How should you select a TVS device? PCB layout is critical for TVS performance.

To enhance understanding of TVS diodes, this article will discuss the selection criteria for TVS diodes and the impact of PCB layout on their surge‑protection performance.

I. TVS Device Selection

Having understood the basic parameters of a TVS diode, we can now proceed to the most critical stage: TVS selection. Before choosing a TVS device, it is essential to first clarify the ultimate objective of the selection.

1. Proper voltage can protect downstream circuits;
2. The junction capacitance of the introduced TVS must not affect the circuit;
3. The TVS has ample power margin, meets the test standards, and must not trip before the fuse.
The selection process can be carried out according to the following steps:
1. Select the TVS’s maximum operating voltage, Vrmw,
2. Select the TVS clamping voltage VC:
3. Select the TVS power rating:
4. Evaluating the impact of leakage current lr:
5. Evaluate the impact of junction capacitance;

When selecting the TVS’s maximum operating voltage, Vrmw, it is essential that the TVS remains non‑conducting under normal circuit operation—i.e., in its cutoff state. Therefore, the TVS’s clamping voltage should exceed the highest operating voltage of the protected circuit to ensure that the TVS does not interfere with normal circuit performance. However, the TVS’s operating voltage also determines its clamping voltage: even when the clamping voltage is higher than the circuit’s normal operating voltage, the TVS’s operating voltage should not be set excessively high. If it is too high, the standoff voltage will also increase, potentially leading to premature conduction or avalanche breakdown and compromising normal circuit operation. When choosing Vrmw, a comprehensive assessment must be made, taking into account both the operating voltage of the protected circuit and the withstand capability of downstream circuitry. Vrmw should be greater than the operating voltage; otherwise, if the operating voltage exceeds Vrmw, the TVS may experience increased reverse leakage current, approach conduction, or undergo avalanche breakdown, thereby disrupting normal circuit behavior. For a balanced selection, Vrmw can be estimated using the following formula: Vrwm ≈ 1.1–1.2 × VCC, where VCC represents the circuit’s maximum operating voltage. Regarding the TVS clamping voltage, VCTVS, it should be lower than the maximum transient safe voltage that the downstream protected circuit can tolerate. The clamping voltage, VC, is directly proportional to the TVS’s avalanche breakdown voltage and to the peak pulse current (IPP). For TVS devices of the same power rating, a higher breakdown voltage corresponds to a higher clamping voltage. Consequently, the selected TVS’s maximum clamping voltage, Vc, must not exceed the maximum voltage that the protected circuit can safely withstand; otherwise, when the TVS clamps at Vc, it could damage the circuit. The clamping voltage, Vc, can be estimated using the following formula: VCPactual, which takes into account the TVS’s junction capacitance and leakage current to evaluate the maximum allowable voltage. If the TVS is used for protecting high‑speed I/O ports, analog signal sampling, or low‑power devices, the effects of junction capacitance and leakage current must be carefully considered; smaller values for these parameters are preferable.

II. The Impact of PCB Layout and Routing on TVS Protection Performance

TVS diodes are the most commonly used devices for overvoltage protection. But does proper device selection alone suffice to complete the design? How can we ensure that a TVS diode delivers optimal performance in a circuit? The layout and routing of the printed circuit board (PCB) are critical. By strategically positioning the TVS diode, selecting appropriate grounding schemes, and carefully managing parasitic inductance and loop areas, a well‑thought‑out PCB layout and routing can maximize the TVS diode’s effectiveness.
Place TVS devices as close as possible to the noise source to ensure that surge voltages are clamped before they can couple into nearby PCB traces.

The protection circuit should shunt surge voltages to the chassis ground; routing surge voltages directly to the signal ground of an integrated circuit can cause ground bounce. Reducing impedance by using relatively short and wide grounding traces can improve the clamping performance of TVS diodes on a single‑ground PCB.

TVS wiring should avoid self‑inductance. ESD is a high‑energy, transient pulse that can easily induce parasitic self‑inductance in the circuit, leading to severe voltage spikes that may exceed the IC’s withstand capability and cause damage. The self‑induced voltage generated by the load is directly proportional to the magnitude of the power‑supply change; the abrupt nature of ESD events readily triggers strong self‑inductance.
The fundamental principle for minimizing parasitic self‑inductance is to shorten shunt loops as much as possible, taking into account all relevant paths—such as the ground loop, the circuit connecting the TVS device to the protected line, and the path from the interface to the TVS. Therefore, the TVS device should be placed as close as possible to both the interface and the protected line, thereby reducing the likelihood of self‑inductive coupling into nearby traces. Reducing the loop area formed by high‑speed data lines and ground traces can help mitigate radiated emissions and RF interference. In particular, when trace lengths are significant, an effective approach to addressing loop issues is to incorporate a ground plane in the PCB design and to increase the footprint of the TVS diode, providing isolation between integrated circuits. However, this also tends to enlarge the loop area.

In addition, the following principles may be applied to optimize the line:
1. Arrange the interfaces on the same edge whenever possible.
2. Avoid paralleling protected circuits with unprotected ones, and refrain from routing critical signal traces near protection circuits.
3. When an interface incorporates both filtering and protection circuits, the principle of “protection before filtering” should be followed. The protection circuit is designed to suppress external overvoltage and overcurrent; if it is placed after the filtering circuit, the latter could be damaged by such transient conditions.
4. Keep reset, interrupt, and control signals away from the input/output ports, as well as from the PCB edges and discharge points.
Using TVS diodes to provide ESD protection for electronic products is a convenient, effective, and highly reliable approach. Component selection and PCB layout and routing are equally critical. We hope this brief overview will help you deepen your understanding of TVS diode PCB layout design, enabling you to optimize your board early on and achieve a lasting, robust solution.

That concludes our discussion of TVS devices. We hope this article has provided you with a solid understanding of TVS diodes.