Feature: Power
Figure 1: Basic star wiring options
Figure 3: Parallel operation, droop-share method
Figure 2: Parallel operation, internal (top) and external (bottom) current share control
power needs, so two or more are joined in parallel, and each is always loaded. Here, the control circuit responsibility shifts to balancing the load sharing among the connected supplies as equally as possible. Balancing can be achieved either internally in each supply or with external control units. An example for internal control implementation is the load-sharing IC, UCC29002, from Texas Instruments. Power supplies in parallel with internal control require an additional current-share signal line; see Figure 2, top. On the other hand, external sharing control, such that of
Analog Devices’s LTC4370, is achieved by modulating the MOSFET voltage drops to offset the mismatch in the supply voltages; see Figure 2, bottom. This circuit allows connecting any power supply in parallel, and placing the balancing control on a separate PCB. The droop-share method is suitable in applications where
an output voltage drop can be tolerated. Here the power supplies are designed to decrease their output voltage with increasing load current. This allows two or more power supplies to “meet” at the same voltage level (enabled by the
Figure 4: Basic wiring of a series operation
increasing load current – see Figure 3) and provide the power in parallel. V1 and V2 power supplies are identical but due to
manufacturing tolerances often slightly differ in the output voltage. V1 has a higher output voltage and will be the first to support the load. With increasing current, and hence decreasing V1 voltage, at some point will meet the V2 level and start sharing the load with the other power sources. Key points to consider here are:
• Power supplies connected in parallel should have the same output voltage.
• This type of configuration is targeted to increase the total output current.
• Balancing of the output power supplies is recommended, to distribute the load between the power supplies as equally as possible.
• The trade-off to control the current share internally or externally.
• The trade-off to using the droop-share method without a feedback loop but with a voltage drop at higher currents.
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