Avtech 14258, Инструкция по эксплуатации, Страница: 23 / 48

SETTING THE AMPLITUDE LEVELS
The amplitude of the positive and negative portions of the PULSE waveform may be set
from the front panel of the instrument, or by computer command. These settings are
expressed in terms of the voltage present on the test jig input.
The positive voltage V
1
("AMP1" on the front panel display) is related to the forward
diode current by:
I
F
≈ (V
1
– V
DIODE
) / (R
1
+ (R
A
|| R
C
))
I
F
≈ (V
1
– V
DIODE
) / 200 Ω
where V
DIODE
is the forward voltage drop of the diode (typically 0.7V). The 200 Ohm
resistance is the sum of R
1
in the figure above and the 25 Ohm resistance formed by R
A
and R
C
.
The reverse current is given by:
I
R
≈ I
F
+ (V
2
/ (R
2
+ (R
A
|| R
C
)))
I
R
≈ I
F
+ V
2
/ 50 Ω
Most test procedures for measuring recovery time will use a particular ratio of forward to
reverse currents - for example, I
REVERSE
/ I
FORWARD
= -2. Some Schottky diodes have
negligible amounts of stored charge resulting from the forward bias, compared to non-
Schottky devices. For these Schottky diodes, the reverse transient will be governed by
the capacitance of the device, and the reverse transient may be largely unaffected by
the amplitude of the forward transient. (In other words, the I
REVERSE
/
IFORWARD
ratio is
irrelevant). The capacitance may be so small that it becomes impossible to obtain the
full -1 Amps of reverse current.
V
1
may be set b1V and +120V.
V
2
may be set between -7.5V and -84V.
AMPLITUDE ACCURACY
Due to the variations in V
F
and the diode parasitic resistance as a function of operating
conditions, the V
1
(AMP1) and V
2
(AMP2) settings should not be relied upon for any
degree of accuracy – especially V
1
. Instead the voltage at the OUT terminal on the test
jig should be monitored with a calibrated oscilloscope. As mentioned above,
VOUT = IDUT × (R
A
|| R
C
)
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