Power Supply for Miniature Quadrupole Mass Spectrometer
- Tuesday, 26 February 2008
This lightweight, low-power system is designed to operate unattended for a long time.
A lightweight, low-power-consumption power supply has been developed to generate a combination of radio-frequency (RF) and dc voltages needed for the operation of a miniature quadrupole mass spectrometer that could operate unattended in the field for a long time, possibly using a battery as an energy source. More specifically, the circuit is designed to supply large, variable, frequency- and amplitude-stable RF voltages, variously floating or superimposed on positive or negative dc voltages, for application to primarily capacitive mass-spectrometer loads with capacitances of the order of 50 pF.
The figure is a simplified block diagram of an ac section of the power supply. This section puts out a signal with a frequency of 10 MHz, though in general, the frequency could be extended to as much as 100 MHz. The basic high-frequency clock signal for this section is generated by oscillator U1 at a frequency of 20 MHz. Flip-flop (FF) U2 divides the frequency by two, producing two 10-MHz pulse trains that differ in phase by a half cycle. Integrator and summing amplifiers U3 and U4 convert the 10-MHz clock pulses to sawtooth waveforms.
Q1 is a high-speed metal oxide/ semiconductor field-effect transistor (MOSFET) that turns on when the sawtooth waveform applied to its gate goes beyond its threshold voltage. The conduction angle and the conducted current can be increased by increasing the sawtooth voltage. Q3 operates similarly to Q1, but a half cycle out of phase with Q1. Q1 and Q3 drive the primary windings of air-core transformer T1.
The air-core transformer design is chosen over a ferromagnetic-core design to obtain a desired combination of low weight, low loss, and low capacitance, and the ability to generate a large RF output voltage. The primary/secondary flux coupling, and thus the gain, can be adjusted in the design by choice of the winding ratio or can be changed by mechanical adjustment of the distance or overlap between the primary and secondary windings. These adjustments are inextricably coupled with the adjustment of the inductance of T1 to resonate with the capacitance of the affected portion of the quadrupole mass spectrometer.
U6 is an RF-detector circuit that generates a voltage proportional to the amplitude of the RF output signal. U5 is an error amplifier and compensation circuit, the output of which is a control signal proportional to the difference between (1) the actual RF output amplitude and (2) the commanded RF output amplitude as represented by the output of digital-to-analog converter U7. This control loop maintains a stable RF output amplitude during long-term operation.
One basic problem in the design of power-supply circuits like this one is to obtain a large dynamic range for the output signal. At a low level, the drive signal is coupled through the gate-to-drain capacitances of the MOSFETs, giving rise to an output signal much greater than the desired minimum. To counteract this effect and thereby extend the lower limit of the dynamic range of the mass spectrometer to below one atomic mass unit, a diode (D1) and a cascode stage (Q2 with bias VB), is incorporated into the branch that contains Q1, and a similar combination (D2 and Q4) is added to the branch that contains Q3.
Another problem is obtaining sufficient sawtooth amplitude to drive the MOSFETs at the highest desired power levels; the peak amplitude (3 to 4 V) generated by currently available operational amplifiers is too low. In the present circuit, the return of the main dc source (U8) for driving the primary of T1 is referred to a negative voltage ( -V). Thus, the apparent peak of the sawtooth signal applied to the MOSFET is increased by V, making it possible to reach correspondingly higher power levels.
This work was done by Ara Chutjian, Dean Aalami, Murray Darrach, and Otto Orient of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Technology Reporting Office
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
Refer to NPO-20493, volume and number of this NASA Tech Briefs issue, and the page number.