Re: Power Supply for Gecko 320 and servo motors for 3 axis router

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Ed Gilbert" <co2man@n...>
wrote:

> Hi Gregory,
>
> What type of machine are you running them on? When you de-

accelerate an axis

> the motor produces voltage back into the power supply, know as back

EMF. A

> DC motor whether servo on not is a generator when being turned by

the load.

> A switching supply will not like it when voltage is back induced

into its

> output. Makes smoke in most cases! (:/
>
>

Most switching power supplies do not have active power components on
the output side of the supply. (other than the voltage feedback
components). They are basically DC to DC converters. The primary
side of the transformer is switched (normally some form of PWM) and
the secondary is rectified and filtered by a small choke and cap low
pass filter. Since the frequencies are from 25K to 100 khz the
magnetics can be much smaller.
The output diodes will block any DC higher than the applied voltage
and with no series pass or other devices in the chain there is
nothing to back EMF into.

I'm not sure what the purpose of a OVP SCR across the output is for
since under most failure conditions the primary switching components
will shutdown rather than run wild. In most supplies if there is an
OVP it is for the voltage that supplies the logic (+5 or +12) So it
would never see the higher reverse voltage. Maybe if you lost the
feedback loop to the output, but we used to design in shutdown if the
loop feedback was out of normal ranges.

I think the biggest reason not to use switchers is that you just
can't find them in voltages above 48VDC (and those are less common
than the 5, 12 and 24 V units.) Linear regulated power supplies are
actually less desirable for running inductive loads than switchers.
All power supplies have to be designed for transient loads (non-
linear current surges). The output filter section of a switcher
takes that into consideration based on its loop charactersitics and
the electronics in a switcher can make some pretty quick adjustments
when the switching times are sub millisecond.

There are hybrid designs that have switching front ends and use
active regulators on the some of the secondary voltages. You often
see this on multi-output supplies where there have to be several
tightly regulated voltages. You have to treat them as a linear type
regulated supply with all of the reverse (back EMF) warnings.

One last observation: If the outboard caps are too large the inrush
current will be more that the current limit will allow and the supply
will never stay on long enough to get them charged. I doubt under
most circumstances if more than a few hundred MFD are needed if any
at all. Just enough to keep the feedback loop dampened below
oscillation.

Okay, I'll duck back down and wait for the slings and arrows :-O