How to choose TVS diode? What's clamping voltage? If in case TVS diodes clamping voltage 57V if anything more than 57V what will happen to diode and circuit? Would it break? Or pass the voltage?
How to choose TVS diode? The threat characteristics must be understood and the needs of the circuit to be protected must be understood.
If the circuit can only cope with 5.5V and the threat potential is 1500V then the upstream protection must ensure no more than 5.5V is presented to the circuit.
- Packet shaping By means of Resistors, Capacitors, Inductors... aspects of the strike waveform are attenuated to limit what the downstream circuitry is presented with
- Clamping. Using TVS, Gas discharge tubes or MOV's the energy is dissipated in these devices to protect downstream circuitry.
- Combination of above.
What's clamping voltage?
The voltage at which the TVS will clamp at. TVS's generally have a minimum breakdown, maximum breakdown and a maximum clamping voltage. These are all current dependent so the specific clamping voltage is dependent on what is being presented to the TVS (source impedance, clamping voltage etc...)
If in case TVS diodes clamping voltage 57V if anything more than 57V what will happen to diode and circuit?
The voltage at the cathode will be held at some voltage between minimum breakdown and maximum clamping. The interaction of the source impedance and this voltage determines the resultant clamping voltage.
Would it break? Or pass the voltage?
Well... IF the source impedance is such that the pulse current is below the maximum stated then it will clamp the voltage at the maximum clamping voltage. If the source impedance permits more current to flow OR the duration is longer then the power dissipating capability then the TVS will break. They tend to fail open-ciruit due to the power blowing them off the card.
In short. Understand the threat characteristics (double-exponental, source impedance etc...), understand your circuit (can normal operation cope with slewed signals) and finally run the calculations on the dissipation needs of the TVS to ensure it is suitable
Assume the normal voltage range through your wire is max 57 V (maybe a bit more for some headroom). Than you should use a diode with a clamping voltage just above that. If you chose it too high (e.g. 80 V), than signals from upto 80 V will pass through the wire without any problem, causing damage maybe, if you chose it too low, 57 V signals will not pass.
If higher voltages goes through the diode, it will not break, but it will pass a lower voltage (probably upto the clamping voltage).
So chose the value of the TVS just above the maximum expected voltage.