When using certain higher performance power supplies from
Agilent, like the N678xA series source-measure modules, you may discover that
the output has shut down and an annunciator displaying “Osc” shows up on the
front panel meter display, like that shown in Figure 1 for the N6705B DC Power
Analyzer mainframe.
Figure 1: DC Power Analyzer front panel meter displaying
“Osc” on channel 1 output
As you would likely guess, Osc stands for oscillation and
this means the output has been shut down for an oscillation fault detection. In
this particular instance an N6781A high performance source measure module was
installed in channel 1 of the N6705B DC Power Analyzer mainframe.
The N678xA series source measure modules have very high
bandwidth so that they can provide faster transient response and output slew
rates. However, when the bandwidth of the power supply is increased, its output
stability becomes more dependent on the output wiring and DUT impedances. To
provide this greater bandwidth and at the same time accommodate a wider range
of DUTs on the N678xA modules, there are multiple compensation ranges to select
from, based on the DUT’s input capacitance, as shown in the advanced source
settings screen in Figure 2.
Figure 2: DC Power Analyzer front panel displaying
advanced source settings for the N678xA
Note that “Low” compensation range supports the full
range of DUT loading capacitance but this is the default range. While it provides
the most robust stability, it does not have the faster response and better
performance of the “High” compensation ranges.
As long as the wiring to the DUT is correctly configured
and an appropriate compensation range is selected the output should be stable
and not trip the oscillation protection system. In the event of conditions leading
to an unstable condition, any detection of output oscillations starting up
quickly shut down the output in the manner I captured in Figure 3. I did this
by creating an instability by removing the load capacitance.
Figure 3: Oscillation protection being tripped as
captured in companion 14585A software
In rare circumstances, such as with some DUTs drawing
extremely high amplitude, high frequency load currents, which may lead to false
tripping, the oscillation protection can be turned off, as shown in Figure 4.
Figure 4: N678xA oscillation protection disable in N6705B
DC Power Analyzer advance protection screen
Oscillation protection is a useful mechanism that can
protect your DUT and your power supply from an excessively high AC voltage and
current due to unstable operating conditions. Now you know what it means next
time you see “Osc” displayed on the front panel of you Agilent power supply and
what you need to do to rectify it!
.