Constant-Power Driver is Simple and Efficient

Apr 1, 2008 12:00 PM
By Alfredo H. Saab, Applications Engineering Manager, and Ahmad Ayar, Member of Technical Support, M

Actuators and sensor systems sometimes include a resistive load that requires a controllable, constant-power drive, regardless of the load's resistance value. If that value changes with operating conditions, and perhaps with its recent operating history as well, then a simple control and regulation of the applied voltage or current is not sufficient to assure the delivery of constant power.

The circuit in the figure provides constant power by exploiting the resistive properties of such loads. It delivers a chopped-dc drive of variable duty cycle, implemented with simplicity, low cost and high efficiency.

A combination current-sense amplifier and multiplier (MAX4211F) senses both the current through and voltage across the load, and then generates a voltage (P_OUT) that is proportional to the continuous product of those variables. In other words, that voltage is proportional to the load's instantaneous power.

One-half of a dual op amp (the upper half in the MAX4163) generates a pseudo-saw-tooth signal of constant frequency (about 300 Hz), which connects to the noninverting input of an auxiliary comparator in the MAX4211F. The other op amp (the lower half) serves as an error amplifier that averages the power signal and then compares it with the control reference, while amplifying the difference.

The error-amplifier output connects to the inverting input of the auxiliary comparator, which in turn generates a PWM output signal. This signal drives a p-channel MOSFET in series with the load.

Circuit applications are simplified because the load has one side to ground, and because the control input is a low-level dc signal (or a microcontroller-generated PWM control signal). The control circuitry demands about 1 mA, which is delivered by a 5-V power supply.

Power limits for the circuit are defined by the power source operating-voltage range, the maximum allowable peak load current under which it can operate and the load-resistance values. The power source voltage range is 8 V to 24 V, set by the MOSFET characteristics and the dynamic voltage range of the MAX4211F input (IN). (That range covers most dc supplies for industrial and instrumentation systems.) The peak load current is 4 A, fixed by the combined effect of the voltage dynamic range at the MAX4211F current-sense input and the chosen value of the current-sense resistor. In this example, that value is 25 mΩ.

Given a desired power level, the voltage and peak-current limits set boundaries for the load resistance in each case. The minimum allowable load resistance is given by the ratio of maximum-power voltage expected to the peak current limit (4 A). The maximum load power the circuit can regulate is approximately the square of the minimum-power voltage expected divided by the maximum load resistance expected.

Under any conditions, the maximum power loss in the circuit is 0.4 W in the sense resistor plus 0.8 W in the power MOSFET, for a total of 1.2 W. With the control voltage input (V_CONT) at 0.5 V, the circuit as shown delivers a regulated 10 W to a 6-Ω load, and is stable within ±1% as the supply voltage is swept from 8 V to 24 V.

The circuit can deliver a regulated 60 W ±0.2% to the same load with V_CONT = 3 V, but only through a supply-voltage range of 19 V to 24 V. From 22 V to 24 V and with V_CONT = 4 V, it delivers up to 80 W ±0.2%. With a supply voltage of 16 V and VCONT = 0.5 V, the output power remains at 10 W ±1% while the load varies from 4 Ω to 12 Ω. Most of the changes in power-regulation value caused by supply voltage or load changes are attributable to nonlinearity in the MAX4211F multiplier, and are within the error limits specified for that IC. This circuit delivers constant power to the load, within the limits described in the text.

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