Low-Level Imaging Needs Low-Noise, High-Stability Electronics

Author:
Gary Bocock, Technical Director, XP Power

Date
03/31/2022

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When imaging signals are measured in single digit electron counts, any background noise is a problem

Click image to enlarge

Figure 1: Examples of high voltage DC-DC converters for sensitive imaging applications, featuring low ripple, noise, and EMI. Shielded internal transformers and plated steel housings additionally achieve low levels of EM radiation

We’re all perhaps familiar with the difficulties of recording images in low light conditions, with our cell-phones producing grainy images in the dark as their electronics winds up the gain of the signal chain to capture as many photons as possible amongst the background electrical noise. The sensors in cell-phone cameras are typically CMOS types with the older CCD technology still used in high-end imaging equipment, but specialized detectors using avalanche photodiodes (APDs), microchannel plates (MCPs) and photomultiplier tubes (PMTs) are used for the most demanding professional applications. These include radiation detection, spectroscopy, laser rangefinders, night vision, blood analyzers, positron emission tomography, long-range fiber-optic telecommunication, particle physics and astronomy.

Image sensors use the photoelectric effect by which electrons are liberated from a photosensitive surface in a number proportional to incident photons of light. The electrons are collected as current, the signal is amplified and then is passed through a D-A converter for subsequent processing. The quantum efficiency of sensors is now such that with good probability, single photons can be registered, producing single electrons, so to exploit this, any extraneous noise in the electronics signal chain and power rails must be extremely low. The sensor is usually biased with a negative voltage, typically between 100V to 6kV, which must also be highly stable and low-noise for optimum imaging performance.

Bias Voltage from a Low-EMI Source

Radiated Electromagnetic Interference (EMI) can be a particular problem for PMTs, distorting electron paths and causing detector gain degradation. Radiated EMI can also affect other detector types directly or indirectly and the problem is exacerbated by the high voltage DC-DC power converter typically used, with its internal high-frequency switching waveforms. Typical small high voltage DC-DC converters for the application are shown in Figure 1.

The converter operates usually with a 5, 12 or 24VDC input, which is switched at high frequency, perhaps several hundred kHz. The resulting waveform is passed through a transformer to increase the voltage, followed by a multiplier stage, then rectified and filtered to produce high voltage DC. Feedback to the input switching circuit sets and stabilizes the output voltage through pulse-width and/or frequency modulation of the switching waveform. Figure 2 shows a common arrangement.

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Figure 2: High voltage can be generated by a DC-DC converter

 

High Voltage DC-DC Converters can Generate Significant Noise

While any design of DC-DC converter can include input/output filtering and shielding to reduce EMI, in sensitive imaging applications it is preferable to utilize circuit topologies that are inherently low noise. Switched-mode DC-DC converters are the only option for increasing voltage and the traditional design approach is ‘hard-switching’, in which the power semiconductors are driven hard on and off as fast as possible at high frequency. This can be a simple arrangement with good efficiency, but waveforms with high edge-rates are naturally rich in harmonics with potential EMI problems that can be difficult to suppress and shield. The high voltages present in sensor applications only make the situation worse, not just because of their amplitude, but also because large insulation spacings are required, increasing parasitic effects such as leakage inductance. This directly produces voltage spikes and radiated EMI from rapidly changing currents. Also, with high voltage transformers, turns number is inevitably high, leading to excessive winding self-capacitance, which then leads to energetic resonances that cause ringing with large voltage overshoots. The resultant component stresses have to managed and the spike in output noise at the resonant frequency can be particularly difficult to attenuate.

Resonant Converter Topologies Minimize Noise

An alternative to the hard-switched’ approach with all of its problems is to use a ‘resonant’ converter. There are different variants that could be used, but they have the common feature that parasitic elements are actively controlled by design and form part of networks that intentionally resonate in the power transfer phase of switching operation. Waveforms are now sinusoidal rather than square minimizing EMI along with efficiency benefits as well; the power semiconductors operate with ‘zero voltage’ or ‘zero-current’ soft-switching, depending on topology, minimizing dynamic losses. Examples of products using the topology are seen in the XP Power range of high voltage DC-DC converters, additionally featuring zinc-plated steel housings for effective shielding. Note that other commonly used materials perform very poorly in this regard. Plastic and non-ferrous materials such as copper, brass and aluminum have a magnetic permeability close to that of ‘free space’ or a relative permeability of unity, which provides no magnetic screening, whereas steel has a relative permeability of around 2000 forming an effective magnetic barrier. Plating on the steel protects against corrosion and enables solder connections to be made for grounding purposes.

Well-designed high voltage converters using low noise topologies and comprehensive shielding internally and externally can be well suited for sensitive imaging applications. At low power, miniature converters like this can be located close to the imaging sensor so that connections are short for best regulation and to maintain good EMI performance. High voltage cable runs can also be kept to a minimum for safety concerns. High efficiency of the converters is also a benefit when fitted close to the sensor – ultra-sensitive image sensors have to be cooled for lowest thermal noise and therefore any adjacent electronics should dissipate as little power as possible.

Commercial High Voltage DC-DC Converter Modules are Cost Effective

Commercially available high voltage DC-DC converters can also include features that make them easy to integrate into overall system control such as remote programmability and monitoring. These modular solutions can be relied on to provide precise output control with load, line, and temperature variations, along with low EMI. Compared with discrete solutions, commercial modules are ready-to-use, safety-certified and often in a common PCB-mount footprint, allowing interchangeability between models. Overall, modules with a proven track record from reliable suppliers can save development time, cost and risk.

External Connections are Key to Optimum Performance

Noise-free power lines are a pre-requisite for sensitive imaging applications but there are still pitfalls to avoid in the external power connections; high performance signal amplifiers in an imaging system will have special local grounding and shielding arrangements for best performance and this should be carefully segregated from the power grounds. There will be separate ground return paths for the input to the high-voltage DC-DC converter, its output, any low voltage control and monitoring and the image signals. These should be separated, and any common connection should be formed with a ‘star ground’ technique to avoid interaction. Figure 3 shows how this might be achieved with a typical modular converter.

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Figure 3: ‘Star’ connection of grounds to avoid interaction

 

Enabling Optimum Sensor Performance

When imaging signals are measured in single digit electron counts, any background noise is a problem. Electronics needs power and the specialized converters necessary to generate high voltages for the application can be a source of EMI. However, good low-noise converter design techniques coupled with comprehensive screening, filtering and careful segregation of external connections can yield optimal detector performance. Component-level, PC board-mounted, high voltage DC-DC converters with the right specifications are available from XP Power.


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