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EETE MAY 2013

voltage generated can degrade load (or DUT) performance or cause load-monitoring measurement inaccuracies. Sources of common-mode noise include voltage transients from rectifier diodes (on the secondary) turning on and off and either the 60Hz line movement or the abrupt voltage transient common with a switching power supply’s primary circuit. Figure 2 shows a simplified block diagram of a power supply. The quality of the transformer’s construction, including sufficient shielding between the primary and secondary windings, can minimize the stray capacitance between primary and secondary. With minimal coupling capacitance, the noise current flowing through the load won’t generally affect the load’s operation Generally, the higher the isolation, the lower the noise coupled through the supply from the AC power line. The problem becomes more complex when the application involves other instruments. In this case, insufficient DC isolation in the power supply can provide a conduction path for a high common-mode current from one of the other instruments. For any particular power supply application, it’s crucial to understand the effect of the power supply isolation resistance and capacitance on the DUT, and the path or loop where the primary and secondary common-mode currents flow in order to determine if a noise voltage (common-mode current × impedance) will be developed and whether the noise will be excessive. One less hat to wear. Let us be your power expert. We understand that you don’t have the time to master every aspect of electronic design. As a leading manufacturer of power supplies we are here to collaborate with you to ensure your next project is a success. Novum® Advanced Power Ac-Dc Power Supplies Dc-Dc Converters www.cui.com/PowerExpert or impact measurements on the load. If the transformer’s primary and secondary aren’t sufficiently shielded from each other, then the coupling capacitance can be large and milli-amps of current can flow into the load, creating performance problems and load current measurement errors. For low power and sensitive components, modules, or end products, evaluate the power supply for low common-mode performance. Keithley’s Series 2200 power supplies have common-mode currents of less than 10μA. Check isolation from Earth ground One further indication of the quality of a power supply is the isolation of its output is from the power line. A power supply with high isolation further minimizes noise on the supply’s output. A good level of isolation impedance includes parameters greater than 1GΩ in parallel with less than 1nF and shielded well enough to support less than 5μA of common-mode current. Unfortunately, few instruments meet or exceed these guidelines. Low frequency 60Hz designs may meet the common-mode current specification but fall short of the DC resistance and capacitance figures; switching designs may have low capacitance and higher DC isolation but excessive common-mode current. In some applications, the DC isolation resistance and capacitance are more important than common-mode current. One case in which the high impedance is important is when a supply is powering a circuit driven by a linear amplifier. In this situation, the power supply is part of the load on the linear amplifier and a large power supply capacitance can create stability problems for the amplifier. Alternatively, a supply being used to power a low voltage resistive divider or a very low current measurement circuit may need low common-mode current, regardless of the isolation impedance. www.electronics-eetimes.com Electronic Engineering Times Europe May 2013 25


EETE MAY 2013
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