ESD portable equipment is very important, while the TVS diode is a very effective protection device, has its unique advantages compared with other devices, but in the application for the protection of different objects to use devices should be, because the electrostatic different port the possible impact of different degree of protection devices have different requirements different. We should pay attention to the corresponding parameter identification and the different design of each manufacturer, at the same time, we should make a reasonable PCB layout. This paper introduces how to use TVS diode device in ESD protection of portable equipment.
Portable devices such as notebook computer, mobile phone, PDA, MP3 players, due to frequent contact with the body susceptible to electrostatic discharge (ESD) impact, if there is no choice suitable protection device, may cause the machine performance is not stable, or damage. Even worse, it is impossible to find out the exact reason, so that users mistakenly believe that the product quality problems and damage the credibility of the enterprise.
Under normal circumstances, exposed to the outside of the device may be in contact with the human body ports are required to anti-static protection, such as keyboard, power interface, data port, I/O port, etc.. Now the more common ESD standard is IEC61000-4-2, the application of human static mode, the test voltage range of 2kV ~ 15kV (air discharge), the peak current of 20A/ns, the pulse duration of not more than 60ns. The total amount of energy produced under such a pulse is not more than a few hundred tiny, but it is enough to damage sensitive components.
IC devices used in portable devices are mostly high integration, small volume products, precision processing technology so that the silicon oxide layer is very thin, and therefore more prone to breakdown, and some will be damaged at around 20V. The traditional protection methods are no longer applicable, and some may even cause interference to the performance of the equipment.
Characteristics of TVS diodes
Can be used for ESD protection devices for portable devices are many, for example the designer can use discrete devices to build the protection circuit, but due to limited space for portable devices and avoid loop inductance, this method has gradually been replaced by more integrated devices. Multilayer metal oxide devices, ceramic capacitor and diode can effectively guard, characteristics and their performance are different, unique performance of TVS diode in such applications to win more and more market for the.
The most significant characteristic of TVS diode is a rapid reaction, the instantaneous pulse in the absence of damage before it was effectively curb on the line or device, two is the cut-off voltage is low, low voltage circuit environment more suitable for battery powered. In addition, the improved design of the TVS diode has lower leakage current and junction capacitance, so it has better performance in dealing with the electrostatic shock of the high speed conduction circuit.
Advantages of TVS diodes
TVS and zener diode: compared with the traditional zener diode, the TVS diode P/N junction is larger, high pressure bearing capacity improvement of the structure of the TVS has stronger, but also reduces the voltage cut-off rate, so it has better effect to protect the handheld low voltage circuit equipment safety.
VS and ceramic capacitors: many designers are willing to use surface mount ceramic capacitors for ESD protection, not only cheap and easy to design, but this kind of device to withstand the pressure is relatively weak. 5kV impact will cause about 10% ceramic capacitor failure, to 10kV, the damage rate of up to 60%, while the TVS can withstand 15kV voltage. In the process of using handheld devices, due to frequent contact with the human body, each port must be able to withstand 8kV contact impact (IEC61000-4-2), visible using TVS can effectively guarantee the qualified rate of the final product.
TVS and MLV: multilayer metal oxide structure device (MLV) can effectively inhibit the impact of instantaneous high pressure, this device has a nonlinear voltage current (impedance performance), cut-off voltage up to the initial suspension voltage 2 ~ 3 times, this characteristic is suitable for protection is less sensitive to the voltage of the circuit and device, such as the power supply circuit. The voltage of the TVS diode has a better cut-off factor, but also has a relatively low capacitance, this point for the handheld high frequency port is very important, because the high capacitance may affect the data transmission, resulting in distortion or degradation. All kinds of surface packaging of TVS diodes are suitable for the assembly line, and the chip structure is easy to integrate other functions, such as EMI and RFI filter protection, which can effectively reduce the cost of the device and optimize the overall design.
One can not ignore the characteristics of LED can be easily with other devices in a single chip, many existing EMI filtering and RFI protection functions and TVS together, not only reduce the number of devices used in the design to reduce the cost, but also avoid the PCB board wiring easily when the associated self induced the.
