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How Electronic DC Loads Work

Electronic DC load

The wide world of testing equipment is something that is not always discussed in PCB design courses or online resources. If you’re designing a prototype power system, then eventually you will need to test its power output, stability, input/output range, and power conversion efficiency. The power output, conversion efficiency, and regulation capability at low load can all be tested with an electronic DC load. This device plugs into the output from your power system and it functions like a programmable power sink. Here’s how these systems work.

What is an Electronic DC Load?

When testing the capabilities of a power system, a load must be connected to the output. Rather than using precision resistors, an electronic DC load enables programmable DC power delivery and limiting in an electronic unit. An electronic DC load is a versatile instrument that can also be used to test batteries, solar panels, and any other components in power systems that are involved in power delivery.

These systems have a bench power supply form factor as shown above, and they typically connect to a wall outlet. AC versions of electronic loads are also available.

The key to using an electronic DC load is to select the correct operating mode for your particular application.

Constant Current (CC) Mode

In this mode, the unit adjusts its equivalent DC resistance in order to ensure a constant current is received at the input, regardless of the voltage output from the DUT. This would commonly be used for testing battery discharge currents, measuring current limiting from a power system or active protection circuit, or a power regulator running in constant current mode.

The current value set in constant current mode is essentially an input current limit. By adjusting the value of the target voltage and maintaining constant current at the inputs, the power delivered from the DUT can be measured; this would just be the product of the DUT’s output current and the voltage limit set of the DC load’s front panel.

Constant Voltage (CV) Mode

In constant voltage mode, the system is setting a maximum input voltage limit for the DUT, and the DUT is sourcing variable current into the unit. By adjusting the current setting in CV mode, the equivalent DC resistance also adjusts so that the DUT can only source that level of current. During the test, the DC load maintains a constant voltage across its terminals, irrespective of the current drawn.

This mode is useful for testing voltage regulators or power supplies that need to maintain a specific output voltage under varying load conditions. This is the most common form of testing with an electronic DC load.

Electronic DC load

Constant Resistance (CR) Mode

In this mode, the DC load simulates a fixed resistance value while the input power (voltage and current) are allowed to vary. In this mode, the DUT output is being adjusted by the user or automatically; the voltage can be set by the user and the current converges to the value determined by Ohm’s law, or vice versa. This mode is useful for simulating real-world resistive loads, like heaters or bulbs.

Constant Power (CP) Mode

The electronic DC load attempts to maintain a constant power draw in this mode. This mode is similar to CR mode in terms of how the voltage and current inputs adjust. By setting the power draw to a specific value, the DUT can be used with manual voltage adjustment, and the current will adjust to ensure that the power (P = VI) remains constant. This mode would be used to test devices like solar panels, where the output power might be of primary interest.

Transient Mode (or Dynamic Mode)

This mode allows the DC load to alternate between two preset levels at a rate defined by the user. This mode is used to simulate how a power supply can respond to sudden changes in loading, as well as how the DUT responds to these changes.

Short Circuit Mode

In this mode, the DC load simulates a short circuit condition by drawing a very high current. In other words, it provides a highly reliable simulation of a very low resistance load. This would be used to measure short circuit discharge rates, such as from a solar cell or battery. It can also be used to test short-circuit protection features or thermal protection features in a power supply.

In addition to these modes, modern electronic DC loads come with various features like over-voltage protection, over-current protection, over-temperature protection, and more. These features ensure safe operation and provide additional diagnostic tools when testing power supplies and other devices.

Comprehensive Testing

When testing the capabilities of a power system, an electronic DC load should be used in conjunction with other measurements. This will allow determination of stability, efficiency, and noise on a single bench. The other equipment needed for these measurements are an oscilloscope (monitoring output voltage) and a multimeter (monitoring input power). Using an input regulated power supply is also helpful for limiting the input voltage/current is helpful for preventing damage to the regulator as it nears short circuit conditions.

Electronic DC load

Power supply test setup with an electronic DC load and two precision multimeters.

After you test your power system with an electronic DC load, you can implement any design changes and verify these in simulation with the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of thermally sensitive chip and package designs.

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