March 14, 1997

Controller provides constant-power dc load

John Wettroth, Maxim Integrated Products, Sunnyvale, CA

Circuits that draw constant power regardless of input voltage usually rely on voltage multipliers or other exotic analog functions. The constant-power load in Figure 1, though, combines a resistive load with a simple switching power supply. The load relies on the relatively flat characteristic of efficiency vs input power for the IC controller. Power in the output load is constant (using a regulated voltage across a resistor). The power-supply efficiency is also relatively constant, so the input power remains almost constant with changes in input voltage (Figure 2). The circuit in Figure 1 is a standard application for the MAX797--a PWM step-down controller with a synchronous rectifier. The IC provides a regulated 5V at 6A maximum from an input of 6 to 30V.

As the table in Figure 1 shows, the circuit's constant-power level is a function of the load-resistor value. Operation depends on low variations in conversion efficiency rather than on high absolute efficiency, though both parameters are inherently good (Figure 3). In general, all switching supplies draw a relatively constant power vs input voltage, but the circuit in Figure 1 takes the constancy a step further. Line regulation is just 0.06% output change per input-voltage variation, the efficiency is high and varies little with input voltage, and the variation of quiescent current with input voltage is minimal.

An input step change from 10 to 16V produces an output disturbance that settles within 50 mV in an approximately 20-µsec ac-transient response time. The component values shown accommodate high power levels and a range of input voltages. You can obtain better performance by restricting the input range. The data sheet for the MAX797 gives design equations that govern other combinations of input and output voltages. (DI #1991)

| EDN Access | Feedback | Table of Contents |