Micronote 108 - Determining Clamping Voltages for Pulse Currents (269.29 kB)

MicroNote #108
By
Mel Clark
and
Kent Walters
Determining Clamping Voltage
Levels for a Range of Pulse Currents
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.
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 value can be calculated with the data sheet
parameters using the formula:
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.
VC = (IP/IPP)(VC max – V(BR) max) + V(BR) max
Where:
IP = actual test pulse current
IPP = maximum rated peak pulse current
VC = clamping voltage at IP
VC max = maximum specified clamping voltage
V(BR) = upper limit of breakdown voltage
24
Clamping Voltage - Volts
Based on previous data, a linear increase in VC can be
assumed between V(BR) and VC max for this formula.
The VC versus IP relationship for the SMCJ15A for a
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.
SMCJ 15A
Clamping Voltage vs. % Peak Current
22
20
d
late
lcu
Ca
l
Typica
ured
Meas
18
16
If only V(BR) min is listed on the data sheet, V(BR) max
can be approximated. For “A” suffix parts, multiply
V(BR) min by 1.20 and for non-suffix parts, multiply by
1.25 to obtain V(BR) max.
Figure 1
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 10/1000 µs waveform.
Corporate Applications Engineering Department,
480-941-6300, Ext 433 or 524
www.
.com
1 mA
25%
50%
Percent of IPP
75%
100%
1