Operating
power supplies in parallel is commonly used to increase the available output
power or to provide system redundancy in the event of a power supply
failure. Not all power supplies can be
connected in this manner, so please consult the manufacturer’s documentation
first.
One
method is “active” current sharing and is available as standard, or as an
option, on most power supplies rated 1,000W or higher. An internal high impedance circuit monitors
the output current and compares its voltage level to the other units via a parallel
connection (PC) wire. A common
connection (COM) serves as the return wire. The power supply output voltage is
automatically adjusted up or down to balance the output currents between the
paralleled power supplies. See Figure 1.
Figure 1: Two power supplies
connected in parallel
At
first glance, this looks perfectly acceptable. When the system is operating
however, it may be observed that the power supplies are oscillating very
slightly. This is not a function of the
power supplies malfunctioning when in parallel, but actually may be due to
noise pick-up or noise currents circulating in the wiring.
The
first thing to check is that the wiring between the PC and COM connections is
twisted, short as possible and routed away from any potentially noisy cables.
If
that does not solve the problem it may be due to how the negative remote sense
is wired. Looking at Figure 2, we can
see that the COM connection is internally connected to the negative sense
connection and as a result creating a potential noise loop. The oscillation may be the result of the currents
flowing in the negative load cables affecting the sensitive parallel and
sensing circuitry.
Figure 2: Noise loop within the parallel,
sensing and load system wiring
The
solution for this is to disconnect the wire between the two COM pins and connect
the negative remote sense wiring at the load. It will also compensate for voltage
drops in the negative load cables. See
Figure 3.
Figure 3: Negative remote sense
cables connected to the load and COM link removed
With
this configuration, the internal signal common is isolated from noise on the
power cables and any voltage differentials. If desired, the positive remote
sense terminals can be connected to the positive of the load to compensate for
voltage drops in those cables too.
Power
Guy
