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WEBENCH® Tools/TPS2491: TPS2491

amos barash
Prodigy 140 points
Part Number: TPS2491

Tool/software: WEBENCH® Design Tools

Hi,

i like the TPS2491 Power Limiting feature, because sometime Current limit is not enough protection.

i would like to use the TPS2491 in a 1275D/E Surge protection.

the 1275 standard requires accomodating with +/-7V around the 28V, frequency 50Hz-200KHz

my question is : does the TPS2491 knows to handle also this "AC" signal. becuase the data sheet assumes all voltages are DC.

What is the response/behavior of the TPS2491 under Voltage Input with Quite high ripple ?

thanks

amos

  • 0
    TI__Guru* 95911 points
    Hi Amos,

    Thanks for reaching out!
    Let me check whether TPS2491 device suits in such application and get back to you.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Guru* 95911 points
    Hi Amos,

    Can you please share the details of end equipment where this is considered.

    When +/-7V peak on 28V with frequency 50Hz-200KHz is applied, the device may go into current limit and can lead to system shut-down. I would suggest a circuit breaker device for this application. My recommendation is TPS2477x device. It has independent current limit and fast-circuit trip function (both are programmable) which provides freedom to allow transients during high Vin ripple case.
    Please order an EVM and try.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    TI__Guru* 95911 points
    Hi Amos,

    Did you get chance to look at the recommended device TPS2477x ?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 140 points
    i need a device that can withstand mil-std-1275D. the device you suggested is limited to 20VDC
    nevermind, i gave-up looking for a better solution.
    kindly regards
    amos
  • 0 in reply to amos barash
    TI__Guru* 95911 points
    Hi Amos,

    I overlooked on the voltage range, Apologies...From the existing product portfolio, devices from TPS249x family are the closest ones to serve the specs but only concern is excessive power dissipation during high Vin ripple. This may lead the device into current limit and then to system shut-down. Saying so, this event depends on external FET, Vin ripple, environmental conditions.
    Are you in touch with any of TI sales team? to get an EVM for testing.

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 140 points

    Hello sir

    thanks for trying to help.

    i have to admit, i graduated 31 years ago, i forgot a lot of theory, since i did not use it. but i am stubber and trying to learn and remember.

    will you be kind to explain me, why a ripple in VIN will cause excessive power consumption.

    let me mention that my load is not resistive, it is an active load consuming a constant power, so if the Vin is in Peak (because of the ripple,) than the active load will draw less current, because it is a dc/dc.

    so why a peak in Vin due to ripple will cause excessive power consumption: The Vin will be higher, but the current will be lower. The FET is suppose to be ON with low RdsON, and Vout suppose to track Vin, and the FET Gate Voltage suppose to track Vout and Vin + 14 to 16V

    my question is also does the the charge pump for the FET Gate is capable to track the Vin and Vout (since the FET is ON with very low RdsON). because the mil-std-1275d specifies Vin as 28V with ripple +/-7Volt at frequencies range 50Hz to 200KHz. in the tps2491 no frequency specification are given.

    kindly regards

    amos

  • 0 in reply to amos barash
    TI__Guru* 95911 points
    Hi Amos,

    Yes, the FET is supposed to be ON with low RdsON, and Vout is supposed to track Vin. I understood that DC-DC converters are constant power loads but my only doubt is whether standard DC-DC converters would be able to track 200kHz Vin ripple well.? Any lag in tracking would establish potential difference across (fully turned ON) external FET of TPS2491, which would push the device into over current condition.
    Does I make sense?

    Best Regards,
    Rakesh
  • 0 in reply to Rakesh
    Prodigy 140 points

    Rakesh hello,

    thanks for your effort to help.

    you are right, what you describe is one of the reasons and maybe the main reason for power dissipation over the FET.

    The dc/dc has in the input capacitors that try to keep the voltage stable. this means there will be developed a potential difference between the Source and drain of the FET, called Vds.

    and Vds x I is power dissipation over the FET.

    i have come to a conclusion that  the only way (and the safe way) to supervise the FET Heat and to protect it is by using a thermistor mounted on the Thermal PAD of the FET. and any other solution like TPS2491is not good.

    thanks

    kindly regards

    amos