The electronic load serves a function that is the opposite of a power supply. While a power supply provides energy to electronic devices, an electronic load absorbs or consumes power. Despite this functional difference, both devices often operate in similar modes—constant voltage (CV) or constant current (CC). In practice, the operating mode of the electronic load typically contrasts with that of the power supply. For example, a CV power supply usually requires a CC electronic load, and a CC power supply pairs well with a CV electronic load. Additionally, most electronic loads also support 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 the previous section on how DC power supplies achieve CV and CC output, as the underlying principles are quite similar. Whether it’s a power supply or an electronic load, the implementation of CC and CV modes follows comparable design logic.
Figure 1 shows a block diagram of the electronic load in CC mode. When operating in this mode, the load typically uses a voltage source. The current amplifier compares the voltage across a sense resistor with a reference voltage, adjusting the FET’s RDS to maintain the desired current level. Figure 2 illustrates the corresponding IV curve for CC mode, where the operating point is determined by the intersection of the voltage source and the set current.
In CV mode, the setup is similar, as shown in Figure 3. The key difference is that the voltage across the load, rather than the sense resistor, is used for feedback. This ensures that the voltage remains stable, allowing the FET to draw as much current as needed from the external source. A typical lithium battery is a good example of a CV source, and during charging, a constant current source is required. Figure 4 displays the IV curve for CV mode.
The CR mode is slightly more complex. As shown in Figure 5, it involves maintaining a fixed ratio between voltage and current (V/I = R), effectively simulating a resistive load. Both the voltage and current loops are compared against the sense resistor, ensuring that the equivalent resistance remains consistent. For instance, if the current is 1 A and the voltage is 5 V, the equivalent resistance would be 5 Ω.
Electronic loads in CR mode are commonly used to test power supplies that can operate in either CV or CC mode. They are also useful for simulating real-world resistive components in various applications. Figure 6 presents the IV curve for CR mode, demonstrating how the load behaves under these conditions.
From this explanation, it’s clear that electronic loads closely resemble power supplies in their CV and CC control mechanisms. However, many modern electronic loads also offer the ability to simulate pure resistive loads, making them versatile tools for testing and analysis.
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