Circuit protection is an important part of the design
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The most important design work in electronics is circuit design. Circuits can be divided into two categories: functional circuits and protection circuits. Functional circuits (such as power supply circuits, oscillator circuits, A/D conversion circuits, etc.) are used to implement various functions of electronic products; and protection circuits (overcurrent protection circuits, overvoltage protection circuits, and overcurrent and overvoltage protection circuits) ) is used to protect functional circuits to achieve the safety, reliability and durability of electronic products.
In recent years, portable electronic products have developed rapidly, such as MP3, MP4, PMP, DVD, digital cameras and camcorders, notebook computers, netbooks (a small laptop) and smartphones. The main features of these portable electronic products are more functions, complicated circuits, high operating frequency, high frequency bandwidth, high signal transmission rate, and small size (all using SMD components), which puts higher design requirements for circuit protection. In portable electronic products such as mobile phones and notebook computers, due to their high operating frequency, frequency bandwidth, and high data transmission rate, the advantages and disadvantages of circuit protection design not only involve the safety, reliability and durability of electronic products, but also It involves the performance quality and cost of electronic products. Therefore, circuit protection is an important task in the design of electronic products.
The importance of circuit protection
In today's portable electronic products, the range of circuit protection is extremely wide and numerous. Here are two typical portable electronic products to illustrate the importance of circuit protection.
1 mobile phone
The development of mobile phones ranges from a single call to a smart phone that can listen to music, take pictures, surf the Internet, watch TV, GPS, etc. The circuit is more complicated, and the corresponding protection circuit range is also expanding. Figure 1 shows the various parts of the phone that need to be protected.
Figure 1 Parts of the phone that need protection
2 laptop
Laptops have become a fashionable portable electronic product, and the recently developed "netbook" has been favored by young people. Laptops are more complex than mobile phones and require a wider range of components to protect (see Figure 2).
Figure 2 Components that need to be protected in a laptop
It should be noted that the portable electronic products in Figures 1 and 2 are not the latest products, but it can be seen that the scope and number of circuit protections are numerous, and the importance of protection circuits in these products can be seen.
It is these protective components that achieve the safety, reliability and durability of electronic products, and reduce the number of user repairs, maintenance costs and the impact of having mobile phones and laptops on the work.
Types of protection components and working principle
Protection components are divided into three categories: overcurrent protection, overvoltage protection, overcurrent, and overvoltage protection. Their basic working principle is as follows.
1. Overcurrent protection
The overcurrent protection component is a nonlinear positive temperature coefficient thermistor (PPTC), as shown in Figure 3. Once an overcurrent condition occurs, an excessive current causes the protection element to heat up to change the resistance value, from a low resistance state to a high resistance state, thereby limiting the current and protecting the protected circuit from overcurrent damage; After the fault is removed, the protection component cools and automatically changes from a high-impedance state to a low-resistance state, returning to normal operation. This type of protection component is called a self-resetting overcurrent protection component, such as Tyco Electronics' PolySwitchTM component.
Figure 3 PolySwitchTM component operating circuit
2, overvoltage protection
The overvoltage protection component is connected in parallel to the protected circuit (see Figure 4). When the input voltage is not over-voltage, the over-voltage protection component is in a high-impedance state, and its leakage current is very small; once the input voltage is over-voltage, the over-voltage protection component provides a low-impedance path in an instant, and the voltage clamp is placed in a safe state. Low voltage to protect the circuit from overvoltage damage. When the input voltage drops to the normal operating voltage, the overvoltage protection component will automatically return to the high impedance state. Tyco Electronics' multi-layer varistor (MLV) and ESD protection component PESD are commonly used overvoltage protection components.
Figure 4 Overvoltage protection component working circuit
3, overcurrent, overvoltage protection
The overcurrent and overvoltage protection device is a module (integrated circuit) composed of an overcurrent protection component and an overvoltage protection component, as shown in FIG. It protects the circuit from damage when the input voltage is overvoltage or an overcurrent fault is caused by a short circuit.
Figure 5: Overcurrent, overvoltage protection device operating circuit
Tyco Electronics' PolyZenTM components are commonly used overcurrent and overvoltage protection devices.
Features of protective components for portable products
In order to meet the requirements of portable electronic products for protective components, the protective components should have the following characteristics.
