By Alexander Mezin, Senior Field Application Engineer EMEA, SL Power Electronics
There are many instances when more than one power supply is required in a system setup – governed by the need for more power, greater system reliability or mechanical constraints. Here are a few scenarios to consider:
In critical applications where power must not fail, redundancy can be achieved by connecting two or more power supplies together. This setup requires same-type power supplies to be connected in parallel to ensure identical operation, regardless of which unit will be connected to the load.
Redundant sharing allows to switch only the desired number of power supplies in parallel at the same time. In case of power failure, the control circuit automatically switches over to another power supply in the setup, to ensure continuous power delivery. Active redundant configuration allows to keep some of the connected power supplies in parallel not loaded, to avoid stressing critical components. Such an approach also extends the life of the spare power supplies. Also, the current- or load-sharing circuit helps manage the output current evenly across all active power supplies, greatly reducing stresses on each unit as well as allowing it to run cooler, resulting in higher reliability of the units.
Although there are different ways to wire load sharing with parallel power supplies, one most recommended is the star wiring method; see Figure 1.
The first configuration in Figure offers a basic level of redundancy, without any extra components, which are not strictly necessary for the load-sharing functionality. However, many safety-critical systems go a step further, demanding advanced redundancy that requires additional components, such as OR-ing diodes or MOSFETs; see Figure 1, middle and right.
OR-ing diodes are a successful method in their simplicity, but they have power losses, which most of the time can be neglected. The power loses are even higher when using MOSFETs, but this setup is considerably more efficient.
There are many reasons why several power supplies should be connected in parallel instead of using higher-power units in modular configurations: when extended wider power range is required, when there are mechanical limitations or even lack of products that will meet the desired specification.
The selection requirements of power supplies in parallel operation are similar to those for redundancy, but the control function differs. A single unit is clearly not sufficient here to provide the desired power needs, so two or more are joined in parallel, and they are always loaded. Here, the control circuit responsibility shifts on to balancing the load sharing among the connected power supplies as equally as possible. Balancing can be achieved either internally in the power supply or with external control units. An example for internal control implementation is the additional load-sharing IC, UCC29002, from Texas Instruments. Power supplies in parallel with internal control require an additional current-share signal line.
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, right. 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 increasing load current) and provide the power in parallel, as shown in Figure 3.
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.
A somewhat easier technique to increase the total power of a system is to connect the power supplies in series. The assumption is that there are power supplies with lower voltage available to achieve the desired output voltage by stacking multiple power supplies in series. The output voltage of the overwhelming majority of power supplies is below 60Vdc, so designs with voltage requirements higher than that might need this type of solution.
The main considerations for power supplies connected in series are:
- Stacking power supplies with different voltages is possible. However, the user must review the safety standards, especially if the total output voltage should rise above 60Vdc.
- The maximum possible total current is defined by the smallest unit, which will run into the overcurrent protection in case of an overload event; the designer should plan exact recovery scenarios.
- Additional features such as inhibit, or DC_OK signals very often share the same output return (DC ground) of the corresponding power supply. As shown in Figure 5, the output return of the stacked power supply B in series connection is elevated to the output voltage of power supply A. Connecting the control signals of both power supplies shortcuts the output of one of the power supplies.
If the control features of the power supplies in series are nevertheless required, this can be achieved with signal isolation boards such as ADuM6422A from Analog Devices.
In case of an accident load shortcut or defective condition, the in-series-connected power supplies will be connected backward. If the power supply is not fitted with reverse polarity protection, additional external reverse-bias diodes are recommended to be installed.
In summary, whether the connection of the power supplies has to be in parallel or in series, it is always recommended to consider worst-case scenarios, such as load short circuit or power supply failure, and hence account for potential problems.