A Comparison of Varistors and Diodes

Special Note:These comparisons hold whether comparing discrete chips

(TransGuard vs. diode) or arrays (MultiGuard vs. diode)

Varistors and Diodes

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.

110

1001000

10000100000

0.047 0.06 0.073 0.1

SOT23 Unidirectional

SOT23 Bidirectional

Transguard

80000+

Energy

Pulses to Failure

Reliability

• A very small 0402 varistor provides more strike protection than a large SOT-23 diodeSuppression Method # Strikes to failure

0402 TransGuard >10000

0603 TransGuard >80000

0805 TransGuard >80000

1206 TransGuard >80000

1210 TransGuard >80000

SOT 23 Diode 23 Unidirectional

678 Bi-directional

ReliabilityCurrent

The ceramic material is doped ZincOxide where every grain is a SchottkyDiode. The structure between the platesgives series/parallel diodes. The entirevolume dissipates energy.

High Temperature Performance

• Varistors maintain their energy handling capabilities at higher temperatures than diodes

0%

20%

40%

60%

80%

100%

120%

0 50 100 150 200

Temperature [°C]

% o

f R

ated

Po

wer

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

Reduction of 37% in board space required!

0508 MultiGuard

0.080”

0.067”

0.087”

0.098”

Total Area = 0.00536 in2

SOT363 Diode Array

Total Area = 0.00853 in2

Board Space Consumption

• Varistor arrays consumer much less boards space than diode arrays

Reduction of >90% in board space required!

0.080”

0.067”

.197”

0.298”

0508 MultiGuard

Total Area = 0.0587 in2

SO-8 Bi-directional

Diode Array

Total Area = 0.00536 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

0603 TransGuard 673

0805 TransGuard 756

1206 TransGuard 818

1210 TransGuard 798

SOT 23 Diode 1380

Sub-nanosecond turn on time

Turn On Time

0

20

40

60

80

100

0.1 1 10 100

Time (ns)

% P

eak

Cu

rren

t

TransGuard Turn-on Time ~ 400psSilicon 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