If your work has been primarily with more basic DC power sources, you will be familiar with them having an adjustable voltage limit, to set the voltage needed to power your device, and an adjustable current limit, to protect your device from too much power draw. You may not realize that some more advanced DC power sources may feature having a selectable operating mode priority to optimize them for either voltage or current operation. Having selectable operating priority modes makes them better suited for a wider variety of different applications. Here we will explore what DC power source priority modes are about, what their characteristics are like, and how priority modes make a DC power source better suited for a particular application.
The I-V output characteristics typical of many adjustable single-quadrant DC power sources is depicted in Figure 1. It features a voltage limit setting that can be typically adjusted from zero to up to the DC power source’s maximum output voltage rating, and a current limit setting that can be typically adjusted from zero to up to the maximum output current rating. Figure 1 is from the Keysight N6700C Low-Profile Modular Power System User’s Guide. The N6700C is a 400W, 4 slot mainframe that accommodates a variety of basic and specialized high performance DC power modules. The N6700C, along with its companion N6702C 1,200W, 4 slot mainframe, are extremely versatile for a variety of test system applications. More information can be found by clicking on the following hyperlinks: N6700C and N6702C. The N6700C is pictured in Figure 2.
Figure 1: Single quadrant DC source I-V characteristics
Figure 2: Keysight N6700C Modular Power System
Constant voltage mode is defined as an operating mode in which the dc source maintains its output voltage at the programmed voltage setting despite changes in load, AC line, or temperature. In comparison, constant current mode is defined as an operating mode in which the dc source maintains its output current at the programmed current limit despite changes in load, AC line, or temperature.
While not usually explicitly called out, most single-quadrant DC power sources are optimized for constant voltage, or voltage priority, operation. This is because most devices require being powered by a voltage source. For example, the single-quadrant DC power source modules for the N6700C Modular Power System are optimized as voltage sources. Note that these power modules cannot be programmed to operate in a specific mode. At turn-on, the operating mode will be determined by the voltage setting, current setting, and the load resistance. As shown in Figure 1, the I-V characteristics for three resistive load lines have been superimposed on the DC power source’s I-V characteristics. Depending on DC power source voltage and current settings, and the load’s resistance:
- For RLOAD > VSET/ISET, DC power source operation is constant voltage.
- For RLOAD < VSET/ISET, DC power source operation is constant current.
- For RLOAD = VSET/ISET, DC power source operation is at mode crossover.
Some more advanced DC power sources feature selectable operating mode priority to let them be optimized for either constant voltage or constant current operation. While applicable to single-quadrant DC power sources, it is even more advantageous for multi-quadrant DC power sources. One key example is rechargeable cell and battery testing. As cells and batteries are predominantly voltage devices, the preferred operation of a DC source used for charging and discharging them is two-quadrant sourcing and sinking with current priority mode operation. Rechargeable batteries usually incorporate advanced battery management systems (BMSs) to manage the charging and discharging of the cells, to prevent them from being over-charged or over-discharged. A second key example is testing these BMS devices. This calls for more advanced DC sources to emulate the cells in the battery. The preferred operation in this application is two-quadrant sourcing and sinking current with voltage priority operation instead.
Characteristics of voltage priority operation may include:
- Output optimized for low impedance typical of a voltage source.
- Voltage control bandwidth optimized for fast voltage transient response and stability.
- Default presets are typically zero volts and maximum current limits to assure start up in constant voltage mode.
- Minimizes voltage transients at turn on and turn off.
- For multi-quadrant operation it will source or sink current as needed to regulate the output at the voltage setting.
- A constant-voltage status flag indicates the unit is in regulation.
- Current limit flags indicate a current limit was reached and the unit is no longer in regulation.
In comparison, characteristics of current priority operation may include:
- Output optimized for high impedance typical of a current source.
- Current control bandwidth optimized for fast current transient response and stability.
- Default presets are typically zero current and maximum voltage limits to assure start up in constant current mode.
- Minimizes current transients at turn on and turn off.
- For multi-quadrant operation it will increase or decrease voltage as needed to regulate the output at the current setting.
- A constant-current status flag indicates the unit is in regulation.
- Voltage limit flags indicating a voltage limit was reached and the unit is no longer in regulation.
The Keysight N678xA series SMU modules for the N6700C/N6702C mainframes are good examples of multi-quadrant DC power sources that have selectable operating priority modes. The Keysight N678xA series are available as either 2-quadrant (+ voltage, +/- current) or 4-quadrant ((+/- voltage, +/- current) DC power sources, making them well suited for a wide range of applications, such as the two that were discussed previously. An N678xA series SMU module is shown in Figure 3.
