How to choose TVS devices

ESD and surge problems are often the biggest headaches for baseband engineers because of the stringent testing standards and the unpredictable nature of the problems. ESD problems in particular, there is no standard path to solve the problem, but only by repeatedly conceptualizing the program and verification.

Want to try to avoid the above problems, you must choose the right protection devices, design to do enough protection measures. This article tells you the ESD and surge test standards, test methods, and how to choose TVS devices.

1. TVS tube test standards

TVS tube here, including anti-ESD with low-power TVS tube and anti-surge with high-power TVS tube.

The test standards mentioned here refer to the test standards indicated in the specification of TVS devices: IEC61000-4-2 Level 2

IEC61000-4-2 Level 4 ESD Protection - electrostatic test standard, must pass, test data provided

IEC61000-4-4 Level 4 EFT protection - some manufacturers will only provide data

IEC61000-4-5 (8/20us)--Surge test standard, must pass, test data provided

This is the information provided in the specification of the ESD tube used in general signal lines for the ON Semiconductor ESD5481MUT5G, and completed the three standards.

2. TLP test

Transmission Line Pulse test, with a 100ns pulse width square wave, measuring the current value at different voltage amplitudes, the voltage value increases until the tube is damaged. 100ns is much larger than the IEC61000-4-2 static waveform 1ns, which is more able to test the performance of the ESD tube, but also more responsive to the ESD tube's clamp capacity, the following chart is the surge Test and TLP test comparison, it is clear that the TLP energy is greater.

When we see two ESD tube can pass the contact ± 8kV electrostatic test, how do we go to choose it, this time we have to look at the ESD tube clamp voltage that is lower, if they are about the same, then use TLP to measure the I-V curve, the following figure, compare the same current clamp voltage, the lower one is the best.

3. ESD tube contact discharge waveform measured

An ESD tube contact ± 8kV measured waveform, you can see that the ESD tube can clamp the 8kV voltage to about 30V, or very powerful.

4. surge tube measured waveform

5. TVS tube technical indicators

Ipp: maximum reverse peak current, is in accordance with the IEC61000-4-5 standard, 8/20us current waveform, capacitance / inductance to coupling network test. The larger this value is, the better the TVS tube performance.

Vc@Ipp: Clamp voltage, when the maximum current Ipp, the clamp voltage value. This value, the smaller the better

Vrwm: maximum reverse operating voltage, the signal being protected, its normal operation of the voltage can not exceed this voltage. In fact, this voltage is the voltage value when the TVS tube reverse leakage current is 1uA, if the voltage is even larger, the reverse current will quickly increase. the number on the TVS model is this voltage.

Vbr: reverse breakdown voltage, the voltage value when the reverse current of the diode reaches 1mA. When the reverse voltage exceeds this value, the reverse current will increase sharply.

Cj: junction capacitance, generally pF level, when used in high-speed signal lines, such as USB2.0 D+ and D-, the junction capacitance should be less than 1pF.

Pppm: in accordance with IEC 61000-4-5 standard test out the maximum Ipp multiplied by the clamp voltage Vc. surge tube selection, do not just look at this value, but also to see what the clamp voltage is.

6. ESD tube and surge tube selection

Vrwm shall not be less than the working voltage of the protected signal.

From the perspective of reducing leakage considerations, Vrwm than the working voltage of the protected signal, the greater the leakage current is smaller. Do not underestimate the leakage current of 1uA, the standby current of many devices are nA level oh.

From the point of view of protection strength, the closer Vrwm to the signal operating voltage, the lower the clamp voltage Vc of the tube, the better the protection of the signal.

ESD tube and surge tube electrostatic protection level to be greater than the protection level required by the product itself.

ESD tube and surge tube of the electrostatic clamp voltage, the lower the surge clamp voltage, the better.

The clamp voltage of the surge tube should be less than the maximal rating of the protected circuit.

Surge tube power requirements: assume that to pass 300V surge, the required clamp voltage is 20V or less, because the internal resistance of the surge tester is 2ohm, so the maximum output current is (300V-20V)/2ohm = 140A, it is necessary to choose the tube power in 140A * 20V = 2800W or more. Of course, if the clamp voltage of this surge tube is less than 20V, the power requirements can also be lower.

The protection performance of the tube, mainly related to the clamp voltage, do not look at parameters such as Ipp.

In short, first look at Vrwm, then look at Vc and power, it is easy to make a choice.

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