Avtech AVR-EB4-B, Instructions Manual

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VA = IDUT × 50Ω
where I
DUT
is the current through the device under test. A 450 Ohm resistance (R
B
) is
present in series with the measurement output. When a 50 Ohm resistance (R
C
) is
installed on the output (by the user), the output voltage will be one-tenth of V
A
due to
the resistor-divider effect. That is:
VOUT = VA / 10 = IDUT × 5Ω
This is the key equation for relating the observed voltage waveform to the DUT current.
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 ("AMP1" on the front panel display) is related to the forward diode
current by:
I
FORWARD
≈ (AMP1 - V
F
) / 50Ω
where V
F
is the forward voltage drop of the diode (typically 0.7V for the classic silicon
PN junction diode, and usually somewhat lower for a Schottky diode).
The negative voltage ("AMP2" on the front panel display) is related to the reverse diode
current by:
I
REVERSE
≈ AMP2 / (50Ω + R
DIODE-REVERSE
).
where R
DIODE-REVERSE
is the effective resistance of the diode under reverse bias. R
DIODE-
REVERSE
may change during the transient. Furthermore, depending on the design of the
diode under test, it is possible that R
DIODE-REVERSE
may be so high that it is impossible to
achieve the full 4 Amps of reverse current. (The ideal diode would of course have
R
DIODE-REVERSE
= ∞). The reverse voltage can actually be increased to -240V (rather than
the nominal maximum of -200V) to increase the likelihood of obtaining the full 4 Amps
of reverse current.
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
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