The difference in the relative position of the TVS and the resistor will affect the anti-surge performance of the interface circuit. I believe most people have not studied it in detail. Today we will discuss this issue theoretically.

According to the relative position of TVS and resistance, there are two connection methods as shown in Figure 1 and Figure 2. The following two connection methods are found in real tests: Figure 1 The interface failure is caused by the damage of the resistor R1 connected in series to the interface circuit, and the TVS is not damaged at this time; Figure 2 The interface failure is caused by the breakdown of the TVS, the resistance No damage. A conclusion can be drawn from this: when the resistor is close to the chip, the TVS is more protective when placed close to the interface.

Figure 1 Model-Circuit schematic diagram with the resistance outside the TVS

Figure 2 Model-Circuit schematic diagram of resistance inside and outside TVS

For Figure 1: When TVS is operating, I=Ia+Ib, I>Ia, and a larger current will flow through the resistor R1 than in TVS. Although the protected circuit is protected to a certain extent in this case, the resistor R1 is burned out because it cannot withstand the large current, which leads to interface failure and protection failure.

For Figure 2: When TVS is in action, the impedance of TVS is very low, most of the current flows through TVS, and the current flowing through R1 only occupies a small part. The effect of Ib< is better than that in Figure 1. Figure 1 is mainly because the resistance is burned out by instantaneous high current. If the resistance in Figure 1 is selected with a higher power, a good anti-surge effect can also be achieved. Therefore, in the design of the interface protection circuit, if we want to reduce the inrush current in the surge through series resistance, we must consider the power of the resistance. If the power is not enough, it is recommended to use the design in Figure 2, that is, place the resistor close to the chip and the TVS close to the interface.

From this we can also sum up some experiences:

Resistors can be used to suppress surge currents and limit currents. They are usually connected in series with current. But you need to pay attention to the selection of resistance power when using it to avoid burning out due to excessive current;

For the two surge protection principles in the above figure, the model can be simplified. First, if the input impedance of the protected circuit is higher (RL>>R1), the model shown in Figure 1 is used, because at this time the series connection of R1 and RL It can not improve the current limiting or voltage division, and the impedance of the TVS is low when it is effective, so the current limiting resistor can be connected outside the TVS to achieve a good current limiting effect. If the input impedance of the protected circuit is low (RL is equal to or smaller than R1), the model shown in Figure 2 is used. At this time, R1 will further divide the residual voltage of the second-stage surge after TVS, greatly reducing the two levels of the protected circuit. Surge voltage at the end.