1. Introduction of Unidirectional and Bidirectional signals
Generally, signals can be summarized into two categories: The 1st category is Unidirectional signal which means the voltage level is above 0V (Figure 1). The 2nd category is a bidirectional signal which contains above 0V and below 0V contents (Figure 2).
Figure 1 : Unidirectional signal
Figure 2 : Bidirectional signal
2. Characteristic of Unidirectional TVS and Bidirectional TVS
Due to the signal which need to be protected maybe is unidirectional or bidirectional, therefore we need to clarify signal’s voltage level is all positive (above 0V) or positive and negative (above 0V and below 0V). If the signal is unidirectional then you need to choose to use unidirectional TVS (Figure 3). About TVS selection, firstly need to make sure is there positive and negative voltage level in the signal to select unidirectional or bidirectional TVS. After that to make sure the voltage level of signal would higher than the breakdown voltage of TVS or not, if the voltage level of signal is higher than breakdown then will cause TVS breakdown conduction. So that when selecting TVS we need to compare the voltage level of signal with the I-V curve of TVS, the voltage level of signal would always less than the breakdown voltage of TVS.The characteristics of unidirectional TVS is that when a positive transient voltage comes and over trigger voltage of TVS, TVS will go into breakdown status to bypass transient current to ground. If negative transient voltage comes and reach s the forward voltage (normally about 0.7V) then TVS will forward conduction and the negative transient voltage will be bypassed to ground to protect internal circuit of IC, the I-V curve (DC characteristics) of trigger voltage and TLP curve (Transient characteristics) of unidirectional TVS is shown in Figure 4, Figure 5.
Figure 3 : Symbol of unidirectional TVS
-k6xe00.png)
Figure 4 : I-V curve of unidirectional TVS (DC characteristics)
Figure 5 : Trigger voltage and TLP curve of unidirectional TVS
(Transient characteristics)
If the signal is bidirectional (signal contains above 0V and below 0V), then you need to select bidirectional TVS for this signal to avoid negative signal be clamped at -0.7V by forward-basing, the symbol of bidirectional TVS is shown in Figure 6. The I-V curve (DC characteristics) of bidirectional TVS is shown in Figure 7.
Figure 6 : Symbol of bidirectional TVS
-cyi354.png)
Figure 7 : I-V curve of bidirectional TVS (DC characteristics)
3. Application difference of unidirectional and bidirectional TVS
Unidirectional TVS is suitable for the signal which above 0V, and according to the voltage level of signal to select a suitable reverse-standoff voltage (VRWM) of TVS is very important. For example, if voltage level of signal is +5V then it is recommend to select a TVS whose VRWM is equal to +5V for this signal. If you select a TVS whose VRWM is below +5V then TVS probably go into breakdown region and finally burn-out damage. If you select a TVS whose VRWM is above +5V then TVS will go into breakdown region lately so that the performance of ESD clamping is not good. If unidirectional TVS be used on bidirectional signals, it will cause -5V negative signal be clamped on -0.7V by TVS, so that this will result in signal distortion or system hang-up, waveform refer to Figure 8.
Figure 8 : Unidirectional TVS applied on bidirectional signal
Figure 9 : Negative signal be clamped at -0.7V
Conversely, if apply bidirectional TVS on unidirectional signal (+5V ~ 0V) there will have no negative signal clamped at -0.7V issue but this situation can’t express TVS’s performance in negative clamping, refer to Figure 10. Due to bidirectional TVS has positive and negative breakdown voltage so that there is no difference in I-V curve between unidirectional and bidirectional TVS. Unidirectional TVS has a -0.7V forward voltage but the breakdown voltage of bidirectional TVS is about -6V in negative voltage region, so unidirectional TVS’s performance is better than bidirectional TVS in negative voltage region, refer to Figure 11.
