By Mel Clark and Kent Walters
Clamping voltage (VC) is specified only at the maximum limit on most silicon transient voltage suppressor (TVS) data sheets. Often the designer needs to determine the VC at some intermediate level between breakdown voltage (V(BR)) and maximum VC.
This value can be calculated with the data sheet parameters using the formula:
VC = (IP/IPP) (VC max - V(BR) max) +V(BR) max
Where:
IP = actual test pulse current
IPP = actual rated peak pulse current
VC = clamping voltage at IP
VC max = maximum specified clamping voltage
V(BR) = upper limit of breakdown voltage
Based on previous data, a linear increase in VC can be assumed between V(BR) and VC max. for this formula. The VC vs IP relationship for the SMCJ15A 1.5 kW TVS between V(BR) and VC as calculated by this method is shown in Figure 1. Results are as expected. This calculation assumes the TVS to be at the V(BR) upper limit, hence it would be conservative for most of the distribution. Note that when IP equals IPP, VC equals VC max.
If only V(BR) min. is listed on the data sheet, V(BR) max. can be closely approximated. For "A" suffix parts, multiply V(BR) min. by 1.11 and for non-suffix parts, multiply by 1.22 to obtain V(BR) max.
An example of a calculated curve compared to one derived from test measurements (Figure 1) illustrates the feasibility and conservative aspects of this method. Surge tests were performed on a 20 piece sample at 25 degrees C with a 10/1000us waveform.
[Figure 1]
The curve based on surge test data has a more shallow slope than the curve interpolated through calculation. This indicates that the devices are conservatively rated and the formula given is adequate for interpolating intermediate values of VC for a fractional part of IPP.
The linear relationship between IP and VC can be applied in determining greater IPP ratings for applications requiring lower than normal values of VC. In the equation above, insert the desired value for VC and solve for the higher IPP value. This often requires upgrading to a higher peak pulse power (PPP) rated device.
108.HTM