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Power consumption has become a crucial problem in modern circuit design. Multiple Supply Voltage (MSV) design is introduced to provide higher flexibility in controlling the power and performance trade-off. One important requirement of MSV design is that timing constraints of the circuit must be satisfied after voltage assignment of the cells. In this paper, we will show that the voltage assignment task on a given netlist can be formulated as a convex cost dual network flow problem and can be solved optimally in polynomial time using a cost-scaling algorithm when the delay choices of each module are continuous in the real or integer domain. We can make use of this approach to obtain a feasible voltage assignment solution in the general cases with power consumption approximating the minimum one. Furthermore, we will propose a framework to optimize power consumption and physical layout of a circuit simultaneously during the floorplanning stage, by embedding this cost-scaling solver into a simulated annealing based floorplanner. This is effective in practice due to the short running time of the solver. We compared our approach with the latest work  on the same problem, and the experimental results show that, using our framework, significant improvement on power saving (18% less power cost on average) can be achieved in much less running time (7times faster on average) for all the test cases, which confirms the effectiveness of our approach.