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Compere traditional SMA/SMB/SMC/ to TI flat-clamp surge protection device

d_zero
Genius 4805 points
Other Parts Discussed in Thread: TVS0500

Dear *,

i was going through SLVAE37 

and have some questions regarding two calculations :

1) 

"

The SMAJ33A has a RDYN of 884 mΩ, so a nominal VCLAMP = 38.6 V + (24 A × 0.884 Ω) = 59.8 V. This is
the nominal clamping and is very close to the clamping voltage requirement, however we want to ensure
that there will be no issues when surges are applied over temperature. The data sheet does not specify
clamping or breakdown voltage over temperature; however it does provide a temperature coefficient that
we can use to calculate. The temperature coefficient is 10 × 10-4 /°C, so we can calculate VCLAMP at 85°C =
VCLAMP at 25°C × (1 + 10 × 10-4/°C × (TJ
-TA
)) = 59.8 V + (1 + 10 × 10-4/°C × (85°C – 25°C)) = 63.4 V. This
clamping voltage violates the 60 V absolute maximum of the input circuitry.

"

1a) i was trying to find Rdyn in SMA TVS datasheet without success, how to get this value?

1b) VCLAMP = 38.6 V + (24 A × 0.884 Ω) = 59.8 V -> what is the 38.6V and what is the 24A from TVS SMA datasheet?

2)

"

The SMAJ33A data sheet lists nominally that it can withstand 400 W of 10/1000 µs transients, which we can translate to 2300 W of 8/20 µs power by using the data sheet curves."

How can we do that?

Best Regards,

David.

  • 0
    TI__Genius 10006 points

    Hello David,

    You are probably referring the the Littlefuse part since that is the first part that shows up when you search "SMAJ33A". However, there are multiple companies that produce parts under this part number. It seems to me that when this document was made, the author was referring to the STMicro part. Here is a link to the data sheet. However, regardless of the part, the principles listed in the document are still valid. Now, I shall answer your questions.

    1a.) On page 3 in the second to last column Rdyn 8/20 us is spec'd at 0.884 ohms.

    1b.) This actually seems to be a typo. 38.6 V is the typical breakdown voltage, but 24 A should be 33 A. However, the Vclamp is slightly different since STMicro uses a slightly different equation to take the surge current in the application into effect.

    2.) If you look at figure 4 on page 4, simply take the power at 20 us instead of 1000 us, and you will get around 2300 W. It is also stated at the top of the datasheet.

    Please let me know if you need any more information. If this answered your questions, please click the "Resolved" button.

    Regards,

    Matt Smith

  • 0 in reply to Matt Smith
    Genius 4805 points

    Dear Matt,

    thank you for the fast response.

    I have one more question: and it is related how to choose the right TVS of flat clamp from TI regarding to Ipp and Ppp ?

    Because the TI flat clam for given Vrm will have  eg. for 5V TVS0500 Ppp= 400W and Ipp 43A , and a SMA diode SMAJ5.0A will have Ppp=2300W and Ipp = 137A, and SMB diode SMBJ5.A will have Ppp=4kW and Ipp= 275A

    I have to comply with the IEC 61000-4-5 standard and i have to follow the 8/20 us curve.

    and i found this statement " The peak pulse current, IPP, should be above the current in the IEC 61000-4-5 standard that requires an 8 µs/20 µs response."

    and I can't find in the standard what is the current in the IEC 61000-4-5 in 8/20 us pulse ?

    How do i know that the 400W is enough and i don't need the 2.3kW Ppp ?

    Best Regards,

    David.

  • 0 in reply to d_zero
    TI__Genius 10006 points

    Hello David,

    The only thing varying the current and power ratings for these parts is just the length of the pulse. If you look at the SMAJ datasheet I linked in my pervious response, you'll see that it is rated for 400 W at 10/1000 us and 2300 W at 8/20 us. Since the 10/1000 us pulse is much longer than the 8/20 us pulse, the power rating is lower to keep the area under both curves(energy) similar. However, the 10/1000 us pulse is going off of the IEC61643-123 standard and not IEC61000-4-5 standard (8/20 us). 

    As far as choosing a device is concerned, the power rating itself can be misleading. It does not give a clear indication of how well the device will protect your system. As outlined in this guide, power is calculated by P = Vclamp * Isurge (Ipp). Our devices provide a flat-clamp technology, which means once the device begins clamping, the clamping voltage barely changes, regardless of current. However, the SMAJ products' clamping voltages keep increasing with current after the device starts clamping. You would end up with high power but worse protection. I encourage you to read through that guide if you are still confused on this matter.

    Please let me know if you need any more info and if this answered your questions.

    Regards,

    Matt