Howard Peat, Microlease, & Giacomo Tuveri, Keysight Technologies
Having 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 the following article we will look specifically at how it relates to modern power sources.
Correctly 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 (see Figure 1).
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Figure 1: Schematic Showing Power Source Output Characteristics
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).
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. 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 (see Figure 2). When connecting remote sense terminals to the 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.
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Figure 2: Series Connection with Remote Sense
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 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
Connecting Power Supply Outputs in Series/Parallel
Each power supply output should be set independently so that the voltages sum up to the total desired value. To do this, each output should be set to the maximum current limit the load can safely handle. The voltage of each output can then be set to sum to the total desired voltage (see Figure 3). When connecting outputs in parallel for higher current, it is critical that:
1. One output operates in CV and the other (or others) in CC
2. The output load draws enough current to keep the CC output(s) in this mode
3. Only connect in parallel outputs with identical voltage and current ratings
4. Set the current limit of all outputs equally
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Figure 3: Parallel Connection with Remote Sense
Minimizing Noise from the Power Supply to the DUT
If the DUT is sensitive to noise on its DC power input, everything possible should be done to minimize noise on the input. Since filtering noise from the power source can be difficult, a unit with very low noise should be selected to begin with. Linearly regulated supplies can accomplish this; however, they can be large and generate considerable heat. Modern switch mode supply technology has reached a stage where outputs are comparable with linear supplies (see Table 1).
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Table 1: Noise Comparison for Linearly Regulated & Switched Supplies
It is recommended to select a supply with low RMS and peak-to-peak output voltage noise specifications, but noise can be further combatted by addressing the lead connections to the DUT. These connections can be susceptible to noise pick-up via inductive coupling, capacitive coupling, RF interference, etc. The most effective way to reduce noise is by ensuring load and sense connections use shielded two-wire cables. In addition, balancing the cable impedance can be beneficial. In order to mitigate the effect of common-mode current, the impedance to ground from the output terminals on the supply should be equalized.
Safeguarding the DUT through Built-In Power Supply Protection Mechanisms
Most DC power supplies have features that protect sensitive DUTs and circuitry from exposure to potentially damaging voltages/currents. When the DUT trips a protection circuit in the power supply, this circuit turns off the output and displays a notification. The most common protection functions are over-voltage and over-current protection.
Most power supplies have an output voltage setting and a current limit setting. The current limit setting determines the value at which the power supply will prevent excessive current from flowing. This CC mode regulates the output current at the current limit but will not turn off the output. Instead, the voltage decreases below the voltage setting and the power supply continues to produce current at the current limit setting in CC mode.
Over-current protection shuts off the output to prevent excessive current flow to the DUT. When this protection is enabled, if the supply enters CC mode, a protection will trip and turn off the output. The current limit should be set low enough to protect the DUT, but high enough to prevent nuisance tripping due to normal fluctuations in the output current that occur during output transient conditions.
Employing Output Relays to Physically Disconnect the DUT
It may be assumed that the power supply output is completely open when set in an ‘output off’ state, this may not actually be the case. The output impedance will vary from model to model while in this setting and can depend upon the options installed in the power supply. The ‘output off’ state will typically set the output voltage/current to zero and disable internal power-generating circuitry.
However, this does not guarantee that no current will flow in/out of the DUT (as it would if the output terminals were physically disconnected from the DUT). Some power supplies have an internal output relay option for disconnection purposes, but even with this installed, certain models may still have output capacitors connected from the output terminals capable of effecting the DUT. For critical applications where complete disconnect between the power supply output and the DUT is mandated, check with the power supply vendor if the output relay provides complete disconnection. If not, then incorporation of external output disconnect relays will be required (see Figures 4 & 5).
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Figure 4: Power Supply with Internal Relays on Output Terminals - With Relays Open the DUT is Completely Disconnected
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Figure 5: Power Supply with Internal Relays Located Inboard of Some Output Components - With Relays These Components Remain Connected to the DUT
While most power supplies can measure DUT steady-state voltage/current, some units can also measure the dynamic voltage and current. These supplies will feature a built-in digitizer. When making a digitizing measurement, the following parameters can be set:
1. Time interval between samples
2. Number of samples acquired
3. Acquisition time
Once 2 of these parameters are set, the remaining parameter is determined via the following equation:
Acquisition Time = Time Interval x (Number of Samples -1)
In a similar manner, a power supply’s built-in digitizer can be configured to trigger and capture power supply output voltage or current waveforms. The digitizer will store a buffer of readings with the waveform data points. The data can be retrieved and analyzed using any standard software package. Using the power supply in place of a battery enables acquisition of dynamic information on the current flowing into the DUT. This in turn allows design adjustments to be made to optimize DUT power management.
Typically, power supplies are used to bias circuits that require a constant voltage. However, more advanced applications may call for a time-varying voltage (or current). Modern power supplies can easily manage both, using the list mode to address time-varying applications. It is normally possible to program a PC to change the power supply’s output voltages for discrete periods of time.
The program is thus able to control transitions between voltages so that the DUT can be tested at different voltages. List mode enables generation of such voltage sequences and their synchronization of internal/external signals without tying up computing resources. It is simply a matter of setting individually programmed steps of voltage (or current), along with an associated step duration.
To create a list, set the following:
1. One or more voltage or current steps
2. Dwell times
3. Repeat count (number of times the list will repeat)
To ensure that engineers can get access to 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.