[Gb-ccb] Power supplies for CCB

[Richard Lacasse]

Randy, Jason, et al.,

I've done a bit more thinking about power supplies for the CCB.  I'd like 
to run my thoughts by all of you and get whatever feedback you deem 
appropriate.

I think what would work best would be a system of two supplies and a 
regulator mounted on a bracket just below the CCB.  The two supplies would 
include a Power One HN5-9/OVPA (5 volts at 9 amps and 6.6 lbs) and an 
HC12-3.4-A (12 volts at 3.4 Amps and 4.25 lbs).  The regulator would be 
built around an LM338 5A linear regulator and would provide 8 volts at 2 
amps to the linear circuitry.

The following considerations drive me to this configuration electrically. 
The embedded computer is quite sensitive to the power supply level.  Having 
the 5-volt supply mounted very close by, with short wires will minimize the 
IR drop.  It eliminates the need for remote sensing.  With regards to the 
8-volt supply, since it is not an industry standard voltage, we seem to 
have a choice between using an adjustable lab supply or adding a regulator 
to an existing supply.  Post-regulating the 12-volt supply down to 8 volts 
does not seem too unreasonable except for the wasted power, but hey this is 
analog!  One concern is the isolation of the analog supply from any noise 
generated by the fan.  We should look at this.  One thing that goes in our 
favor is the isolation gained from two regulators, one in the power supply 
chassis which regulates 12 volts down to 8 and a second in the CCB chassis 
which regulates 8 volts down to 5.  Another advantage of this approach is 
RFI.  We have used Power-One supplies in the past with no problems I am 
aware of.  If we were to buy an adjustable lab supply the RFI qualification 
could be a problem (not to mention extra weight).

There are some mechanical considerations we need to think through.  A power 
supply chassis like I'm proposing about would add about 15 pounds to the 
receiver package.  Does this have any negative consequences, e.g. pointing? 
  I don't think it makes a whole lot of difference to the moment generated 
by the supply weight, as compared to the traditional power supply plates. 
The supplies are mounted at about the same radius either way.  We would 
need to come up with an additional bracket to support the supplies below 
the CCB.  I don't think this would be very hard.  Access to the supplies 
would be a little better with them mounted below the CCB.  Installation 
would be easier.

Clearly, we need to get some decisions made fairly soon.  These supplies 
are not that expensive.  The 5-volt/9 amp for instance is $78 in stock at 
Digikey.  My feeling is that we ought to just go ahead and buy the 
supplies, design and build the bracket and give it a try.  If we have to 
backtrack, it will not have cost us very much.  What does everybody else think?

Rich