A Comparison of Varistors and Diodes

Varistors and Diodes Special Note: These comparisons hold whether comparing discrete chips (TransGuard vs. diode) or arrays (MultiGuard vs. diode)

Summary of Advantages Varistors have many advantages over diodes. These include: Reliability High temperature performance Board space consumption Bipolarity Turn On Time Filtering characteristics

Reliability Varistors survive many more ESD strikes than do diodes. 80000+ Pulses to Failure Energy

Reliability A very small 0402 varistor provides more strike protection than a large SOT-23 diode Suppression Method # Strikes to failure 0402 TransGuard >10000 0603 TransGuard >80000 0805 TransGuard 1206 TransGuard 1210 TransGuard SOT 23 Diode 23 Unidirectional 678 Bi-directional

Reliability Current The ceramic material is doped Zinc Oxide where every grain is a Schottky Diode. The structure between the plates gives series/parallel diodes. The entire volume dissipates energy.

High Temperature Performance Varistors maintain their energy handling capabilities at higher temperatures than diodes Varistor Diode Better temperature performance

Board Space Consumption Discrete varistors are much smaller than discrete diodes: 0.067” 0.112” 0.020” 0.120” Total Area = Total Area = 0.00134 in2 0.01340 in2 0402 TransGuard SOT 23 Diode Reduction of 90% in board space required!

Board Space Consumption Varistor arrays consumer much less boards space than diode arrays 0508 MultiGuard SOT363 Diode Array Reduction of 37% in board space required! 0.067” 0.098” 0.080” 0.087” Total Area = 0.00536 in2 Total Area = 0.00853 in2

Board Space Consumption Varistor arrays consumer much less boards space than diode arrays SO-8 Bi-directional Diode Array 0508 MultiGuard Reduction of >90% in board space required! 0.067” 0.298” 0.080” .197” Total Area = 0.00536 in2 Total Area = 0.0587 in2

Bipolarity Diodes protect in only one polarity Many of the diode array packages protect only in one polarity In order to protect in both polarities like the varistors, the number of elements is cut in half! A 4-element unipolar diode array is only a 2-element bipolar diode array!

Turn On Time The performance of an ESD protection device is measured by the amount of energy that the device allows to be passed to the circuit (or device) being protected. The key measure of a device’s ability to reduce the amount of energy passed is measured by the turn of time of the device. The turn on time of varistors is much less than the tune on time of a diode

Turn On Time Device Type Turn on Time (pS) 0402 TransGuard 417 673 0805 TransGuard 756 1206 TransGuard 818 1210 TransGuard 798 SOT 23 Diode 1380 Sub-nanosecond turn on time

Turn On Time ESD Waveform TransGuard Turn-on Time ~ 400ps Silicon TVS Turn-on Time ~ 1200ps ESD Waveform Unprotected Energy passed to circuit by the Diode

Filtering Characteristics Diodes have very low capacitance, so they do not have filtering characteristics. In fact, to achieve filtering and bipolar protection, 3 components must be used for the diode solution as compared to a single component for the varistor solution.

Filtering Characteristics Again, varistors provide EMC filtering as seen in these S21 curves Varistors can be manufactured to filter at different frequencies while still providing transient protection

Summary of Advantages Varistors have many advantages over diodes. These include: Reliability High temperature performance Board space consumption Bipolarity Turn On Time Filtering characteristics