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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Those are good questions, and I don't have a single definitive answer for them. I'd start with putting an appropriate TVS (or maybe separate diodes to ground and 3.3 V) between C3 and S1. My reason for clipping there is because then this gets to work against the impedance of the inductor.
Look carefully at the capacitance of the TVS or diodes, and reduce C2 accordingly. This is where separate diodes might be necessary if you can't find a suitable TVS that has less capacitance than C2 needs to be.
In the end, you need to test yourself. Fortunately, the high voltage discharge models are usually no more than a capacitor and resistor, so you can make your own with a high voltage supply. Several places I've worked over the years made jigs like that. At HP we called it the "fickle finger" test. Testing it yourself lets you do fast turn arounds, and gives you confidence you'll pass the real test at the certification company.
It sounds as we will need to run this in a simulator or we'd be fumbling around in the dark
Seems the opposite to me. You need some real experimentation. There are too many unknowns to allow sufficiently realistic simulation of high voltage transients. There are parasitic capacitances all over the place. Parasitic inductances also matter. Capacitors can be quite non-linear at high voltages, and leakage in various places may not be resistive anymore either. The biggest unknown is how exactly the RF switch input reacts to short term out-of-range spikes.
All in all, this is a case where you need to use experience, intuition, and something called a "brain". Let the new kid play with the simulator while you actually fix the problem the old fashioned way by doing some real lab work.
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