Optoisolation needed for 90V dc motor control?

[Mike V.]

Hi,

I've seen microcontrollers coupled to a DC motor controlled by
H-bridge drivers (from vendors like International Rectifier, Intersil,
SGS-Thomson, etc.) When it comes to low voltage motors (e.g. 24V), the
motor, microcontroller, bridge driver, and MOSFETs/IGBTs all share a
common ground.

I always thought that when you moved up to higher voltage motors, it
was desirable to isolate your micro's 5V ground from the
driver/fet/motor's ground with optoisolators.

Now, i've seen reference designs and reference boards that have up to
600V DC voltage rails with FETs/IGBTs and bridge drivers NOT isolated
from the digital 5V microcontroller ground.

What is the proper way to do things??

 

[Frank Bemelman]

Hi,

I've seen microcontrollers coupled to a DC motor controlled by
H-bridge drivers (from vendors like International Rectifier, Intersil,
SGS-Thomson, etc.) When it comes to low voltage motors (e.g. 24V), the
motor, microcontroller, bridge driver, and MOSFETs/IGBTs all share a
common ground.

I always thought that when you moved up to higher voltage motors, it
was desirable to isolate your micro's 5V ground from the
driver/fet/motor's ground with optoisolators.

Now, i've seen reference designs and reference boards that have up to
600V DC voltage rails with FETs/IGBTs and bridge drivers NOT isolated
from the digital 5V microcontroller ground.

What is the proper way to do things??

A proper layout of the PCB and/or cabling helps. There is
no harm in connecting two grounds if both are 0V in respect
to ground

OTOH opto's can help to reduce further damage when something
terrible happens in the power stages.

 

[Spehro Pefhany]

Hi,

I've seen microcontrollers coupled to a DC motor controlled by
H-bridge drivers (from vendors like International Rectifier, Intersil,
SGS-Thomson, etc.) When it comes to low voltage motors (e.g. 24V), the
motor, microcontroller, bridge driver, and MOSFETs/IGBTs all share a
common ground.

I always thought that when you moved up to higher voltage motors, it
was desirable to isolate your micro's 5V ground from the
driver/fet/motor's ground with optoisolators.

Now, i've seen reference designs and reference boards that have up to
600V DC voltage rails with FETs/IGBTs and bridge drivers NOT isolated
from the digital 5V microcontroller ground.

What is the proper way to do things??

Cheaper controls don't tend to put the isolation in. As well as cost,
(depending on technology and cost) the isolators slow down the signals
to one degree or another, which may or may not have any real effect on
performance. Having the microcontroller at mains voltage means you can
have nasty common-mode voltages on the microcontroller rails wrt
earth. For reliable operation you may need more shielding. You may
need to isolate communications ports. Front panel controls may have to
be rated for mains operation, which can push cost up. The housing may
have to be more complex or expensive if displays etc. have to be
safely insulated from the operator's fingers.

Bottom line: There is no "proper" way, whatever meets the written and
unwritten specs and is lower cost is best. Personally, I prefer to see
the micro galvanically isolated from the mains in most cases, but cost
constraints may prevent it.

Best regards,
Spehro Pefhany

 

[Mike V.]

Thanks Spehro and Frank for your input.

I do have to put some buttons for user input. I'll most likely end up
using no more than 2-3 optocouplers (at a buck each), which is
negligible in cost.
-Mike