Inductors, resistors, and wires are not protective devices in themselves but can play a role in combining several different protective devices to form a protective circuit.
Protection devices, gas discharge tube, is characterized by a large flux, but the response time is slow high impact breakdown voltage; TVS tube flux is small, the fastest response time, the best voltage-clamp characteristics; varistor characteristics between the two, when a protection circuit requires a large overall flux to achieve fine protection, the protection circuit often requires these types of protection devices to work together to achieve more ideal protection characteristics. However, these protective devices cannot simply be used in parallel.
For example, suppose a high-flux varistor and a low-flux TVS tube are directly connected in parallel. In that case, the TVS tube will be damaged first under the action of overcurrent, and the advantage of high-flux varistors cannot be exploited. Therefore, several protection devices used in conjunction with the occasion often need inductors, resistors, wires, etc., between the different protection components to cooperate. The following is an introduction to each of these components.
Inductor: In the series DC power protection circuit, there should not be a large voltage drop on the feed line, so the inter-pole circuit can be used with a hollow inductor, as follows.
Inductance plays a role: the protection circuit to reach the design flux, the TVS on the overcurrent should not reach the maximum flux of the TVS tube, so the inductor needs to provide sufficient current limiting capacity for lightning overcurrent.
In the power circuit, the design of the inductor should pay attention to several issues.
The inductor coil should operate normally without overheating when flowing through the full operating current of the device.
Hollow inductors should be used as much as possible. Inductors with magnetic cores can become magnetically saturated under overcurrent, and the inductance in the circuit should only be calculated as the inductance without a magnetic core.
The coil should be wound single layer as far as possible. This can reduce the parasitic capacitance of the coil and, at the same time, can enhance the tolerance of the coil to transient overvoltage.
Winding inductor coil wire on the insulation layer should have sufficient thickness to ensure that the transient overvoltage effect between the coil turns does not occur in the breakdown of the short circuit.
In the company's power port protection circuit design, the inductor usually takes the value of 7 to 15uH.
Resistance: In the signal line, the line is connected in series to the components of the high-frequency signal inhibition to be as little as possible, so the inter-pole fit can be used resistance, as follows.
The resistor should play the same role as the role above of the inductor is the same. In the above figure, for example, the resistance value is calculated as follows: the measured impact breakdown voltage value of the air discharge tube U1. Check the TVS device manual to get the TVS tube 8/20us inrush current under the maximum flux I1 and the highest TVS tube clamp 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 resistor's power should be large enough to avoid damage to the resistor under the action of overcurrent.
2, try to use linear resistors so that the impact of resistance on normal signal transmission is as small as possible.
Conductor: Some AC / DC equipment with full working current is very large, more than 30A. In this case, the protection circuit between the poles using inductors will be too large. The protection circuit can be divided into two parts to solving this problem. The front and rear protection are not designed on the same circuit board, while the two levels of the circuit can be used between the specified length of the feeder line.
This combination forms the protection circuit. The role of the specified length feeder line and the role of the inductor is the same because the inductance of the 1-meter-long wire between 1 and 1.6uH, feeder line to a certain length, can play a good role with the feeder line diameter can be selected flexibly according to the size of the full working current, overcoming the use of inductors to do interpolate with the inductor can not flow through a large working The shortcomings of using inductors for inter-pole coordination.