SLYT862 March 2025 LM5066I , TPS25984B

4 TVS diode selection

A TVS diode is designed to protect electronic components from voltage spikes. The TVS diode begins to operate once the voltage on the diode exceeds the avalanche breakdown potential. Figure 4 is a graph of a TVS diode’s current-voltage curve.

Figure 4 TVS diode characteristics.

As you can see in Figure 4, the final clamping voltage is a function of the current, which the TVS diode shunts, and the dynamic resistance (R_d) of the TVS diode. Again, the dynamic resistance is a function of diode package size and the time duration (t_P) that the TVS diode shunts the current.

For example, the SMAJ diode (13.52mm²) has a higher R_d than the SMBJ diode (19.44mm²), so the SMAJ diode leads to a higher clamping voltage at a given shunt current.

Use these values of R_d to calculate the clamping voltage, these values are available from the TVS diode manufacturer’s data sheet.

For t_p ≤ 20μs:

Equation 3.

R_{d (t_{P})} = R_{D (8 / 20 μ s)}

For 20μs < t_p < 1ms :

Equation 4.

R_{d (t_{P})} = \frac{R_{D (10 / 1,000 μ s)} - R_{D (8 / 20 μ s)}}{980} [t_{P} - 20 μ s] + R_{d (8 / 20 μ s)}

For t_p ≥ 1ms:

Equation 5.

R_{d (t_{P})} = R_{D (10 / 1,000 μ s)}

This multiparameter dependency leads to a challenging iterative design process. To ease design, TI released an online tool for TVS selection [5]. Figure 5 illustrates the design methodology as a flow chart, while Table 1 lists the typical specifications of a rack server.

Figure 5 Flow chart for TVS diode selection.

Table 1 Typical system specifications.

Parameter	Value
Nominal operating voltage (V_IN)	12V
Maximum operating voltage (V_{DC_}max)	13.2V
Circuit breaker current (I_P)	200A
Parasitic inductance (L)	100nH
Maximum tolerable voltage (V_C(max))	20V
Maximum operating temperature	75°C