Transient diode is a kind of high efficiency protection device with fast response time and high surge absorbing capacity. Both ends when the TVS diodes are reverse transient overvoltage pulse, to a very high speed high impedance between the two ends of the transition to a low impedance, to absorb the instantaneous large current, and the voltage clamped at a predetermined value, so as to effectively protect the circuit components from damage. This paper describes the main characteristics of the TVS device parameters and selection of attention, and gives the application of TVS in the circuit design method.
1 TVS diode characteristics and main parameters
Device characteristics of 1.1 TVS diodes
Under the specified reverse conditions, the TVS diode has a high impedance state to the protected line. When the voltage exceeds the breakdown voltage, the TVS diode will provide a low impedance path, and shunt diode TVS through high current to flow is the instantaneous current protection components, while the voltage protection components at both ends of the clamping voltage limit in TVS. When the overvoltage condition disappears, the TVS diode returns to the high impedance state. Compared with the ceramic capacitor, TVS can withstand the voltage of 15 kV, but the ceramic chip capacitors are relatively weak capacity of the high voltage. 5 kV impact will cause about 10% ceramic capacitor failure, and to 10 kV, the damage rate will be as high as 60%.
1.2 main parameters of TVS diode
(1) minimum breakdown voltage VBR
When the TVS flows through the specified current, the voltage at both ends of the TVS is called the minimum breakdown voltage. In this region, the TVS is in a low impedance path. At 25 C, below this voltage, TVS is not going to happen.
(2) rated reverse turn off voltage VWM
TVS is VWM in the normal state can withstand the voltage, this voltage should be greater than or equal to the normal operating voltage of the protected circuit. But it also needs to be as close as possible to the normal operating voltage of the protection circuit, so that it will not work in the TVS before the entire circuit to face the threat of overvoltage. According to the VBR TVS and the standard value of the degree of dispersion, can be divided into 5% and two VBR 10%, for 5% of the VBR, VWM=0.85VBR legs; and for 10% of the VBR, VWM=0.81VBR.
(3) maximum peak pulse current IPP
IPP is TVS in the reverse state when working in the prescribed pulse conditions, the device allows the maximum peak current through the pulse.
(4) clamp voltage Vc
When the pulse peak current flowing through the Ipp TVS diode, the maximum voltage appears across the value called clamp voltage Vc. Vc and Ipp reflect the surge suppression capability of TVS. Usually the Vc and VBR is called the clamping factor (coefficient), its value is generally at 1.2 ~ 1.4. In practical use, the Vc should not be greater than the maximum allowable safe voltage of the protected circuit, otherwise the device will face the possibility of being damaged.
(5) maximum peak pulse power consumption PM
PM is usually the product of maximum peak pulse current and voltage clamp Ipp Vc, which is the maximum peak pulse power. It is TVS able to withstand the maximum peak pulse power consumption. Under a given maximum clamping voltage, the greater the power consumption of PM, the greater the ability to withstand the surge current. In addition, the peak pulse power consumption is also related to the pulse waveform, pulse duration and environment temperature. Moreover, TVS can withstand the transient pulse is not repeatable.
(6) capacity C
The capacitance of the TVS diode is determined by the cross-sectional area and the bias voltage of the silicon wafer, which is measured at a specific frequency of 1 MHz. The size of C is proportional to the current capacity of TVS, and the C is too large, which will cause the signal attenuation. Therefore, the capacitor C is an important parameter of the TVS data interface circuit.
(7) leakage current IR
IR is the maximum reverse operating voltage applied to the TVS when the leakage current of the TVS diode. When TVS is used for high impedance circuit, the leakage current IR is an important parameter.
2 selection of TVS diode matters needing attention
In the selection of TVS diode, according to the specific circumstances of the circuit, the following main factors should be considered in general:
First of all, because of the bidirectional TVS can absorb large instantaneous pulse power in two positive and negative direction, and the voltage clamped to a predetermined level. Therefore, if the circuit is likely to withstand the impact from the two direction of surge, it should be a two-way TVS. Bidirectional TVS is generally suitable for AC circuit, one way TVS diode is generally used for DC circuit. In addition, the clamp voltage Vc is not greater than the maximum allowable safe voltage of the protected circuit.
