The electronic load functions in the opposite way to a power supply. While a power supply provides energy to electronic devices, an electronic load absorbs or consumes it. Despite this functional difference, both devices often operate in similar modes—constant voltage (CV) or constant current (CC). In practice, their operating modes are typically inverse: a CV power source is usually paired with a CC electronic load, and a CC power source is matched with a CV electronic load. Most electronic loads also include a constant resistance (CR) mode, which allows them to simulate real-world resistive characteristics.
This article explores how electronic loads can operate in CV, CC, or CR modes. It’s recommended to review previous content on how DC power supplies achieve CV and CC outputs, as the underlying principles are quite similar. Whether it's a power supply or an electronic load, the mechanisms for maintaining constant voltage or current are fundamentally alike.
Figure 1 shows a block diagram of the CC mode in an electronic load. In this mode, the device typically uses a voltage source. A current amplifier compares the voltage across a sense resistor with a reference value, adjusting the FET’s RDS to maintain the desired current. The IV curve in CC mode, shown in Figure 2, illustrates the operating point where the set current intersects with the voltage from the source.
Figure 3 demonstrates the CV mode, which is similar to CC but with a key difference: instead of monitoring the voltage across the sense resistor, the system tracks the voltage across the load. This keeps the voltage stable, allowing the FET to draw as much current as possible from the external source. For example, a lithium battery acts as a CV source, and during charging, a constant current source is required. Figure 4 shows the corresponding IV curve in CV mode.
To implement the CR mode, both CV and CC controls are used simultaneously. The ratio of voltage to current (V/I = R) is maintained by comparing the sensed voltage and current. For instance, if the current is 1V/A and the voltage is 0.2V/V, the equivalent resistance is 5Ω. This is illustrated in Figure 5.
Electronic loads in CR mode are commonly used to simulate real-world resistive loads, especially when testing power supplies that can operate in either CV or CC mode. Figure 6 displays the IV curve under CR conditions.
In summary, while electronic loads may seem different from power supplies, their control mechanisms in CV and CC modes are very similar. Many modern loads also support CR mode, making them versatile tools for various testing applications.
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