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EETE MAY 2013

POWER COMPONENTS Identifying the best power supply for your test application By Robert Green, James Niemann, and Qing D. Starks Most electrical engineers believe they have a good understanding of power supplies because they are relatively simple, single-function DC devices designed to output controlled voltages. However, there is much more to them than this description would suggest. Although a review of a power supply’s specifications should always be a part of the selection process, other characteristics should also be considered. Investigate the power envelope The most significant decision is ensuring that sufficient power is available to energize the device under test (DUT). Different types of power supplies have different power envelopes. A power supply with a rectangular power envelope as shown in figure 1a, the most versatile type, allows supplying any current to the load at any voltage level. A supply with multiple rectangular envelopes for multiple ranges (such as the two-rectangular envelope shown in figure 1b), permits higher values of one parameter at the expense of the other parameter, so it can output a higher level of current but at a lower maximum voltage. Supplies that output a hyperbolic envelope offer a more continuous transition than a multi-range power supply, with one parameter inversely proportional to the other – see figure 1c. High power output supplies tend to have multi-range or hyperbolic envelopes. Determine the noise performance Noise from external sources may cause problems when powering a circuit that operates at a very low voltage or a circuit that uses or measures very low currents. The supply itself is one source of noise, which can be broken into two components: normal-mode noise and common-mode noise. Normal-mode noise is generated across the supply’s output terminals due to the supply’s internal circuitry. Common-mode noise is earthreferenced noise originating from the power line and stray capacitance across the main transformer. For sensitive circuits, linear power supplies provide much lower normal-mode output noise than supplies designed using switching technology but have lower power-conversion efficiency and can be bulkier and heavier. Switching supplies typically offer more output power in a smaller enclosure. For noise-sensitive circuits, a linear supply can have just one-fifth to one-tenth of the noise (5mVp-p vs. >50mVp-p) of a switching supply. Whenever normal-mode noise is a crucial consideration, use a linear supply, such as Keithley’s Series 2200 single- and multi-channel power supplies, if possible. Assess common-mode noise current Linear power supplies generally have lower common-mode noise than switching supplies. Common-mode noise is generated whenever changing voltages, such as AC voltages and transients (dv/dt) on either the primary or the secondary windings of an isolation transformer, couple current across the barrier. Whenever this current flows through an impedance, the noise Robert Green is a Senior Market Development Manager focusing on low-level measurement applications. James Niemann is a staff engineer responsible for designing instrumentation used in low-level measurements. Qing D. Starks is a staff applications engineer. All three work at Keithley Instruments - www.keithley.com Fig. 1a: Rectangular power supply envelope. Fig. 1b: Multi-range output. Fig. 1c: Hyperbolic output characteristic. Maximum voltage and current follow a curve. 24 Electronic Engineering Times Europe May 2013 www.electronics-eetimes.com


EETE MAY 2013
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