As the complexity and integration of electronic systems are getting higher and higher, and the working voltage is getting lower and lower, the requirements for reliability, stability and safety of the electronic systems are getting higher and higher, and the importance of circuit protection design is getting higher and higher. Come stronger. In the circuit protection design, whether the selection and application of circuit protection devices are reasonable will directly affect the protection effect of the circuit protection scheme of the electronic system.

In order to help engineers choose circuit protection devices correctly, and rationally use circuit protection devices to design efficient circuit protection solutions, the editor of Yuhuaxin Electronics has compiled some experience and will introduce to you:

The first part introduces the selection skills of common circuit protection devices;

The second part focuses on the actual application of fuses, transient voltage suppressors, ESD protection devices, lightning protection devices, etc.;

There are three main forms of circuit protection: overvoltage protection, overcurrent protection and overtemperature protection.

Choosing an appropriate circuit protection device is the key first step to realize an efficient and reliable circuit protection design. So, how to choose a circuit protection device reasonably? Different protection devices have different protection principles, so the selection should be combined with their protection Consider the principle, working conditions and use environment.

This article will introduce several commonly used over-voltage, over-current and over-temperature protection devices selection techniques to help engineers correctly select circuit protection devices.

1. Key points for selection of overvoltage protection devices

Overvoltage protection devices (OVP) are used to protect subsequent circuits from load dumping or instantaneous high voltage damage. Commonly used overvoltage protection devices include varistors, transient voltage suppressors, electrostatic suppressors, and discharge tubes. The following four points should be paid attention to when selecting overvoltage protection devices:

1) Selection of the turn-off voltage Vrwm. Generally, the cut-off voltage is at least 10% higher than the maximum working voltage of the line

2) The choice of clamping voltage VC. VC refers to the voltage that passes through the TVS in an ESD impact state, and it must be less than the maximum transient voltage that the protected circuit can withstand

3) Selection of surge power Pppm. Different power, the protection time is different, such as 600w (10/1000us); 300W (8/20us)

4) The choice of capacitance between electrodes. The higher the working frequency of the protected component, the smaller the capacitance of the TVS is required

1.1 ESD suppressor

Choosing the right ESD protection device, the biggest difficulty lies in how to easily determine which device can provide the greatest protection. System suppliers generally compare the quality of ESD protection devices through the ESD ratings (or nominal values) on the data sheet. In fact, from these ratings, it is impossible to see how strong the device protection system is, and the key depends on its diode parameters. The main reference coefficients should be:

Fast response time

Low clamping voltage

High current surge withstand capability

The selection of ESD devices should comply with the following requirements:

(1) Attention when choosing electrostatic protection devices:

The clamping voltage should not exceed the maximum withstand voltage of the protected device

The circuit voltage does not exceed the working voltage of the protection device

Low capacitance value, leakage current to reduce interference and loss as much as possible

(2) The electrostatic protection device should be installed as close to the electrostatic input as possible, far away from the protected device

(3) The ground wire that the electrostatic protection device must be connected to, not the digital ground wire

(4) The return line should be as short as possible, and the distance between the electrostatic protection device and the protected line should be as short as possible

(5) Try to avoid routing the protected and unprotected lines side by side

1.2 Varistor

Varistor is one of the most used voltage-limiting devices. Widely used in automotive electronics, communications, computers, consumer electronics, military electronics, etc., especially in LCD, keyboard, I/O interface, IC, MOSFET, CMOS, sensor, mobile phone, DVD, AV, ABS , Motor control board, MP3, PDA, USB interface and high-speed data signal line for protection, etc.

When selecting a varistor, it should be noted that the power supply voltage continuously applied to both ends of the varistor cannot exceed the maximum continuous working voltage listed in the specification table. Full consideration should also be given to the fluctuation range of the working voltage of the power grid (or circuit), and sufficient margin should be left when selecting the varistor‘s varistor voltage value. The general domestic fluctuation range is 30%. The maximum surge current through the varistor should not exceed the maximum inrush current ±± value in the technical specification (that is, the maximum flow rate). Considering the need to withstand multiple shocks, the surge current value that can withstand more than 10 shocks should be selected. The clamping voltage of the varistor must be less than the maximum voltage that the protected component or equipment can withstand (ie, safe voltage).

1.3 Transient Voltage Suppressor TVS

A transient voltage suppressor (TVS) is a high-efficiency protection device in the form of a diode. When the two poles of the TVS diode are impacted by the reverse transient high energy, it can change the high impedance between the two poles into low impedance at a speed of 10-12 seconds, absorb up to several kilowatts of surge power, and make the two poles The middle voltage clamp is located at a predetermined value, which effectively protects the precision components in the electronic circuit from damage by various surge pulses.

Transient Voltage Suppression Diodes (TVS) are widely used in semiconductors and sensitive electronic parts over-voltage, ESD protection, including: consumer products, industrial products, communications, computers, automobiles, power supplies, signal line protection and military, Aerospace navigation system and control system. The maximum clamping voltage VC must not be greater than the maximum safe voltage of the protected equipment, and the reverse working voltage (reverse off-state voltage) must be greater than the normal working voltage of the line. This is a problem that must be paid attention to when using TVS tubes. In addition, the AC voltage can only be Use two-way TVS.

The following points should be paid attention to when selecting TVS tube:

Determine the maximum DC or continuous working voltage of the protected circuit, the rated standard voltage of the circuit, and the "high-end" tolerance.

