Inductors, resistors, and wires are not protection devices themselves, but they can play a cooperative role in the protection circuit formed by the combination of multiple different protection devices.
Among the protective devices, the gas discharge tube is characterized by large flux, but slow response time and high impact breakdown voltage; TVS tube has small flux, the fastest response time, and the best voltage clamping characteristics; the characteristics of varistor are medium. Between the two, when a protection circuit requires a large overall flow rate and can achieve fine protection, the protection circuit often requires the cooperation of these types of protection devices to achieve ideal protection characteristics. However, these protective devices cannot be simply used in parallel. For example, a varistor with a large flux and a TVS tube with a small flux are directly connected in parallel. Under the action of overcurrent, the TVS tube will be damaged first, and the varistor cannot be used. The advantage of large traffic flow. Therefore, when several protective devices are used together, it is often necessary to cooperate with different protective components such as inductors, resistors, and wires. These components are described below:
In the series DC power supply protection circuit, there cannot be a large voltage drop on the feeder, so the coordination of the inter-pole circuit can use an air-core inductor, as shown in the following figure:
Use inductors to realize the cooperation of two-level protection devices
The role of inductance: when the protection circuit reaches the designed flow rate, the overcurrent on the TVS should not reach the maximum flow rate of the TVS tube, so the inductor needs to provide enough current limiting capability for lightning strike overcurrent.
In the power supply circuit, the design of the inductor should pay attention to several issues:
1. The inductance coil should be able to work normally without overheating when the full working current of the equipment flows;
2. Use air-core inductors as much as possible. Inductors with magnetic cores will have magnetic saturation under the action of overcurrent. The inductance in the circuit can only be calculated by the inductance when there is no magnetic core;
3. The coil should be wound in a single layer as much as possible, which can reduce the parasitic capacitance of the coil and enhance the tolerance of the coil to transient overvoltage;
4. The insulating layer on the wire of the wound inductance coil should have sufficient thickness to ensure that there is no breakdown and short circuit between turns of the coil under the action of transient overvoltage.
In the protection circuit design of the power port, the inductance is usually 7~15uH.
In the signal line, the components connected in series on the line should suppress the high-frequency signal as little as possible, so the resistance between the poles can be used, as shown in the following figure:
Use resistors to realize the cooperation of two-level protection devices
The resistor should play basically the same role as the inductor previously described. The above figure is taken as an example, the calculation method of the resistance value is: measure the impulse breakdown voltage value U1 of the air discharge tube, check the TVS device manual to obtain the maximum flow rate I1 of the TVS tube under the 8/20us impulse current, and the maximum clamp of the TVS tube Bit voltage U2, the minimum value of the resistance is: R≥(U1-U2)/I1.
In the signal line, the use of resistors should pay attention to several issues:
1. The power of the resistor should be large enough to avoid damage to the resistor under the action of overcurrent; 2. Try to use a linear resistor to minimize the impact of the resistor on normal signal transmission.
The full configuration working current of some AC/DC equipment is very large, exceeding 30A. In this case, the use of inductance between the poles of the protective circuit will cause the problem of excessive volume. To solve this problem, the protective circuit can be divided into two In some cases, the front-level protection and the rear-level protection are not designed on the same circuit board, and a feeder with a specified length can be used for coordination between the two circuits. Buy components in stock on the only sample mall.
Use wires to realize the cooperation of two-level anti-device
In the protection circuit formed by this combination, the function of the feeder with a specified length is the same as that of the inductance, because the inductance of a 1-meter-long wire is between 1 and 1.6uH, and the feeder can reach a certain length. It plays a good role in coordination, and the wire diameter of the feeder can be flexibly selected according to the size of the working current of the full configuration, which overcomes the disadvantage that the inductance cannot flow a large working current when the inductance is used for inter-pole coordination.