“In systems that are required to operate with high reliability, it is often necessary to operate multiple power supplies in parallel,” said Lambda CTO Andy Skinner. “Should one power supply fail, a sufficient number of power supplies remain in operation for the system to continue to function – this is known as N+1 redundancy.”
Individual supplies must automatically share the load evenly.
There are two established techniques used to improving the load sharing.
One is to use a ‘share bus’, “but this is not suitable for high reliability applications because of the failure of the share bus can cause the whole system to fail”, said Lambda.
‘Droop sharing’ is the second technique, in which the load regulation of the power supply (variation of output voltage with load current) is deliberately made slightly soft, which allows output voltage to reduce as the power supply is loaded.
“Unfortunately, in many implementations of droop share, the load regulation needed to provide good current sharing is unacceptably high,” said Lambda.
The firm’s invention, enabled by digital control and for which it now has a UK patent, is to apply droop over the full load range of each power supply, but with a two-stage profile, which is less steep at lighter loads than at heavier loads.
“As a result, high sharing accuracy can be achieved at high loads, whilst still allowing for some load sharing at lighter loads,” explained Skinner. “By improving sharing accuracy at high loads, the number of power supplies required to achieve N+1 redundancy is reduced. And by allowing for load sharing at lighter loads, a reduced spread in MTTF of the power supplies is achieved.”