Noise Suppression Effect of Low Ripple High Voltage Power Supply in Precision Measuring Instruments
Precision measuring instruments including electron microscopes, mass spectrometers, and spectroscopy systems require stable high voltage bias for their detectors and electron optics. Ripple and noise on the high voltage degrade the measurement resolution and accuracy, limiting the instrument performance. Low ripple high voltage power supplies specifically designed for precision applications provide the noise suppression needed to achieve the full capability of the instrument.
Ripple refers to the periodic variation in the output voltage, typically at the power line frequency or the switching frequency of the power supply. Noise refers to the random fluctuations in the output voltage across a broad frequency range. Both contribute to the instability of the instrument operating point, affecting the measurement quality. The acceptable ripple and noise levels depend on the instrument type and the measurement requirements, with high precision applications requiring ripple below parts per million of the output voltage.
In electron microscopes, high voltage ripple causes variations in the electron beam energy, affecting the focus and the image stability. The electron wavelength depends on the accelerating voltage, and voltage variations cause focus variations that blur the image. High resolution electron microscopy requires voltage stability better than one part per million to achieve atomic resolution. Chromatic aberration in the electron lenses amplifies the effect of energy variations, making voltage stability critical for high resolution imaging.
Mass spectrometers use high voltage to accelerate ions into the mass analyzer. Voltage variations cause variations in the ion energy, affecting the mass resolution and the mass accuracy. In time of flight mass spectrometry, the flight time depends on the square root of the accelerating voltage, and voltage variations cause peak broadening that degrades the resolution. In magnetic sector instruments, the ion trajectory depends on the accelerating voltage, and variations cause peak shifts and broadening.
Optical spectroscopy instruments may use high voltage for photomultiplier tubes or other detectors. Voltage variations on the photomultiplier cause gain variations that modulate the signal. In photon counting applications, gain variations affect the pulse height distribution and the counting efficiency. In analog mode, gain variations appear directly as noise in the signal. Stable high voltage preserves the detector calibration and the measurement accuracy.
Sources of ripple in high voltage power supplies include the rectification of line frequency AC, the switching frequency of switch mode converters, and the beat frequencies between multiple switching stages. Line frequency ripple can be reduced through multi phase rectification and large filter capacitors. Switching frequency ripple can be reduced through filtering and through operating the switching frequency above the bandwidth of interest for the instrument.
Noise sources in power supplies include thermal noise in resistors and semiconductors, shot noise in current flows, and flicker noise in active devices. The noise is amplified by the gain of the power supply from the input to the output. Low noise design requires selection of low noise components, minimization of gain where noise is generated, and filtering of the output noise.
Output filter design for low ripple and noise must balance the noise suppression against the response speed and the stability. LC filters provide attenuation that increases with frequency, with the attenuation proportional to the square of the frequency above the cutoff. Multiple filter stages provide greater attenuation. The filter inductance and capacitance values determine the cutoff frequency and the attenuation characteristics. The filter must not cause oscillation with the feedback loop of the power supply.
Linear post regulation can achieve extremely low ripple by using a series pass element to regulate the output after the switching converter. The linear regulator rejects the ripple from the preregulator, providing clean output at the cost of additional power dissipation. The power dissipation equals the dropout voltage times the output current, requiring thermal management for the pass element. Linear post regulation is effective when the output current is moderate and the efficiency loss is acceptable.
Measurement of ripple and noise requires instrumentation with sensitivity comparable to the specification. Oscilloscopes with high sensitivity and appropriate bandwidth measure the ripple waveform. Spectrum analyzers measure the noise spectral density across frequency. True RMS voltmeters measure the total noise voltage. The measurement technique must avoid pickup from the environment and must account for the instrument noise floor.
System integration of the low ripple power supply with the precision instrument requires attention to grounding and shielding. Ground loops can couple noise into the instrument signal paths. The power supply and instrument grounds must be connected appropriately to prevent noise currents from flowing through signal references. Shielding contains the electromagnetic interference from the power supply and protects the supply from external interference. The integration must preserve the power supply performance in the system context.
