Before
economically priced DIN rail power supplies gained popularity, many 100W or
higher power rated units provided users with a parallel operation function in
the form of switch fitted on the front panel.
This switch allowed the user to select between “droop mode” current
share between two or more supplies and single unit operation. Droop mode
allowed for balancing of the output currents between the units.
If power supplies without a parallel operation function are being used
and additional current is required for the system load, a second power supply
could be connected in parallel, although many manufacturers do not recommend this. Unless the output voltages of both power
supplies are set to the same voltage, the unit with the greatest output voltage
may deliver the majority of the current and operate in an over current
condition. This will reduce the unit’s field life due to excessive internal heating. Ideally the output current of each power
supply should be routinely measured during preventative maintenance to ensure
they are balanced, which is both time consuming and cumbersome.
One alternative is to use a “redundancy” module with a load balance
option, like TDK-Lambda’s DRM40. Figure
1 shows a DRM40 used to connect two 20A power supplies to deliver up to 40A and
Figure 2 the DRM40 with its current balancing LED.
Figure 1: Two units connected in
parallel with the DRM40
Figure 2: DRM current balance LED
If the LED is not illuminated, measure the output voltage on both power supplies.
Adjust the voltage of the power supply with the lowest voltage higher until the
LED turn on. Alternatively, one can adjust
the voltage of the power supply with the highest voltage lower. The current balance LED will be illuminated
when the difference between the voltages is less than 50mV and the output currents
are balanced.
The DRM40 has internal MOSFETs, used to block reverse currents in the
event of one power supply failing short circuit. They also allow the measurement of each power
supply’s output voltage without disconnecting any load cables.
The DRM40 can also be used in redundant power systems, where two power
supplies are used in a 1+1 configuration as shown in Figure 3. If one power supply fails the other will
continue to provide current to the load.
The load balancing function can be used in same manner.
Figure 3: DRM40 used in a 1+1 redundant
configuration
Power Guy
