Specifications

Under light or no load conditions,
a power supply’s output capacitor
discharges slowly. If you’re using
the supply as a static voltage
source, this is not problematic, but
when you’re making tests at vary-
ing voltage levels, slow discharge
means slow tests.
Down programming circuits in
power supplies rapidly decrease
the output voltage, reducing dis-
charge times by hundreds of mil-
liseconds. Hewlett-Packard power
supplies use two types of down
programming circuits:
In Figure 1, an FET is placed
across the output terminals.
Whenever the output voltage is
higher than the programmed
value, the FET activates and
discharges the output capacitor.
The FET can sink currents
ranging from 10% to 20% of the
supply’s output current rating.
The maximum load at low volt-
ages is limited to the On resis-
tance of the FET plus the series
monitoring resistor, resulting
in a slight degradation of the
down programming current
near zero volts.
In Figure 2, the down program-
mer lies between the power
supply’s positive terminal and a
negative source. This configura-
tion pulls the output completely
down with no degradation near
zero.
Some power supplies, such as the
HP 662xA and 663xB series, can
sink currents equal to their full
output current rating. In the HP
663xB series, this sink current is
programmable, so you can use the
supply both as a programmable
source and load—very useful in
applications such as charging and
discharging batteries.
Use Down Programming
to Increase Test Speed
Tip
5
Figure 1: A down programming cir-
cuit with an FET across the output
terminals.
Figure 2: A down programmer situat-
ed between power supply’s positive
output and a negative source.
http://www.hp.com/go/power
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