Avoiding noise issues when operating power supplies in parallel

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