RTC Magazine

Providing high voltage that can be remotely controlled in a small, confined area is a problem faced by many laboratories around the world. This problem became apparent for Sandia National Laboratories (SNL) when working with the world’s most powerful X-ray generator named the Z-Machine. The Z-Machine is capable of releasing 290 trillion watts for a billionth of a second. That’s 80 times the world’s electrical power usage. Due to the nature of these large scientific machines, the systems need to be placed behind Faraday shield screen boxes inside the Z-machine. And because of the limited space, Sandia needed a power solution that could stack 20 channels of high-voltage power in an area no larger than 19” x 40” x 20” and be controlled and monitored remotely via computer control.

Physics experiments often require many (up to 1000) continuously variable high-voltage (2kV to 6kV), low-current power supplies for various types of experimental apparatus, such as photo multipliers, ion chambers, tracking chambers, etc. In the past, this has often required a variety of units from different manufacturers with different voltage-current ratings, control interfaces, voltage, current and state monitoring circuitry and output protection features.

The physical packaging and I/O connections were also dissimilar. Such lack of uniformity has caused problems with system implementation, maintainability and control/monitor software because of the need to custom-tailor for each type. The work required to implement large, multi-channel high voltage systems with these system was often difficult and expensive.

Traditional high-voltage power systems were offered in NIM, VME or CAMAC architecture. Implementing a full 64-bit bus in VME requires a 6U form-factor, where in CompactPCI, both 32-bit and 64-bit can be implemented using a 3U form-factor. Unlike the older VME-based high-voltage power systems, where the main electronic design constraint was cost, newer designs must meet additional requirements for size, power consumption, remote control and monitoring capability. Newer designs should also be able to seamlessly integrate with other manufacturers off-the-shelf products in an environment containing a large variety of electronic equipment.

The CompactPCI architecture allows easy integration of needed functionalities into a single system. Instrumentation, data acquisition, machine vision, motion control and bus interface modules are just a few types of CompactPCI modules available. It also provides a high-performance and rugged industrial form-factor along with high compute density and higher-quality components. Taking these points into consideration, the CompactPCI architecture has been found to meet Sandia’s high-voltage system requirements.

Major system requirements are as follows:

1) DC supplies should be modular with the modules supplying an array of voltage ranges (typically -100V to -5KV)

2) Current draw in a quiescent state should be very small (typically < 1 mA)

3) Individual modules should be stable to 5% of the set value over a period of time

4) The ability to control and monitor the voltage and current characteristics remotely using a computer

5) Small and robust design, which is easy to maintain

6) Scalable in nature

System Architecture

Bi Ra Systems has three versions of these high-voltage module boards. Model 4720 has four high-voltage channels each having a voltage range from 0 to -300 volts; Model 4720A (Figure 2) has three channels with each channel having a range of 0 to -300 volts and Model 4730 has two channels, each having a range of 0 to -5KV. Since robustness and remote control and monitoring of these high-voltage systems were a priority, the next consideration was the selection of an appropriate, modular computer-interface bus system.

Once again CompactPCI proved to be the bus backplane of choice. The chassis selected for this purpose is National Instruments PXI-1044, which accepts both PXI and CompactPCI 3U modules. The PXI-1044 offers 467W of available power across 14 slots with 25W of cooling for each slot provided by three fans running at 140 cfm. The chassis is PXI specification Revision 2.1-compliant. It is equipped with NI-8196, which is a high-performance Pentium M 760-based embedded controller for use in PXI and CompactPCI systems. The PXI-8196 includes high-performance peripheral I/O such as 10/100/1000 Base TX Ethernet, four USB ports, an RS-232 Port and an IEEE 1284 ECP/EPP parallel port. This allows the user to have a choice in selecting the peripheral for remote communications with the chassis. Additionally, it comes with 512 Mbyte dual-channel DDR2 RAM and Windows XP Professional already installed.

At the heart of the system for control and monitoring is Quicklogic’s 32-bit PCI chip running at 33 MHz, with 14 RAM blocks comprising 32,256 RAM bits and 97 I/O pins. The chip is PCI v2.3-compliant with zero wait states to provide up to 264 Mbyte/s transfer rates. It also offers full PCI Configuration Space and flexible target addressing. It supports retry, disconnect with/without data transfer and target abort requested by the user-programmable logic. Any number of 32-bit BARs may be configured as either memory or I/O space. The boards also have a CMOS FLASH-based 8-bit microcontroller chip from Microchip that acts as a slave, and assists in controlling and monitoring the feedback voltage. The microcontroller features 256 bytes of EEPROM data memory (self-programming), a USART and a synchronous serial port, which can be configured as either 3-wire Serial Peripheral Interface (SPI) or the 2-wire Inter-Integrated Circuit (I²C) bus.

The Models 4720 and 4720A utilize DC to high-voltage DC converters that require an input current of 70 mA, with an input impedance of 10K ohm. These modules have less than 10 mV peak to peak of ripple and are protected against continuous output short circuits. These supplies do not have Input/Output isolation; hence Isolation amplifiers were used to effectively isolate the high voltage. At full running capacity, each of these high-voltage modules uses 0.7 watts of power.

On the other hand, Model 4730 utilizes DC to high-voltage DC converters that require 380 mA of input current and supply 3W of output power with a 5% typical regulation. The modules also have a 6KV input/output isolation. The finished Model 4720 and 4730 boards have eight layers with impedance-controlled traces and split ground planes for high and low voltages respectively. Special care has been taken to prevent noise from coupling onto the digital data, address and control lines, which interface with the CompactPCI bus.

A 3U form-factor of these boards presents significant challenges for noise coupling, heat dissipation and arcing. When running at full capacity, the 4730 boards produce 384 volts/inch-sq’ even at low output current; this has a potential of arcing over the air. The 4730 is a

two-slot-wide 3U form-factor board. The output voltage is brought down to a more manageable range of 0 to 5 volts using a 60:1 voltage divider in the case of 4720 and 4720A, and 1000:1 divider in the case of 4730, which was then monitored using 10-bit ADCs. The boards also use many of Linear Technology’s Power Management chips to juggle the available power to be used on the board.

To control the power supply cards, LabVIEW from National Instruments was chosen because of its easy to understand graphical interface controls. LabVIEW also provides a deterministic, real-time performance along with LabVIEW Real Time for data acquisition and control. LabVIEW can be readily deployed and the cards can easily be controlled and monitored over Ethernet (Figure 3).

The intensive computing prowess of today’s chips implies that fewer discrete components need to be used to achieve almost identical results, which results in a drastic reduction of PCB real estate and cost. Apart from that, it is much easier to maintain and scale these systems.

Possible applications of these high-voltage power modules will be in Photo Multiplier Tubes (PMT), which are used for analytical applications like emission-spectroscopy, fluoroscopy, atomic-absorption-spectroscopy as well as bio and chemo-luminescence. Ionization chambers, CRT systems testing, high-voltage biasing for Avalanche Photodiodes and Photo detectors, X-ray tubes and pulse generators (used in radars, lasers, EMC testing and other imaging applications) are other potential users of this technology.

Bi Ra Systems
Albuquerque, NM.
(505) 881-8887.
[www.bira.com].