More Than Just a DC Load
by Tom Lecklider, Senior Technical Editor
Electronic loads present a varied current path to ground. For a simple constant resistance (CR), the relationship between voltage and current is linear. However, other types of loads also exist, such as constant voltage (CV), constant current (CC), and constant power (CP). The most sophisticated loads have complex dynamic current profiles.
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Figure 1. Typical Electronic Load Single-Quadrant |
Electronic loads are used to simulate real loads and typically comprise an array of MOSFET power transistors, a heatsink, and control circuitry. All electronic loads are constrained to operate within a safe operating area (SOA) similar to that shown in Figure 1, bounded by maximum power, voltage, and current limits.
Corner 1 of the SOA characteristic is determined by the minimum voltage at which the load will operate. This depends on the load’s design but typically is a few tenths of a volt. In Figure 1, it’s shown as 0.1 V. Even with several large MOSFETs and heavy copper bus bars and terminals, a load will have a minimum resistance of at least a few milliohms. This resistance multiplied by the load’s maximum rated current defines corner 2 of the SOA.
Increasing the voltage at the maximum current eventually causes the maximum power limit to be reached at corner 3. If the current is reduced, the voltage simultaneously can be increased to maintain the same maximum power dissipation. This results in the slanted line in the upper part of Figure 1 and eventually hits the maximum voltage limit at corner 4.
Performance Details
Operation is allowed at any I-V combination within the SOA and can be controlled in real time. For example, according to Kepco, the EL Series Electronic Load can use test patterns from an arbitrary waveform generator via the external analog input.1
AMETEK Programmable Power’s recent catalog includes a very good introduction to electronic loads that highlights the dynamic characteristics of the Sorensen brand loads. According to the company, there are many applications that require either fast, random slew rates or an AC signal on top of the DC level. These are encountered when developing rechargeable battery controllers, fuel cells, and magnet control. These applications require both a high bandwidth electronic load and the capability to generate an arbitrary waveform.
The company’s SL Loads have a 20-kHz bandwidth full scale across all DC models and up to 40 to 50 kHz for small signal AC on a DC setpoint. These frequencies can be applied through analog control from either an arbitrary waveform generator or a signal generator depending upon the waveform desired. In fuel cell testing, a high-frequency, low-amplitude current can be drawn to perform electrochemical impedance spectroscopy for characterization of cell impedances.2
Wiring resistance and inductance both affect performance, resistance causing undesirable voltage drops and inductance potentially leading to control instability. Resistance is minimized by using the heaviest wire that is practical and keeping the leads as short as possible. Nevertheless, it’s difficult to avoid adding several milliohms to the load’s internal resistance value.
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