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AN5370 Application Note
www.dynexsemi.com
AN5370
Using Avalanche Diodes Without Sharing Capacitors
Application Note
AN5370-1.2 July 2002
Replaces July 2000, version AN5370-1.1
Reverse voltage
R
Avalanche voltage
Avalanche diode
Normal diode
0.1
1
10
100
1
10
100
1000
10000
Surge duration - (
μ
s)
I
T
j
= 25C
T
j
= 150C
Duty Cycle
<0.001% Square Wave Pulse
Fig.1 Reverse characteristics
Fig.2 Non-repetitive peak reverse power
1. INTRODUCTION
The important property of the avalanche diode is its ability to
safely handle, without damage, relatively large reverse powers
levels. Normally diodes are limited to a reverse current of the
order of mA at room temperature and mA at elevated temperatures
but with avalanche diodes reverse currents of greater than 1 A
are possible.
The reverse characteristic of an avalanche diode is compared
in figure 1 with the characteristic of a normal diode. The
avalanche diode has low leakage current up to the avalanche
voltage and does not show the same increase in leakage current
as the normal diode. The silicon of avalanche diodes is regular
across the cross section therefore the majority of the reverse
current is conducted within the bulk of the silicon. In a normal
diode the majority of the reverse current is conducted close to
the edge of the silicon where the electric field strength is highest.
As a consequence of this, the reverse power handling capability
of the avalanche diode is expressed in kW for rectangular shaped
pulses of defined width. Normal diodes may conduct the reverse
current in a very small localised area so the reverse current has
to be limited.
The avalanche rating curve is shown in fig 2 for the MZ0409W,
a 2A, 1400V Vrrm (repetitive rating) and 1500V V(AB)R (avalanche
voltage) diode. The curve is valid for a case temperature of 25
o
C
and 150
o
C. The permissible avalanche power is 9kW for a
rectangular pulse of 10us and 0.9kW at 1 millisecond width.
Provided the diode is operated within the limits specified on the
data sheet, the excursion into the avalanche region is non
destructive.
2. DESIGN VOLTAGE
Avalanche diodes are used in series to make high voltage
strings. Due to their avalanche capability they can be used
without static sharing resistors or dynamic sharing capacitors.
For these strings, it is customary to allow a voltage safety margin
of 2.4 to 3 between the rated repetitive voltage of the string and