In the design, a TVS diode is used. If you haven't paid much attention to the TVS tube in your previous designs, let's take a closer look at it through this detailed explanation. The TVS, or Transient Voltage Suppressor, is a protective component designed to safeguard circuits from voltage surges. Its working principle is similar to that of a Zener diode, but with a much faster response time. When a high-energy transient voltage occurs, the TVS quickly switches to a low-impedance state, effectively clamping the voltage and absorbing the surge energy. This helps maintain a stable voltage level across the protected components and prevents damage from unexpected voltage spikes.
Comparing TVS and Zener diodes, both can limit the voltage at their terminals within a certain range and have similar long-term current handling capabilities. However, there are key differences. First, the voltage regulation of a Zener diode is more precise, while the TVS operates within a voltage range. Second, Zener diodes have a much lower surge current capability compared to TVS, which can handle hundreds of amperes. Third, Zener diodes rely on tunneling or avalanche effects, while TVS mainly uses avalanche effect. Fourth, Zener diodes are used for voltage regulation, whereas TVS is used for transient voltage protection. Lastly, Zener diodes typically operate between 3.3V and 75V, while TVS tubes can work from 6.8V up to 550V. The response time of TVS is also significantly faster, often in the picosecond range, making them more suitable for fast transient protection.
Now that we understand the basic principle, let’s look at some key parameters of the TVS tube as described in its datasheet. These values help determine the appropriate TVS for your circuit. The first two columns usually relate to specific device models, so we’ll skip those. The Reverse Stand-off Voltage (VRWM) is the maximum voltage the TVS can withstand without conducting. It ensures that the TVS remains in a high-impedance state during normal operation. The Breakdown Voltage (VBR) is the voltage at which the TVS enters the breakdown region. The Test Current (IT) is the DC current used to measure VBR. The Maximum Clamping Voltage (Vc) is the highest voltage the TVS will allow during a surge, and the Peak Pulse Current (IPP) is the maximum current it can safely absorb. The Reverse Leakage Current (IR) is the small current that flows when the TVS is under VRWM.
Other important parameters include Junction Capacitance (Cj), which affects signal integrity in high-speed applications, and Insertion Loss, which may be specified for high-speed IO protection. Understanding these parameters allows you to choose the right TVS for your application. A general selection rule includes: 1) The TVS’s VRWM should be greater than or equal to the circuit’s maximum operating voltage. 2) The minimum breakdown voltage VBR should be around 0.8–0.9 times VRWM. 3) The maximum clamping voltage VC must be less than the circuit’s damage threshold, typically 1.3 times VBR. 4) The peak pulse power (PM) of the TVS must exceed the expected surge power. Finally, the peak pulse current should be higher than the expected transient surge to ensure reliable protection.
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