High-current and low-voltage step-down converter for feeding cold superconducting magnets

1. High-current and low-voltage step-down converter for feeding cold superconducting magnets P. Varela Design, prototype and manufacture of a large series of 10V/50A DC/DC power converters for driving superconducting magnets into the 4km X-Ray free-electron laser tunnel, located in the research facilities of DESY, Hamburg XFEL Project: the Application Project sponsored by What for? Smaller, faster, more intense: The European XFEL will open up areas of research that were previously inaccessible. Using the X-ray flashes of the European XFEL, scientists will be able to map the atomic details of viruses, decipher the molecular composition of cells, take 3D images of the nano-world, film chemical reactions and study processes such as those occurring deep inside planets. How it works? To generate the X-ray flashes, bunches of electrons will first be accelerated to high energies and then directed through special arrangements of magnets. In the process, the particles will emit radiation that is increasingly amplified until an extremely short and intense X-ray flash is finally created. Magnets The Power Supply ±10V / 50A 1/2 Redundant module Design inputs η, size, T, … Bipolar output Redundancy (2 power modules per power PCB) THE LOAD: ELECTROMAGNET Normal operation: L(2H) + verylow R Vsec 1. Design Planar technology Enhances repeatability & reduces assembling cost (500 units) Interwinding  Low RAC Low Cu loss (50A) Low Lleak For low ringing  Bipolar Synchronous Rectification (4 MOSFETs) 2 LF switches (define polarity) 2 HF switches (rectify Vsec waveform) TOPOLOGY Usual operation: free-wheeling ≈0V/50A Current-doubler rectifier Saves transformer losses when running at almost 0V TRANSFORMER 3. Adaptation to control interface Control PCB CONTROL  Outer loop: iout control, which creates the field  Inner loop: Vout control, which defines the magnet di/dt  Variable fsw control for optimal losses balance (switching vs on) iref, PI param. Vgs Power PCB v2 T, iout, Vout… (raw data) T, iout, Vout… 1st prototype with FPGA open-loop control Vgs3 Vds3 IL2 IL1 50A/10V, detail of the current share among both inductors in a module 2. Initial prototype Externaldetection PCB 4. Protections QUENCH Failure in the magnet, detected by a voltage rise in the magnet terminals Planar transformer RM transformer Quench transient evolution (voltageacrossthemagnet) η up to 92,5% (planar transformer, no driving nor control losses) Vgs3 Vds3 Iout Vout 50A/1.4V, detail of a secondary MOSFET turn-on 2nd version of the power PCB Conclusions • A DC/DC SMPS intended for cutting-edge research is under development in CEI-UPM. • Its main singularity is the load nature to be fed: a superconducting magnet, which makes necessary to provide very high current output at low voltage. For that reason, variable switching frequency is applied, in order not to drive very short on times in steady-state (low duty cycle). • Planar transformer design is applied as long as current-doubler with 4 SR for low losses under free-wheeling operation mode allowing bipolar output. Glosary Cold magnet: Electromagnet whose superconductivity comes from its low temperature (<10K). The low temperature is achieved by using liquid He cryostats. Hot magnet:Superconductingelectromagnet at “muchhighertemperature” (>77K) Quench: A quench is an abnormal termination of magnet operation that occurs when part of the superconducting coil enters the resistive state, mainly due to magnetic field excess. That part sinks power which increases T in the surroundings, making them enter the resistive state as well.