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Hello,
i have an issue with the BTS50085-1TMA power switch.
I have no Z-Diode in my design implemented. When i am doing a reverse voltage test (-28V) the normal diode will be destroyed.
I have already tried a Z-Diode in serie with the normal diode but the Z-Diode will be destroyed again.
Therefore, i have no idead how to design (parameters) the Z-Diode to be alive in this test.
I also have Rin = 330 Ohm and Ris = 1kOhm. But there is no Ds.
Could you help me please.
Thanks!
BR
Solved! Go to Solution.
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Hello,
You can keep two resistors in parallel but make sure the overall resistance is less than 500Ohm as specified in the datasheet for the proper turn-on of the switch.
One more hint, How long you are expecting this reverse battery condition? It's not a continuous event, right? Can you please check whether the resistor will withstand the reverse battery condition for that specified duration?
The ambient temperature for the reverse battery condition is also an interesting point here.
Regards
Vishnu N
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Hello,
When you apply the reverse battery, it gets a very low ohmic path through the normal diode and RDson of the smart FET. Channel will be turned on by the device in negative battery conditions to reduce the power dissipation in the smart FET. In order to provide protection in reverse battery, there should be a zenor diode in series with the normal diode.
Coming to your second scenario, You have connected a zenor diode and zenor diode was destroyed.
Can you please let me know the breakdown voltage of the zenor diode?
It will be great if you can share the datasheet link.
Regards
Vishnu N
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Hello Vishnu,
i had the zener diode BZX84-C15 (https://assets.nexperia.com/documents/data-sheet/BZX84_SER.pdf).
This was actually a fail, with a working voltage > 28V (for example MM3Z33VB https://www.onsemi.com/pdf/datasheet/mm3z9v1b-d.pdf) it works fine.
But in this case the Rin (=330Ohm) is heating up to over 200°C (seen with thermal camera).
BR
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Hello,
BZX84-C15 has a 15V breakdown voltage which cannot survive 28V reverse battery condition. MM3Z33VB has a 33V breakdown voltage and it survived the 28V reverse battery.
Please make sure the current going through the IS and IN pin together is less than 120mA in your reverse battery condition. Use the formula given in the datasheet for calculating the R_IN.
In your case, (R_IN = 330Ohm, Ris = 1kOhm, and I assume no RV) you have sufficient resistance to limit the current.
Can you please check what is the voltage drop through the R_IN during the negative battery condition?
How much power you can dissipate in R_IN? (better if you share the datasheet)
Regards
Vishnu N
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Hello Vishnu,
the design looks like:
Its a standard 330R - 0402 SMD-Chip, like the 1k.
Do you have any suggestion to change it to 0603 or other values ?
BR
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Hello,
In this schematics, I can also see RV.
Please calculate the current through Rin and decide the package for Rin.
In my rough calculation, 0402 is not sufficient.
As I said in the earlier post, please measure the current in Rin as well in your application to double-check the calculations.
Regards
Vishnu N
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Hello Vishnu,
i have measured the current through Rin. Its about 45mA.
Therefore i need a resistor which can withstand about 1W thats Chip 2512.
For me it wasn't clear, maybe it would be nice to give an advice in the datasheet or application note about the consumption through this resistor.
Is it possible to have 2x resistors (e.g. 600R with 0.5W) at Rin because of placement issues.
Thanks.
BR
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Hello,
You can keep two resistors in parallel but make sure the overall resistance is less than 500Ohm as specified in the datasheet for the proper turn-on of the switch.
One more hint, How long you are expecting this reverse battery condition? It's not a continuous event, right? Can you please check whether the resistor will withstand the reverse battery condition for that specified duration?
The ambient temperature for the reverse battery condition is also an interesting point here.
Regards
Vishnu N
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Hello Vishnu,
okay, i´ll check it.
Actually the reversed voltage (-28V) will be hold for 1min at room temperature (20-25°C).
Therefore i think a 0.5Watt resistor will be enough for 1min holding.
BR