Figure 1-1 shows the block diagram of a typical MRI system. Magnet is the key part and also the most expensive part in such a system. There are mainly two methods to generate a constant magnetic field for an MRI system. One method uses a permanent magnet and the other one uses a superconducting coil with high current to generate a constant magnetic field. Permanent magnet-based system is mainly used for magnetic strength less than 0.5T (Tesla) application. For magnetic strength more than 1T such as 1.5T or 3T system, superconductor coils are commonly used and this type MRI is widely used in hospitals nowadays. The internal magnetic strength of an infinite length solenoid is decided by Equation 1.

Here, μ is permeability of air and equal 4π×10^-7, n are coils of one meter, I is current flow the coils. Assume n=4000, B=3 T then I=597 A. MRI application also ask a high stable and constant magnetic. It is necessary for MRI designers to design or purchase a precise high-output current power supply which can drive the superconductor coils.