TDK-Lambda FPS-TB Directives D'installation page 21

Fig 5-1
6. SERIES OPERATION
Up to 3 units of the same voltage and current rating can be used for increased output voltage
It is recommended that diodes be connected in parallel with each unit output to prevent reverse voltage.
Each diode should be rated to at least the power supply rated output voltage and output current.
Series operation is not applicable
Fig 6-1
for units with I2C bus option.