Design technology of high-voltage multi-megawatt polyphase resonant converter modulators

Los Alamos has developed a new methodology and technique to generate high voltages (pulse or "DC") with a high average power, from a low voltage input source (e.g. +/-1.2 kV). We are using this recently developed technology for the Oak Ridge National Laboratory (ORNL) Spallation Neutron Source (SNS) accelerator klystron RF amplifier system. These klystrons operate up to 140 kV, 11 MW pulses with a 1.1 MW average power. The technology is easily scalable to other performance regimes whether pulsed or DC. The polyphase resonant converter-modulator borrows many components from the traction motor industry such as Insulated Gate Bipolar Transistors (IGBT's) and self-clearing metallized hazy polypropylene capacitors. The use of "large" cut-core amorphous nanocrystalline transformer alloy (also developed for this effort) permits high power, high frequency (20 kHz) conversion techniques with low loss and extremely small size. Other techniques engineered for the converter-modulator topology are polyphase resonant voltage multiplication and resonant rectification. These techniques additionally reduce system size and improve electrical efficiency. The nanocrystalline boost transformers are wound for leakage inductance not turns ratio. The secondary windings generate multiple volts per turn as compared to the primary. This is not typical of pervious power transformer techniques. With the appropriate resonant conditions, significantly reduced IGBT switch losses result due to the zero-voltage-switching characteristics. Additionally, because of the resonant conversion techniques, "crowbars" and other load protective networks are not required. A shorted load de-tunes the resonance and little power transfer can occur. This provides for a high-power and high-voltage system that is inherently self-protective, with fault "ride-through" capabilities. To provide regulated voltages, Pulse Width Modulation (PWM) of the individual 20 kHz IGBT pulses is used to regulate the output voltage. A Digital Signal Processor (DSP) is used to control the IGBT's with feedback and adaptive feedforward control learning algorithms that improve pulse fidelity. The converter-modulator has many attributes that make it attractive to various high power and high voltage applications. This paper will review the design as used for the SNS accelerator and examine in detail the design techniques and high-average-power performance.