POWER
allel or series de power supplies?
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)
Figure 3: parallel operation, droop current share method
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 60Vdc. System designs with voltage requirements higher 60Vdc might want to grip to this type of solution. The main considerations for
power supplies connected in series are: • Stacking power supplies
Figure 4: series operation, basic wiring
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
with different voltages is possible. However, the user must review the safety standards especially if the total output voltage may rise higher 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
Figure 6: series operation, reverse bias diodes protection
options 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. Feel free to contact us for further information or even just for knowledge exchange.
SL Power Electronics T: 01769 581311
https://slpower.com/
References: 1
Figure 5: The reference voltage of power supply A shares the same signal ground as of the load. The
signal ground of power supply B is elevated to the output voltage potential of power supply A
. [
https://www.ti.com/product/UCC29002?qgpn=ucc29002] 2. [
https://www.analog.com/en/technical-articles/novel-
current-sharing-ic-balances-two-supplies-with-ease.html] 3
.[
https://www.analog.com/en/products/
adum6422a.html# product-overview]
DECEMBER/JANUARY 2022 DESIGN SOLUTIONS 41
• 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).
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[3]
.
FEATURE
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