TVS rated reverse turn-off VWM should be greater than or equal to the maximum working voltage of the protected circuit. If the selected VWM is too low, the device may enter an avalanche or the normal operation of the circuit may be affected by too much reverse leakage current. Connect the sub-voltage in series, and connect the sub-current in parallel.

The maximum clamping voltage VC of TVS should be less than the damage voltage of the protected circuit.

Within the specified pulse duration, the maximum peak pulse power consumption PM of the TVS must be greater than the peak pulse power that may appear in the protected circuit. After determining the maximum clamping voltage, its peak pulse current should be greater than the transient surge current.

For the protection of the data interface circuit, attention must also be paid to selecting a TVS device with a suitable capacitance C.

Select the polarity and package structure of the TVS according to the application. It is more reasonable to use bipolar TVS for AC circuit; it is more advantageous to use TVS array for multi-line protection.

Temperature considerations. The transient voltage suppressor can work between -55~+150℃. If TVS needs to work at a changing temperature, because its reverse leakage current IR increases with increase; power consumption decreases with the increase of TVS junction temperature, from +25°C to +175°C, approximately a linear decrease of 50%. The through voltage VBR increases with a certain coefficient as the temperature increases. Therefore, it is necessary to consult the relevant product information and consider the influence of temperature changes on its characteristics.

1.4 Ceramic gas discharge tube

The ceramic gas discharge tube is a switch component. It is used in the common mode circuit of the power surge protector to bleed the lightning current into the ground. It can also be used in series with the varistor in the differential mode circuit to block its leakage current. In the signal lightning protection device, it is often used in the first stage to discharge the surge current. Because of its slow response speed, the second stage is also used for voltage limiting protection.

Attention should be paid when choosing ceramic gas discharge tube:

The ceramic gas discharge tube cannot be directly used for differential mode protection on the power supply;

The breakdown voltage should be greater than the maximum signal electrical frequency voltage on the line;

The withstand current cannot be less than the maximum abnormal current that may appear on the line;

In addition, the pulse breakdown voltage must be less than the voltage of the protected circuit.

2. Selection of overcurrent protection devices

Overcurrent protection devices mainly include disposable fuses, self-recovery fuses, fusing resistors and circuit breakers, among which the most important overcurrent protection devices are fuses, also called fuses. It is generally connected in series in the circuit, and its resistance is required to be small (low power consumption). When the circuit is working normally, it is only equivalent to a wire, which can conduct the circuit stably for a long time; current fluctuations occur due to power supply or external interference It should also be able to withstand a certain range of overload; only when there is a large overload current (fault or short circuit) in the circuit, the fuse will act, and the safety of the circuit is protected by disconnecting the current to avoid the risk of product burning .

In the process of the fuse breaking the circuit, due to the existence of the circuit voltage, an arc will occur at the moment the melt is disconnected. A high-quality fuse should try to avoid such arcing; after the circuit is opened, the fuse should be able to withstand The circuit voltage applied to both ends. The pulse damage of the fuse will gradually reduce the ability to withstand the pulse. The necessary safety margin should be considered when selecting; this safety margin refers to the total fusing (action) time of the fuse, which is the pre-arcing time and the arcing time with. Therefore, you need to pay attention to the basic conditions of its fusing characteristics and rated current when choosing; in addition, consider the environment surrounding the fuse when installing. The fuse will only fuse when it reaches its own melting heat value, if it is in a cold environment. Under the circumstance, its fusing time will change, which must be paid attention to when using it.

In general, the fuse selection should pay attention to the following ten elements:

Rated current;

Rated voltage;

Ambient temperature;

Voltage drop / cold resistance;

Fuse characteristics: overload capacity, time / current characteristics;

Breaking capacity;

Melting heat value;

Durability (life);

Structural features: shape / size, installation form;

safety certificate

Combining the above ten fuse selection considerations, the selection of the fuse can be carried out according to the process shown in the figure below.

3. Selection of over-temperature protection devices

Over-temperature protection devices mainly include thermistors, temperature switches, and temperature fuses. NTC thermistor type surge suppressors are often used for over-temperature protection in power supply design, because their ability to suppress surge current is equivalent to that of ordinary resistors, but the power consumption on the resistor can be reduced by tens to hundreds of times.

NTC thermistor, that is, negative temperature coefficient thermistor, its characteristic is that the resistance value decreases non-linearly with the increase of temperature. The selection of NTC thermistor should consider the following points:

1) Maximum rated voltage and filter capacitor value

The size of the filter capacitor determines what size NTC should be used. For a NTC thermistor of a certain size, there are strict requirements on the size of the filter capacitor that is allowed to be connected, and this value is also related to the maximum rated voltage. In power supply applications, the start-up surge is caused by the charging of the capacitor, so the allowable access capacitance under a given voltage value is usually used to evaluate the ability of the NTC thermistor to withstand the surge current. To put it simply, the greater the input voltage, the smaller the maximum capacitance allowed to be connected, and vice versa. The specification of NTC thermistor products generally defines the maximum capacitance value allowed to be connected under 220Vac.

2) The maximum allowable starting current value of the product and the long-term working current loaded on the NTC thermistor

The maximum starting current value allowed by the electronic product determines the resistance value of the NTC thermistor. When the product is working normally, the current loaded on the NTC thermistor for a long time should not exceed the current specified in the specification.