How should TVS tubes and ESD static protection tubes be selected?

TVS tube and ESD electrostatic protection tube selection guide: a complete analysis from principle to practical application
In electronic circuit design, TVS tube (transient voltage suppressor) and ESD electrostatic protection tube are two core protective devices. The former is good at dealing with high-energy impacts such as lightning strikes and surges, while the latter focuses on precise protection against electrostatic discharge (ESD). The following provides practical selection solutions based on three dimensions: selection logic, parameter matching, and scenario adaptation, combined with actual cases.
1、 Core logic of selection: differentiated protection based on threat types
Protective object TVS tube ESD tube
Main threats include lightning strikes, power surges, inductive load switching, human body static electricity (HBM), and equipment discharge (MM)
Energy range of several hundred ampere level pulse current, duration of microsecond level nanosecond level pulse, typical value ± 8kV~± 30kV
Response speed 1-10ns (focusing on energy absorption)<1ns (focusing on fast clamping)
Typical scenarios include power input terminals, motor drives, USB/HDMI interfaces for industrial equipment, chip I/O, and high-frequency signal lines
Selection principle:
TVS is preferred for surge protection: it needs to withstand IEC 61000-4-5 standard testing (such as 8/20 μ s waveform).
ESD is preferred for electrostatic protection: it must meet the IEC 61000-4-2 standard (such as contact discharge ± 8kV).
Mixed scenario combination use: TVS serves as the primary protection to absorb high energy, while ESD serves as the secondary protection to handle residual pulses.
2、 Key parameter analysis: "hard indicators" during selection
1. Reverse Cut off Voltage (VRMM)
Definition: The maximum reverse voltage at which a device operates normally.
Selection rules:
TVS: VRWM ≥ 1.2 times the maximum operating voltage of the circuit (such as selecting 15V devices for 12V systems).
ESD: VRMM ≥ peak voltage of signal line (such as selecting 5.5V devices for 5V USB bus).
Misconception: Low VRWM can lead to false triggering, while high VRWM can reduce protection sensitivity.
2. Clamp voltage (VC)
Definition: The maximum voltage at both ends of a device after it is turned on.
Selection rule: VC ≤ the withstand voltage value of the protected device (with a 20% margin).
case
Chip withstand voltage 20V → TVS selects VC ≤ 18V (such as SMBJ15CA).
High speed signal line withstand voltage 10V → ESD select VC ≤ 8V (such as ULC0502P3).
3. Peak Pulse Current (IPP)
Definition: The maximum transient current that a device can withstand.
Selection rules:
TVS: IPP ≥ 1.2 times the actual surge current (e.g. IPP ≥ 200A is required for lightning testing at 6kV).
ESD: IPP ≥ 30A (IEC 61000-4-2 contact discharge current).
4. Junction capacitance (CJ)
Impact: Excessive CJ can cause signal distortion (e.g. USB 3.0 requires CJ<0.5pF).
Selection rules:
TVS: The power end can accept CJ>100pF.
ESD: Select CJ<0.3pF for high-frequency signal lines (such as ESD7004).
5. Packaging and heat dissipation
TVS: Select SMC/DO-214 package (such as SMBJ24A) for high-power scenarios.
ESD: Choose 0201/0402 package for portable devices (such as PESD15VS2UT).
3、 Scenario based selection practice
Case 1: Mobile USB 3.0 Interface Protection
Requirement:
Protection against ± 15kV ESD, signal rate 5Gbps.
Compatible with USB 3.0's 90 Ω characteristic impedance.
Plan:
Vbus power cord: Choose ESDA05CP (VRMM=5V, VC=10V, IPP=8A).
D+/D - Data cable: Choose ULC0524P (CJ=0.3pF, VC=12V).
Verification: The signal eye diagram shows no distortion after passing the IEC 61000-4-2 air discharge ± 30kV test.
Case 2: Industrial power surge protection
Requirement:
Dealing with lightning surge (8/20 μ s, 6kV).
Backend DC-DC withstand voltage of 40V.
Plan:
Select TVS tube SMBJ15CA (VRMM=17V, VC=24V, IPP=66A).
Paired with GDT (gas discharge tube) to discharge and amplify current, forming multi-level protection.
Verification: According to IEC 61000-4-5 Level 4 testing, the surface temperature of TVS is less than 125 ℃.
Case 3: Automotive Electronic Pulse 5A Protection
Requirement:
Compliant with ISO 7637-2 Pulse 5A (peak voltage 200V).
Working temperature -40 ℃~+125 ℃.
Plan:
Choose the TVS SM8S36A (VRWM=36V, VC=60V, AEC-Q101 certified) according to the vehicle specifications.
Verification: Through the load drop test, the clamp voltage remains stable below 60V.
4、 Misconceptions and Solutions in Selection
Misconception 1: Blindly pursuing low VC and neglecting power tolerance
Consequence: The device fails after multiple surges.
Improvement: Calculate transient power (Ppp=VC × IPP) to ensure it is within a safe range.
Misconception 2: Choosing High CJ Devices for High Frequency Signal Lines
Consequence: USB 3.0 signal jitter increases.
Improvement: Choose an ESD array with CJ<0.5pF or adopt a multi-channel integration solution.
Misconception 3: Ignoring the Effect of Temperature on VRMM
Consequence: VRWM drift in high temperature environment leads to false triggering.
Improvement: Select wide temperature range devices (such as -55 ℃~+150 ℃) and reserve 20% voltage margin.
5、 Summary: Selection Priority and Process
Determine the scenario: electrostatic discharge protection (ESD) or surge protection (TVS).
Parameter matching:
Prioritize ensuring that VRMM and VC meet the requirements.
CJ is preferred for high-frequency scenarios, and IPP is preferred for high-power scenarios.
Verification and testing: Confirm reliability through actual surge/ESD testing combined with thermal simulation.
Through systematic selection, the anti-interference ability of the circuit can be significantly improved, avoiding system failures caused by improper selection.