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Isolated DC/DC converter or non isolated DC/DC converter for PoE application?

javi marco
Intellectual 360 points
Other Parts Discussed in Thread: TPS2379, TPS23861, TPS2379EVM-106, PMP7499

Hi,

I am developing a PoE application where I am going to use the controller TPS2379. My question is about the DC/DC converter which has to be attached to the output of the TPS2379. When this DC/DC converter has to be isolated and when no?

Regards

javier

  • 0
    TI__Mastermind 41760 points

    Hi Javier,

    Please consult IEEE802.3at, IEC60950, and any other applicable regional requirements.  

    Generally speaking, the IEEE wants the ethernet network isolated form all other potentials, and also from high-surge environments.  The safety rules treat the ethernet like a telecom potential and have requirements for eliminating personal exposure to hazzardous telecom potentials including direct touch and metallic interfaces exposed to the telecom potentials. Many safety-conscious customers use the isolation when in doubt to be "safe."

    If there are no externally accessible metallic interfaces except the MDI (ethernet cable), an isolated power supply design is not required.

  • 0
    TI__Mastermind 42795 points

    The IEEE standard requires that the MDI (ethernet cable wires) be isolated from all external conductors including frame ground. You can find the detailed wording in section 33.4.1 of IEEE802.3at. Generally speaking, if your product has any external connectors (USB for example), then those wires require isolation from the ethernet cable (POE) side. An example of a product not requiring isolation would be a fully self-contained POE camera with no external connectors except for the POE RJ45 connector.

  • 0 in reply to Eric Wright
    Intellectual 360 points
    Thanks for your reply.
    In my case, my final product just will have the PoE RJ45 connector and one USB port. But this USB port will be used just for update the firmware once in the year for example. So in this case I have to isolate the DC/DC converter?, because once the product is working properly , just the PoE RJ45 will be used.
    And if I want to put my product in the market, the product has to be compatible with the standard IEEE802.3? or what is the purpose of this standard?
    Looking forward to hearing from you
  • 0 in reply to javi marco
    TI__Mastermind 42795 points

    I would think this case is possible as long as the USB connector is protected from user access in the finished product (ie, maybe it will need an access cover to prevent your customer from touching the housing or plugging something into it when powered by the PSE). You should check with your safety engineer as well about any manufacturing flow related aspects but in principle, this should be fine.

    You could use a synchronous buck to convert the PSE input voltage to 5V (or whatever your DC voltage is).

    That being said, since there is no existing "standard" for 4 pair-high power POE, you will have to choose one of the existing pre-standard methods of getting this amount of power down the ethernet cable. You'll have to choose a vendor specific approach for now (TI, LTC, Micro-semi) and know whose PSE you will be plugged into.

    If you need information on TI's TPS23861 based high power approach, let me know. 100W (at the PSE) can be demonstrated easily using TPS23861EVM and TPS2379EVM.

  • 0 in reply to Eric Wright
    Intellectual 360 points
    Ok thanks for the information.
    In my case I need to power a PD which contains 4 components by PoE with the following supply requirements:
    Component 1: 7W(12-19V)
    Component 2: 4W(12-24V)
    Component 3; 1W(3.3V)
    Component 4: 13W (10-30V)

    So what is the best option to power these devices?. With DC/DC converters or with LDO regulators?. I know that any DC/DC converter is much more efficient than a LDO but they need more passive components in order to work properly.
  • 0 in reply to javi marco
    TI__Mastermind 42795 points

    The output power requirement is about 25W right? The requirements seem quite custom, but you may find a solution at the link below.

    http://www.ti.com/general/docs/refdesignsearchresults.tsp?powerlabflag=Y

    If you can get the input power (at RJ45 connector) down to 25.5W, then the PD can meet the standard type 2 power level. You will need a switching regulator to do this and get good efficiency.

  • 0 in reply to Eric Wright
    Intellectual 360 points
    My power requirements in the PD is around 40W. And I want to use the TPS2379 to deliver the power in my PD. The thing is that my PD is compose for several components with different requirements of power. So my idea is to use two DC/DC converters for the components which consume more power (component 1 and 4) and two LDOs for the other components which consume less power (component 2 and 3).
    Is my idea correct?.Are there other options more efficient to give the power to the components?. The TPS2379 will work with this high power?
  • 0 in reply to javi marco
    TI__Mastermind 42795 points

    You have the right idea. Switch down for the higher power loads and you can use LDO's for lower power loads. Basically, the power loss in an LDO is (Vin-Vout) x Iout so a lower load current and/or lower Vin-Vout drop can minimize the LDO loss.

    TPS2379 can support 25.5W loads w/o the external boost FET but can work much higher with a boost FET. For example, the TPS2379EVM-106 can support currernts up to ~2A (2A at 50V is 100W).

  • 0 in reply to Eric Wright
    Intellectual 360 points
    And what about the efficiency of the TPS2379 when it is used for high power applications?(like in the application report SLVA498).
    The TPS2379 has Rds_on of pass-FET 0.75 Ohm max, while Microsemi's PD70210 has 0.3 Ohm max. So for example at 40W (some 0.8A at 50V) this translates to 280 mW less losses and 0.7% higher efficiency with the PD70210 of Microsemi.
    So what is the advantage of using the TPS2379 instead of the PD products from Microsemi or Linear Technology?
  • 0 in reply to javi marco
    TI__Mastermind 41760 points

    Hi Javi,

    The TPS2379 uses an external FET for higher power applications so you have flexibility of designing your effective RDSon. For example, PMP7499 is a 60W solution that uses the TPS2379 in high power configuration.

    http://www.ti.com/tool/pmp7499

    You can see its test report in the link above. It's efficiency is 90% end to end (most of the power loss coming from the diode bridge as you can see in the thermal image).