Some differences between constant current (CC) and constant resistance (CR) loading on your DUT’s performance

Most electronic loads provide constant current (CC), constant resistance (CR) and constant voltage (CV) loading. Some also offer constant power (CP) loading as well. The primary reason for this is this gives the test engineer a choice of loading that best addresses the loading requirement for the DUT, which invariably is some kind of power source.

Most usually the device should be tested with a load that reflects what the loading is like for its end use. In the most common case of a device being predominantly a voltage source the most common loading choices are either CC or CR loading, which we will look at in more detail here. Some feel they can be used interchangeably when testing a voltage source. To some extent this is true but in some cases only one or the other should be used as they can impact the DUT’s performance quite differently.

Let’s first consider static performance. In Figure 1 we have the output characteristics of an ideal voltage source with zero output resistance (a regulated power supply, for example) and a non-ideal voltage source having series output resistance (a battery, for example).  Both have the same open circuit (no load) voltage. Superimposed on these two source output characteristics are two load lines; one for CC and one for CR. As can be seen they are set to draw the same amount of current for the ideal voltage source. However, for the non-ideal voltage source, while the CC load still continues to draw the same amount of current in spite of the voltage drop, not surprisingly the CR load draws less current due to its voltage-dependent nature.

Figure 1: CC and CR loading of ideal and non-ideal voltage sources

CC loading is frequently used for static power supply tests for a key reason. Power supplies are usually specified to have certain output voltage accuracy for a fixed level of current. Using CC loading assures the loading condition is met, regardless of power supply’s output voltage being low or high, or in or out of spec. Non-ideal voltage sources, like batteries, present a little more of a problem and are often specified for both CC and CR loading as a result, to reflect the nature of the loading they may be subjected to in their end use. Due to a battery's load-dependent output voltage, trying to use one type of loading in place the other becomes an iterative process of checking and adjusting loading until the acceptable operating point is established.

Let’s now consider dynamic performance.  CC loading generally has a greater impact on a power supply’s ability to turn on as well as its transient performance and stability, in comparison to CR loading. When the power supply first starts up its output voltage is at zero. A CR load would demand zero current at start up. In comparison a CC load still demands full current. Some power supplies will not start up properly under CC loading. With regard to transient response and stability, CR loading provides a damping action, increasing current demand when the transient voltage increases and decreases demand when the transient voltage decreases, because the current demand is voltage dependent. CC loading does not do this, which can negatively influence transient response and stability somewhat. Whether CC or CR loading is used depends on what the power supply’s specifications call out for the test conditions. Batteries have some dynamic considerations as well. Their output response can be modeled as a series of time constants spanning a wide range of time. This presents somewhat of a moving target for an algorithm that uses an iterative approach to settling on an acceptable operating point.

This is just a couple of examples of how a load’s characteristic affects the performance of the device it is loading, and why electronic loads have multiple operating modes to select from, and worth giving thought next time towards how your device is affected by its loading!

Safeguarding your power-sensitive DUTs from an over power condition

Today’s system DC power supplies incorporate quite a variety of features to protect both the device under test (DUT) as well as the power supply itself from damage due to a fault condition or setting mishap. Over voltage protect (OVP) and over current protect (OCP) are two core protection features that are found on most all system DC power supplies to help protect against power-related damage.

OVP helps assure the DUT is protected against power-related damage in the event voltage rises above an acceptable range of operation. As over voltage damage is almost instantaneous the OVP level is set at reasonable margin below this level to be effective, yet is suitably higher than maximum expected DUT operating voltage so that any transient voltages do not cause false tripping. Causes of OV conditions are often external to the DUT.

OCP helps assure the DUT is protected against power-related damage in the event it fails in some fashion causing excess current, such as an internal short or some other type of failure. The DUT can also draw excess current from consuming excess power due to overloading or internal problem causing inefficient operation and excessive internal power dissipation.

OVP and OCP are depicted in Figure 1 below for an example DUT that operates at a set voltage level of 48V, within a few percent, and uses about 450W of power. In this case the OVP and OCP levels are set at about 10% higher to safeguard the DUT.

Figure 1: OVP and OCP settings to safeguard an example DUT

However, not all DUTs operate over as limited a range as depicted in Figure 1. Consider for example many, if not most all DC to DC converters operate over a wide range of voltage while using relatively constant power. Similarly many devices incorporate DC to DC converters to give them an extended range of input voltage operation. To illustrate with an example, consider a DC to DC converter that operates from 24 to 48 volts and runs at 225W is shown in Figure 2. DC to DC converters operate very efficiency so they dissipate a small amount of power and the rest is transferred to the load. If there is a problem with the DC to DC converter causing it to run inefficiently it could be quickly damaged due to overheating. While the fixed OCP level depicted here will also adequately protect it for over power at 24 volts, as can be seen it does not work well to protect the DUT for over power at higher voltage levels.

Figure 2: Example DC to DC converter input V and I operating range

A preferable alternative would instead be to have an over power protection limit, as depicted in Figure 3. This would provide an adequate safeguard regardless of input voltage setting.

Figure 3: Example DC to DC converter input V and I operating range with over power protect

As an over power level setting is not a feature that is commonly found in system DC power supplies, this would then mean having to change the OCP level for each voltage setting change, which may not be convenient or desirable, or in some cases practical to do. However, in the Agilent N6900A and N7900A Advance Power System DC power supplies it is possible to continually sense the output power level in the configurable smart triggering system. This can in turn be used to create a logical expression to use the output power level to trigger an output protect shutdown. This is depicted in Figure 4, using the N7906A software utility to graphically configure this logical expression and then download it into the Advance Power System DC power supply. As the smart triggering system operates at hardware speeds within the instrument it is fast-responding, an important consideration for implementing protection mechanisms.

Figure 4: N7906A Software utility graphically configuring an over power protect shutdown

A glitch delay was also added to prevent false triggers due to temporary peaks of power being drawn by the DUT during transient events. While the output power level is being used here to trigger a fault shutdown it could have been just as easily used to trigger a variety of other actions as well.