bipolar stepper driver IC with 4n7 caps connected to switching nodes of internal HBridge:
http://i54.tinypic.com/23sc0eo.jpg


Internal view of stepper driver IC:…….
http://i56.tinypic.com/30bpxue.jpg

in the immediately above diagram, the 4n7 caps are connected to the points DR1A1, DR1B1, DR1A2, etc etc…..



Anyway, please notice that the “protection” capacitors are connected to the switching nodes of the H Bridge, ….
-this obviously means that a large current spike flows out of these caps and through the respective HBridge mosfet when that (lower) mosfet turns on…………
-I thought that this current spike would be damaging to the mosfet, but ignored it because…….

1. the transistors are mosfets and mosfets are better able to handle high current spikes than BJT’s for example.
2. There doesn’t “appear” to be a high number of stepper driver IC fails in the field.



Anyway, it gets more complicated here…
..because the MC34920 chopper style stepper driver IC has just gone obsolete…
…and so we are now being told to use the NJM2673 stepper driver IC instead.

-The point is, the NJM2673 stepper driver IC has internal HBridge transistors which are BJTs……
…...and I am doubtful if these BJTs will be able to handle the current spike that occurs due to the prescence of our 4n7 protection capacitors. (?)
-on the simulator, these current spikes have a peak of 4 Amps, and the NJM2673 BJTs are only rated up to 1A.

Here is the diagram of the NJM2673 Stepper driver:
http://i54.tinypic.com/s150y9.jpg

$$$ QUESTION: $$$
So, please could you tell me how we can protect against accidental stepper motor removal whislt running, without using these 4n7 capacitors?
-Also, do you suspect that putting capacitors on the switching nodes of the HBridge is a bad idea?

(even though the capacitors are connected to the switching nodes of the HBridge, they are actually positioned right next to the stepper motor windings connector)


………………………………….. ………….. ……………..
Here are relevant datasheets to this question:

NJM2673 Stepper driver IC datasheet weblink:
**broken link removed**

NJM2673 stepper driver DATASHEET to Download:
2shared - download NJM2673_E[1] _STEPPER DRIVER.pdf

Stepper motor datasheet web link: (SAIA UBB1N03M10)
**broken link removed**



Stepper motor datasheet web link: (SAIA UBB1N03M10)
2shared - download SAIA UBB.pdf

OLD Stepper driver IC (MC34920) (now obsolete)
2shared - download MC34920 freescale stepper driver.pdf

When the motor is energized some current I is flowing through the windings. The windings are inductive and store some energy = 1/2L(sqr I). When you disconnect the cable this energy MUST be dissipated (thats what causes arcing in switches and relays). To do this usually a circuit called snubber is required (I suggest your using a simple RC snubber, thats better than using only a capacitor). Take a look here... **broken link removed** or just google for snubbers.