Transient voltage suppressors (TVS) have the advantages of fast response time, large transient power, low leakage current, small breakdown voltage deviation, easy to control clamping voltage, no damage limit, and small size. It has been widely used in computer systems, communication equipment, AC/DC power supplies, automobiles, household appliances, instrumentation and other fields. This article will introduce several typical application circuits of TVS based on the characteristics of TVS applications and precautions for use, and discuss in detail how to apply TVS correctly through examples of TVS application in hot-swap circuit protection and automotive power line protection.
In actual application circuits, a better way to deal with damage to devices caused by transient pulses is to divert the transient current away from sensitive devices. To achieve this purpose, the TVS is connected in parallel with the protected line on the circuit board. In this way, when the instantaneous voltage exceeds the normal operating voltage of the circuit, the TNS will undergo avalanche breakdown, thereby providing an ultra-low impedance path for the instantaneous current. As a result, the instantaneous current is diverted through the TVS to avoid the protected device, and Keep the protected circuit at cut-off voltage until the voltage returns to normal. After that, when the instantaneous pulse ends, the TVS diode automatically returns to the high resistance state, and the entire circuit enters the normal voltage state.
1 Three major features of TVS applications
1) Adding TVS diodes to signal and power lines can prevent microprocessors or microcontrollers from malfunctioning due to instantaneous pulses, such as electrostatic discharge effects, AC power surges and switching power supply noise.
2) The electrostatic discharge effect can release pulses of more than 10,000V and 60A, and can last for 10ms; while general TTL devices will be damaged when encountering 10V pulses of more than 30ms. The use of TVS diodes can effectively absorb pulses that can cause device damage and eliminate interference (Crosstalk) caused by switching between buses.
3) Placing TVS diodes between signal lines and ground can prevent data and control buses from being affected by unnecessary noise.
2 Things that should be paid attention to when using TVS tubes
The relationship between the absorbed power (peak value) of transient voltage and the pulse width of transient voltage. The manual only gives the absorbed power (peak value) under a specific pulse width, but the pulse width in the actual line is unpredictable and needs to be estimated in advance. Derating should be used for wide pulses.
To protect small current loads, you can consciously add a current-limiting resistor to the line. As long as the resistance of the current-limiting resistor is appropriate, it will not affect the normal operation of the line, but the current generated by the interference caused by the current-limiting resistor will be greatly reduced. . This makes it possible to use TVS tubes with smaller peak power to protect small current load lines.
3.2 Use TVS to protect DC regulated power supply
Figure 2 is a DC regulated power supply. Adding TVS to its regulated output can protect the instruments and equipment using the power supply. It can also absorb the peak voltage between the collector and emitter of the transistor in the circuit, thereby protecting the transistor. It is recommended to add a TVS tube at the output end of each voltage regulator source, which can greatly improve the reliability of the entire machine.
Figure 2 TVS protects DC regulated power supply
3.3 Use TVS to protect transistor circuits
Various transient voltages can cause the EB junction or CE junction of the transistor to breakdown and be damaged. Especially when the collector of the transistor has an inductive (coil, transformer, motor) load, a high-voltage back electromotive force is usually generated, which may damage the transistor. In practical applications, it is recommended to use TVS as a protection device. Figure 3 shows four circuit examples for TVS protection transistors.
Figure 3 TVS protection transistor circuit
3.4 Use TVS to protect integrated operational amplifiers
Integrated operational amplifiers are very sensitive to external electrical stress. Therefore, in the process of using the op amp, if due to operational errors or abnormal working conditions, excessive voltage or current, especially surges and electrostatic pulses, will often occur, which can easily cause damage or damage to the op amp. Invalid. Figure 4 shows a protection circuit using TVS at the differential mode input end of the op amp to prevent overvoltage damage.
Figure 4 TVS protection circuit to prevent overvoltage damage at the differential mode input end of the op amp
4 TVS application examples
4.1 TVS is used for hot-swap circuit protection
In hot-swap applications, the TVS is primarily used as a ground shunt path for differential-mode currents that need to be interrupted. Hot-swappable systems are often used in distributed power systems to provide reliable system protection and electrical management. The line card interface and hot-swappable circuit schematic diagram of a typical server system is shown in Figure 5.
Figure 5 Schematic diagram of the line card interface and hot-swappable circuit of a typical server system
TVS selection applied in hot-swap circuits can be carried out according to the following steps:
1) Select a unidirectional TVS with a cut-off voltage VR that is equal to or greater than the DC or continuous peak operating bus voltage level. 14V or 15V TVS suitable for low impedance 12VDC±10% server system input bus.
2) Determine the peak pulse current level IP based on the hot swap controller circuit breaker threshold voltage, response time and selected shunt resistor.
