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PROTEK公司TVS器件常见问答
发布时间:2006-9-7 阅读:8984

1.      As a circuit design engineer, when/why should I consider designing in a TVS component or protection network?

 

The optimum time for considering the use of TVS components in a particular circuit/system is during the first stages of design.  It is easier to allot space for planned components at the beginning a design, rather than after the design is complete.  Also, it is easier to calculate the effect the TVS component will have on signal and system performance.  Lastly, today's transient immunity standards (IEC or EN Standard) require a system/circuit to have TVS protection.

 

2. Do I need multiple devices for a given equipment, system or circuit?

 

Yes.  The need for multiple protection devices depends upon the transient threat environment, protection coordination (I/O Port & Power Bus) and the location of the TVS components.  For example, there are three major threat environments: Lightning (Surge), Electrical Fast Transients (EFT) and Electrostatic Discharge (ESD).  Any one of these threats can be induced in to the equipment, system or circuit directly or indirectly at multiple entry points.  Thus, all entry locations will need to be protected, which requires the use of multiple protection devices.

 

3. Do I need to use a TVS component to between two different ground connections on a printed circuit board (PCB)?

 

Yes, at the farthest point on the PCB away from a common point connection.  At the circuit board level, there is generally some isolation between grounds (analog and digital).  At some point of connection within the system, these two grounds are common.  At the circuit extremes, a low voltage differential must be maintained in order to prevent ground bounce (GPR), which will affect sensitive IC components.

 

4. Will a protection device/network affect the signal during normal operation of the circuit?

 

Yes.  TVS components or networks are across-the-line devices and will affect insertion losses or causes signal distortion.  Selection of these components/networks is based on their capacitance.  The presence of any component element in the line will affect the capacitance value of the TVS device for a given frequency of operation.  However, it is important to factor into TVS device selection any parasitic or transmission line capacitance.  

 

5. Are handheld products required to be protected against the threat of lightning transients?

 

The primary environmental threat for handheld products is ESD.  However, when the product is placed in a cradle for power charging or down loading into a computer, it can become susceptible to the effects of induced lightning and possibly EFT events.

 

6. Can a reflected signal, the result of mismatch in line impedance, be suppressed by filtering?

 

No.  A filter is designed for low voltage noise suppression.  Reflected signals are usually twice the amplitude of the incident pulse and twice the frequency.  Therefore, a TVS/Filter combination network is recommended to clamp the high voltage and reduce the residual noise.

 

7. Where is the dividing point between low and high frequencies?

 

For TVS component selection, the dividing points is within the range of 10 to 30 MHz.  A low capacitance device should be above the 30 MHz operating frequency.  The actual capacitance value will also depend upon the parasitic components on the line or in the circuit.

 

8. When and why do we use different types of waveforms (i.e., 10/1000µs, 8/20µs, etc. . .)?

 

The waveform to be used for TVS selection depends upon the specific standard that a system is required to meet.  

 

Waveforms are a result of the threat environment, point of entry, and actual system being tested.  For example, 10/1000µs is the waveform for lightning on long transmission lines, the 8/20µs waveform is for near lightning strikes, and the 1/30ns waveform is for ESD.

 

Some waveforms are dependent upon the source of the threat.  For example, ESD has a 1/30ns waveform and EFT has a 5/50ns waveform with multiple bursts.

 

9. Why is 50% decay used in the description of an impulse waveform to indicate the pulse time?

 

An impulse waveform can be measured in two ways.  The most popular form of measurement is at the 50% point in the decay of the waveform.  This 50% point represents the integrated area under the curve for that waveform.

 

10. Why is the capacitance of a TVS component or network important?

 

Capacitance of a device can affect performance, insertion loss, reflections and signal integrity.  With the speed of data transmission increasing, more attention is being given to the capacitance of the TVS component or network. 

 

11. How do I know what space should be allotted to the TVS component or network?

 

Transient threat standards will define the transient conditions in terms of voltage, current or both for a given waveform.  Accordingly, TVS components or networks are selected to match the peak pulse current of a threat for a give waveform.  If a low capacitance device is required, these TVS components will typically require a slightly larger package size.  Small packages, such as a flip-chip device are sufficient for ESD transient events and low frequency applications.  

 

12. How does one properly mount a Flip-Chip device onto a printed circuit board?

 

A general rule of thumb is to use the minimum amount of solder in the mask area defined by the manufacturer's mounting instructions.  Please consult the factory for a copy of the mounting instructions.

 

13. What device would be acceptable protection in Aircraft Electronics?

 

It is important to define the transient threat conditions or know which standard is required.  Generally, the industry standard for commercial aircraft is the DO-160 document.  There are several waveforms and test levels within this document.  Depending upon the waveform and test level, the two most applicable device types are the 5KP and the 15KPA Series devices.  Voltage selection is based upon the operating voltage of the system.  For military aircraft, there are several standards that identify the transient environment.  Once these requirements have been identified, device selection is based upon the peak pulse current and waveform matched to a specific TVS device.


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