Hello All,

I'm designing a 10port PoE based GigabiEthernetet switch for outdoor application. The device will be powered up using -48v which is commonly used for telephonic devices.

I would require 48v for my octal port PSE controller and some other positive voltages for internal circuitry. The total output power requirement is around 300Watt.

I'm confused with -48v as I have never used negative inputs in past. Can anyone please let me know how can I convert -48v to +48v? Will I have to give any isolation between grounds? Is there any particular IC which I can use?

Any help would be highly appreciated.

 

You could use an off-the shelf DC-DC converter, use an isolated type, just ground the positive output terminal.

http://www.vicorpower.com/dc-dc

 

If nothing else shares teh -48 volt power source just turn the inputs around and you have +48 VDC.

Without knowing more about your system it's impossible to know what you may or may not actually need.

 

If nothing else shares teh -48 volt power source just turn the inputs around and you have +48 VDC.

Without knowing more about your system it's impossible to know what you may or may not actually need.

Hello tcmtech,

Thanks a lot for your response.
Positive 48v will be going to PSE controller and some other buck converters in the design. I think I can just reverse the polarity and everything should work fine. Should I use any kind of isolation for this purpose?

Have you ever done this before?

 

If this is the same supply that is powering the telephone exchange then the positive is at about ground potential. It is probably connected to ground in the exchange. All the network interfaces I have seen on network switches have transformer coupling to each port so it should not matter i that the "ground" for the electronics in you switch sits at -48 volts. As you are designing the switch you can mage sure that you use port transformers that have suitable isolation specs. If you switch is a managed switch and the management is done over an RS232 post then you would have to provide isolation on this. As you are designing the switch you have control over the specifications.

Les.