Design of a Robust Cold Plate for High-Power Gradient Drivers in Magnetic Resonance Imaging Systems

Abstract

As radiologists demand increased power, speed, and duty cycle from their magnetic resonance imaging (MRI) systems, thermal management of the gradient driver sub-system becomes more challenging. MRI systems contain three IGBT-based gradient drivers that apply current waveforms to an inductive load located inside a cylindrical magnet. The effectiveness of the gradient driver cooling system determines the power, speed, and duty cycle of the MRI system. MRI gradient drivers are unique in the world of power electronics in that they must be able to generate arbitrary current waveforms with extremely high fidelity. This high fidelity requirement means that the IGBTs must switch at frequencies greater than >10 kHz, resulting in die-level heat fluxes approaching 100W/cm/sup 2/, dictating the use of liquid cooling. The gradient driver cooling system design is further challenged by a continuing loss of die-level thermal time constant as IGBT manufacturers incorporate thinner die with each new generation of IGBTs. The loss of die-level thermal time constant results in higher IGBT junction temperature excursions, placing more demands on cold plate performance. Numerous experiments were performed to characterize IGBT switching losses and transient thermal behavior. The data was used to create an electrical/thermal model to predict heat generation, junction temperature rise, and reliability. Monte Carlo simulations were performed on the electrical/thermal model to assess the impact of variations in IGBT manufacturing, cooling system performance, and scan sequences in order to derive requirements for a cold plate design. Cold plate performance was experimentally verified and the design was deemed robust, or insensitive to sources of variation.