VBUS054B-HS3
Vishay Semiconductors
Background knowledge:
A zener- or avalanche diode is an ideal device for "cutting" or "clamping" voltage spikes or voltage transients
down to low and uncritical voltage values. The breakthrough voltage can easily be adjusted by the chip-
technology to any desired value within a wide range. Up to about 6 V the "zener-effect" (tunnel-effect) is
responsible for the breakthrough characteristic. Above 6 V the so-called "avalanche-effect" is responsible. This
is a more abrupt breakthrough phenomenon. Because of the typical "Z-shape" of the current-voltage-curve of
such diodes, these diodes are generally called "Z-diode" (= zener or avalanche diodes). An equally important
parameter for a protection diode is the ESD- and surge-power that allows the diode to short current in the pulse
to ground without being destroyed.
This requirement can be adjusted by the size of the silicon chip (crystal). The bigger the active area the higher
the current that the diode can short to ground.
But the active area is also responsible for the diode capacitance - the bigger the area the higher the
capacitance.
The dilemma is that a lot of applications require an effective protection against more then 8 kV ESD while the
capacitance must be lower then 5 pF! This is well out of the normal range of a Z-diode. However, a Protection
diode with a low capacitance PN-diode (switching diode or junction diode) in series with a Z-diode, can fulfil
both requirements simultaneously: low capacitance AND high ESD- and/or surge immunity become possible!
A small signal (V < 100 mV) just sees the low capacitance of the PN-diode, while the big capacitance of the
pp
Z-diode in series remains "invisible".
D
= 0.4 pF
C
D
C
TOT
ZD
CZ = 110 pF
D
20400
I/O
Such a constellation with a Z-diode and a small
PN-diode (with low capacitance) in series (anti-serial)
is a real unidirectional protection device. The
clamping current can only flow in one direction
(forward) in the PN-diode. The reverse path is
blocked.
D
ZD
Gnd
20401
I/O
Another PN-diode "opens" the back path so that the
protection device becomes bidirectional! Because the
clamping voltage levels in forward and reverse
directions are different, such a protection device has
a Bidirectional and Asymmetrical clamping behaviour
(BiAs) just like a single Z-diode.
D
1
D
2
ZD
Gnd
20404
Document Number 81586
Rev. 1.4, 07-Oct-08
www.vishay.com
5
For technical support, please contact: ESD-Protection@vishay.com
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