A year ago my colleague here, Gary, provided a posting
“How can I get more power from my power supplies?” (Click here to review). He
describes connecting power supplies in series for higher voltage or in parallel
for higher current. Along with suggested set ups a list of requirements and
precautions are also provided.
Connecting multiple power supplies in parallel operating
as voltage sources is always problematic as there will be some imbalance of
voltage between them. That’s why, in this previous posting, one unit operates
as a voltage source and the remaining paralleled units operate in constant
current. The compliance voltage limit of all the units operating in constant
current need to be set higher than the master in operating in constant voltage
in order to maintain this operation. This is illustrated in Figure 1.
Figure 1: Operating power supplies in parallel for higher
power
As long as a high level of loading is maintained the
paralleled units remain in their respective operating modes (in this case at
least 2/3 loading). However, what happens if you cannot maintain that high
level of loading? It is possible in practice to operate at lighter loads with
this approach. In this case it is important to set the voltage levels of all
the units the same. Now what happens is when the units are fully loaded they
operate as already described, with the lowest voltage unit remaining in
constant voltage. But when they are unloaded the lower voltage units transition
to unregulated operation and the highest voltage unit then maintains the
overall output in constant voltage. This is shown in Figure 2, for 0 to 1/3
loading.
Figure 2: Conditions of power supplies connected in parallel at light loading
There is a bit of performance compromises as a result.
The transition between the lowest and highest voltage limits adds to the
voltage regulation. Also, due to different units experiencing mode crossover
transitions between constant voltage, constant current and unregulated
operating modes transient voltage performance suffers considerably.
An improvement on this direct paralleling approach is
having a master-slave arrangement with control signals to maintain current
sharing across units. Our N5700A and N8700A series power supplies use such a
control arrangement as depicted in Figure 3, taken from the N5700A user’s
guide.
Figure 3: N5700A Connection for parallel operation (local sensing used)
With this arrangement the master unit, operating in
constant voltage, provides an analog current programming output signal to the
slave unit, operating in constant current. In this way the two units equally
share the load current across a wide range of load current.
Still, having multiple units with only one in constant
voltage does not provide as good of dynamic performance as a single voltage
source of higher power. A unique and
innovative approach was taken with our N6900A / N7900A series Advance Power
System (APS) to support seamless parallel operation without compromising
performance. The paralleling arrangement for our N6900A / N7900A series APS is
depicted in Figure 4.
Figure 4: N6900A / N7900A series APS Connection for
parallel operation
