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EETE MAY 2013

POWER COMPONENTS Fig. 2 : Normal-mode and common-mode noise currents Ensure sufficient isolation between channels of a multi-channel power supply If a DUT requires individual isolated power supply sections, then either a number of individual isolated supplies or a single multichannel output power supply will be required. If using a multichannel power supply, always ensure that the isolation between the power supply channels is greater than the isolation required between the DUT circuits. However, that’s not always easily determined just by reading a multi-channel power supply’s data sheet. Some power supplies don’t actually provide isolation between channels. However, Keithley’s Model 2220-30-1 dualchannel programmable DC power supply and the 2230-30-1 triple-channel programmable DC power supply shown in figure 3 have two and three fully isolated channels respectively. When the isolation between circuits in a DUT is critical, consider actually measuring the power supply’s isolation between its channels. Maximize output accuracy If tight control of voltage at the load is essential for research experimentation, device characterization or production testing, then a careful review of the power supply’s output accuracy and read-back specifications are important. However, that accuracy can be compromised if the supply is controlling the voltage at its output terminals. What’s needed is feedback control right at the DUT, which means the supply should include sense connections (remote sensing) that can be connected to the DUT where the power leads are connected. The sensing circuits measure the voltage at the DUT so that the supply can compensate for any voltage drop in the test leads – see figure 4. No matter how accurate the power supply output is, there’s no way to guarantee that the programmed output voltage is the same as the voltage at the DUT’s load. This is because a power supply with two source terminals regulates its voltage only at its output terminals. However, the voltage that is important to regulate is at the DUT load, not at the power supply’s output terminals. The power supply and the load are separated by lead wires that have a resistance (RLead) determined by the length of the lead, the conductivity of the conductor material, and the geometry of the conductor. The voltage at the load, without remote sensing, is: Fig. 3: Model 2220-30-1 dual-channel programmable DC power supply and the 2230-30-1 triple-channel programmable DC power supply. If the load requires high current, then ILoad is high and VLead can easily be a few tenths of a volt, especially if the power supply leads are long, as can be the case in an automated test rack. A voltage at the load could easily be 80mV to 160mV lower than the desired voltage (with 2A to 4A flowing through a five-foot length of 0.004Ω/foot, 16-gauge wire). The remote sensing technique solves the problem of voltage drop in the leads by extending the power supply’s feedback Fig. 4: Remote sensing overcomes voltage drops in the source leads to apply the programmed voltage to the DUT. loop to the input of the load. Two sense lines from the power supply are connected to the DUT power inputs. These sense leads are voltage measuring lines that connect to a high impedance voltage measuring circuit in the power supply. Given that the voltage measuring circuit is a high input impedance circuit, the voltage drop in the sense leads is negligible. The sense lead voltage measurement circuit becomes the feedback control loop for the power supply. The voltage at the load is fed back to the power supply by the sense leads. 26 Electronic Engineering Times Europe May 2013 www.electronics-eetimes.com


EETE MAY 2013
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