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Overcoming Electronic Load Low Voltage Limitations

Electronic loads provide greater flexibility than other methods to dissipate power (such as resistors) by allowing you to sink various levels of power profiles in multiple modes. Electronic loads enable you to emulate easily various scenarios and actual devices that connect to your power source. Using a fixed resistor to sink power makes it difficult to automate and emulate the dynamic behavior of a real device. Fixed resistors also lack flexibility to adapt to changes in test requirements. For these reasons, an electronic load is a more efficient solution to test power sources than using a fixed value resistor.

When using electronic loads, it is important to understand their limitations. Electronic load inputs consist of multiple power MOSFETs connected in parallel as shown in the following illustration.

A diagram showing several power MOSFETs placed in parallel.

Feedback loops within the electronic load’s circuitry control these MOSFETs to maintain whatever setting the user has specified. If the electronic load input voltage is high enough to keep the MOSFETs in saturation mode then the electronic load can sink full current and maintain its specified slew rates. However, if the electronic load input voltage decreases to the point where the MOSFETs are operating in their linear region then the electronic load’s current sinking capabilities diminish.

A plot of transistor drain current (Id) versus drain to source voltage (Vds) at different values of gate voltage (Vg). At high values of Vds the Id value is largely independent of Vds, but at low Vds values the current exhibits a linear dependence on Vds.

For this reason, all electronic loads provide a graph like that shown below to specify the current sinking capability of the electronic load at low voltages.

A plot showing the voltage versus current sinking capabilities of a typical electronic load at low values of input voltage. At low voltages (<2 V) the electronic load cannot sink full current and slew rate limitations apply.

Although this limitation is innate to all electronic loads, you can overcome this constraint using a boost power supply. A few years ago this technique usually specified the use of a “linear” boost power supply because linear supplies had lower noise than similarly rated switching power supplies. However, the noise performance of modern switching power supplies has improved to the point where their noise levels are comparable to linear power supplies.

The essence of this technique is easy to understand. Using the boost supply, you raise the ground reference point of the power source under test several volts above the ground reference point of the electronic load. This means that, from the point of view of the electronic load, even when the DUT is sourcing current near its zero volt reference the electronic load is still operating in a region where all its current sinking capabilities apply. The following illustration shows the connections necessary to implement this technique.

A diagram showing how using a boost supply, you can raise the ground reference point of the power source under test several volts above the ground reference point of the electronic load. This means that, from the point of view of the electronic load, even when the DUT is sourcing current near its zero volt reference the electronic load is still operating in a region where all its current sinking capabilities apply.

To operate correctly in constant voltage mode, the electronic load must have its remote voltage sense leads connected across the power supply under test. The auxiliary supply can be a low-cost fixed output 3 V to 5 V power supply, but it must have a current rating at least as high as the maximum peak load current needed. While this configuration can compensate for the load minimum voltage requirement and voltage drop in the power leads, it does have some caveats.

  1. Any current noise from the boost power supply will affect noise measurements made on the power source under test. You can minimize this by selecting a supply with suitably low noise specifications.
  2. The electronic load now must dissipate the power from both the power supply under test and the boost supply. Therefore, a higher power load may be necessary if the original load was operating at full rated power. For example, if you want to test a 300 Watt power source, then a 300 Watt load would not have enough capability to dissipate the power generated by both power sources. You need to select an electronic load capable of dissipating the power from both the power source under test and the boost power supply.
  3. The possibility exists that the boost supply could reverse bias the power source under test as the voltage across the load decreases. This can occur, for example, when the power source under test can no longer maintain its output voltage because (if it is a power supply) it is in overcurrent protection mode. To protect against potential reverse biasing of the power supply under test you might need to add a reverse protection circuit to protect the power supply under test when using an auxiliary boost power supply.

In conclusion, electronic loads have many advantages over other methods of sinking power (such as using fixed value resistors). However, electronic loads do have restrictions when operating at low voltages due to their use of power MOSFETs to sink current. You can overcome these limitations by using a suitable boost power supply and following the guidelines outlined in this blog post.