1, performance
â— The response time should be short;
â— Its own power consumption is small, which can extend battery life;
â— Stable performance and long life;
â— Does not affect the performance of the protected circuit;
â— Meet environmental requirements (lead-free, RoHs compliant).
2, packaging and size
â— SMD package to meet SMT requirements;
â— Small package size (small PCB area) and low profile height.
3, other
â— Reflow soldering process can be used;
â— Low prices and reduced product costs.
PolySwitchTM components and their application circuits
The PolySwitchTM component is a self-resetting polymer nonlinear positive temperature coefficient (PPTC) thermistor. In 1995, Tyco Electronics developed the SMD miniSMDTM overcurrent protection component. In recent years, it has developed smaller series such as microSMDTM, nanoSMDTM and picoSMDTM. In addition, new varieties of 24V and 60V have been added to the miniSMDTM series and 2.0A has been added to the microSMDTM series.
The main parameters of the PolySwitchTM component: when the current is not overcurrent, the resistance value is very small (the resistance value is different at different rated voltages, generally a few Ω to several Ω); when the overcurrent occurs, the resistance value can reach mΩ. The current is very close to "off" and the time-to-trip is shorter.
The package size of the component is small; the current is 0.05~3A; the voltage is 6~60V; the resistance value is small in the normal state; the UL, CSA and Tå“£ arguments are passed.
The components are used in a wide range of applications, mainly computers, portable electronics, multimedia, game consoles, mobile phones, as well as automotive, industrial control and batteries.
Figure 6 Overcurrent protection with PolySwitchTM components
The overcurrent protection of PolySwitchTM components in portable electronics adapters (external power supplies) is shown in Figure 6. The structure uses two PolySwitchTM devices to prevent damage to the adapter during a short circuit.
Figure 7 Protecting the charger with PolySwitchTM components
The charger is an essential accessory in portable electronics, and Figure 7 shows the application of the PolySwitchTM component in the charger. Inside the dotted line in Figure 7, there is a battery pack in which the PolySwitchTM component is used for charge and discharge overcurrent protection.
MLV, PESD and its application circuit
MLV and PESD are overvoltage protection devices. They are also electrostatic discharge (ESD) protection components in addition to overvoltage protection. When plugging and unplugging various plugs of portable products, each port is easily damaged by electrostatic discharge, so ESD protection components are required for plugging and unplugging ports.
Adding an ESD protection element to the port that transmits the signal can prevent electrostatic discharge damage, but because the ESD protection element has a certain capacitance, it will cause signal attenuation and distortion during high-speed signal transmission. Therefore, the correct choice of ESD protection components for high-speed data transmission ports is very important.
The MLV has a large capacitance (40 to 220 pF) and is mainly used for ports with low DC and data transmission rates. However, the "E" series in the MLV has a small capacitance of only 3 to 12 pF, and can be applied to a port with a higher data transmission rate for ESD protection.
PESD is a protection component developed specifically for ESD protection of high data rate ports. Due to its extremely low capacitance (0.25pF), it meets the requirements of IEEE1394 and HDMI1.3, and satisfies the signal transmission quality during high-speed data transmission. Therefore, PESD is used as an ESD protection component in products such as high-definition televisions, LCD plasma TVs, notebook computers, and mobile phones.
Figure 8 PESD inhibition characteristics
The main characteristics of ESD: leakage current is small, response time is short (<1ns), phase voltage is low, and capacitance is small.
There are two types of PESD: PESD0603-240 and PESD0402-140. The electrical characteristics are shown in Table 1. The suppression characteristic curve is shown in Figure 8.
The most important parameter of an ESD component is the capacitance. Generally, the capacitance of the MLV is 40 to 220 pf, and the capacitance of the E series is 3 to 12 pf; and the capacitance of the PESD is only 0.25 pf.
Figure 9 Eye diagram of the ESD protection component at 2.25 GHz
Figure 10 PESD eye diagram at 3.4 GHz
Figure 11 shows the port protection circuit
The relationship between the different operating frequencies and the capacitance requirements of the ESD protection components is shown in Table 2. A comparison of commonly used ESD protection components is shown in Table 3.
The eye diagram of the silicon ESD protection component at 2.25 GHz is shown in Figure 9, and the eye diagram of the PESD component at 3.4 GHz is shown in Figure 10. The comparison of these two figures illustrates the excellent performance of PESD.