Figure 3: Keysight N678xA series SMU module
First, for voltage priority mode operation, the output voltage should be programmed to the desired positive (2 or 4-quadrant) or negative (4-quadrant only) value. A positive current limit value should also be set. The current limit should always be set higher than the actual output current requirement of the external load. With tracking enabled, the negative current limit tracks the positive current limit setting. With tracking disabled, you can independently set different values for the positive and negative current limits.
Figure 4 shows the voltage priority operating locus of these SMU modules. The area in the white quadrants (1 and 3) shows the output as a source (sourcing power). The area in the shaded quadrants (2 and 4) shows the output as a load (sinking power).
Figure 4: Keysight N678xA SMU module voltage priority operation
The heavy solid line illustrates the locus of possible operating points as a function of the output load. As shown by the horizontal portion of the line, the output voltage remains regulated at its programmed setting as long as the load current remains within the positive or negative current limit settings. A CV (constant voltage) status flag indicates that the output current is within the limit settings.
When the output current reaches either the positive or negative current limit, the unit no longer operates in constant voltage mode and the output voltage is no longer held constant. Instead, the power supply will now regulate the output current at its current limit setting. Either a LIM+ (positive current limit), or LIM− (negative current limit) status flag is set to indicate that a current limit has been reached.
As shown by the vertical portions of the I-V characteristic line, when the unit is sinking power, the output voltage may continue to increase in the positive or negative direction as more current is forced into the unit. When the output voltage exceeds either the positive or negative over-voltage setting, the output will shut down, the output relays will open, and either the OV or OV- and the PROT status bits will be set. Either the user-defined over-voltage setting, or the local over-voltage function can trip the over-voltage protection.
In comparison, when current priority mode operation is set, the output current should be programmed to the desired positive or negative value. A positive voltage limit value should also be set. The voltage limit should always be set higher than the actual output voltage requirement of the external load. For 4-quadrant models, with tracking enabled, the negative voltage limit tracks the positive voltage limit setting. With tracking disabled, you can again independently set different values for the positive and negative voltage limits. For 2-quadrant models, zero volts is the inherent lower voltage limit for current priority mode operation.
Figure 5 shows the current priority operating locus of the SMU power modules. The area in the white quadrants (1 and 3) shows the output as a source (sourcing power). The area in the shaded quadrants (2 and 4) shows the output as a load (sinking power).
Figure 5: Keysight N678xA SMU module current priority operation
The heavy solid line illustrates the locus of possible operating points as a function of the output load. As shown by the vertical portion of the I-V line, the output current remains regulated at its programmed setting as long as the output voltage remains within the positive or negative voltage limit settings. A CC (constant current) status flag indicates when the output voltage is within the limit settings.
If the output voltage reaches either the positive or negative voltage limit, the unit no longer operates in constant current mode and the output current is no longer held constant. Instead, the power supply will now regulate the output voltage at its voltage limit setting. Either a LIM+ (positive voltage limit) or LIM− (negative voltage limit) status flag is set to indicate that either the positive or negative voltage limit has been reached.
As shown by the horizontal portions of the load line, when the unit is sinking power, the output current may continue to increase in the positive or negative direction as more current is forced into the unit. Once the current exceeds the overcurrent rating the output will shut down, the output relays will open, and the over current (OC) and PROT status bits will be set.
Not all DC power sources are the same! The N678xA series SMU modules are one example of how voltage and current priority operation are implemented. Other DC power sources will likely implement voltage and/or current priority operation, and multi-quadrant operation differently. So, it is important to review the DC source’s operation to understand how to optimize it for performance in your application and not cause surprises.
In Closing:
If working primarily with basic DC power sources featuring only a programmable voltage limit, to set the operating voltage for the device to be powered, and a programmable current limit, to set it to a safe level to protect the device, one may not realize more advanced DC power sources may feature priority modes that will better optimize them for specific applications. Or that some more advanced DC power sources may feature programmable operating priority modes, allowing them to be optimized for an even greater range of applications. This becomes even more important with DC power sources featuring multi-quadrant operation, such as that used for testing rechargeable cells and batteries.
Here we covered what DC power source priority modes are about, provided examples of what their characteristics are like, and how different priority modes make a DC power source better suited for a particular application, such as that for charging and discharging cells and batteries.
In the future, look for me to provide additional details and examples of different priority operating modes and how their I-V characteristics interact with specific devices, such as cells and batteries, leading to the characteristic performance over time that you may be more accustomed to seeing!
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