Working principle of TVS diode
Transient voltage suppression diode (transient) referred to as the TVS device in the reverse application of prescribed conditions, when subjected to a transient overvoltage of high energy pulse, the impedance immediately dropped very low turn-on value, allowing high current through and voltage clamp to a predetermined level, so as to effectively protect the precision components in the electronic circuit from damage.
The transient pulse power of TVS can reach up to kW, and the clamping response time is only 1ps (10^-12S).
TVS allows forward surge current at T =25 C, T=10ms conditions, up to 50 ~ 200A.
Bidirectional TVS can be instantaneous pulse power absorption in two positive and negative direction, and the voltage clamped to a predetermined level, double Main parameters of TVS devices
1 breakdown voltage V (BR) device in the area where the breakdown occurs, under the specified test current I (BR), the voltage measured at both ends of the device is called the breakdown voltage, in this region, the diode becomes a low impedance path.
2 maximum reverse pulse peak current IPP
In reverse operation, the maximum peak pulse current of the device is allowed under the specified pulse condition. IPP is the product of the maximum clamping voltage Vc (MAX), which is the maximum value of the transient pulse power. When used, the TVS should be selected correctly, so that the rated transient pulse power PPR is greater than the maximum transient surge power of the device or circuit. When the peak value of the instantaneous pulse current occurs, the TVS is punctured, and the value of the breakdown voltage increases to the maximum clamping voltage. With the exponential decrease of the pulse current, the clamping voltage also decreases, and then returns to the original state. Therefore, TVS can suppress the impact of the pulse power, which can effectively protect the electronic circuit.
3 the maximum reverse operating voltage VRWM (or position voltage) reverse operation, under the specified IR, the voltage value of both ends of the device is called the maximum reverse operating voltage VRWM. Usually VRWM = (0.8~0.9) V (BR).
At this voltage, the power consumption of the device is very small. When used, the VRWM shall not be less than the normal operating voltage of the device or circuit to be protected.
4 maximum clamping voltage Vc (max) is the maximum voltage at the two ends of the device under the peak current Ipp.
When used, the Vc (max) should not be higher than the maximum allowable safety voltage of the protected device.
The ratio of the maximum clamping voltage to the breakdown voltage.
That is, the clamping coefficient =Vc (max) /V (BR) is about 1.3.
5 the peak pulse power PPR TVS of the PPR depends on the pulse peak current IPP and the maximum clamping voltage Vc (max), in addition to the pulse waveform, pulse time and ambient temperature.
When the pulse time Tp is certain, PPR =K1, K2 (max), Ipp (Vc), K1 is the power factor, K2 is the temperature coefficient of power.
The typical pulse duration TP is 1MS, and the pulse peak power will increase with the shortening of TP when the pulse time TP is applied to the transient voltage suppression diode. TVS reverse pulse peak power PPR and withstand the pulse waveform of the surge, with the power factor K1 said that the K1 value of various surge waveforms as shown in table 1.
E= formula I (T).V (T) DT
In type:
I (T) pulse current waveform,
V (T) is the clamping voltage waveform.
This nominal energy value is not repeatable for TVS in a very short period of time. However, in practical applications, the surge is usually repeated, in this case, even if the pulse energy of single pulse energy than TVS devices can withstand much smaller, but if applied repeatedly, the single pulse energy accumulated, in some cases, will be more than TVS pulse energy device bear. Therefore, the circuit design must be carefully considered and selected in this part of the TVS device, so that it can be repeated at the specified interval of time, the cumulative pulse energy does not exceed the pulse energy rating of TVS devices.
6 the capacitance of the capacitor CPP TVS is determined by the area of the silicon wafer and the bias voltage. When the bias voltage increases, the capacitance of the capacitor decreases. The size of the capacitor affects the response time of the TVS device.
7 leakage current IR when the maximum reverse operating voltage is applied to the TVS, the TVS tube has a leakage current of IR, which is an important parameter when TVS is used in the high impedance circuit.Classification of TVS devices
Polarity can be divided into: unipolar and bipolar two;
According to the use can be divided into: general and dedicated;
According to the packaging and internal structure can be divided into: Axial Leaded diode, dual in-line TVS array, SMD and high-power modules, etc..
Axial lead product peak power up to 400W, 500W, 600W, 1500W and 5000W.
