TVS tube parameters, selection and application
First, first understand the parameters of TVS tube, we take littelfuse's 5.0SMDJ series as an example.
figure 1
figure 2
Maximum Peak Pulse Power Maximum Peak Pulse Power
The Pppm peak pulse power is the maximum power instantaneously passed through the ESD device, which is determined by the maximum clamp voltage Vc and the peak current Ipp at this time.
From the above figure, we can see that the maximum pulse peak power of the 5.0SMDJ series is 5000W. It can also be seen that the Vc*Ipp of each model under this series is basically 5000W. The larger the Vc is, the smaller the Ipp is.
Maximum pulse peak current Ipp
The maximum pulse peak current that TVS can withstand. The maximum pulse current of 5.0SMDJ13A is 233A.
Maximum clamp voltage Vc (residual pressure)
When the impulse peak current Ipp with a duration of 20 mS flows through the TVS, the maximum peak voltage appearing across the TVS is Vc. Also called the maximum protection voltage.
5.0SMDJ series different types of Vc is not the same, 5.0SMDJ13A maximum protection voltage is 21.5V.
Maximum reverse leakage current Ir
TVS maximum reverse leakage current, we can see 5.0SMDJ13A maximum reverse leakage current is 500uA
Maximum reverse working voltage Vrwm or Vr.
When the maximum reverse leakage current Ir flows through the TVS, the voltage across the TVS is the maximum reverse operating voltage. 5.0SMDJ13A maximum reverse voltage is 13V.
Test current It
The test current It determines the reverse breakdown voltage Vbr of the TVS.
Reverse breakdown voltage Vbr
The voltage at which the TVS tube passes the specified test current It is a sign voltage indicating that the TVS tube is conducting, that is, from this point the device enters avalanche breakdown.
Junction Capacitor Cj
When protecting high-speed signals, consider the effect of junction capacitance.
Second, TVS tube selection and application
1, TVS is used to protect inrush current, if you know the maximum current Ipp to protect is best. First determine the TVS Ipp.
2. Then determine the DC voltage or continuous operating voltage of the circuit to be protected. If it is AC, the maximum value should be calculated, which means that the effective value is 1.414.
3. According to the known operating voltage in 2 , selecting Vrwm of TVS requires Vrwm to be greater than the working voltage, otherwise working voltage greater than Vrwm will cause TVS reverse leakage current to increase, close to conduction, or avalanche breakdown, affecting the normal circuit work. .
4. The maximum clamp voltage Vc of the selected TVS cannot be greater than the maximum voltage that the protected circuit can withstand. Otherwise, when the TVS clamps at Vc, it will damage the circuit.
5, which 2-4 steps can be preceded by 1, if you do not know Ipp, Ipp can be determined based on the power of the signal.
6, unipolar or bipolar - often misunderstood that bidirectional TVS is used to suppress reverse surge pulse, in fact, it is not the case. Bidirectional TVS is used for alternating current or from positive and negative bidirectional pulses. TVS is also sometimes used to reduce the capacitance. If the circuit has only a forward-level signal, one-way TVS is sufficient. The TVS operates in the following manner: In the forward surge, the TVS is in the reverse avalanche breakdown state; in the reverse surge, the TVS conducts and absorbs the surge energy like the forward bias diode.
This time you need to refer to the TVS forward conduction peak current, Ifsm. The maximum forward current of the 5.0SMDJ series is 300A.
Third, the surge waveform 10/1000us and 8/20us.
10/1000us and 8/20us are two different surge test waveforms. The two waveforms have different energy.
The 10/1000us has a duration of 1000us and a rising edge time of 10us.
The duration of 8/20us is 20us, and the rising edge time is 8us.
It can be seen from Fig. 2 that the maximum power of the TVS is 5000W, which is tested under the wave energy of the surge 10/1000us. Ipp is based on 10/1000 waveform energy.
When the surge waveform is 8/20us, the corresponding parameter will become larger.
It can be seen from the figure that when the waveform is 1000us, the maximum power that can be tolerated is 5KW.
When the test waveform is 20us, the maximum power that can withstand is 20KW. If Vc does not change, the maximum peak current that can be tolerated will become 4 times larger than the original Ipp. 20/5=4
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