In the semiconductor electronics industry, if you want to become a qualified electronic, in addition to understanding the basic knowledge, but also know the key points of circuit protection device selection. Users buy electronic products more likely to take a fancy to the function of the product, and manufacturers to rely on product reputation to occupy the market, it needs the best product quality, this time product circuit protection has become the key to ensure product quality and safety. As we all know, circuit protection is mainly to protect the components of electronic circuits from damage under the circumstances of overvoltage, overcurrent, surge, electromagnetic interference, and so on, and the selection of appropriate circuit protection devices becomes the key factor to determine the effectiveness of the protection scheme.
Overvoltage, overcurrent, surge, electromagnetic interference, electrostatic discharge, etc., has always been the first circuit protection, therefore, the mainstream circuit protection devices in the market are mainly lightning protection/overvoltage/overcurrent/anti-static, etc. Shuokai Electronic research and development of the production of protective devices are ceramic discharge tube, gas discharge tube, solid discharge tube, transient diode, varistor, self-recovery fuse and ESD electrostatic diode, a qualified electronic understanding of the key points of circuit protection device selection can quickly according to the product protection grade, application port and specification parameters to determine the circuit protection device model.
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As circuit structures and the physical size of electronic products become smaller, it becomes increasingly important to design circuit protection early in the design cycle. Although performance and performance are important in a design, if circuit protection is not incorporated early in the design cycle, it can lead to delays in the development cycle and, in a bad case, failure in the market.
At the beginning of designing circuit protection scheme, we should first determine what damage the product needs to protect; Decide what outcome you want when a failure situation arises; We need to make reasonable assumptions about what "normal" and "abnormal" operating conditions are; ** Be aware that no protection is possible if you design protection for a specific event, but there is always a chance that something more serious will happen; The circuit protection scheme should be planned at the beginning of the design. Although the circuit protection device is much smaller than in the past, it is impossible to add the circuit protection device without sufficient space after the PCB design is completed.
The circuit protection device is divided into overvoltage type and overcurrent type according to the different protection effects. Traditional overcurrent protection is generally the application of fuses, but because of its disadvantages are obvious, can only play a protective role, therefore, in Shuokai Electronics reusable PTC self-recovery fuse research and development after the successful mass production, in many overcurrent protection applications instead of fuses, especially outdoor communication, security equipment overcurrent protection, Almost all of them are PTC self-restoring fuses. The following are the main points and points for attention in selection of PTC self-restoring fuses of overcurrent devices arranged by Shuokai Electronics:
Key points of selection of PTC self-restoring fuse:
1. Keep the current slightly higher than the user's normal working current.
2.Vmax must be greater than or equal to the user's large operating voltage.
3.Imax should be larger than the short-circuit current of major faults.
PTC self-restoring fuse selection points for attention:
PPTC is actually a positive temperature coefficient thermistor, so it is greatly affected by temperature. The parameters in the specification are measured at around 25℃. As the temperature increases, the holding current and trigger current will decrease, so it is important to know the temperature of the environment in which PPTC is applied.
2.PPTC is used to prevent back-end equipment from short-circuiting and abnormal current, and cannot be used to prevent transient surge, because its operation time is relatively slow.
3. The operation time of PPTC is controlled by the short-circuit current. The larger the short-circuit current is, the faster the operation time will be.
The overvoltage device is divided into clamp type and switch type according to the different mode of action. Switching type overvoltage devices are the lightning protection devices we are familiar with: ceramic gas discharge tube, semiconductor discharge tube and glass discharge tube. Another class of clamp type overvoltage devices are clamp type overvoltage devices with transient suppression diode, varistor, patch varistor and ESD discharge diode.
The following four points should be noted in the selection of overvoltage protection devices:
1) Selection of turn-off voltage Vrwm. Generally, the turn-off voltage should be at least 10% higher than the line high working voltage;
2) Clamp voltage VC selection. VC refers to the voltage passed by TVS in ESD impact state, it is less than the protected circuit can withstand the large transient voltage;
3) The selection of surge power Pppm. The protection time varies with the power, such as 600w (10/1000μs); 300W (8/20μs);
4) Selection of intermediate capacitance. The higher the operating frequency of protected components, the smaller the capacitance of TVS is required.