Power supplies are put in parallel for two main reasons; to increase the available output power or for redundancy to keep the system running if one supply fails. For optimum life expectancy, having the power supplies share the load equally is strongly advised. Without sharing, one unit could deliver full power and run hot, and one could be off load and run cold. With load sharing, both would run “warm” and last longer.

One method to balance the load is active current share. This involves a signal connection between power supplies, feeding back information to the internal control circuits. If power supply “A” is supplying more current than power supply “B”, the output voltage of “B” is raised slightly until it is the same as “A” so both deliver the same output current.

In many telecom and datacom applications, power supplies are plugged into a backplane, guaranteeing the length and position of the current share connection. In an industrial application where the use of DIN rail mount power supplies is common, that connection could be a long copper wire and may be subject to interference from motors drives or relay switching.

Droop mode current share is probably the simplest way of ensuring power supplies share a load.  There is no wire interconnection (and potential for noise pick-up); instead the output voltage drops in proportion to the current drawn from the power supply. If power supply “A” is supplying more current than “B”, the output voltage of “A” will fall to that of “B” and balancing will occur. This feature is often standard on DIN rail power supplies above 100W and is enabled by either a switch, or like on TDK-Lambda’s DRF series (shown below), the removal of a wire link or jumper on a connector.

DRF DIN Rail power supplies from TDK-Lambda

When the DIN rail power supply is operating as a stand-alone unit, the switch is closed (or wire link in place) and the internal control circuit will compare the output voltage to a reference (Fig 1).

Figure 1

Any change in the output voltage due to load will be compensated for, and the output regulation will be minimal – in the order of 10mV.

When two or more power supplies are connected in parallel, the switches (or links) are opened (Fig 2).

Figure 2

As the output load is increased, the voltage across the shunt will add to the voltage sensed across the output, causing the control circuit to react and lower the power supply’s output voltage.

As an example, for the 24V 10A TDK-Lambda DRF240-24-1 power supply, the droop characteristic is set at 64mV / A. Table 1 shows the output change against load.

Table 1

When using power supplies in droop mode current share, care must be taken to:

• Ensure the output voltages of the power supplies are set at the same voltage. The output voltage can be adjusted slightly higher if needed, to compensate for the droop voltage*
• Use the same length and same gauge of wire from the output to the load for each unit
• Note any additional derating stated by the manufacturer. This avoids tolerances from overloading a unit
• Do not exceed the manufacturer’s recommendation for the number of power supplies that can be paralleled.
• Make sure the parallel switch is in the right position, or the wire link is removed!

* If the power supplies are being used in a redundant mode with ORing diodes, set the output voltages higher to compensate for the forward voltage drop (Vf) of the diode. The function of these diodes is to prevent the bus voltage being pulled down due to an internal short in a faulty power supply. The diodes should be rated to carry the full output current of the power supply.

Redundant mode connection with ORing diodes

Although certain electronic loads can be sensitive to the supplied voltage (3.3V or 5V for logic ICs), typically 12V, 24V or 48V loads (driving relays, DC-DC converters or motors) are more tolerant.