Pulsed current gradient power supply for microcoil magnetic resonance imaging

Magnetic resonance imaging (MRI) microscopy requires very strong gradients (>10 T/m) that can be rapidly switched between positive and negative gradient fields. A gradient power supply is presented that is capable of pulsing a custom built triaxial microcoil gradient system to an excess of 15 T/m (and as high as 50 T/m), representing pulsed currents in excess of 150 A into loads of ∼200 mΩ (gradient coil plus cables). The gradient power supply is switchable (from positive to negative gradient fields) within a time of ∼10 μs. An H‐bridge design, consisting of four metal‐oxide semiconductor field‐effect transistors (MOSFETs), is used for changing both the current and magnetic field gradient directions. In series with the H bridge is an additional MOSFET that is optimized to be a voltage‐controlled (adjustable) resistor to send the current through the gradient coils. Because MOSFETS require special driving circuits to overcome parasitic capacitances, the MOSFET fundamental design parameters are also presented to complete the circuit schematics. The gradient power supply presented allows for the high gradient fields required for microcoil MRI of small samples.