High power products are mainly used in the power supply line, low power products are mainly used in high-density installation occasions. In the case of high density installation, you can also select the dual in-line and surface mount package.
Selection of TVS devices
In the selection of TVS, the following factors should be considered:
(1) if TVS is likely to withstand impulse voltage (surge voltage) from the two directions, it should be bipolar, or it may be unipolar.
(2) the Vc value of the selected TVS shall be lower than the maximum voltage of the protected element. Vc is a diode in the state of the cut-off voltage, that is, in the ESD state of the impulse through the TVS voltage, it can not be greater than the protected circuit can withstand the limit voltage, otherwise the device is facing the risk of damage.
(3) TVS in normal working condition, not in the state of breakdown, preferably in the following VR, should take into account the requirements of VR and VC two to choose the appropriate TVS.
(4) if you know the more accurate inrush current IPP, you can use VCIpp to determine the power; if you can not determine the approximate range of IPP, then select the power of the larger TVS as well. PM is the maximum peak pulse power dissipation of TVS. Under the given maximum clamping voltage, the greater the power consumption PM, the greater the capacity of the surge current. Under the given power consumption PM, the lower the clamping voltage VC, the greater the capacity of the surge current. In addition, the peak pulse power is also related to the pulse waveform, duration and ambient temperature. (5) TVS can withstand transient pulse is not repeated, the device provides the pulse repetition rate (duration and intermittent time ratio) is 0.01%. If there is a repetitive pulse in the circuit, the pulse power should be considered, otherwise it may damage the TVS.
(6) for the protection of small current load, can consciously in the line to increase the current limiting resistor current limiting resistor, as appropriate, generally does not affect the normal work of the circuit, but the current flow resistance of interference is greatly reduced. However, it is possible to select a smaller peak power TVS tube to protect the small current load line.
(7) the capacitance C is determined by the TVS avalanche cross section, which is measured at a specific 1 MHz frequency. The size of the C is proportional to the current carrying capacity of the TVS, and the C is too large to signal attenuation. Therefore, C is an important parameter for the selection of TVS in data interface circuits. The data / signal higher frequency circuit, the diode capacitance interference on the circuit is formed, noise or attenuation of signal intensity is large, therefore, need according to the characteristics of the capacitance range loop to determine the selected device. High frequency circuit generally choose the capacitor should be as small as possible (such as LCTVS, low capacitance TVS, the capacitance is not more than 3 pF), and the capacitor requirements of the loop is not high, the capacity of the capacitor can be higher than 40pF.
(8) in order to meet the international standard IEC61000-4-2, TVS diode must reach a minimum of 8 KV can handle (MB, contact) and 15 kV (BM air) ESD impact, some semiconductor manufacturers use a higher standard impact on their products. For some special requirements of portable device applications, designers can select the device as needed.
Application of TVS devices
1 bottom connector applications
Bottom connector design is widely used in mobile consumer products, the market is currently the main products for mobile phones, PDA, DSC (digital camera) and MP3 and other portable products.
Because it is a DC circuit, high capacitance devices can be selected. This port may be subject to high energy impact, you can choose to integrate the TVS and over-current protection device.
2 RJ-45 (10/100M Ethernet network)
RJ-45 interface is widely used in the network interface device, the typical application is 10/100M Ethernet network.
3 video line protection
At present, the design of the video output ports are D-SUB, DVI (28 lines), SCART (line 19) and D-TERMINAL (the main Japanese products in use). Video data line has a high rate of data transmission, data transmission rate up to 480Mbps, some video data transmission rate can reach more than 1G, thus to choose low capacitance LCTVS, it is usually a low capacitance diode and TVS diode series capacitor to reduce the whole line (less than 3pF), to achieve high-speed circuit demand.
4 SIM card data line protection
SIM card data line protection has been the company‘s key products, and specifically for the design of such port set ESD (TVS) /EMI/RFI protection on a chip device, fully embodies the infinite integration of chip devices.
In the choice of devices for different purposes, to avoid the device works in the design parameters near the limit, should also be based on the characteristics of the protected circuit device characteristics and can withstand ESD shock response speed is fast enough, the sensitivity is high enough, the protection device to effectively play the crucial role, another integrated device other functions should also be considered first.
5 USB protection
USB protection of the general ESD uplink and downlink two cases.