SUPPLEMENT POWER ELECTRONICS ESSENTIAL ELEMENTS OF MODERN DC
POWER SOURCE IMPLEMENTATION Howard Peat, Microlease & Giacomo Tuveri, Keysight Technologies explores some of the essential elements of modern DC power source implementation and the impact this is having on the latest designs
H
aving a good grasp of how measurement tools actually operate
can provide valuable insight into how test methods can be improved. Setups can subsequently be made simpler and more effective, plus test performance parameters can be significantly raised. This is true for any sphere of technology that requires the capturing of test data, but in this article we will look specifically at how it relates to modern power sources.
CORRECT PROGRAMMING OF POWER SUPPLIES FOR CC OR CV OPERATION A power supply’s output can operate in either constant voltage (CV) mode or constant current (CC) mode depending on its load resistance, voltage setting and current limit setting. In the majority of cases a power supply output operates in either CV or CC mode, but under some unusual circumstances it may be caused to go into an unregulated (UNR) mode. By understanding these different modes it will be much easier for the power supply to be correctly programmed. Power supplies will operate in CV mode as
long as the load does not need more current than the current limit setting allows. In accordance with Ohm’s law, to maintain a constant voltage while changing the load resistance will require an increase/decrease in the current. As long as the output current (Iout) = Vs/RL is less than the current limit setting, the power supply will regulate the output at the voltage setting. Should the load resistance decrease -
such as if a device under test (DUT) component fails - and the load resistance (RL) is less than RC (which is the ratio of the power supply voltage setting to the current limit setting), the power supply will regulate the current instead. Again, Ohm’s law dictates a change in voltage if the current stays constant at the current limit setting - leading to CC operation. If the supply is unable to regulate its
output voltage or output current, then UNR will ensue. Possible causes of UNR include an internal fault in the supply, the AC input line voltage being below the specified range, another source of power connected across the supply’s output, or the output transitioning between CV and CC (or vice versa).
S6 NOVEMBER 2016 | ELECTRONICS
EMPLOYMENT OF REMOTE SENSING A power supply’s lead connections would ideally have no resistance, but in reality their lead resistance increases with lead length and wire diameter, so when a supply delivers current through the wire it may decrease the voltage at the load. Power supplies are typically shipped with the sense leads connected locally at the output terminals.
Figure 1: Schematic Showing Power Source Output Characteristics
load, the internal feedback amplifier will see the voltage directly at the load, rather than at the output terminals. Since the control loop senses the voltage directly at the load, the supply will keep the load voltage constant, regardless of voltage drops already discussed.
USING THE POWER SUPPLY FOR DUT CURRENT MEASUREMENT Accurate DUT current measurements can be obtained via an ammeter, a current shunt, or the built-in read-back on the power supply. Each of these methods has advantages and disadvantages. The current read-back function can
Figure 2: Series Connection with Remote Sense
provide the accuracy needed for measurement while avoiding the difficulties associated with connecting current shunts. It means that connection equipment (relays, switches, etc.) can be kept to a minimum and measurements can be triggered to start with other power-related events. It is possible to connect multiple power
supply outputs in series to get more voltage, or connect outputs in parallel to get more current. When connecting outputs in series for higher voltage, the following precaution should be taken: 1. Never exceed the floating voltage rating of any of the outputs
2. The power supply outputs should not be subjected to reverse voltages
3. Only connect outputs with identical voltage/current ratings in series To ensure that engineers can get access to
However, for setups with long load leads, the voltage at the output terminals will not accurately represent the load voltage. Remote sensing can be used to compensate for this, thereby correcting for voltage drops. When connecting remote sense terminals to the
Figure 3: Parallel Connection with Remote Sense
both the superior technical expertise and the highly advanced test equipment required for the type of power supply analysis discussed in this article, Keysight and Microlease have partnered together. This collaborative effort is certain to prove highly beneficial to engineers, enabling them to carry out more effective test procedures. There are a broad variety of equipment sourcing options that can be chosen from, which allow engineers to find the best fit for their specific financial and logistical requirements. In addition, all of this is backed up by calibration and maintenance services.
Keysight Technologies
www.keysight.com T: 0800 026 0637
/ ELECTRONICS
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