Figure 11 is a typical display port protection circuit. The 2.7GHz high-frequency signal line uses eight PESD protection components. The auxiliary channel operates at 1MHz and uses five MLV protection components. In the low-voltage DC power supply, PolySwitchTM components are used for overcurrent protection. This design meets the circuit's energy requirements and reduces costs.
Figure 12 USB2.0 interface protection circuit
Figure 13 IEEE1394 interface protection circuit
12 and 13 are respectively the protection circuit design of the USB2.0 interface circuit and the protection circuit design of the IEEE1394 interface circuit.
PolyZenTM components and their application circuits
PolyZenTM component devices are integrated circuits that combine a precision Zener diode with a polymer positive temperature coefficient (PPTC) component. It is a protection device for preventing induced spike voltage, instantaneous high voltage, misuse of power adapter to overvoltage and overcurrent of the circuit. The internal structure is shown in Figure 14. Its typical application circuit is shown in Figure 15. In Figure 15, RLOAD is the protected downstream circuit. 
Figure 14 PolyZenTM component internal structure
Figure 15 Typical Application Circuit for PolyZenTM Components
During normal operation, the VIN input voltage is higher than the breakdown voltage VZ of the Zener diode, and the IFLT current is passed through the Zener diode to ground, and VOUT outputs a stable voltage. When there is an abnormal overvoltage input VIN, the Zener diode's IFLT will generate an overcurrent. When there is an overcurrent on the device, its resistance will change from a low-resistance state to a high-impedance state, resulting in a large voltage drop across it. As the VOUT output is substantially unchanged, the current IFLT flowing through the Zener diode is reduced, as shown in FIG. The increase in voltage drop across the device protects both the Zener diode and the downstream circuitry. In addition, if there is a partial short circuit or short circuit fault in the protected downstream circuit, IOUT will increase, and the PPTC component will change from a low resistance state to a high resistance state, which can make the circuit get overcurrent protection.
During normal operation, the VIN input voltage is higher than the breakdown voltage VZ of the Zener diode, and the IFLT current is passed through the Zener diode to ground, and VOUT outputs a stable voltage. When there is an abnormal overvoltage input VIN, the Zener diode's IFLT will generate an overcurrent. When there is an overcurrent on the device, its resistance will change from a low-resistance state to a high-impedance state, resulting in a large voltage drop across it. As the VOUT output is substantially unchanged, the current IFLT flowing through the Zener diode is reduced, as shown in FIG. The increase in voltage drop across the device protects both the Zener diode and the downstream circuitry. In addition, if there is a partial short circuit or short circuit fault in the protected downstream circuit, IOUT will increase, and the PPTC component will change from a low resistance state to a high resistance state, which can make the circuit get overcurrent protection.
Figure 16 Voltage or current changes when PolyZenTM components are operating
The main features of the device: protect downstream electronic components from overvoltage and reverse bias (eg 5V adapter misused 12V adapter or polarity reverse); due to the role of PPTC components, Zener is reduced Diode heat dissipation requirements reduce PCB area and cost; in the event of overvoltage faults, the VOUT output is essentially unchanged; Zener diodes have a high withstand capability of up to 30W; small-size patches or 4mm x 4mm packages.
The device is mainly used for portable multimedia player (PMP), GPS, 5V and 12V hard drive bus protection, USB5V bus protection and so on.
The device has nine varieties and its electrical characteristics are shown in Table 4. Among them, ZEN132V230A16LS and ZEN056V075ALS are new products to be launched soon.
Conclusion
Since circuit protection components appear to be simple components, they are often ignored. In many circuit books, there is no introduction to protection circuits or only a few old products (such as fuses, zinc oxide varistor and TVS). The technical books in this area cannot keep up with the development of products, so the source of the data is mainly Data sheet and application guide for circuit protection component manufacturers.
Due to space limitations, this article simply introduces several of the original protection devices used by Tyco Electronics in portable electronic products. In addition, the application range of protective components is extremely wide, in addition to portable electronic products, there are communications, automotive, medical equipment, household appliances, industrial control and other fields.
Connector 1.5Mm Pitch,Zh Connector,Zh Terminal,Pitch Connector
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