Main parameters and selection of TVs diodes

TVS main parameters of diodes--reprint

The best way to deal with the damage to the device by instantaneous pulses is to draw the instantaneous current from the sensitive device. The TVs diode is connected in parallel with the protected line on the circuit board, and when the instantaneous voltage exceeds the normal operating voltage of the circuit, the TVs diode has an avalanche that provides an ultra-low resistance path to the instantaneous current, which results in instantaneous current being induced by the diode to avoid the protected device, And the protected circuit keeps the cutoff voltage until the voltage returns to its normal value. When the instantaneous pulse is over, the TVs diode automatically recovers a high-impedance state and the entire loop enters the normal voltage. Many devices are subjected to multiple shocks and their parameters and performance degrade, and the diode will not be damaged or degraded as long as the work is within the limits.

As can be seen from the above process, in the selection of TVs diodes, you must pay attention to the following several parameters of choice:

1. Minimum breakdown voltage VBR and Breakdown current IR. VBR is the smallest breakdown voltage of TVs, and at 25 ℃, TVs are not avalanche-less than this voltage. When TVs flows through the specified 1mA current (IR), the voltage added to the TVs poles is the minimum breakdown voltage VBR. VBR can be divided into 5% and 10% according to the discrete degree of VBR and standard value of TVs. For a VBR of 5%, VWM=0.85VBR; for 10% VBR, VWM=0.81VBR. To meet IEC61000-4-2 international standards, TVs diodes must meet ESD shocks that can handle a minimum of 8kV (contact) and 15kV (air), and some semiconductor manufacturers use higher impact resistance standards on their products. For some portable device applications with special requirements, the designer can select the device as needed.

2. Maximum reverse leakage current ID and rated reverse shutdown voltage VWM. VWM This is the voltage that the diode can withstand in normal condition, which should be greater than or equal to the normal operating voltage of the protected circuit, or the diode will continue to cut off the loop voltage, but it needs to be close to the normal operating voltage of the protected circuit, so that the entire loop is not exposed to overvoltage threats before the TVs work. When the rated reverse-shutdown voltage VWM is added to the TVs bipolar, it is in reverse shutdown state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID.

3. Maximum clamping voltage VC and Maximum peak pulse current IPP. The maximum peak voltage that occurs at both ends of the pulse peak current of 20mS when the IPP flows through TVs is VC. VC, IPP reflects the surge suppression capability of TVs. The ratio of VC to VBR is called clamping factor, usually between 1.2~1.4. VC is the diode in the cut-off state of the voltage provided, that is, in the ESD impact state through the TVs voltage, it can not be greater than the protected circuit can withstand the limit voltage, or the device is exposed to the risk of damage.

4. PPPM rated pulse power, which is based on the maximum cutoff voltage and the peak current at this time. For handheld devices, 500W TVs in general are sufficient. The maximum peak pulse power consumption PM is the maximum peak pulse power consumption value that TVs can withstand. At a given maximum clamping voltage, the larger the power consumption pm, the greater the withstand capacity of its inrush current. Under a given power consumption pm, the lower the clamp voltage VC, the greater the withstand capacity of the inrush current. In addition, peak pulse power consumption is also related to pulse waveform, duration and ambient temperature. Also, the TVs can withstand transient pulses that are not duplicated, and the device specifies a pulse repetition rate (the ratio of duration to interval) of 0.01%. If repetitive pulses are present in the circuit, the accumulation of pulsed power should be considered and the TVs may be damaged.

5. Capacitance c. The capacitance C is determined by the TVs Avalanche Junction section and is measured at a specific 1MHz frequency. The size of C is proportional to the current withstand capacity of TVs, and C too large causes the signal to decay. Therefore, C is an important parameter of selecting TVs for data interface circuit. Capacitance for the higher the data/signal frequency of the circuit, the capacitance of the diode to the circuit more interference, the formation of noise or attenuation signal strength, it is necessary according to the characteristics of the circuit to determine the capacitance range of the selected device. The general selection of high-frequency circuit capacitor should be as small as possible (such as Lctvs, low capacitance TVs, capacitance is not greater than 3pF), and capacitance requirements of the circuit capacitor selection can be higher than 40pF.

Note: TVS Selection of
The maximum clamping voltage VC is less than the maximum allowable safety voltage of the circuit.
The cut-off voltage VRWM is greater than the maximum operating voltage of the circuit, generally can choose VRWM equal to or slightly larger than the maximum operating voltage of the circuit.
The rated maximum pulse power (given in the TVs parameter) is greater than the maximum transient surge power.

Examples of DC power use:
The whole machine DC working voltage 12V, the maximum allowable safety voltage 25V (peak), the surge source impedance 50mω, its interference waveform is square wave, tp=1ms, maximum peak current 50A. Choose:

1, first from the operating voltage 12V Select the maximum reverse operating voltage VRWM 13V, the breakdown voltage:

V (BR) =vrwm/0.85=15.3v

2, from the breakdown voltage value to select the maximum clamping voltage VC (max) =1.30XV (BR) =19.89v, take

vc=20v

3, then from the clamping voltage VC and the most peak current IP to calculate the square wave pulse power:

ppr=vcxip=20x50=1000w

4. Calculate the peak power of the tp=1ms exponential wave, the coefficient of k1=1.4,

ppr=1000w÷1.4=715w

From the manual can be found 1n6147a wherein the ppr=1500w, the deflection voltage vrwm=12.2v, the breakdown voltage V (BR) =15.2v, the maximum clamping voltage vc=22.3v, the most inrush current ip=67.3a. Can meet the above design requirements, and left a margin, regardless of the square wave or exponential waves are applicable.

Examples of AC circuit applications:
A unidirectional transient voltage suppressor diode is used in a DC line, and a bidirectional transient voltage suppressor diode is required for AC communication. AC is the grid voltage, the transient voltage generated here is random, sometimes also encountered lightning (lightning induced transient voltage) so it is difficult to quantitatively estimate the instantaneous pulse power PPR. However, the maximum reverse operating voltage must be properly selected. The general principle is that the AC voltage is multiplied by 1.4 times times to select the maximum reverse operating voltage of the TVs tube. The DC voltage is 1.1-1.2 times times to select the maximum reverse operating voltage of the TVs tube VRWM.

Figure 2-8 shows a microcomputer power supply using TVs for line protection schematic diagram, the diagram is visible:

1. The 220v~ of the input line is added to suppress the spike interference in the 220v~ AC power grid.
2. In the transformer input line coupled with interference filter, filter out small spike interference.
3. At the v~=20v of the voltage-transformer output, the TVs tube is added and the interference is suppressed again.
4. The DC 10V output is also coupled with a TVs tube to suppress interference.

Wherein: two-way TVs tube D1 vrwm=220v~x1.4=308v around
Two-way TVs tube D2 vrwm=20v~x1.4=28v around
VRWM=10V~X1.2=12V about one-way TVs tube D3

After four times of inhibition, become so-called "purification power", but also can add other measures to suppress interference more effectively to prevent interference into the computer's CPU and memory, thereby improving the application of microcomputer system reliability.
From the probability of failure statistics: microcomputer system generated 100 times, of which 90 times from the power supply, 10 times is the microcomputer itself, the reliability of the visible power is the most important, to improve the reliability of the machine, first of all should improve the reliability of the power supply.

Main parameters and selection of TVs diodes