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Protection capacitors for stepper driver IC?

Protection capacitors for stepper driver IC?

Protection capacitors for stepper driver IC?

(OP)
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:
http://semicon.njr.co.jp/njr/hp/productDetail.do?_isTopPage=false&_productId=1690&_moveKbn=PRODUCT_DETAIL_MOVE_SPEC

NJM2673 stepper driver DATASHEET to Download:
http://www.2shared.com/document/xlTjxTHv/NJM2673_E1__STEPPER_DRIVER.html

Stepper motor datasheet web link:  (SAIA  UBB1N03M10)
http://www.johnsonelectric.com/mediando/index.php?IdTreeGroup=97&IdProduct=394



Stepper motor datasheet web link:  (SAIA  UBB1N03M10)
http://www.2shared.com/document/BIXACc2M/SAIA_UBB.html

OLD Stepper driver IC  (MC34920) (now obsolete)
http://www.2shared.com/document/w5v9IRE4/MC34920_freescale_stepper_driv.htmland
 

RE: Protection capacitors for stepper driver IC?

Loading the H-bridge with 4.7 nF capacitors isn't what I would do. Putting free-wheeling diodes from 0V to each output is better, but you will probably slam those diodes closed with the switching and that causes a lot of EMI (reverse recovery current being snapped off generates lots of HF).

The standard way of doing it is to use RC snubbers. It is much better than capacitors because the 'R' part reduces current while still reducing inductive kick-backs.

Are you even sure that there is a risk? Check what happens when you pull the connector and have a scope connected. If you break one of those ICs - so what? It takes a few eggs to make an omelette...

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Protection capacitors for stepper driver IC?

I agree.  Hook up a scope and then plug and unplug your stepper motor a few dozen times.  I can't think of any reason why that would be particularly hard on the driver.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Protection capacitors for stepper driver IC?

(OP)
Hi,

I see what you mean. Definetely.....

though i am wondering if the following is possible.....

The thing is, when the energized stepper is removed, the stepper's phase winding suffers a massive sudden high voltage....this breaks through the insulation of the phase winding, giving a stepper phase winding with significantly less inductance.................but , crucially, the same energy, since energy cannot be destroyed............

....so then the stepper phase winding current is massively high and flows through the stepper motor driver IC and damages it....

at least, this is what i presume happens.

do you agree its possible?


googling "stepper driver damage when running stepper motor removed" gives a load of links to articles warning of the danger to the driver IC of removing energized stepper motors.  

RE: Protection capacitors for stepper driver IC?

So, put your protection on the motor end.
<als>
 

RE: Protection capacitors for stepper driver IC?

(OP)
third paragraph down of the following warns about disconnecting a running stepper motor.............


http://www.sparkfun.com/products/9402


"Caution: Do not connect or disconnect a motor while the driver is energized. This will cause permanent damage to the A3967 IC"  

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