PoE: Swapping of PSE & PD

Hello Harish. I think you are looking for guidance in designing a so-called "PoE Pass Through" system.

In order to help us better support you, would you please advise on the power levels you are looking to achieve? As an example, perhaps the system is intended to receive 25.5W at the input of the CAT-5 cable (it's "load rating" if you will) and then, in turn, offer up 4 potential IEEE802.3af rated ports of power - utilizing port power management software to ensure the system stays under budget.

The power levels (and port counts) you are looking to achieve will influence what device suggestions we place into some of the blocks.

Also, please note that due to Independence Day our response time might be a little longer than usual upon receipt of your answer.

Best Regards,
Thomas Lewis
Applications & Marketing Manager

Hi Thomas,
Our load requirement is less than 12.95W (Type 1) on CAT5 cable, classifiaction is optional in our case.

For our better understanding, I'm explaining the question in detail:
There are two power supplies on our system, one is Main power supply and second is external power supply (call it as auxiliary power supply). During normal operation the main power supply will be functional & device will act as PSE. If main supply gets failed then auxiliary power supply will function and device will act as PD.

For now, concept diagram and some application notes will be fine.

Regards,
Harish Kumar

Thank you for the clarity Harish.

Someone from our applications team will get back to you following the US Holiday.

Regards, Thomas

Hi Harish,

Could you please provide a block diagram of your system to help us better understand your requirements?
My understanding is you want to build a system on both PSE and PD side and the PD can receive power from sources: main power supply + PSE or a second DC power supply. PD will be supplied by the main power supply +PSE most of the time and if anything happens on this path, then the backup power supply should turn on and supply power to the PD. Is this correct?
Thanks.

Best regards,
Penny

Hi Penny,

I don't have any block diagram at this time.
I want to build a system which will work as PSE for one port and PD for another port.
During PSE operation, the system will receive power from main power source and will provide power to other PDs on first Ethernet port.
When Main power source is not available then this system must work as PD and it will receive power from an external DC source on another Ethernet port.
So I want to understand the configuration how we can switch b/w two.

Regards,
Harish

Hi Harish,

I tried to create a system block diagram based on your description. Please take a look and verify if my understanding in correct. If so, I would recommend TPS23861 on the PSE side. TPS23861 can output both Type1(15.4W) and Type2(30W) depending on the type of PD is connected and it has autonomous mode which doesn't need any external MCU. 

On the PD side, please tell us what the voltage input to the PD, whether you need to add a DCDC after the PD, what prototype you want and the expected efficiency. Our PD expert will assist you to find the right part. Thanks.

Best regards,

Penny

Hi Penny,

Thanks for your response.
Voltage input to PD is 37-57V will prefer to use a DC-DC after PD controller, efficiency must be > 85%.
And also, how to switch between these two interface?

Hi Harish,

Are your main power supply and external DC power supply turned on at the same time or only one is turned on at a time?

Best regards,
Penny

Hi Penny,

External DC supply will be available all the time but requirement of this will depend on availability of main power supply.

Thanks,
Harish

Hi Harish,

Will you power the PSE in the box through the PD in the box while the main power supply is not available? Is your second power supply a PSE device? if that's the case, it makes more sense. You connect the PD's output to PSE's input through a diode Oring circuit, and the voltage of PD's output needs to be lower than the main power supply's output. Thanks.

Best regards,
Penny

Hi Penny,

PSE in the box will not be powered up by PD, it will get input supply through main power source. External DC source is a PSE device.

Thanks,
Harish Kumar

Hi Harish,

I think the previous block diagram already meets your requirement. TPS23861 is the right PSE part for your application. I will let our PD expert to recommend PD part for you. Thanks.

Best regards,
Penny