FEATURE
POWER
making the decision: para operation of switch-mod
A single power supply is, most of the time, sufficient in applications connected to an AC power source. However, sometimes there are
additional boundaries set to fulfill either higher power needs, system reliability, or even mechanical constraints. In the following article, Alexander Mezin, senior field application engineer EMEA at
SL Power Electronics, explains the typical scenarios of connecting more than one power supply in the same system and the reasons behind it
T
he connection of two or more power supplies for redundancy is important in critical applications, where power source
fails cannot be tolerated. The current sharing circuit, or load sharing circuit, is the ability to manage the output current evenly across all active power supplies to greatly reduce stresses on each power supply and allow them to run cooler, which results in higher reliability of the active power supplies. Redundant sharing is the control of the power
supplies internally or externally by switching only the desired number of the power supplies in parallel at the same time. In case of a power failure, the control circuit will automatically switch to another redundant power supply for continuous power delivery. Active redundant configuration allows you to keep some of the connected power supplies in parallel not loaded, so the critical components are not stressed. Such a system approach extends the lifetime of the spare power supplies. When selecting the power supplies for
redundancy, choosing the same type of power supplies connected in parallel will ensure identical operation, no matter which unit will be connected to the load. Although there may be more than one way to
wire load sharing with parallel power supplies, the star wiring method is typically the one most recommended. Demonstrated below are the three typical star wiring schemes that provide options for any desired level of redundancy. This first method in Figure 1, left diagram,
demonstrates a basic star wiring scheme that offers a basic level of redundancy without the inclusion of extra components that are not strictly
Figure 2: parallel operation, internal (left) and external (right) current share control
necessary for a load-sharing function. However, many safety-critical systems demand reliability that evolves an advanced redundancy that requires additional components to this load share wiring scheme as shown in Figure 1, middle. Although OR-ing diodes are a successful and
redundant method in their simplicity, they may be neglected for their power losses impact. This level of redundancy is still achievable without the impact of such power losses as Figure 1, right, exemplifies. This elaborate scheme consists of the same basic OR-ing star wiring method, however it is considerably more efficient. It demonstrates the introduction of a relevant OR-ing system with the use of MOSFETs driven by a compatible integrated circuit controller.
higher current (parallel operation)
The reasons for connecting several power supplies in parallel instead of using higher power units can be, for example, modular configurations or a variety of applications in a design house with an extended wider power range, mechanical limitations, or even lack of products
in the market with the desired specification. A common example in modular applications is when the system designer adds more and more power supplies in parallel as the system building blocks are added. In a wide variety of scenarios, the purchasing team might prefer to have one single power supply on their bill of materials in simple designs, and stacked power supplies in parallel from the same type in more powerful ones. The selection of power supplies for parallel
operation are similar to those for redundancy, but the control function differs. It is obvious, in this type of application, that a single unit is not sufficient to provide desired power needs, so two or more power supplies in parallel are expected to always be loaded. The control circuit responsibility shifts here to balancing the load sharing among the connected power supplies equally as possible. The balancing circuitry implementation can be
achieved both internally in the power supply or also with external control units. An example for internal control implementation would be an additional load-sharing IC such as UCC29002 from TI [1]
. Power supplies in parallel with internal
control will require an additional current share signal line for this method as shown in Figure 2, left diagram. External sharing control, such for example offered by Analog Devices LTC4370, is achieved by modulating the MOSFET voltage drops to offset the mismatch in the supply voltages (Figure 2, right) [2]
. This circuitry allows use of any Figure 1: redundant operation, basic star wiring options 4 DESIGN SOLUTIONS DECEMBER/JANUARY 2022 0
power supplies in parallel and takes the balancing control on additional independent PCB. In applications where the output voltage drop
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