Performance of a High Voltage Power Supply incorporating a Ceramic Transfomer
Yoshiaki Shikaze(Department of Physics, Faculty of Science, University of Tokyo), Masatosi Imori(ICEPP, University of Tokyo), Hideyuki Fuke(Department of Physics, Faculty of Science, University of Tokyo) Hiroshi Matsumoto(ICEPP, University of Tokyo), Takasi Taniguchi(National Laboratory for High Energy Physics(KEK))
Abstract:
This paper describes the performance of a high-voltage power supply incorporating a ceramic transformer. Since the transformer doesn't include any magnetic material the power supply can be operated under a strong magnetic field. In the article, the efficiency of the power supply is studied against various parameters. It was found that the efficiency reaches more than 50 percent when zero-voltage switching was realized. From a voltage source of 2V, the power supply can supply 3000V at a 21 megohm load. A voltage source of 5V is enough to supply 4000V at the same load.
Summary:
This paper describes the performance of a power supply incorporating a ceramic transformer which uses the piezoelectric effect to generate high voltage. By using the ceramic transformer and a air-core coil, the power supply can produce high voltage under a strong magnetic field. The output high voltage is stabilized by feedback. A feedback loop includes divider resistors, an error amplifier, a voltage controlled oscillator (VCO) and a driver circuit. An output high voltage is produced by a Cockcroft-Walton (CW) circuit. The driver circuit generates a sinusoidal carrier the frequency of which is genertaed by the VCO. The driver circuit drives the transformer, applying the sinusoidal carrier. The amplitude ratio of the transformer has dependence on the frequency, which is utilized by the feedback.
The transformer shows a sharp resonance in the vicinity of 120kHz. From a view point of efficiency, it is favorable to drive the transformer at efficient frequencies ranging from 120kHz to 124kHz. When 3V is supplied to the driver circuit, the power supply produces 1500--3000V at a 21 megohm load with the above efficient range of frequency. The driver circuit includes FETs and the air-core coils. The inductance of the coil and the input capacitance of the transformer composes a oscillation circuit, by which the sinusoidal carrier is produced. The inductace is adjusted so that the frequency of the oscillation can be in the efficient range. So the FETs are switched while the voltage applied to the FETs is zero. The zero-voltage switching of the FET was realized, which contributed to improving the efficiency.
On-resistance of the FET is important for the efficiency. Yet the efficiency depends on characteristics of the FET intricately. Several kinds of FET are tested. For each FET, many plots on the efficiency were drawn against various parameters such as output high voltage, frequency, inductance, and capacitors of CW circuit. The efficiency increases as the frequency moves to the resonant frequency, being saturated at the level of more than 50 percent at the frequency in the efficient range with zero-voltage switching. When the voltage supplied to the driver circuit is 2V, 3V and 4V and output high voltage is 2000V at a load of 21 megohm, the current to the driver circuit is about 200mA,130mA and 110mA, respectively.
The amplitude of the sinusoidal carrier is about three times the input voltage to the driver circuit. The output voltage of the transformer is furthermore multiplied by about six times in the CW circuit. When the voltage to the driver circiut is 1.5V, the maximum output high voltage of the power supply reaches 3000V at a load of 18 megohm. It can be seen that the amplitude ratio of the transformer reaches a hundred at the resonance frequency. So Feeding 5V to the driver circuit is enough to supply 4000V. Hence the power supply could be available for any of Thin Gap Chamber, Monitored Distributed Tube (MDT) and Resisted Plate Chamber (RPC) of ATLAS experiments.
Id: 66
Corresponding Author: Claudio RIVETTA
Experiment: CMS
Sub-system: Calorimetry
Topic: Low Voltage And High Voltage Distribution
Design Considerations of Low Voltage DC Power Distribution for CMS Sub-Detectors
B.Allongue, F. Fontaine, F. Szoncso, G. Stefanini CERN Switzerland S. Lusin, P. Robl University of Wisconsin, Madison, USA J. Elias Fermilab, USA C. Rivetta ETH Zurich/CERN Switzerland
Abstract:
A distinguishing feature of LHC detectors is the enormous number of front-end electronics (FE) channels in all of the sub-detectors. Low-voltage power supply systems in the range of multi-kilowatts are required to bias such electronic read-outs. Several configurations has been proposed and analyzed by the different groups showing particular advantages and disadvantages. For the CMS detector, the Hadronic Calorimeter (HCAL) and the Muon End-caps (EMU) have proposed a DC power distribution system based on DC-DC power switching converters.
