[Matt Smith]

Hello,

I have just purchased a 5 x MMC boards for a PTZ project I am working on.

The pan/Tilt (Datasheet attached) unit we are using is rated at 24V 1.5A

My test board has 1 x MMC set up in analog mode and is driving both the pan and tilt motors.

I have not implemented any protection stated in AN703 as it was assumed that we would not be driving the system at any speed or load during the initial testing. (Perhaps this was a mistake?)

The MMC was working until the pan motor hit its limit switch (As far as I can tell from the PT unit this simply breaks the flow of current in that direction).
The MMC failed to drive the motor in the other direction giving an error signal from the LEDs.
(It is worth noting at this point that the bench supply was limited to 1.5A at this stage)

After this event the MMC would only drive negative voltages out of that channel. (0 to 5V input related to 0 to -24V on the output) The other channel worked as expected.

I swapped the motor outputs to check it was not the pan tilt at fault and managed to break the other channel in the same way.

I'm assuming I have blown the H- Bridge devices, unfortunately I am finding it difficult to source the Infineon parts, Any ideas where I can get them? (DigiKey are out of stock)

Can anyone advice where i'm going wrong? I would not have expected the MMC to Blow with a bench supply limited to 1.5A. The device didn't get hot it simply stopped working. I have tried to detect any back EMF in the system driving the motors directly but so for have seen nothing other than a 2.4A in rush (which would not have happened with the MMC testing because of the current limit on the PSU)

My current (back up) plan is to use 1 x MMC for each motor.

Any advice would be greatly received.

Kind regards,

Matt

Attached File(s)

•  DM_2000.pdf (283.86K)
  Number of downloads: 30

[Lon]

Matt,

Sorry to hear about the problem. It is likely one of two issues, or both. Hitting the limit switch could cause a transient voltage spike that exceeds the ratings of the Infineon part. The application note on protecting the MMC when using 24V systems can limit these transients, but will not always remove the problem. The other possible problem is overcurrent spikes. This is less likely to cause a failure, but even a current limited supply can provide large amounts of current for very small periods of time.

You could try running your system at 12V to see if the problem goes away. If so, you should include external components to reduce transient voltage spikes.
To limit the current you could try placing a small valued power resistor between the supply and MMC. 2 ohms would drop 3V across it at 1.5A (about 5W) and limit a short circuit to 12A. It's still high, but it could limit currents until the power supply or MMC protection kicks in.

Your backup plan will not work. The MMC is not designed to have the H-bridges paralleled. And even if it were, if the problem is voltage transients it won't be solved by running the devices in parallel.

I'm not sure who, other than Digi-Key carries the H-bridge. We probably buy our components from Future Electronics. The Infinion web site might point you in the right direction.

Lon

[Matt Smith]

Lon,

Thank you for your quick reply.

I have confirmed that both H - bridges have popped due to back EMF from the motor.

We have recorded a spike of -200V to +100V when the motor is disengaged through the switch!

I have added an array of schottky diodes to the outputs to strap the motor to the supply rail, this seem to be doing the trick.
I will add this and your suggestions in the final design.

Not sure I follow your comment about connecting the H - bridge devices in parallel. (I think we must have misinterpreted the datasheet)
If the devices can not be configured in this way, why is there a single motor mode(pin)?

No luck with Future Electronics on the Infineon part, Is there any chance we could buy some stock from you? (only need two.)

Thanks again for your help,

Regards,
Matt

[Lon]

Matt,

There was an ealier version of the product that had a set of H-bridges that could be paralleled. That IC was discontinued, and we replaced it with 2 Infineon parts. The single motor option reflects that earlier design, and we just carried it over for compatibility.

We do have stock of the Infineon part, and could sell you two I don't know the price or shipping cost. If you call our office tomorrow (530-891-8045) and ask for Lisa she can take the order. I'll send her an email letting her know you may be in contact.

Lon

[Matt Smith]

Ok Thanks