Choosing an appropriate TVS – 9 step approach
When deciding which device use in a design, is a great tool to have a step-by-step guideline to make easier and quicker the process. Here is an approach on how to choose TVS devices in a HOT SWAP application.
Fig. 1. * Block diagram 54V DC-based power supply architecture.
-
First step: select a minimum standoff voltage VR. In this case a 5% tolerance was considered:
V_{IN}=54 \rightarrow V_R=105\%V_{IN}=1.05(54V),
\;\;V_{R}=56.7V. -
Obtain maximum expected pulse current I_P based on the circuit worst case characteristics.
In this example, a peak current of 480 A is used.
-
Designer should preselect some TVS with the stablished working voltage in Step 1. The MCC’s SMC 5kW series offers robust TVS options and the following set of part numbers meet the standoff voltage requirement. We will evaluate them for this application:
• 5.0SMLJ58A
• 5.0SMLJ60A
• 5.0SMLJ64A
• 5.0SMLJ70A -
Next step is finding the clamping voltage 𝑉𝐶 for the transient. The following
equation is used:V_C=I_P\left(\dfrac{V_{C(Max),8/20\mu s}-V_{BR}} {I_{PP,8/20\mu s} }\right)+V_{}BR.
V_C=480 A\left(\dfrac{125V-70.37V} {320A }\right)+70.37,
V_C=152.15V.
Fig. 2. Table with the important parameters from the datasheet for the 5.0SMLJxxxx
TVS series. -
After that, the calculated 𝑉𝐶 from previous step and the estimated 𝐼𝑃 are used to
find power across the TVS during the transient pulse:P_P=V_CI_P=152.15V\left(480A\right)=73.03W.
-
The pulse duration ∆𝑡𝑑 will be estimated using the input parasitic inductance in
the equation below.V_L=L_{parasitc}\dfrac{di_L}{dt}\approx L_{parasitc}\dfrac{\Delta i_L}{\Delta t},
\Delta t_d=\dfrac{L_{parasitic} I_P}{V_C-V_{IN}},and using the values obtained in previous steps, the following time is obtained
for this example:t_d=\dfrac{L_{parasitic} I_P}{V_C-V_{IN}}=\dfrac{500nH\cdot4890A}{152.15V-54V},
t_d=2.45\mu s. -
In this and the next step, the power capabilities of a single TVS will be found. The
allowable peak pulse power 𝑃𝑃𝑃 for the pulse duration ∆𝑡𝑑 of step 6 is shown in
Figure 3. The power that the component can dissipate is slightly greater than 80
kW.
Fig. 3. Peak pulse power rating curve for the 5.0SMLJxxxx TVS. -
Up to this point, the next step is to derate the allowable P_{PP} to account for the
maximum ambient temperature expected in the application. It is suggested to
include heating effects caused by nearby components. In Figure 4, a maximum
temperature of 125°C is shown.
Fig. 3. Peak pulse derating curve for temperature for the 5.0SMLJ60A TVS.In this example, the final maximum allowable power for the component after
derating is:P_{PP@125°C}=35\%P_{PP}=35\%82kW=28.7kW.
-
For a TVS to be successful the total derated power of step 8 must be above the
calculated TVS peak power from step 5 with a margin of at least 50% to ensure
the reliability of the component and the application:\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;P_{PP@125°C}>1.5P_P.
In this example the result is the following:
P_{PP@125°C}=28.7 kW,\\ \;\;\;\;\;\;1.5P_P=109.6kW.
The power condition is not met and a new iteration of this 9-step process for TVS
selection should follow. However, there are two solutions:
a. Another device with greater power capabilities is analyzed.
b. Use a few of the same TVS component in parallel to divide power.
Table 1 summarizes the calculation for three different I_P values, 120A, 160A and 480A.
Cases in green are successful in meeting the power condition given in step #9 with a single
device.
Table 1. Datasheet parameters and calculated P_P and td values for TVS devices from the 5.0SMLJxxxx MCC ‘s series showing the parameters found in the datasheet and from calculations shown in this work.
For a deeper approach, please check Application Note: Hot-Swap Controller Application Note: Enhancing Data Center Reliability