Weak boundary condtitions in magnetic field calculations

Hey,

I have an easy but large model, which can be made smaller by identifying a unit cell. In this unit cell I want to implement periodic boundary conditions for the magnetic field (B) . The only periodic boundary conditions i can find are the ones for the magnetic potential (Vm). Does any one now how to implement this condition (B_boundary1 = B_boundary2)?

Thank you in advance,

Sam