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TPS23881: PSE Output Voltage and isolation

Part Number: TPS23881
Other Parts Discussed in Thread: PMP22764, PMP20588, , BOOST-PSEMTHR-007, PSEMTHR24EVM-081

Dear All,

we are designing PoE PSE 802.3bt Type 3 Class 5 45W single port

1) I was going through many TI PSE designs, and the PSE output voltage in some designs is 52V, 54V, 55V, 56V ...

Is the PSE output voltage specified with 802.3bt ? , And what is the recommended PoE PSE voltage for Type 3 Class 5 45W and Type 3 Class 6 60W

2) Regarding isolation of PSE, i found in TI design two different approaches 

2a) Isolation of PSE voltage 56V with flyback (PMP20588, PMP22764)

There is input DC voltage that is boosted with flyback topology to 54V and it is isolated.

2b) Isolation on 3.3V and signal that are going to HOST MCU ( PSIL081+PSIL083A )

Here is also input dc voltage 44V to 57V and this goes directly to PSE vout and it is not isolated? Does this assumes that the input voltage J1 ( 44-57VDC)  on PSIL081 comes from and isolated power supply?

3.3V power that supplies the TPS23881 and 54V that goes to PSE vout are not isolated.

As i see the isolation is only on 3.3V and digital signals that goes to Host MPS430 MCU.

2c) Does the PSE need isolation as per IEEE 802.3bt standard or for Ethernet alliance PoE certification?

3) In our case we design a PSE injector and the power input to our injector will be from 12V to 48V DC from an generic industrial power supply and we don't know if it has isolation.

Best Regards,


  • David,

    Here's the PSE port voltage spec in IEEE802.3bt standard. 45W falls into Type 3 space and the port voltage should be between 50V and 57V.

    Regarding the isolation:

    PSE and PD should provide isolations if there's accessible conductors in your system. 


    Best regards,


  • Dear Penny,

    thank you for the info.

    1) could you please explain how is the isolation achieved on BOOST-PSEMTHR-007 and PSEMTHR24EVM-081? Because i don't see any isolation between input voltage and PoE voltage?

    BOOST-PSEMTHR-007 (SLVUBP0A) -> 48V connector J1 VPWR is directly connected to PoE Connector

    PSEMTHR24EVM-081 (SLVUBN3A, TIDM290) -> J1 44-57V input direct to VPWR and it is not isolated

    Does this assume that the input voltage to J1 comes from and isolated power source?

    Best Regards,


  • David,

    Yes. We just use the bench power supply which is isolated already.  Thanks.

    Best regards,