28V BATTERY TO 5V / 3.3V

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Sambuchi
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28V BATTERY TO 5V / 3.3V

Post by Sambuchi » Mon Apr 27, 2009 5:17 am

Hello everyone.

I've been asked to make a widget for one of my buddies.

The widget has to go on a vehicle that runs on 28V battery.

I need 5V and 3.3V to run what I want to use... micro/GPS/radio.

I dont see this design drawing more than 2 Watts.

My next step now is to go to Linear's website to play with some numbers but I thought
I would ask you guys and (gal if we have one), what precautions I should take when
interfacing a power supply to a battery. I would guess there is something to look into
when dealing with a raw 28V battery.

Thanks in advance!

Tony

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Dave Dixon
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Re: 28V BATTERY TO 5V / 3.3V

Post by Dave Dixon » Mon Apr 27, 2009 10:46 am

Hi Tony,
I'm sure this is obvious, but it's always best not to assume. An in-line fuse or equivalent short circuit protection. In the past, I have skipped that step for what I thought was "just for the time being - I'll add it later" with bad results from waiting too long to add the proper protection! Smoke under the hood - or in the cockpit? - is never a good thing.
Best of luck!
Dave

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Re: 28V BATTERY TO 5V / 3.3V

Post by sofaspud » Mon Apr 27, 2009 8:28 pm

Does your 2 watt estimate include power losses from the power supply itself?
You might give serious consideration to a separate battery supply, despite
what appears here to be a remote controlled vehicle.

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Sambuchi
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Re: 28V BATTERY TO 5V / 3.3V

Post by Sambuchi » Mon May 11, 2009 5:23 am

Ok, I have a twist to put on this.

I have a device that I need to hook up to my embedded design. It will need the 28V input from the battery to run and will need to interface to my micro. The 28V device can draw 1 amp 28W. I am worried now when I activate the 28V device on and off, the current will mess up my ground reference. What should I do here.

I starting to look at isolated regulators but everything I have found is outrageously expensive.

I've also started to look into filter circuits to ground but I'm not fully understanding them.

Let me know what you think!

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Re: 28V BATTERY TO 5V / 3.3V

Post by Externet » Mon May 11, 2009 6:24 am

Hello Sambuchi.
If I understand correctly, use diodes and large capacitors for the controller circuitry supply.

+28V--------./ .--------Load----------Gnd
+28V----|>|-----capacitor------converter to +5.7V-----|>|-----capacitor----controller-------Gnd

The spikes and voltage drop at the +28V rail are less likely to affect the control circuit that way.
I do not know why you aim your concern to 'ground'. Use fat enough wiring and a current-capable power supply.
Miguel
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Re: 28V BATTERY TO 5V / 3.3V

Post by Sambuchi » Mon May 11, 2009 6:55 am

I'm worried about the current surge from the device and the return current messing up my GND.

In these cases I thought there was ways to protect your circuit.

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Re: 28V BATTERY TO 5V / 3.3V

Post by haklesup » Mon May 11, 2009 8:59 am

What do you have connected that is so sensitive to ground noise? Ground usually only requires enough metal so that you don't get voltage drops as the ground current makes its way back to the battery. In most cases, the 5.0/3.3V power supply will sufficiently isolate your low V device from your high V device. Make the ground connection for the new P/S close to the battery and you shouldn't have much if any ground noise.

As for a DC to DC supply, off the shelf can get pricey. Take a look at the ti.com website, look for Webench. it will design a DC to DC converter complete with schematic and parts list for you. The main cost will be a PCB to mount the components on, the other parts cost is often suprizingly low. You might be able to do it with an linear 3 pin regulator (LDO) but thats a pretty large voltage drop and the power you need to heatsink is the drop across that device times the current you deliver to your load.

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Re: 28V BATTERY TO 5V / 3.3V

Post by Sambuchi » Mon May 11, 2009 9:18 am

I have processors on a data bus operating at high speeds. Noise could greatly effect my design.

I was reading this and that is what got me on this subject.
http://www.robustdc.com/library/an005.pdf

Now i'm reading up on Galvanic isolation. Wow... Getting little overwhelmed. :D

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Re: 28V BATTERY TO 5V / 3.3V

Post by haklesup » Mon May 11, 2009 11:28 am

Then just don't connect the ground return of the 3.3V supply to the return for the 28V supply. You probably don't NEED a common ground. You can also connect the two grounds using an inductor. at low DC currents, it will appear like a short but at faster higher current transients, it can show a significant impedance depending on the value.

Adding a large bypass capacitor near to the load will also help to squelch large transients caused by that load. Your problem is not unlike that encountered in any automotive design. The objective in automotive is not to eliminate the transients (that's virtually impossible in a running car) but to use a power supply in the front end of sensitive equipment that provides sufficient isolation. The input to a DC-DC supply shares ground with the car but its output provides a separate ground return path for local circuitry.

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Re: 28V BATTERY TO 5V / 3.3V

Post by Externet » Mon May 11, 2009 1:02 pm

Plain printers power supplies and circuits run several stepper motors (inductive and sustantial loads) from a 28V rail and also the 5/3.3V controller circuitry with not much interference.
Miguel
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Re: 28V BATTERY TO 5V / 3.3V

Post by Bob Scott » Tue May 12, 2009 6:34 am

Some high switching frequency RC speed controllers use opto isolators between the control logic and the MOSFET. It is quite important when your radio receiver is an inch away from this fast high current switch.
-=VA7KOR=- My solar system includes Pluto.

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Re: 28V BATTERY TO 5V / 3.3V

Post by Engineer1138 » Wed May 13, 2009 9:17 am

Tony, it looks like you need a "star ground." The 28V circuit should have its own large gage ground wire and the 3.3V circuit will have its own small ground return. These grounds will connect to the battery at one point only. This will avoid the current in one ground return affecting the other.
It's a standard configuration when powering digital and high resolution analog circuits from the same supply.

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