Help With a Project: Controlling Current Flow Without Diodes

[ElusivePastry2]

Hello, I'm currently working on a project and I'm having a hard time solving a problem.  Essentially I've got a number of microcontrollers running simultaneously within a corresponding number of fixtures.  The position and orientation of a given fixture is not static relative to the other fixtures, and while the fixture is moving it needs to retain power.  Because of the nature of this movement, power can potentially enter the fixture from a number of different places (same bus, different entry points).

The issue I'm currently facing is I want to be able to turn on a specific group of fixtures, but since all of the fixtures share the same power bus, if I turn it on in one place I get it everywhere.  What I would like to do is be able to control the direction power can travel. I've included a schematic below to try and illustrate what I'm trying to accomplish.  In that example, there are 9 MCUs.  MCUs 2-5 can receive power from two ports.  MCUs 6-9 can receive power from three ports. At a given time, if the main bus (PS1) is powered, I do *not* want power to travel to the V_Ext path.  As you can see, this can be accomplished by incorporating diodes as I have in the schematic, however I believe this will cause a few issues.  The main issue I see is that I'm going to see multiple voltage drops before I reach each different grouping of MCUs, and this voltage drop isn't going to be consistent as the current required by each MCU fluctuates.

Long explanation, short questions: How can I achieve what I'm looking for without the voltage drop? Is there an alternative that doesn't include a PMOS (as I'm trying to keep the budget low as well)?

[alm]

Add high-impedance sense lines that connect to the cathode side of the diodes? That should cancel any voltage drop. Obviously you would have to limit the voltage so it does not go crazy when the sense lines are not connected.

[hlavac]

Maybe have the power bus be just a dumb wire through, and control the power state locally

[trampas]

A solution is to have all Power inputs diode OR'd  for each board.  Hence power at MCU will always be one diode drop below highest power voltage.

To control which power source powers on the MCU use a high side power switch.  For example assume that all power sources are dioded OR'd into Vmain, Vmain is then connected to a relay (high side switch) however the relay is powered powered by a second diode OR with PS1 and V_EXT1 so only when PS1 or V_EXT1 is powered does the relay turn on.

Of course if you have PS1 powered up all the time you only need the diode OR'd on the high side switch (relay).  The board only powers on when select pins have power.

Trampas
www.misfittech.net

[chris192]

Hi ElusivePastry2,

It's possible get MOSFETs to "act as a diode" but with zero voltage drop.
See attached configuration.

However you will still get a voltage drop, up to the MOSFET parasitic diode voltage drop in the event where both voltage supplies are at the same voltage.

You need to select a MOSFET with threshold voltage less than your (power supply voltage - MOSFET parasitic diode voltage drop).

Cheers,
Chris.

[ElusivePastry2]

Hi everybody. Thanks for the responses.

At a given time, the MCUs with 3 incoming connections can be powered by 0, 2, or 3 incoming directions. The MCUs with 2 incoming connections can be powered by 0, 1, or 2 incoming directions.  Within the entire system, there are upwards of 48 connections that need to be managed this way.

Is there some modification to the circuit suggested by Chris that could allow multiple incoming connections without affecting the incoming voltage?

Also, while I'm not against incorporating diodes, how power is fed through the fixtures requires voltage to be passed from one fixture to another like a chain, much like:

Ex: SystemPower->Fixture1->Fixture2->Fixture3. 

If Fixture3 receives voltage from a path other than Fixture2, that voltage cannot pass back to Fixture2.  Note, in the entire circuit there is only one power source. The source of power for the alternate path would still originate from SystemPower, it would just follow a different path to get there. Therefore, to receive the same voltage at every MCU in every fixture, there would have to be extra diodes added earlier in the chain just to make the voltage drop.  This will work, but it would add a lot of cost and eat into my already limited space.