By Mel Clark and Kent Walters
The maximum transient power and current capability of a silicon transient voltage suppressor (TVS) can be derived for conditions other than the 10/1000us pulse specified on data sheets by using its peak pulse power vs pulse time curve.
Most TVSs are rated for 10/1000us non-repetitive pulses (10ms being the rise time and 1000us being the time to decay to one-half peak value) based on an early telecom specification. Real world conditions produce a vast array of waveforms having pulse widths ranging from tens of nanoseconds to tens of milliseconds in duration. For example, IEC 801-5 describes 1.2/50us lightning threats to signal lines.
The graph in Figure 1 relates peak pulse power vs time for 600W suppressors. Similar graphs depict power vs time for TVSs rated at other power levels. At 1000us, the maximum peak pulse power (PPP) for this series is 600W (the device rating).
For short pulse widths, the TVS will sustain higher peak pulse currents (IPP). The graph illustrates at 20 us the PPP rating is 3.2kW, or 5.3 times its rating at 1000us. Hence the IPP rating of an SMBJ12C would increase by a factor of 5.3 from 27.3A to 145A for a 20us pulse.
For longer pulses, the TVS will withstand lower IPP values. At 10,000us (10ms), PPP rating is down to approximately 200W, one-third of its 1000us rating. The IPP for an SMBJ12C would be reduced by a factor of .33, from 27.3A to 9.1A for a 10,000us pulse. This applies to all devices in the 600W series regardless of their operating voltage.
In the same manner, the PPP and/or IPP can be derived for a component of any other TVS series using its associated power vs time curve.
Most silicon TVSs are rated for 10/1000us double exponential waveforms, including the example shown here. For one-half sine wave pulses, derate to 75% of the peak exponential value and for square wave pulses derate to 66%.
[Figure 1. Peak pulse power vs pulse time]
Copyright Microsemi Corporation 1996
104.HTM