The topology of this DC power distribution is as follows: AC/DC converters in the control room are used to rectify the three phase mains and generate the primary 311 VDC voltage. Each rectifier supplies several DC-DC converters located in the cavern near the FE. The switching regulators convert the high voltage into appropriated low voltages that are locally distributed to the detector read-outs. Local regulation is performed in the FE at the board level using special linear low-dropout voltage regulators developed by CERN RD-49 collaboration.
The main advantage of this topology is the reduction in volume of the distribution cables due to the relative low primary currents. Locating the DC-DC converters in the hostile environment of the detector cavern is a disadvantage due to the presence of magnetic fields and radiation. Analysis and tests are necessary to characterize the behavior of those units under such conditions and find acceptable solutions. Also, further studies and tests are necessary to mitigate the radiated and conducted noise generated by the switching converters, to ensure stability of multi-converter systems against interactions between units, etc.
In this paper, tests conducted to validate the application of commercial units are reported and future tests are described. Also, an analysis of the overall system performance is presented along with guidelines for design and selection of the components are presented.
Summary:
This paper describes the tests performed to validate switching converter units to be applied in the DC power distribution of CMS-EMU and CMS-HCAL sub-detectors.
Modularity of the DC-DC converters is the primary requirement. It will facilitate replacement of failed units during short-period scheduled access to the cavern providing a reduction in the time that a part of the sub-detector is down. Each modular unit will thus include not only the basic power converters to attain the required output low-voltages but also protection, filtering, monitoring system and interface for remote operation. The tendency is to use commercial units (COTS) to fulfill this design but it is difficult to satisfy all of the requirements with such units. Instead, a 'semi-custom' design has been used based on COTS with the collaboration of manufactures, assembly companies and the universities and laboratories involved.
The primary stage of the design is the search for suitable units that can operate in environments with radiation. A radiation test has been conducted and future tests are under evaluation. The idea is to characterize the radiation tolerance of candidate converters and analyze, in case of failure, critical component to be replaced in the prototypes.
These tests include total dose effects and Single Event Effects (SEE) (single event up-set, single event latchup, single event breakdown, etc.). The first test was performed at the Commisarat d'Energy Atomic (CEA), Dijon, France. A low energy neutron reactor was used to test commercial DC-DC units for total dose effects. Two Vicor converters with 300V input and 5V / 12 V output, 400W, were radiated up to a level of 3x10^11 neutrons/cm^2 during an 8 hrs exposure which represents the total dose for continuous operation over 10 years. The results were satisfactory measuring only less that 0.5% of drift in the output voltage of the converters. This behavior was the similar to the one experienced in previous tests performed on other Vicor units with different characteristics. In future tests, higher energy particles will be used to study the SEE performance of these units and of new prototypes acquired from different vendors.
The magnetic field, in the areas assigned for the DC-DC converters, is about 200mT (2000 Gauss). This level is inappropriate for good operation or performance of converters with magnetic components. This problem involves a study of the maximum levels of magnetic field in different directions that the converter can tolerate operating with good performance. Studies and tests of the inductance and transformer of Vicor converters have been performed using a constant magnetic field at CERN. In the experiment, the level of magnetic field around the converter units will be reduced to an adequate level using a soft-steel magnetic shielding.
Switching power supplies, in general, are noisier than equivalent linear power supplies. In the CMS application it is very important to keep the noise level below values that do not compromise the operation and performance of the FE and neighbor systems. Conductive and emitted noise tests are scheduled on the Vicor units and input/output cables. The first test is performed on Vicor units connected through standard input/output cables to allow a determination of the level of filtering necessary and the shielding necessary in the cables to reduce both the conducted and emitted noise. Similar tests are planned on new prototypes.
Due to the negative input impedance characteristic of the DC-DC switching regulators at low frequencies, interaction between switching regulators and the others part of the input system may result in system instabilities. Small and large-signal models of both converters and line conditioners have been evaluated to analyze the performance and stability of the whole system. Based on this analysis, guidelines to design the proper input filtering of the DC-DC converters is presented.
The Front-end electronics will use on board radiation tolerant linear regulators. This choice simplifies the design of the connection between the converters and the FE because remote sensing is not required. The regulators will absorb the voltage variation due to the drop in the line resistance by changes in the load current. The only consideration on this link is to provide enough damping on the lines with passive elements to avoid big voltage excursions at the FE input.
An additional, but important, last consideration is the reliability of the complete system. As presented above, the DC-DC converters should be modular to assure the easy replacement in case of failure. That allows consideration of units with industry standard mean time between failure (MTBF) performance. If it is not possible to replace units during scheduled weekly accesses, then units with longer MTBF or N+1 redundant units are necessary.