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This paper first introduces the design of an ultrahigh efficiency 50-kW bidirectional dc-dc converter at zero-voltage-switching operation, and then, a high-precision efficiency measurement method using a regenerative approach. The ultrahigh efficiency bidirectional dc-dc converter is achieved with 1) the use of CoolMOS as the main switch under zero-voltage soft switching condition; 2) multiple-phase legs for current sharing to reduce the conduction loss; and 3) coupling inductors between each two-phase legs to reduce the core loss. Two identical hardware prototypes were designed, fabricated, and tested for performance evaluation. In order to precisely measure the converter efficiency, the two identical bidirectional dc-dc converters are tested with one as the device under test and the other as the regenerative unit. With the use of ?? 0.5% current shunt and regenerative measurement, the relative efficiency error stays below ??0.025%. Measured efficiency with load from 20% to 100% consistently shows above 97.50%. At the 50 kW full-load condition, the efficiency is 99.05% with ??0.01% efficiency relative error.