EDN -- 04.11.96 Load simulator maintains constant power

Design Ideas: April 11, 1996

Load simulator maintains constant power

Khy Vijeh,
Analog Devices, Santa Clara, CA

The circuits in Figures 1 and 2 show how you can emulate a constant-power load for cases when a simple load resistor is unacceptable. For example, using a simple resistor as a load is unacceptable for applications that require you to accurately monitor generated or consumed power. One such application is in battery-energy measurement. You measure battery energy density (watts/hour) by measuring the total output power (W=V×I) over time. This task is relatively simple if you use an automated system to take frequent and periodic measurements of the discharge current and battery voltage while keeping track of time.

However, monitoring discharge time using a simple resistor or a constant-current sink as a load yields an inaccurate result, particularly when you want to compare the performance of different battery chemistries. The various battery types have many discharge-voltage characteristics, and power output varies as the output voltage changes. For example, NiCd batteries have a relatively flat discharge curve after the initial voltage drop, whereas lithium-ion batteries exhibit larger voltage variations during a normal discharge cycle.

To maintain constant power, adjust the discharge current to compensate for battery-voltage variation during discharge. As the battery voltage drops, load current must increase to maintain a constant V×I.

The simplified circuit in Figure 1 uses an analog multiplier to produce a voltage proportional to the product of input voltage and current. The X₁ and X₂ inputs monitor the battery voltage; the Y₁ and Y₂ inputs monitor the discharge current as a voltage across R_SENSE. You can select the value of R_SENSE for maximum accuracy for a given range of discharge current. Output voltage is directly proportional to the battery's instantaneous power output; 1V is equivalent to 1W.

The circuit in Figure 2a adjusts the discharge current so that the instantaneous power that the battery draws equals the power level set by the control pin, where 1V=1W. (Figure 2b is the corresponding block diagram.) Q₁ should have appropriate power-dissipation capability; you can substitute a Darlington transistor if higher power is necessary. However, when using Darlington transistors, allow for the higher V_SAT of the Darlington. The error amplifier must have enough output current to drive the base of the pass transistor. The OP-50 is a good choice for high-current applications. (DI #1856)