15 Jun 2012
The Art of Selecting Power Supplies (Page 2 of 2)
Another area of importance is the isolation and its associated parameters.
The three most important power supply isolation parameters are 1) insulation resistance between the primary and the secondary windings of the power supply transformer, 2) stray capacitance between the windings, and 3) common-mode current.
The insulation resistance measurement is a DC characterization of the isolation between the low terminal of the power supply and the chassis. A supply such as one of Keithley’s new Series 2200 power supplies (Figure 2), can output voltage between the high and low terminals at least 100V above its chassis ground. This performance is a good indication of the insulation resistance between the line input on the primary winding and the low terminal on the secondary.
Recently, as a comparison, the insulation resistances of power supplies from two different vendors were characterized, which involved sourcing 200V to each supply’s low terminal while keeping the supply unplugged or floating, then measuring the resulting current with an electrometer (as illustrated in Figure 3). One supply allowed 130nA, which translated to 1.5 giga-ohms of insulation resistance; the second supply passed 65nA of current was drawn through the competitive supply for 3 giga-ohms of insulation resistance. Both results are quite good.
Figure 3. Configuration for characterizing the insulation resistance between the primary and the secondary windings of a power supply’s transformer.
Stray capacitance is an AC characteristic of a power supply’s isolation. If not properly designed, the operation on the primary side of the transformer will disrupt the operation of the secondary side. This capacitance can be measured using an LCR meter connected between the low and the chassis ground with the power supply floating (Figure 4a). At 1kHz, the stray capacitance on the first supply was measured at about 18nF; the second supply measured 800pF. At 100Hz, these capacitances were measured at 20nF and 1.15nF respectively.
Figure 4a. Configuration for characterizing the stray capacitance in a power supply.
Figure 4b. Configuration for characterizing the common-mode current in a power supply.
Common mode operation
The common-mode current characterizes the noise path between the low terminal and the chassis plane of the power supply while the power supply is in operation. A well-designed power supply with proper shielding on both the primary and the seconding windings of the transformer will exhibit very small (often micro-amp-level) common-mode current.
This low current can be characterized by using a low noise oscilloscope and a wideband current amplifier, as shown in Figure 4b. The scope outputs in Figures 5a and 5b display voltages representing common-mode currents through the two power supplies compared previously. This corresponds to about 4 micro-amps of common-mode current for the first power supply and 20–30 micro-amps of common-mode current for the second supply. Both supplies have good isolation, but the individual parameters can be quite different.
Figure 5a. Power supply #1’s common-mode current.
Figure 5b. Power supply #2’s common mode current.
Generally, the higher the isolation, the lower the noise that is coupled through the supply from the AC power line will be. The problem becomes more confusing when the application involves additional 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. The bottom line is that for any particular power supply application, one must understand both the effect of the power supply’s isolation resistance and capacitance on the DUT and the path or loop where the primary and secondary common-mode currents flow. Then the noise voltage (common-mode current multiplied by path impedance) can be estimated and an assessment of whether the noise will be excessive and disruptive can be made.
If your DUT requires individual isolated power supply sections, then you will need either multiple individual isolated supplies or a multi-channel output supply. If you choose a multi-channel supply, always ensure the isolation between the channels is greater than the isolation required between the DUT circuits. That can be difficult to determine from a multi-channel power supply’s data sheet (or even from the application circuit). Some power supplies do not provide isolation between channels, so consider taking the time needed to characterize the isolation between channels when the isolation between circuits in a DUT is critical.
If tight control of the voltage at the load is essential, review the supply’s output accuracy and readback specs carefully. However, that accuracy can be compromised if the supply is controlling the voltage only at its output terminals. Instead, you need feedback control right at the DUT, so your supply should include sense connections (remote sensing) to the DUT at the same place the power leads are connected. In this way, the sensing circuits measure the voltage at the DUT so the supply can compensate for any voltage drop in the test leads (Figure 6).
Figure 6. Remote sensing ensures that the programmed voltage is delivered to the load. Voltage at the sense leads is fed back to the power supply to adjust the power supply output so that VLoad = Programmed Voltage. The power supply output is adjusted to overcome the voltage drop in the leads, Vlead = ILoad * Rlead.
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About the author
Robert Green (photo) is a senior market development manager at Keithley Instruments, Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. During his career at Keithley, Green has been involved in the definition and introduction of a wide range of instrumentation. He holds a BS in electrical engineering from Cornell University and an MS in electrical engineering from Washington University in St. Louis, Missouri. As a staff engineer at Keithley, James Niemann is responsible for designing instrumentation used in low-level measurements. He earned a Bachelor of Science degree in Electrical Engineering from the University of Akron. He has been awarded three patents for his work and has 23 years of experience in instrumentation design. Qing D. Starks is a Keithley staff applications engineer. Prior to joining Keithley in 2006, she served in engineering roles at Infineon Technologies/Qimonda and Cypress Semiconductor. She earned a BSc in electrical engineering at the University of Calgary and an MSc in electrical engineering at Stanford University.
With more than 60 years of measurement expertise, Keithley Instruments has become a world leader in advanced electrical test instruments and systems. Our customers are scientists and engineers in the worldwide electronics industry involved with advanced materials research, semiconductor device development and fabrication, and the production of end products such as portable wireless devices. The value we provide them is a combination of products for their critical measurement needs and a rich understanding of their applications to improve the quality of their products and reduce their cost of test. In 2010, Keithley Instruments joined Tektronix as part of its test and measurement portfolio.
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