DC/DC Converters with High Power Density and Fast Response for Pulsed Load Applications

As a special kind of load that requires periodic high power bursts, pulsed loads are widely being powered using solid state converters in various modern applications, such as radar, nuclear magnetic resonance imaging (NMR), light detection and ranging (Lidar), etc. The related power converters need to handle the following three conditions: 1) high peak to average power ratio (PAPR); 2) small duty ratios under peak power; and 3) fast pulse rise and fall times. With the additional requirement of high power density in modern space-constrained applications, these bring several challenges in the design of such ‘pulsed power supplies’ or PPSs. In applications involving voltages up to 900 V, Gallium Nitride (GaN) devices are increasingly being preferred due to their advantages over Silicon Carbide (SiC) and Silicon (Si) devices. For example, GaN devices’ lowest switching loss helps the PPSs to operate in higher switching frequencies, which increases converters’ response speeds and reduces the size of the passive components. This dissertation bridges the use of GaN devices with new converter architectures and control techniques to improve the power density and response speed of PPSs. First, design and development of a high power density 375 V input, 50 V output, two stage GaN based DC/DC converter prototype for 4 kW/800 W (peak/average) pulsed load application is discussed, along with its hardware, in this dissertation. By the benefits of lower switching loss, the first stage isolated converter can operate in a higher frequency to reduce transformer size. Higher switching frequency of 1 MHz also contributes to a high bandwidth of the second non-isolated converter stage, which improves the transient performance. A novel average current control strategy has been proposed and applied in this PPS, which reduces the input current noise at pulse repetition frequency (PRF), and, therefore, reduces the input filter size and power rating. Some practical issues, such as the failure of zero voltage switching (ZVS) during light load, voltage ringing caused by GaN devices with small Coss, and the limit of DSPs’ processing speed, have also been discussed in this two stage converter. Solutions to these problems have been proposed and verified via experiment results. A novel power converter architecture that can be applied in pulsed NMR applications such as in subsea or downhole formation evaluation has been proposed and verified in this dissertation. The proposed topology reduces the volume of the transformer, input filter, and midpoint capacitor due to a unique average power control strategy. A 150 V to 400 V, 20 kW peak power converter is designed. Simulation results, scaled-down experimental results, and design analysis are also provided in this dissertation to prove the validity of the proposed architecture. Finally, a linear assisted DC/DC converter for PPSs to improve the response speed is also proposed in this dissertation. The main power is transferred by a highly efficient LLC resonant converter and the linear amplifier is responsible to compensate for the current difference during the load transients. The otherwise large output capacitor size gets reduced due to this converter’s fast response speed. A ‘band separation’ solution is proposed and verified by simulation to deal with the practical issues arising from the parallel output connection between the LLC resonant converter and the linear amplifier.