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uA78M33QDCY failed in EN55024 surge tests

Gribo
Intellectual 805 points

Hello,

 We are using the uA78M33Q (Automotive Linear regulator) In our products and encountered a failure during a test. The test involved a 500V surge into the DC supply lines.

The design includes a Schottky diode (MBR0540) in series and a varistor (V18AUML1206 by Littlefuse).

The regulator was damaged after the test and did not supply 3.3V to the circuit.

Is there anything that can be done to improve this design, or do we have to accept that this regulator cannot handle such tests?

Thanks in advance, 

Lior Bilia 

Precyse Wireless Technologies.

 

  • 0
    TI__Prodigy 185 points

    Hi Lior,

    The maximum voltage rating on VIN for uA78M33Q is 25V. I see the varistor (V18AUML1206) clamping voltage is at 40V. This is probably why the regulator was damaged during your surge test. 

    I recommend choosing a lower clamping voltage varistor <25V. There are some options from Littlefuse, however, max DC voltage rating is lower as well. What is your normal VIN to the regulator?

    Regards

    Xiaofan Qiu

  • 0 in reply to Xiaofan Qiu
    Prodigy 180 points

    Hello Xiaofan Qiu,

     

    Normal VIN to the regulator is 12V.

     

    Michael Adelman

    Precyse Wireless Technologies

  • 0 in reply to Michael Adelman
    TI__Prodigy 185 points

    Hi Michael and Lior,

    Sorry that I made a mistake in my previous statement. The abs max voltage on VIN is 35V. 

    I did some research on Littlefuse product. For DC max >12V, it is hard to find a varistor that provides max clamp voltage below 35V.

    I would recommend you to look into SMBJ parts from Diodes Incorporated. Particularly, SMBJ15(C)A, has DC 15V and max clamp voltage at 24.4V, with max peak pulse current at 24A. This should fit your application. We have used SMBJ parts before for transient voltage protection which passed ISO tests. 

    I have attached SMBJ datasheet for your reference. 2352.SMBJ transient voltage suppressor from DIODES.pdf

    Please let me know if you have more questions!

    Regards

    Xiaofan Qiu