Specifications

R
Foldback power supplies protect
connected equipment by reducing
excess current (and thus output
voltage) along a foldback path.
When testing foldback power sup-
plies using an electronic load, you
need to take steps to be sure the
power supply starts properly.
When using an electronic load in
constant resistance mode, the cur-
rent increases as the resistance
decreases, R
1
to R
2
, until the cur-
rent limit is reached; after that, the
supply reduces its output current
along the foldback path (Figure 1).
For constant current load, the
power supply operates in constant
voltage mode while the electronic
load operates in constant current
mode. When the power supply
starts up, its output voltage is zero,
and the electronic load tries to sat-
isfy the programmed current value
(I
1
) by decreasing the load resis-
tance. The power supply interprets
the low load value as an over-cur-
rent condition, and immediately
folds back to a stable operating
point (P
2
or P
3
in Figure 2,
depending on the power supply’s
startup characteristics).
To alleviate this problem, program
the electronic load to a value
below the current limit foldback
point (I
min
). This value can be
close to zero on many supplies.
After applying power to the
source, increase the load’s current
limit to the desired value.
For power supplies that don’t
require a minimum load current at
all times, you can initially program
the load in constant resistance
mode. Then, when the power sup-
ply reaches its nominal operating
voltage, switch the electronic load
to constant current mode. (During
the transition, the load momentari-
ly goes to zero.)
Ensure Proper Power-up
with Foldback Supplies
Tip
6
Foldback
Region
Resistive Load
( < )
R
1
R
1
R
2
R
2
I
1
Foldback
Region
Constant Current Load
I
min
Figure 1: Foldback behavior in constant resistance
mode.
Figure 2: Foldback behavior for constant current
load.
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