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BQ25505: Input Protection

Ellis Cave
Prodigy 120 points
Part Number: BQ25505
Other Parts Discussed in Thread: CSD23202W10

I am designing a bq25505 into a wind harvesting device. The kinetic energy harvesting is done by a turbine connected to a small brushed DC motor used as a generator.

Three issues:

1. The generator output impedance is around 70 ohms. Are there specific input issues I should address when using this fairly low impedance source as an input to the bq25505?

2. The generator output has the potential to exceed the 5.1V input limit to the bq25505. I plan to clamp the bq25505 input to around 5V with a zener diode.

3. What about negative excursions on the input to the bq25505? What kinds of internal protection does the bq25505 have on it's input for negative voltage inputs? I don't expect the input to go negative except in rare situations (turbine reverses, or generator connected backwards) but I want to protect the harvester chip and input cap from those situations. Is there a diode clamp on the bq25505 input to protect from negative input? If so, what is the current limit on that internal diode? I need to determine whether I need to supply an external diode clamp on the bq25505 input to protect from negative excursions. I don't want to use a diode bridge, because it causes to much voltage drop. 

  • 0
    TI__Genius 10385 points

    Ellis,

    1. 70 ohm should actually be fine. We actually recommend 20 ohm series resistance with the power supply in the EVM user guide. I personally use 100 ohm in the lab when using this part.

    2. A 5V zener diode from VIN_DC to GND is a good over voltage protection solution.

    3. I would suggest using a PMOS FET in the power path or NMOS FET in the ground return path as described here. BQ25505 has no built in reverse polarity protection and exceeding the negative VIN_DC absolute maximum value will result in damage to the IC.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 120 points

    Ricardo,

    I'm a bit confused about your suggestion for using a PMOS FET in the power path for polarity reversal protection. The bq25505 has a cold start voltage on Vin(cs) of 600mv, and the Vishay SI2323DDS FET you recommend has a gate-source threshold voltage Vgs of -0.4 to -1V. Won't that potentially prevent the bq25505 from cold starting ?

  • 0 in reply to Ellis Cave
    TI__Genius 10385 points

    Ellis,

    Sorry for the confusion, I am not specifically recommending you use the FETs used in that report. I just wanted to show you that app note to illustrate the concept. You are correct in your assumption and you will need to look for a PFET with the lowest possible Vgs while keeping RDSON as low as possible. I suggest you create a new thread to get a specific recommendation from one of the MOSFET teams.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 120 points

    Ricardo,

    I chatted with the TI MOSFET group today, and they said the following:

    <<<>>>

    Thanks for your interest in TI FETs. I did a search and the CSD23202W10 has the lowest typical VGS(th) = -0.6V. TI tests and specifies threshold voltage at ID = -250uA & T = 25C. Keep in mind, the datasheet limits are -0.4V to -0.9V and that's all we can guarantee. I pulled up the characterization data when this FET was developed and the maximum measured value was just few mV over -0.6V. Please note, these are samples from 3 lots and may not reflect the actual lot-to-lot variation over a larger production sample size. The magnitude of the threshold voltage has a negative temperature coefficient of -7mV/C, nominally. As you heat the part, the threshold is lower and at lower temperatures the threshold increases.

    <<<>>>

    So it doesn't look good for using a MOSFET for protection, since with a -0.6V VGS(th), the bq25505 may not start.

    However, I think the problem is worse than that. Our generator will output voltages as low as 0.1 V in some low-wind-velocity conditions, and the bq25505 can harvest energy from voltages as low as 0.1V after it is started. So I want to be able to harvest energy from the generator, even at these low voltages. This definitely rules out FETs with a VGS(th) of 0.6V, as the bq25505 would never see that voltage.

    So I'm thinking that perhaps a schottky diode across the input in parallel with the 5V zener clamp, would be the best option to protect from reverse voltage. Your thoughts?

  • 0 in reply to Ellis Cave
    TI__Genius 10385 points

    Ellis,

    A schottky is an acceptable solution. Keep in mind that Schottky's are not ideal and have a small forward voltage drop, but this would only apply on a negative voltage swing.

    Thanks,

    Ricardo