Feature Power Electronics
Mass spectrometry demands precision
Gary Byfield at HiTek Power, discusses the demands that mass spectrometry applications place on high-voltage power supplies
mass spectrometer is a precision analytical instrument used for measuring particle mass and charge. It does this by producing an ion source from the sample then sep- arating the ions, which can only then be detected. Knowing and comparing the mass spectra allows the separate elements in the sample to be identified. A number of different sample ioni-
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sation techniques suit different appli- cations. The most popular of these are Atmospheric Pressure Ionisation (API), Atmospheric Pressure Chemical Ionisation (APCI), Electrospray Ionisation (ESI) and Matrix Assisted Laser Desorption Ionisation (MALDI). With the exception of MALDI, a high voltage is used to either charge droplets or create a corona discharge. There are three main methods of sample analysis and processing: Time- of-Flight (TOF), which uses an electric field to accelerate samples to the same potential and then measures their transit time, Quadruple Mass Filter, which uses an oscillating electric field to filter sample mass and Ion Trap, which also filters sample mass in the same way but traps and ejects the sample as required. Once the sample has been through the processing stage, steering and focusing takes place using either magnetic, electric or both methods.
A number of different functions for detectors and high-voltage power supplies can be used in this area of the mass spectrometer. These can be elec- tron multiplier, ion detector, dynode, faraday cup, ring, phosphor, micro channel plate or Reflectron outputs. Most of the required outputs will be below 8kV, some may be floating or require polarity switching.
High-voltage power supplies are
used in most of the key areas within a mass spectrometer. Their performance can be the principal factor governing the performance of the overall system. Although there are many separate uses of high-voltage power supplies within a mass spectrometer, the critical performance parameters are almost always the same:
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• Low ripple voltage • High stability and repeatability • Fast switching speed/settling time • Low radiated noise • High performance consistence from unit to unit
The highest voltages used within a mass spectrometer are normally up to 12kV. Higher voltages can appear, as some outputs are of opposite polarity or reversible. The output current levels are in the
order of a few
microamps for electrostatic outputs and 300µA for some detector outputs. It is important that a supplier of high-voltage power supplies to this demanding market has extensive application knowledge and experi- ence, so that it can discuss the customer’s application in great detail before translating it into an electrical specification. Elements that may not have been considered previously can be discussed in the early stages to reduce project risks, to ensure a cost effective solution and high reliability in the end application.
Factors defining power topology A number of factors define the ideal power topology for a low-noise power supply. The supply should be resonant to provide low dv/dt and minimise rectifier reverse recovery noise. It should have a continuous input current to reduce emissions and ground plane noise and the operating frequency should be stable with tem- perature and between different units to ensure no possibility of beating between different operating frequen- cies within the overall system. Lastly, the topology should be tolerant to component variation – to give repeat- able electrical performance over the product’s lifetime.
Filtering switching-frequency noise from a resonant power converter seems quite simple, that is until the ripple levels reach a certain level, beyond which other effects start to dominate and give unpredictable results. Everything matters when you are trying to achieve below 2mV of ripple with an
Figure 1:
A selection of non- standard mass spectrometry power supplies, custom built for specific customers
8kV output voltage: the stray capaci- tance of the output filter resistors and potting compound, the radiated field from the high-voltage transformer, any- thing but an ideal PCB layout or even a low frequency modulation introduced directly from PWM jitter. As the output filter stage corner frequency is low due to the filtering required, it is sometimes difficult to reject lower frequency noise, so this becomes dominant. Boltzmann constant and 1/F noise then affect component selection and very detailed component knowledge and careful design is required to achieve the desired results. Floating outputs can also create noise issues due to common mode currents in the isolation transformer. Companies such as HiTek Power are employing special techniques to minimise the common mode currents to a point where they do not affect the output ripple voltage or overall system performance. The voltage stress across compo- nents must also be controlled and the components de-rated in the required application to ensure the required per- formance level. Partial discharges can be a problem. Experience once again comes to the fore, being used to minimise the risks to an absolute mini- mum at the design stage.
Careful considerations
Making a high-stability power supply with excellent repeatability between polarity reversals every 2-50ms requires a number of careful considerations. The voltage stresses across components and other
materials must be carefully
Gary Byfield is General Manager at HiTek Power
considered and components selected accordingly. The current levels of repeatability between polarity reversals of Time of Flight power supplies pro- duced by the company can be lower than 5ppm. Static outputs can provide stability of better than 2ppm. Simple changes can be made to any mass spectrometry module if required, such as changing cable lengths and connectors or,
if preferred, a com-
pletely new custom solution can be developed using the company’s power conversation topologies, control methods and mechanical knowledge. HiTek Power
www.hitekpower.com
Enter 208 DECEMBER/JANUARY 2014 Electronics
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