PPUs for high power systems challenge current technology. High voltages and currents require special attention so that undesirable power losses and parasitic transients are not created. For high power applications, power can be processed in smaller fractions to reduce voltage and current magnitudes and allow the use of more efficient semiconductors. These power modules can be used as building blocks by connecting them in series or parallel to produce the desired voltages and currents.

Recently NASA GRC developed a 1 kW power module for Hall thrusters, shown in Figure 1. It consisted of a phase-shifted full-bridge converter operating at a switching frequency of 50 kHz. The module, operated with an input voltage of 100 ± 20 VDC, generated a nominal output of 300 VDC. This module, integrated with a NASA-120Mv2 HET, demonstrated efficiencies in excess of 96 percent. The total component weight of this unit, including printed circuit boards, was 0.765 kg. The reason for developing this module was to create a test bed to evaluate circuits, concepts and designs that could be applied to a higher power module design.

The 1 kW power module was not designed with circuitry to operate multiple modules in a parallel configuration. During this investigation, a load current share circuit was designed and implemented to force the output current to evenly divide between modules. Three 1 kW modules were connected in an arrangement with both inputs and outputs in parallel to increase current output. In addition, this parallel arrangement enabled the use of a phase-synchronization or phase-staggering circuit that shifted the switching phases of the modules reducing input and output ripples. Three 1 kW modules including these additional functions were used to assemble a 3 kW discharge power supply. This unit was successfully integrated with the NASA-120Mv2 HET.

All power and control design concepts implemented in the 1 kW module were then used to develop a multi-kilowatt power module. A power level of 10 kW was selected because of semiconductor parts and magnetic cores availability. The topology used for the 10 kW module was also a phase-shifted full-bridge converter. Three significant design changes had to be implemented relative to the 1 kW design because of the higher power level on this application. First, the switching frequency was decreased from 50 kHz to 20 kHz to reduce switching losses. Second, the power stage that routes input current through the MOSFETs and the transformer was designed to minimize interconnections, current path length and loop inductance. These can introduce parasitic elements to the power circuit resulting in transients detrimental to the performance of a converter. Snubbers are then required to damp these transients at the cost of further power losses. The power stage also included larger heat sinks for improved heat rejection. Third, the gate drive circuit design was changed from a charge-pump based circuit to a transformer-isolated design with high current drivers. This was implemented because the larger gate capacitance in high power transistors requires high current for fast and efficient switching. The bipolar drive from the transformer-isolated design improves turn-off characteristics. In addition, a "Miller killer" circuit, using a bipolar junction transistor, was added to minimize the effect of the Miller capacitance on the MOSFETs and further increase turn-off speed.

Other changes were made to improve efficiency. Two high-power MOSFETs, in an SOT-227 package and with very low on-resistance, were used on each leg of the bridge converter. Also, high-voltage, high-current, ultra-fast, soft-recovery diodes, in a TO-247 packages, were used for the output rectifier. These allowed the use of one single bridge rectification output stage, which minimizes losses and simplifying transformer design. Last, a new high power transformer was assembled by stacking two large ferrite C-cores. The windings utilized an interleaved design, a minimum number of layers and Litz wire to minimize leakage inductance and proximity and skin effects.

As in the 1 kW module design, phase-shifted, peak-current-mode, pulse-width-modulation (PWM) control was implemented using a commercially available integrated circuit (IC). This device included all the necessary functions including the four phase-shifted gate drives, current limit and soft-start. Integrating this IC into the design resulted in a significant part count reduction. The component weight of the 10 kW power module is 6.2 kg not including heat sinks or mounting hardware. A photograph of the breadboard is shown in Figure 4.

The performance of the multi-kilowatt power module was tested using a resistive load. Efficiencies greater than 96 percent were obtained through a wide range of output powers. These efficiencies were attributed to the soft-switching characteristics of the phase-shift bridge converter, fast gate drive circuits and low parasitics losses inherent in the design.

This 10 kW power module will be used to develop a discharge supply for high-power HETs. As proven with the 1 kW module, multiple units will be connected in parallel to scale the design to high power.