3) Analysis of typical applications and examples of TVS diodes using the formula - (where, VC: clamping voltage; IP: peak pulse current; VC(max): maximum clamping voltage; VBR: breakdown voltage; IPP: using 10 /1000 ms waveform at VC(max).) Calculate the circuit clamping voltage VC from the IP level given in step 2 and the relevant datasheet parameters. Is VC low enough? If not, another approach is to use a larger TVS to get a steeper descent. Note that the voltage temperature coefficient of VC is similar to VBR (e.g. at 75°C operating ambient conditions, 0.1%/°C means a 5% increase in the coefficient).
4) Calculate the product of VC and IP to obtain the actual peak power level maintained by the TVS.
5) Use the formula of typical applications and example analysis of TVS diodes – (where L is the parasitic inductance in the circuit) and the known input parasitic inductance to determine the pulse duration td of the triangular pulse waveform (i.e., the time to decay to zero) .
6) Derate PPP using step 5 pulse duration similar to the curve in Figure 6(a). As mentioned before, the triangular pulse current waveform can achieve 33% higher pulse power than the double exponential reference waveform curve.
7) Use an ambient temperature derating PPP similar to the curve in Figure 6(b). The mutual thermal effects of adjacent components should also be considered.
8) Does the net derating PPP in step 7 achieve sufficient design margin (at least 50%) of the actual TVS peak power calculated in step 4? If not, choose a larger TVS and repeat steps 1-8.
Figure 6 (a) Peak pulse power and pulse duration, (b) Thermal derating characteristics
For the suppression of recurring transient voltages, it is particularly worth noting whether the steady-state average power of the TVS tube is within a safe range.
3 Typical application circuits of TVS
3.1 Application of TVS in AC circuits
Figure 1 shows an application circuit of a bidirectional TVS in an AC circuit. The application of TVS can effectively suppress overload pulses caused by the power grid, thereby protecting all components in the rectifier bridge and load. The TVS clamping voltage in Figure 1 should be no greater than the maximum allowable voltage of the circuit.
4.2 TVS is used for primary protection and secondary protection of automobile power lines
Automotive electronics such as electronic control units, sensors, and infotainment systems are connected to a single power cord, as shown in Figure 7. The power sources for these electronic products are batteries and alternators, both of which have unstable output voltages that are easily affected by temperature, working status, and other conditions. Additionally, automotive systems using solenoid loads such as fuel injection systems, valves, motors, electrical and hydrolysis controllers can introduce ESD, noise spikes and other types of transients and surge voltages into power and signal lines. Therefore, automotive designs must protect electronic devices such as control units, sensors and infotainment systems from harmful voltage surges, transients, ESD and noise that occur on power lines. Transient voltage suppressors (TVS) are an ideal solution for automotive electronic protection. The following mainly introduces the primary protection and secondary protection applications of TVS in automotive power lines.
Figure 7 Typical automotive power cord
4.2.1 Primary protection of automobile power lines (load shedding)
There are two types of load shedding TVS used for primary protection of automotive electronics: epitaxial type and non-epitaxial type. In reverse bias mode, the two sets of products have similar breakdown operating characteristics. The difference is that epitaxial TVS has low forward voltage drop (VF) characteristics in forward mode, while non-epitaxial TVS has a relatively high VF under the same conditions.
In the reverse power input mode, the power line voltage is the same as the voltage of the TVS VF. This reverse bias mode will cause electronic circuit failure. The low forward voltage drop of the epitaxial TVS can solve this problem well.
4.2.2 Secondary protection of automotive power lines
The primary target of protection circuits in automotive systems is high surge voltages, but the clamped voltages are still high. Therefore, secondary protection is particularly important in 24V powertrains, such as those in trucks and vans. The main reason is because the maximum input voltage of most voltage regulators and DC-DC converter ICs is 45V~60V. For such applications it is recommended to use the secondary protection in Figure 9. Adding resistor R on the power line can reduce the transient current, so that a TVS with a smaller power rating can be used as secondary protection.
5 TVS diode performance is improved
How to improve the performance of TVS in circuit applications? The wiring of the printed circuit board and the selection of circuit components are very important. Through reasonable placement of TVS, grounding selection, parasitic inductance and loop area processing, as well as burst TVS and diode arrays, unidirectional and bidirectional burst TVS diodes , Comparison of external and internal chip protection circuits, scientific and reasonable selection of PCB wiring and TVS components to optimize TVS performance.
6 Conclusion
This article introduces the application characteristics, typical applications and application examples of TVS in detail, and discusses how to optimize the performance of TVS from two aspects: the wiring of printed circuit boards and the selection of TVS components, which is reasonable for circuit protection design engineers. The efficient application of TVS for circuit protection design is of great reference value.
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