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[FAQ] Question on TVS / DC current rating

Kurumi Hasegawa
Expert 6951 points
Other Parts Discussed in Thread: TVS2700, TVS2701

Hi team,

 

I was having hard time understanding the definitions of DC current rating of TVS datasheet. Could you please help me understand the concept?

 

For the uni-directional TVS it is stated like below (TVS2700);

 

For bi-directional it is like below (TVS2701);

 

Q1: I believe IBR is the current @ VBR (break-down voltage) which would be flowing though the TVS. And the concept is the same for both uni/bi-directional. Is my understanding correct?

Q2: What is the DC forward current rating? Is it the max current that the TVS can handle continuously? Below figure is from TVS2700, showing there is ~0A of current when the voltage is 0V~30V. I thought that means, regardless of the current that tries to flow thought the TVS, TVS can block the current when the voltage is 0V~30V. And the DC forward current rating is for when the voltage is larger than 30V, which need to be less than 500mA. Is my understanding correct? For example, when expected max voltage is 22V and the max DC current is 2A, can TVS2700 be used?

 

Q3: I cannot find IV figure like above for bi-directional TVSs. Can I assume it would the same for positive-voltage region, and rotated 180 degree for negative-voltage region?

 

Best regards,

Kurumi

  • +1
    Guru 287380 points

    1. A unidirectional diode behaves like a normal diode when forward biased (about 0.7 V forward voltage). A bidirectional diode consists of two unidirectional diodes in series, in opposite directions, so when one breaks down, the other is forward biased.

    2. Durig normal operation, no current flows, and a transient is not DC. So the DC current rating is pointless.

    3. Yes.

  • 0 in reply to Clemens Ladisch
    TI__Expert 6951 points

    Thank you so much for the reply.

    Clemens Ladisch said:

    2. Durig normal operation, no current flows, and a transient is not DC. So the DC current rating is pointless.

    So what would be the condition that the DC current rating becomes critical? When voltage larger than Vrwm was applied?

    I was looking for TVS which can be used with supply which might have 1-2A. And worried if using TVS27xx violate its DC current rating.

  • 0 in reply to Kurumi Hasegawa
    Guru 287380 points

    You indeed would have to apply a voltage larger than VBR, but nobody does that with a TVS diode. Therefore, the DC current rating never becomes critical.

    The supply current never flows through the TVS diode, so that current does not matter.

  • +1
    TI__Expert 8486 points

    Hi Kurumi,

    What Clemens is saying is correct. The unidirectional diode (TVS2700) can be forward biased with a max constant DC current of 500 mA while it can only be in constant DC breakdown for up to 12 mA. The bidirectional diode (TVS2701) does not have a forward DC current rating since the only way for it to conduct would be in breakdown mode (reversed biased). The transient current ratings for these diodes will be much higher (ie 27 A for an IEC 61000-4-5 8/20 us pulse with TVS2700) since they are not constantly conducting and heating up during those transient events like surges. You do not have to worry about how much current your power supply produces as long as you are not putting the TVS2700 in series and forward biased with that 1-2 A constantly flowing through it. 

    Regards,

    Matt Smith 

  • 0 in reply to Matt Smith3
    TI__Expert 6951 points

    Hi Clemens and Matt,

     

    Thank you for the explanation, but I’m still not sure if I understand it right.

     

    For the definition of Ibr and If:

    When you say “forward biased” it means the diode is forward biased and the voltage on IV curve (Figure 5 on TVS2700 datasheet) is below 0V.

    When the forward biased is applied, max constant DC current TVS2700 can handle is 500mA.

    On the other hand, when the diode is reversed biased with larger voltage of Vbr, the max constant DC current rating is 12mA.

    When this happened the voltage on IV curve (Figure 5 on TVS2700 datasheet) is positive.

    Is my understanding correct?

     

    For DC forward current rating:

    Matt Smith3 said:
    as you are not putting the TVS2700 in series

    By “in series” you mean in series with the protected system, like MUX, is it correct?

    When there is a risk of negative voltage applied to TVS, is it OK to use bidirectional TVS like TVS2701 then? If my understanding is correct, it can operate unless less than -30V or larger than 30V was not applied with 1-2A constantly flowing through it.

     

    Best regards,

    Kurumi

  • +1 in reply to Kurumi Hasegawa
    TI__Expert 8486 points

    Hi Kurumi,

    When the TVS2700 IV curve was measured, the diode was reversed biased. Therefore, the positive region of the curve would be going towards breakdown (the right) and would only be able to handle 12 mA DC in that direction. The voltage across the reversed biased diode when it is conducting 12 mA will equal Vclamp in the datasheet. 

    When diode is forward biased, it acts as a simple diode with a diode voltage drop of about 0.5 V (VF in the datasheet). This corresponds to the negative region of the IV curve (the left) in the datasheet. However, if the diode was connected in series and forward biased like below and not in parallel and reversed biased like above, the voltage drop would be positive and can conduct up to 500 mA DC. If you were to put 1-2 A DC through it in this configuration, the diode would fail. That is what I meant with my last statement. Like Clemens mentioned earlier, this is not a common use case for ESD/TVS protection diodes. Note that is just for the TVS2700 unidirectional diode. The bidirectional TVS2701 should not be used in series like this, only in parallel like above.

    A bidirectional diode, like TVS2701, is only needed when the normal operating voltage levels go below ground, like in an AC coupled signal. Both unidirectional and bidirectional diodes will protect your system during a negative voltage ESD event. The only difference would be that the unidirectional diode would star conducting sooner. 

    Hopefully this clears things up. If you would like a more in-depth explanation of these topics, I would encourage you to visit our training page and watching the "ESD Essentials" video series.

    https://www.ti.com/interface/circuit-protection/esd-surge-protection-ics/support-training.html  

    Regards,

    Matt Smith 

  • 0 in reply to Matt Smith3
    TI__Expert 6951 points

    Hi Matt,

    Thank you so much for the additional explanation. I understand.

    Best regards,

    Kurumi