A VME Board Prototype of High Voltage Power Supply Incorporating Piezoelectric Ceramic Transformer

Abstract

A high voltage power supply channel produces a stabilized direct current high voltage, utilizing a piezoelectric ceramic transformer to generate high voltage. With the intention of developing a 16-channel VME board, a single channel VME board was prototyped, where a high voltage power supply was implemented on a VME board so that a computer could control the power supply. The power supply is capable of functioning under a magnetic field of 1.5 tesla. The power supply is protected against overload. The computer is enabled to control the power supply in the channel. The computer can turn on and off the power supply, set the output high voltage, monitor the change in the output current and recover the power supply from the overload. I. HIGH VOLTAGE POWER SUPPLY With the intention of developing a 16-channel VME board, a single high voltage power supply channel was prototyped on a VME board so as to be brought under the control of a computer. The computer is enabled to control the power supply in the channel. The computer can turn on and off the power supply, set the output high voltage, monitor the change in the output current and recover the power supply from the overload. The channel includes a high voltage power supply which incorporates a piezoelectric ceramic transformer, where the ceramic transformer takes the place of a conventional magnetic transformer. The ceramic transformer generates high voltage with the piezoelectric effect efficiently. The ceramic transformer does not include any magnetic material, and can be operated under strong magnetic field. An inductance element is required to obtain efficient high voltage generation, being implemented by an air-core coil. The power supply is capable of supplying stabilized high voltage from 2500 V to 3500 V to a load of more than 25 MΩ at efficiency of better than 55 percent from a supply voltage of 3 V under a magnetic field of 1.5 tesla. Noises on the high voltage are around a hundred milli-volts in peak-to-peak amplitude. The power supply is protected against overload such as short-circuiting [1]. II. STABILIZATION OF OUTPUT VOLTAGE The high voltage power supply includes feedback to stabilize the output voltage. The output voltage is fed to the error amplifier to be compared with a reference voltage. The output of the error amplifier is supplied to a voltage-controlled oscillator (VCO), which generates the frequency of the carrier, where the carrier is sinusoidal voltage wave generated by a driver circuit. The carrier drives the transformer, where the carrier is amplified in amplitude and supplied to the CW circuit. The carrier is further multiplied in voltage and rectified by the Cockcroft Walton (CW) circuit. The voltage at the output of the CW circuit is the output voltage of the power supply [2, 4]. The ceramic transformer includes an internal resonance circuit. The amplitude of the carrier at the input of the transformer is amplified at the output, with the input to output voltage ratio of the amplitude being an amplitude ratio that shows a resonance as a function of the driving frequency: the frequency of the carrier. Fig. 1 plots the amplitude ratio against the driving frequency. Voltage amplification at the transformer depends on the driving frequency. The dependence is utilized for stabilization. Controlling the driving frequency, the feedback adjusts the amplification so as to stabilize the output voltage. III. BREAKDOWN OF FEEDBACK The range of the driving frequency is designed to be higher than a resonance frequency of the ceramic transformer as shown in Fig. 1. So the feedback increases the driving frequency when the output voltage is higher than the reference voltage at the input of the error amplifier. Similarly the driving frequency decreases when output voltage is lower than the voltage specified by the reference voltage.