TVS parameters and how to select and apply them

Seven major parameters of TVS

As shown in the above figure, TVS has the following seven parameters:

1. Reverse turn off voltage/reverse operating voltage of VRMM

The maximum operating voltage of TVS can be continuously applied with the highest peak operating voltage or DC peak voltage that does not cause degradation or damage to TVS. For AC voltage, represented by the effective value of the highest operating voltage, TVS is considered non functional, i.e. non-conductive, under VRWM. The maximum operating voltage during circuit design must be less than VRWM, otherwise it will cause TVS operation and circuit abnormalities.

2. IR reverse leakage current

Reverse leakage current is generally standby current. The maximum current flowing through the TVS under specified temperature and maximum operating voltage conditions. The leakage current of TVS is generally measured under reverse operating voltage. For a certain model of TVS, IR should be within the specified value range. The voltage value applied to both ends of the TVS is VRWM, and the current read from the ammeter is the leakage current IR of the TVS. For TVS with the same power and voltage, when VRWM is less than or equal to 10 V, the leakage current of bidirectional TVS is twice that of unidirectional TVS. The general leakage current is less than 100 nA. For some analog ports, the leakage current will affect the sampling value of the ADC, so the smaller the leakage current of the TVS, the better.

3. VBR breakdown voltage

Breakdown voltage, the voltage at which ESD protection begins to operate, is generally the voltage at which the TVS passes through 1 mA. The duration of current application is generally not more than 400 ms to avoid damaging the device, VBRmin and max are a deviation of the breakdown voltage of TVS, usually with a deviation of plus or minus 5%. When VBR falls between min and max during measurement, it is considered a qualified product.

4. IPP peak pulse current

Peak current, the peak value of a given pulse current waveform. TVS generally uses current waveforms of 10/1000 us or 8/20 us.

5. VC clamping voltage

When a clamping voltage is applied to the peak pulse current IPP of a specified waveform, Peak voltage at both ends of TVS. IPP and VC are parameters that measure the ability of TVS to resist surge pulse current and limit voltage in circuit protection, and these two parameters are interrelated. For TVS of the same model, The larger the IPP, the stronger the ability to withstand current surges. If the VC under the same IPP is smaller, it indicates that the clamping characteristics of TVS are better.

6. Cj junction capacitor

The junction capacitance is a parasitic capacitance in TVS, which needs to be given special attention in high-speed IO port protection. Excessive junction capacitance may affect the quality of the signal. Based on the protected signal rate, select the appropriate junction capacitor. The following are common interfaces and recommended ESD junction capacitor size ranges.

 

7. TVS power

TVS power is considered when considering surge protection, TVS will not be damaged by surge current pulses.

TVS selection (taking 5a test pulse as an example)

  

1. Determine the reverse turn off voltage VRMM of the TVS DIODE

For example, if the product is a 24V system, the reverse turn off voltage is generally required to be greater than or equal to the supply voltage. However, considering the need for overvoltage performance testing, 36V is chosen.

2. Determine the peak current of TVS

For example, the highest peak value of the 5a pulse is 202V, and the output internal resistance of the testing equipment is 4 Ω (both of which are related to the testing equipment and testing standards). It can be inferred that the peak current Ipp of TVS should not be less than 50.5A.

3. Determine the clamping voltage of TVS

The clamping voltage Vcmax is generally 1.5~2 times the reverse turn off voltage VRMM, so the range of clamping voltage Vcmax is selected to be 54V~72V.

4. Determine the peak power of TVS DIODE

According to the clamping voltage and peak current, it is known that the peak power Pppm range of TVS DIODE is 2.7kW~3.6kW. It should be noted that the 5a pulse is different from the surge pulse in that it has a large pulse width and a long duration. At 5-10ms, the peak value reaches a maximum of 202V, and the power of the TVS DIODE is required to be above 3.6kW; At 100ms, the peak voltage pulse is 84V, and the TVS power should be required to be above 1.5kW. When selecting, attention should be paid to the power time curve of the device's power.

5. Confirm the selection:

Based on the above parameters, select the TVS DIODE model SM8S 36A 6701 to protect against 5a pulses. The TVS DIODE of model SM8S 36A 6701 is a unidirectional conducting TVS DIODE, with a reverse turn off voltage of 36V and a maximum current capacity of 114A, which meets the requirements.

Looking at the power time curve of SM8S 36A 6701 again, the device protection power is 5.3kW at 10ms, which is higher than the protection requirement of peak power of 3.6kW; The protection power is 1.8kW at 100ms. Above the 1.5kW protection requirement, therefore the selection is complete.