Magnetic modulators are often combined with solid state switching to drive high repetition rate loads. As with solid state switches, the most significant advantage of magnetic switching is that the switching devices can have a virtually unlimited lifetime. In addition, the magnetic switch is a passive device and requires no triggering for normal operation.
Since solid state switches often have limitations regarding high peak currents and/or dI/dt, magnetic pulse compression can often be used to compress the initial (solid state switched) pulse in time such that the peak power and peak current are both increased to the levels required by the load.
Magnetic Modulators and Switching Technologies:
- Magnetic switches for pulse compression stages in magnetic modulators
- Magnetic switch assists to limit the initial dI/dt for solid state switches
- Biased saturable reactor for limiting fault currents until other methods can be implemented
- Typical magnetic core materials: ferrites, tape wound cores using NiFe, Metglas, nanocrystalline materials, etc.
- Solid State Switching (SCR or IGBT) for initial pulse generation and long module lifetime
- 2-3 stages of magnetic pulse compression to achieve fast output risetime
- Low leakage inductance, step-up transformer provides voltage multiplication
- Capacitor charging HVPS or command resonant charging to allow high repetition rate operation
- Advanced thermal management techniques employed to remove heat at high rep-rates
Solid State Pulsed Power Module (SSPPM) Technical Specifications
- Input Voltage: Up to ~2500 V
- Output Voltage: As much as ~45 kV
- Continuous Rep-rate: 1000 – 6000 Hz
- Output Pulse Rise Time: 30 – 150 ns
SSPPM technology utilized a solid state switch, SCRs and (later) IGBTs, to generate the initial ~micro-second pulse long that was then compressed through several stages of pulse compression and amplified through a pulse transformer to generate the final output pulse (up to ~45 kV with output rise times of ~30-150 ns).
A capacitor charging HVPS provides initial charging of the SSPPM. Parallel SCR switches then discharge the pulse energy into a 3 stage magnetic pulse compression circuit and pulse transformer in order to generate the final output pulse delivered to the laser chamber load.
A HVPS and command resonant charging system (with de-qing) provide fast pulse charging of the SSPPM for the high (4000 Hz and above) repetition rates necessary for these applications. Dual parallel IGBTs then discharge the pulse energy into a two stage magnetic pulse compressor and pulse transformer.
The figure shows the HVPS and command resonant charging system (with de-qing circuit) charging up two parallel, identical SSPPM systems (for MO and PA laser channels). The common charging system in this case minimizes timing variation between Master Oscillator and Power Amplifier laser channels due to charging voltage differences which would then translate into timing variation in the magnetic pulse compression stages.
From left to right shows the magnetic switch bias circuitry, the series diode and IGBT trigger hardware, the IGBT switches, the C0 capacitor bank, the C1 capacitor bank, the pulse transformer, and the C2 capacitor bank. The DPF load chamber would be attached on the right end of the machine. The magnetic assist and magnetic switches are buried inside the coaxial assembly structure.
More details on the technical design and performance of these particular modulator systems can be found in the published technical papers on the “Decade of Solid State Pulsed Power Module Development at Cymer Inc.”, “Solid State Pulsed Power Module (SSPPM) Design for a Dense Plasma Focus (DPF) Device for Semiconductor Lithography Applications” and the “Lifetime and Reliability Data of Commercial Excimer Laser Power Systems Modules” for these modulators.
- Customer: Lawrence Livermore National Laboratory
- Application: Atomic Vapor Laser Isotope Separation (AVLIS) extractor (plate) power supply
- 6 kV, 80 A dc power supply
- 12 pulse, 480 V, SCR phase control provides adjustment and regulation
- Air insulated Transformer/Rectifier (T/R) set
- 40 (series) x 60 (parallel) MOSFET series switch array generates output pulses into capacitive load
- 40 (series) x 10 (parallel) MOSFET shunt switch (“tailbiter”) array terminates output pulses and discharges capacitive load
- GTO switched “pre-pulse” generator provides initial “test” pulse
- Biased saturable reactor limits fault currents until MOSFET arrays have sufficient time to turn off
- Computer interface, as well as local/remote control panels, allows operation from LLNL computer
- Diverter system operates in case of a load short circuit fault
- 2 (redundant) ignitron crowbar switches dissipate energy stored in 570 mF capacitor bank
- Diverter also protects MOSFET switch arrays
- Input voltage: Up to -6 kV
- Output voltage: -500 to -5500 V
- Peak current: 700 A
- RMS current: 150 A
- Average current: 80 A
- Current pulse rise time: less than 500 ns
- Current decay time: 3 ms (e-fold)
- Pulse on-time: ~16 ms to dc
- Inter-pulse time: 1.75 ms to 256 ms
- Rep-rate: dc or 5.5 kHz to 30 kHz (60 kHz at reduced power)
- Peak power: 4.2 MW
- Average power: 0.5 MW
60 kHz Modulator Series FET PCB Assembly showing main power board connections to backplane at top. Twenty parallel connected Power MOSFETs are attached to the three water-cooled, cold-plate, heat sink plates running vertically in the photo. The fiber optic trigger receiver and status confirmation transmitter are located in the bottom right side of the picture underneath an aluminum EMI shield. Three sets of additional trigger fan-out drive circuitry are located at the bottom between the heat sinks and the on-board diagnostics circuits located at the bottom edge of the board. Forty of these board assemblies were connected in series in a backplane structure of the modulator to act as the main series switch in providing pulsed energy to the load to charge up the extractor plates.
Close Up View of 60 kHz Modulator Series FET PCB Assembly showing the details of the MOSFET trigger and voltage protection circuitry. The large resistor is utilized to ensure proper dc voltage grading while transzorb devices ensure similar voltage grading under transient conditions. As can be seen, individual fuses are utilized to isolate each MOSFET cell from the parallel neighbors in the case of a device short circuit fault.
Mounting of Power MOSFETS on 60 kHz Modulator Series FET PCB Assembly is shown in this photo.
60 kHz Modulator Shunt FET PCB Assembly showing the four series connected sections of 10 parallel Power MOSFETs. Since the shunt or “tailbiter” switch in this application required less RMS current capability, only 10 parallel MOSFETs were necessary in each series section. As a result, each similarly sized PCB assembly could contain four series sections and a total of 10 Shunt FET PCB assemblies were required in the modulator for the overall Shunt switch.
Overall 60 kHz Modulator Series FET Assembly showing all 40 series-connected Series FET PCB assemblies stacked on edge and installed into the modulator backplane. As can be seen, fiber optic connections are looped down from the top of each board into the transmitter and receiver connections while cooling water inlet and outlet connections are made at the bottom of each PCB assembly.
60 kHz Modulator Overall Enclosures showing the modulator Transformer/Rectifier (T/R) Set and Phase Controller enclosure on the far right. The near enclosure on the left side contains the dc filter capacitor bank and ignitron diverter switch assembly located in the bottom half of the cabinet (the diverter electronics chassis indicators can be seen in the window of the bottom right). On top of that is the modulator portion of the system. The Series FET assembly is located in the back half of the enclosure (on the far side) while the Shunt FET assembly is located in the middle bay of the top near side. To the left of the Shunt FET assembly are several operator control chassis for local control of the HVDC power supply as well as the modulator.
More details on the technical design and performance of this overall modulator system can be found in the published technical papers on the 0.5 MW 60 kHz Solid State Power Modulator and the High Power Switching Using Power FET Arrays for this modulator.