-gp6xur.png)
Figure 10 : Bidirectional TVS applied on unidirectional signal (+5V ~ 0V)
Figure 11 : I-V curve comparison of unidirectional and bidirectional TVS
(DC characteristics)
Normally, we can verify the TVS type is unidirectional or bidirectional TVS by checking the electrical characteristics of TVS. The VRWM of unidirectional TVS only shows positive value, refer to the datasheet of AZ5105-01F (Figure 12). The VRWM of bidirectional TVS will show positive and negative value, refer to the datasheet of AZ5725-01F (Figure 13).
-2ib5hk.png)
Figure 12 : Electric characteristics of unidirectional TVS (AZ5015-01F)
-5l84vv.png)
Figure 13 : Electric characteristics of bidirectional TVS (AZ5725-01F)
4. Four steps for selecting TVS correctly
The 1st step is to make sure the common mode voltage level of signal, if TVS operates under this voltage (VRWM) then the leakage current of TVS will less than 1uA. The breakdown voltage (VBV) of TVS is defined as the voltage when leakage current of TVS is over 1mA, so you need to make sure how big is the common mode voltage of signal ? The VRWM of TVS must bigger than the common mode voltage of signal (common mode voltage of signal (common mode voltage <VRWM<VBV), if leakage current is too big then would cause system abnormal issue.
The 2nd step is to make sure the insertion loss (-3db) frequency of signal, because of there is a parasitic capacitance on TVS and it would cause signal decade in the high frequency. So that using TVS on high-speed signals need to take care the insertion loss o f TVS and total capacitance in this signal. Normally, high-speed signal have a requirement against insertion, the frequency of insertion loss would decrease when the total capacitance of high-speed signal increased. For example : if the insertion loss requirement of a high-speed signal need to meet -3db@5GHz, the total capacitance of high-speed signal is too big after adding TVS (insertion loss change to -3db@2.5GHz) so that we could sure the capacitance of TVS for this high-speed signal is too big and cause this high-speed could not transmit and receive normally. Generally, if we want to meet insertion at 5GHz, the total capacitance limitation of high-speed signal is about 0.5pF. The total capacitance (signal to ground) of AZ1043-04F is 0.45pF and the frequency of insertion loss (-3db) is 8.3GHz, if convert to data-rate is about 16.6Gbps. Since the data-rate of AZ1043-04F is 16.6Gbs, this value is higher than 5Gbps so it is suitable for the transmission of USB3.1 Gen1 (5Gbps), refer to Figure 14.
-gtrm72.png)
Figure 14 : Insertion Loss of TVS (AZ1043-04F)
The 3rd step is to make sure the duration of spike for TVS selecting, because of there are many types of TVS, we catalog those spikes by two types duration (ns, us), we define this spike belong to “ESD (Electrostatic Discharge)” if the duration is “ns” and the specification ESD pulse is described by IEC61000-4-2 standard. This IEC61000-4-2 standard has the specification of rise time, peak voltage, waveform at 30ns, waveform at 60ns of ESD pulse, detail refer to Figure 15.
Figure 15 : Waveform of IEC 61000-4-2
The 2nd type spike has longer duration and about “us”, since the duration is longer so it has bigger destructive force, for example : Surge spike. European Union define this type of spike as Surge, and the Surge waveform and specification is described in IEC 61000-4-5. Surge 8/20us waveform refer to Figure 16. If IC suffers from Surge attack, it would have EOS (Electrical Over Stress) situation and appearance damage, the picture of IC EOS refer to Figure 17.
-ks8xfa.png)
Figure 16 : Surge waveform of IEC 61000-4-5 (8/20us)
Figure 17 : Picture of IC EOS
The 4th step is to select the TVS of low clamping voltage, since clamping voltage of TVS is the performance index, the clamping voltage of TVS shows the voltage level when spikes occur, the lower clamping voltage the higher ESD performance. TVS clamping voltage is seen by internal IC pin then internal IC will not be damaged by ESD spike, Figure 17 is clamping voltage comparison of TVS.
