1. What is a surge?
A surge is an unpredictable transient voltage or current spike that is much longer in length and contains much higher energy than electrostatic discharge; But surges are complex and have many different causes;
Second, the standard of surge
While there are multiple causes for surges, fortunately they can take an approximate standard waveform regardless of the cause;
Similar to electrostatic discharge, the standard for surge can refer to IEC-61000-4-5, which defines the voltage waveform and current waveform of characteristic surges, the rise time of the voltage waveform is 1.2 microseconds, and the half-length is 50 microseconds; The rise time of the current waveform is 8 microseconds and the half-length is 20 microseconds;
To pass IEC-61000-4-5, the system must apply this waveform to the system input without damage to the system;
The peak voltage range where this waveform is applied is usually 500V~4000V, and the surge tester has an output impedance of 2 ohms; The standard defines the resistance of the coupling network as 40Ω, that is, a 500V surge generates a current of 11.9A;
The surge pulse length can be known through the standard, and the coupling network and open circuit voltage determine the testable short-circuit current, and the appropriate TVS tube can be selected with this information;
Experimental Level:
Third, TVS tube parameters
3.1 Maximum inrush current Ipp
This parameter defines the upper limit of the amount of current that the TVS tube can absorb, beyond this limit it will overheat and be damaged, the current that the TVS tube can withstand depends on the length of the current pulse, so the IPP is always defined with reference to the characteristic waveform, such as 8/20us waveform, 10/1000us waveform; When designing, ensure that the TVS tube Ipp is higher than the inrush current that must be withstood according to IEC 61000-4-5 testing, and that this inrush current refers to the actual waveform being tested.
The 8/20us waveform and the 10/1000us waveform are two different surge test waveforms, and the figure below is the peak pulse rated power diagram of a TVS
It can be seen from the figure that when the test waveform is 1000us, the maximum power that can be withstood is 5KW; When the test waveform is 20us, the maximum power that can be withstood is 20KW; If Vclamp does not change, then the maximum peak current Ipp that 20us can withstand is 4 times that of 1000us;
3.2 Clamping voltage Vclamp
Vclamp depends on the current flowing through the TVS tube and will therefore always be defined relative to the inrush current, which is the voltage that the system can see during the surge pulse, ensuring that Vclamp is less than the absolute maximum voltage of the input circuit
In summary, how to choose TVS tubes for surges? Just make sure the following two points
Fourth, the precautions for choosing TVS tubes
4.1 TVS tube parameters will change with temperature
Most surge tests are performed at room temperature, but real systems typically reach temperatures of 85°C or higher at the time of surge events, so derating is required
4.2 IPP and Vcalmp are defined with reference to specific waveforms
Make sure that the reference waveform in the TVS tube datasheet is the same as the system test waveform and can be converted by the peak power and pulse length curves in the datasheet
4.3 Parasitic parameters of TVS tubes: leakage current and junction capacitance
In battery-powered or power-sensitive systems, pay particular attention to leakage current and the effect of temperature on leakage current;
In the communication circuit, it is necessary to pay attention to the influence of the capacitance of the TVS tube junction on the signal quality.