Secondly, the maximum peak pulse power consumption of PM must be greater than the maximum transient surge power in the circuit. However, in the actual application process, the surge may appear repeatedly, in this case, even if the pulse energy of single pulse energy than TVS devices can withstand much smaller, but if repeated applied, these single pulse energy accumulated, it may exceed the pulse energy of TVS devices can bear in in some situations. Therefore, in the circuit design, must seriously consider and choose the appropriate TVS device at this point, to make it at prescribed intervals, repeat on cumulative pulse energy as the pulse energy of TVS devices more than rated value.
Third, in the actual application process, the maximum reverse working voltage must have the correct choice, the general principle is to the AC voltage of 1.4 times to select the TVS tube of the maximum reverse working voltage. DC voltage is 1.1 ~ 1.2 times to select the maximum reverse voltage of the TVS diode.
3 typical applications of TVS in circuit design
In the actual application circuit, instantaneous pulse device best way to damage, is the instantaneous current away from the sensitive device. In order to achieve this goal, the TVS is connected in parallel with the protected line on the circuit board. So, when the instantaneous voltage exceeds the normal working voltage circuit, TNS avalanche breakdown, thus providing instantaneous current of a low impedance path, the result is the instantaneous current is off by TVS, so as to avoid the protected device and restore the normal value before the protected circuit has been cut-off voltage in voltage. After that, when the instantaneous pulse is over, the TVS diode is automatically recovered to a high resistance state, and the whole circuit is in the normal voltage state. The following is a typical example of the application of TVS in the circuit.
Application of 3.1 TVS diode in AC circuit
Figure 1 shows the application of a bidirectional TVS in an AC circuit. The application of TVS can effectively restrain the overload impulse caused by the power grid, thus it can protect the rectifier bridge and all the components in the load. The maximum allowable voltage clamp in figure TVS 1 bit voltage should be not greater than the circuit.
3.2 using TVS diode to protect the DC regulated power supply
Figure 2 is a DC power supply, the output voltage plus TVS, can use the power protection equipment, but also can absorb the collector of the transistor circuit to the emission peak voltage between the electrodes, thereby protecting the transistor. It is suggested that a TVS tube is added at the output of each voltage stabilized source, which can greatly improve the reliability of the whole machine.
3.3 protection transistor circuit with TVS diode
All kinds of transient voltage can make the transistor EB or CE junction breakdown and damage, especially with inductive transistor collector (coil, transformer and motor load), usually have a high EMF, which may damage the transistor. In practical applications, the proposed TVS as a protective device. Figure four shows the 3 circuit examples of TVS protection transistor.
3.4 protection of integrated circuits
As the integration of modern IC is more and more high, and its voltage is getting lower and lower, so it is vulnerable to the impact of transient voltage and damage, it is necessary to take protective measures. The input and output terminals of the CMOS circuit are usually protected by the network. For the sake of reliability, various protection networks have been added to the external interface of the whole machine. Figure 4 shows the circuit measures for the protection of TTL and CMOS devices using TVS.
3.5 TVS diode protection integrated operational amplifier
The integrated operational amplifier to external electric stress is very sensitive. Therefore, in the process of using the operational amplifier, if due to improper operation or take the normal working conditions, often appear excessive voltage or current, especially the surge and electrostatic pulse, so it is easy to make operational damage or failure. Figure 5 is the use of TVS in the differential input amplifier overvoltage protection circuit for preventing damage of the end.
4 the end of this article
This paper describes the characteristics of TVS devices and the main parameters, and the selection of devices when precautions, and gives the typical application circuit in the circuit design, circuit designers in order to enhance the understanding of TVS diode, provides the basis for the design of application circuit with high reliability. Although the application of current TVS diode in China is in the promotion stage, but we have reason to believe that, with excellent performance more and more designers deepen understanding of TVS diode and TVS show, TVS diode will ultimately be widely used.
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