by Jon Dutra
Linear Technology has developed many new switcher-based battery charger ICs. Testing accuracy, regulation and efficiency in the lab with a battery load is inconvenient because the terminal voltage of a battery constantly changes as it is being charged. If much testing is to be done, a large supply of dead batteries will be needed, since one set of cells can quickly become overcharged. This article describes an active load circuit that can be used to simulate a battery in any state of charge. The battery simulator provides a constant-voltage load for a battery-charging circuit, independent of applied charging current. The simulator's impedance is less than 500mQ at all reasonable input frequencies. Best of all, the simulator can never be overcharged, allowing long-term testing and debugging of a charger system without the possibility of battery damage.
The simulator (Figure 236) uses an LT1211 high speed, single-supply op amp to drive the base of a high gain PNP transistor-stage active load. Power for the LT1211—a portion of the charging current—is supplied through a
diode so the op amp and reference can survive brief periods of zero charging current. The op amp is configured for a DC gain of four, so the voltage on its noninverting input is one fourth of the voltage that the load box is set to. With S1 open, the load-voltage adjust range will be from 10V to 20V, and with S1 closed it will be approximately 3.5V-10V. Low voltage operation could be improved by replacing the top LT1004-2.5 with an LT1004-1.2 and reducing R1, the reference bias resistor, to 1k. The 510Q and 1.1k resistors are required for high frequency stability; they suppress a 1MHz oscillation. The 1N5400 diode and 4-amp fuse protect the circuit from reverse voltages.
The battery simulator circuit has been tested "swallowing" currents from 30mA to 3A with the output voltage essentially unchanged. When simulating a battery, the voltage adjust can be increased until the charger thinks the battery is fully charged and reduces the current into the simulator. Conversely, as the voltage is adjusted down, the battery charger may think the battery is becoming discharged and increase the current into the simulator.
Figure 237 shows the circuit's capacity for current absorption at two voltages, 5V and 15V, from 50mA to 3 amps.
S1 CLOSED > 0 TO 10V RANGE S1 OPEN > 10V TO 20V RANGE ALL RESISTORS 5% UNLESS NOTED
* Q1 DISSIPATES MOST OF THE POWER, MOUNT ON AN ADEQUATE HEAT SINK
Figure 236. Schematic Diagram of the Battery Simulator
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