○ The voltage Vmof the circuit using a VRD should be Vm≦Vs
Vss the rated voltage of the VRD.
○ The withstand voltage Vmaxof the device to be protected should be set Vc so that Vmax≧the clamping voltage (Vc).
Vc is VBmax+α.
VBmax is the maximum breakdown voltage tolerance for each product.
α varies with ambient temperature, surge waveform, surge current, thermal resistance, surge frequency, and other conditions.
– Ambient temperature Ta
The power rating of the VRD should be reduced relative to the ambient temperature.
The change is ⊿VB＝VB ×（Ta－25℃）× temperature coefficient.
– Surge waveform, surge current
The figure below shows the clamping voltage characteristics at 10 × 1000 μsec and 8 × 20 μsec.
The clamping voltage VCvaries with the applied surge waveform and current.
Clamping voltage characteristics of SMD-VRD
– Repetitive Surge
If surges are expected to occur repeatedly, use the product such that the average power does not exceed the power rating.
○ The VRD is a device to absorb a surge with steep rise time and is not intended for use in constant voltage applications.
Do not use the VRD in a way that keeps it constantly energized.
