Comments on How to protect RF switches from ESD?
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How to protect RF switches from ESD?
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:
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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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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