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Q&A

Comments on How to protect RF switches from ESD?

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How to protect RF switches from ESD?

+6
−1

Assume that I have some generic antenna connector, followed by a pi-filter/50ohm impedance matching components, then a DC blocking cap (as per the RF switch recommendations) and then a RF switch for split tx/rx semiduplex transceiver, as in this schematic:

antenna schematic

How do I best protect this RF switch from ESD coming in from human fingers touching the antenna connector? I need the highest ESD class (4) of the IEC61000−4−2, meaning 8kV contact discharge, 15kV air discharge.

I'm considering a specialized "ultra-low capacitance" TVS diode such as ESD8472 (rated up to 20kV), which is bidirectional.

Questions:

  • Is this TVS diode the way to go?
  • If so, where do I place it in this schematic? Directly on the antenna or somewhere else?
  • How to determine the suitable breakdown voltage? I'm assuming it should be based on the RF switch supply which is 3.3V.
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2 comment threads

Hello Lundin. I think it would help if you can specify what is the general configuration of your syst... (2 comments)
Are you going for the formal IEC 61000−4−2 certification test? Sometimes passing a test it a harder ... (2 comments)
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+4
−3

Currently there are $\boxed{\color{red}{\text{three}}}$ downvotes so maybe someone can explain why this has happened. Anyway, on to my downvoted answer: -

You have "mentioned" the threat (the ESD level) but, you haven't defined the peak voltage or current limit for the potential victim. Neither have you considered what the ESD pulse source impedance is and how the 8 kV is transformed to a significantly lower level by the capacitors in the pi filter.

Simulation is very effective in these situations but, you need to model the ESD source (there are a few different types) and, you have to choose the one that your device is expected to be able to cope with. So, here are the mitigations: -

  • The ESD source (the threat) has output resistance and, it might be as low as a few hundred ohms or as high as 1.5 kΩ
  • The first capacitor in the pi filter will slow down the ESD pulse and provide voltage limiting over the duration of the pulse
  • The inductor and following capacitor may also have a beneficial effect or, may cause a nasty ringing voltage that might be worse than if they were not there. This is where simulation can help.
  • The peak input current due to ESD into your "victim" might be several tens of mA and that may be good enough to win-the-day

But, without component values and ESD source impedance and details of the potential victim, it's guesswork. If the simulation shows that your victim may be over-stressed then there is absolutely no point hoping that your real circuit will survive the day.

You'll also need a discharge resistor for the pi filter if you are doing a series of ESD pulses because you don't want the charging voltage to form a staircase that progressively rises higher during the testing.

Bottom line: use a sim to get you to the point of deciding whether you need to add a TVS. Then choose the TVS and yes, it needs to be low capacitance to avoid detuning the pi filter but, there might be half a chance you won't need one.

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1 comment thread

Human-body model (4 comments)
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Human-body model
Lundin‭ wrote almost 3 years ago

Any human body model will do for now. (The one defined in MIL-STD-883 is for example available for free). Considering conducted susceptibility from other sources like charges to chassis might be the next step, but for now I'm just trying to narrow down the problem and see if ESD is the cause or not.

Andy aka‭ wrote almost 3 years ago

Did you downvote my answer @lundin?

coquelicot‭ wrote almost 3 years ago

In place of asking Lundin if he downvoted you and complaining in your answer, why did you not come to participate to the debate about downvotes in Meta? after all, you are the trigger of this thread. Where are you when your are needed?

Andy aka‭ wrote almost 3 years ago

coquelicot‭ I choose to complain for sure and, if I'd known about a debate (I received a message just minutes ago from Olin), I'd have both complained and participated. I will of course participate now that I know there's a debate.