As mentioned before, different load conditions can have an impact on the ability of a programmable DC power supply to perform as expected. You should exercise caution when sourcing power to capacitive, inductive, and reverse-current loads. Improper use can cause ringing in your output signal or damage your power supply. Ringing in a power supply signal is an undesired oscillation of the output voltage as the power supply attempts to recover from a transient caused by a sudden change in current. Ringing affects a system’s ability to stabilize, which increases measurement time and, if the oscillation peaks are high enough, can even damage connected circuitry. Below, you can find general guidelines for different load conditions; however, when in doubt, reference your power supply documentation for more information.
Figure 1. Transient response can affect measurement time and accuracy if unstable or too slow.
Generally, a power supply remains stable when driving a capacitive load, but certain loads can cause ringing in the transient response of the device. The slew rate of a power supply is the maximum rate of change of the output voltage as a function of time, which is directly related to transient response. When using a power supply to drive a capacitor, the slew rate is limited to the output current limit divided by the total load capacitance, as shown below.
Using the slew rate formula, you can see that the larger the load capacitance, the slower the change in output voltage. If the transient response is too slow, then measurement time could be negatively affected as you must wait for the system to stabilize before taking accurate measurements. However, if the slew rate is too high, then ringing can occur. Furthermore, capacitors are commonly used to dampen ringing in other load conditions.
A power supply typically remains stable when driving an inductive load in constant voltage mode. If an inductive load is driven by a power supply operating in constant current mode, specifically in higher current ranges, the power supply can become unstable. In these situations, increasing output capacitance may help improve the stability of the system.
Some power supplies have a user-programmable output capacitance option, which gives you the ability to choose a higher capacitance setting to reduce the chance of ringing. Alternatively, you can provide an external capacitance parallel to your load, which dampens ringing. Typical capacitor values used to reduce ringing when driving an inductive load are 0.1-10 µF. However, as described in the previous section, the larger the capacitance, the slower the output response. Therefore, you should use the minimum capacitance required to reduce the effects of ringing. Typically, you want the output voltage to recover from transients as fast as possible to limit the time that your circuit is receiving undesired voltage levels. The quicker your system returns to a stable output level, the quicker you can take your measurements, which results in a shorter overall test time. Refer to your power supply documentation for more information.
Reverse Current Loads
Occasionally, an active load may pass a reverse current to the power supply. Power supplies not designed for four-quadrant operation may become damaged if reverse currents are applied to their output terminals. Reverse currents can cause your power supply to move into an unregulated mode. To avoid reverse currents, you can use a bleed-off load to preload the output of the device. Ideally, a bleed-off load should draw the same amount of current from the device that an active load may pass to the power supply.
Figure 2. Use a bleed-off load to protect your power supply from the damage that reverse currents can cause.
For example, suppose your power supply is operating in constant voltage mode, supplying 10 V to an active load that can produce a 30 mA reverse current. In this case, a parallel resistor serves as a bleed-off load to preload the power supply output. The value of the bleed-off resistor should be such that the current flowing out of the power supply output is greater than, or equal to, the reverse current produced by your active load. Dividing 10 V by 30 mA suggests using a preload resistance of 333 Ω, effectively matching the reverse current and preventing damage to the power supply.
Choosing Hardware with Overload Protection
As described in the previous sections, it’s important to understand the load conditions of your equipment and testing environment, but you can also choose hardware that will help to protect your investment should something happen. NI PXI Programmable Power Supplies include features such as channel output protection, auxiliary power input protect, and overtemperature protection.