Servo loop improves linear-regulator efficiency

-July 08, 2004

Linear regulators are easy to implement and have better noise and drift characteristics than switching approaches. Their largest disadvantage is inefficiency: excess energy dissipated as heat. Several well-known techniques are available to minimize the input-to-output voltage across a linear regulator. I had been looking for an inexpensive, easy-to-implement, and efficient preregulator to reduce the dropout voltage of a linear regulator. Closed-loop, self-oscillating preregulators built around a switching transistor, a comparator, and a filter are difficult to predict in terms of frequency. Thus, the power-mains input filter is also difficult to implement. The best option is a fixed-frequency preregulator combined with a linear, low-dropout regulator. The arrangement shown in Figure 1 fulfills all the requirements. The LM2576T-ADJ, IC1, switcher uses a 52-kHz fixed frequency. The LT1085, IC2, is a good choice for the linear regulator. The preregulator feedback loop uses an operational amplifier, IC3.

When the servo loop is closed, the feedback voltage for IC1 is:

EQUATION 1

If R3=R5 and R4=R6=kR3, Equation 1 becomes:

EQUATION 2

Equation 2 yields the relationship VA–VOUT=VDROPOUT=kVFB. You can set the dropout voltage according to the linear-regulator requirements. If you select an LT1085, maximum VDROPOUT is 1.5V; for the LM2576T, VFB=1.23V and if k=1.5, VDROPOUT=1.89V, slightly higher than the value in the data sheet. The dropout voltage is the same regardless of the output voltage and thus ensures reasonable efficiency. (The overall efficiency is greater than 56% for VOUT=5V at 3A and at least 72% for VOUT=30V at 3A.) The output voltage, VOUT, ranges from 0 to 30V, and VIN must be at least 5V greater than the maximum VOUT. IC3 has no special requirements, and IC2 may be any kind of linear regulator. C6 reduces the output ripple, and C2 filters some of the 52-kHz noise on the control line coming from IC3. The result is a simple, robust, and high-performance laboratory power supply that can supply 3A in a 0 to 30V output-voltage range, using only a small heat sink.

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