POWER DEVICES of switched-mode power supplies Figure 3: Parallel operation, droop current share method
with internal control will require an additional current share signal line for this method as shown in figure 2 (left). External sharing control, such as 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). This circuitry allows usage of any power supplies in parallel and takes the balancing control on additional independent PCB. In applications, where the output voltage drop is allowed to be
tolerated, the droop share method can be used. A basic understanding of such configuration is when the power supplies are designed to decrease the output voltage with increased load current. This allows two or more power supplies to “meet” with increased load current at the same voltage level and provide the power in parallel as seen 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 so decreasing voltage V1, it will meet at some point the V2 level and start sharing the load between two (or more) power sources. The key points to consider for parallel operation of the power supplies 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 equally as possible
• The trade-off to control the current share internally or externally should be considered
• The trade-off to using the droop share method without a feedback loop but with a voltage drop at higher currents should be
considered as well
Higher voltage (series operation) A somewhat easier technique to increase the total power 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 available on the market is lower than 60Vdc. System designs with voltage requirements higher than 60Vdc might want to grip to this type of solution. The main considerations for power supplies connected in series are listed below:
• Stacking power supplies with different voltages is possible. However, the user must review the safety standards especially if the total output voltage may rise higher than 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
Figure 6: Series operation, reverse bias diodes protection DECEMBER/JANUARY 2022 | ELECTRONICS TODAY 23
Figure 5: The reference voltage of power supply A shares the same signal ground as of the load
• In case of an accident load shortcut or defective condition, the connected power supplies in series 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. (figure 6). There are many different options available to increase the total
power of the system or add a redundant power supply for reliability reasons. Whether the connection of the power supplies has to be in parallel or series, it is always recommended to consider worse case scenarios such as load short circuit or power supply failure.
SL Power Electronics
https://slpower.com/
Figure 4: Series operation, basic wiring
voltage of power supply A. Connecting the control signals of both power supplies shortcuts the output of one of the power supplies. The signal ground of power supply B is elevated to the output
voltage potential of power supply A. If the control features of the power supplies in series are
nevertheless required, this can be achieved with signal isolation boards such as ADuM6422A.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42
