How to Select a TVS Diode?

 

Transient Voltage Suppression (TVS) diodes are essential components in protecting electronic circuits from voltage spikes and surges. Selecting the right TVS diode for your application requires careful consideration of several parameters, including breakdown voltage, clamping voltage, worst-case calculations, package, and wattage. This guide will walk you through each of these factors in detail.

1. Understanding TVS Diodes

TVS diodes are designed to protect sensitive electronic equipment from transient voltage spikes. When a transient occurs, the TVS diode becomes conductive, clamping the voltage to a safe level and diverting the excess energy away from the protected circuit.

2. Key Parameters for Selecting a TVS Diode

Breakdown Voltage (V_BR)

• Definition: The voltage at which the TVS diode begins to conduct current significantly.
• Selection Criteria: Choose a breakdown voltage that is higher than the normal operating voltage of your circuit but lower than the maximum voltage your circuit can tolerate.
• Example: For a circuit with a normal operating voltage of 12V, a TVS diode with a breakdown voltage of around 13-15V might be appropriate.

Clamping Voltage (V_C)

• Definition: The voltage at which the TVS diode clamps the transient and starts to divert the surge current.
• Selection Criteria: The clamping voltage should be lower than the maximum voltage your circuit can withstand without damage.
• Example: If your circuit can tolerate up to 20V, select a TVS diode with a clamping voltage below this threshold.

Working Peak Reverse Voltage (V_RWM)

• Definition: The maximum continuous operating voltage the TVS diode can withstand without entering avalanche breakdown.
• Selection Criteria: Ensure that V_RWM is slightly higher than the normal operating voltage of your circuit to avoid unnecessary clamping during normal operation.
• Example: For a 5V circuit, a TVS diode with a V_RWM of 5.5V or 6V would be suitable.

3. Power Dissipation and Wattage

• Definition: The amount of power the TVS diode can dissipate when clamping a surge.
• Selection Criteria: Choose a TVS diode with a power rating sufficient to handle the energy of the expected transient. This is usually specified in watts (W).
• Example Calculation:
  • Estimate the peak pulse current (I_PP) of the transient.
  • Calculate the power dissipation using the formula: $P = V_C \times I_{PP}$.
  • Ensure the TVS diode’s power rating exceeds this calculated value.

4. Worst-Case Scenario Calculations

Peak Pulse Current (I_PP)

• Definition: The maximum surge current the TVS diode can handle.
• Selection Criteria: Determine the highest possible transient current your circuit might encounter and choose a TVS diode rated to handle this current.
• Example: If a 100A surge is the worst-case scenario, select a TVS diode with an I_PP rating higher than 100A.

Energy Absorption (E_AS)

• Definition: The amount of energy the TVS diode can absorb during a transient event.
• Selection Criteria: Ensure the diode can handle the energy of the worst-case transient.
• Example Calculation:
  • Calculate the transient energy: $E = 0.5 \times V_C \times I_{PP} \times t_{pulse}$.
  • Verify the TVS diode's energy absorption rating exceeds this value.

5. Package Selection

• Definition: The physical form factor of the TVS diode.
• Selection Criteria: Consider the available space on your PCB, thermal management requirements, and the mechanical robustness needed for your application.
• Common Packages:
  • SMD (Surface Mount Device): Suitable for compact, high-density PCBs.
  • Through-Hole: Ideal for higher power dissipation and better mechanical stability.

6. Additional Considerations

Response Time

• Definition: The time it takes for the TVS diode to respond to a transient event.
• Selection Criteria: TVS diodes generally have fast response times, but ensure the chosen diode is quick enough for your application's requirements.

Capacitance

• Definition: The inherent capacitance of the TVS diode.
• Selection Criteria: Lower capacitance is critical in high-speed data lines to avoid signal degradation.

7. Practical Example

Application: 12V Automotive Circuit

1. Normal Operating Voltage: 12V
2. Maximum Tolerable Voltage: 24V
3. Estimated Peak Pulse Current: 50A
4. Pulse Duration: 1ms

Step-by-Step Selection:

1. Breakdown Voltage: Choose a TVS diode with a breakdown voltage around 13-15V.
2. Clamping Voltage: Ensure the clamping voltage is below 24V, say around 20-22V.
3. Working Peak Reverse Voltage: Select a TVS diode with a V_RWM of at least 14V.
4. Power Dissipation: Calculate power dissipation: $P = 22V \times 50A = 1100W$. Select a TVS diode rated for at least 1500W.
5. Package: An SMD package for compactness or a through-hole for better power dissipation.

Conclusion

Selecting the right TVS diode involves balancing multiple parameters to ensure optimal protection for your circuit. By carefully considering breakdown voltage, clamping voltage, working peak reverse voltage, power dissipation, package type, and additional factors like response time and capacitance, you can choose a TVS diode that offers reliable and effective protection against transients. Always refer to manufacturer datasheets for precise specifications and consult with engineers if needed to ensure the best choice for your specific application.