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Replace TPS65381-Q1 by standard SMPS for SIL

Niels_D
Prodigy 100 points

Other Parts Discussed in Thread: TPS65381-Q1, TPS65381A-Q1

Hi all,

I am developing a SIL system with the TI RM48 and TPS65381-Q1.

Because of temperature and power restrictions I have to have a power rail with a certain efficiency.
Unfortunately, the end-to-end efficiency of the TPS65381-Q1 is not good enough to meet those requirements.
This means that I have to use standard SMPS (e.g. buck converters) to supply the RM48.

I am quite new to SIL requirements, so I was wondering if there is information available about what should be done to make an equivalent to the TPS65381-Q1 from standard SMPS components (and POR / WD devices).
I noticed that IEC61508-2 has 3 diagnostic techniques/measures for power supplies, but it seems that the TPS65381-Q1 has more features than just that.

Thanks in advance!

  • 0
    TI__Mastermind 20600 points

    Hi Niels,

    What is you input voltage range?  You should easily be able to support the load currents available from TPS65381-Q1 with a supply of 18V or lower and using a DCDC from VDD6 to core rail if the core load current is higher than about 300mA unless you are not using a good thermal PCB and housing design.  If you are using a higher DC input supply you may have to separate out and pre-regulate the input to spread the thermal loading.

    You can theoretically build a SIL level supply from discrete components and this has been done in the past. However all functional safety analysis will be done by you and generally documents such as FMEDA's with FIT rates, etc are not available for catalog devices.  The integrated POR of these catalog devices may not meet the independence needed by the SIL level you are going for and may require additional discrete supervisors.  Depending on the number of rails,etc it may end up being a lot of discrete components to reach the number of voltage rails, independent voltage monitoring and diagnostics.  The TPS65381 indeed has many diagnostic and monitoring features specifically to enable various methods of diagnostics in applications that are beyond what standard components will offer.   

    Can you describe in more detail exactly what you are needing for input supply, rails including loads, etc and maybe we can provide you a better guideline. 

    Thanks,

    Scott

  • 0 in reply to Scott Monroe
    Prodigy 100 points

    Hi Scott,

    Thanks for your reply.

    My input voltage range is 14,6 to 15,4V.
    The loads are about 150mA for VDD3 and VDD5 (VDD3/5 set to 5V), 250mA for VDD1 and 50mA for VSOUT1 (set to 5V).
    With these numbers, the power loss is 400mW for VDD3, 150mW for VDD5, 950mW for VDD1 and 50mW for VSOUT1.
    This results in a total power loss of 1,5 W, which is a lot.

    Considering a solution in which the 1.2V is done with an external switcher and monitored by the TPS65381-Q1, there is still a power loss of 550mW.
    Even with good thermal PCB design, the lost power will still be converted to heat (just spread more).
    As our device is placed in an enclosure with little to no air circulation and has strict self-heating requirements, we have to look for a solution with a better efficiency.

    I am looking into using switchers to make a 5V, a 3V3 and a 1V2.
    A POR is required indeed and also a WD (supervisor and reset IC?).

    As far as I know, FMEDA can be done, considering certain failure modes for a complex IC (like a supervisor IC).
    It is not necessarily required to have specific FIT-rates for an IC. But maybe you can confirm that.

    Regards,
    Niels

  • 0 in reply to Niels_D
    Prodigy 100 points
    Hi,

    Could you tell me what supervisory ICs are recommended for the RM48 microprocessor?
    I found the TMS3702-Q1 for UV and OV monitoring, but it seems very difficult to find a supervisory IC that can do 1V2 monitoring with the accuracy required by the RM48 and that also offers a watchdog feature.

    Also I noticed that the TPS65381-Q1 is NRND, so I guess there should be at least one alternative for implementing the TMS570/RM48 for IEC61508/ISO 26262 with discrete components, right?

    Thanks!
  • 0 in reply to Niels_D
    TI__Mastermind 20600 points

    Hi Niels,

    While the TPS65381-Q1 has moved to NRND, the TPS65381A-Q1 is in development and should be showing as preview on the web shortly.  We would recommend if you are using TPS65381-Q1 for development to continue using it, and move to the A version when it is available.  The current version is only not recommended for new designs, it will still be available. 

    For discrete implementation: as with any functional safety system, the system designer has to make sure the SMPS and monitoring concept will meet the specific system requirements. So any set of regulation and monitoring funcitons meeting the system requirements could be used. 

    Scott