Simple switchers make simple pre-regulators
Linear regulators are used extensively because of their simplicity, low cost, quiet operation, and clean outputs. In applications where the output voltage must be adjusted over a wide range, however, they can dissipate considerable power and require large heat sinks. In these cases, adding a switching pre-regulator can keep the power dissipation down and eliminate the need for bulky heat dissipators.
Building your own switching preregulator from scratch can be a daunting task, and is generally unnecessary because of the availability of simple, widely adjustable switching regulators like those in National Semiconductor’s Simple Switcher line.
Looking at Figure 1, the rightmost outlined block contains a basic (linear) 0- to 25-V at 3-A bench supply circuit. Without the pre-regulator, the drain of the MOSFET (Q1) would typically be connected directly to +33 V. And, with this circuit operating at 3-A output current into a very low-resistance load, Q1 might have to dissipate as much as 99 W. That requires a big heat sink, perhaps externally mounted with electrical insulators to prevent shock. Packaging is difficult because Q1 may be off-board, or the large heat sink may have to be part of the chassis.
Why manage all that wasted power? By adding the LM2576 switching pre-regulator and its associated control to supply power to the MOSFET, the wasted power can be reduced dramatically (Fig. 1, again). The LM2576 switcher output voltage (SOUT) tracks the MOSFET output voltage (REGOUT) and maintains a 4-V differential across the MOSFET for all load conditions. The maximum power dissipated by the MOSFET is now reduced from 99 Watts to 12 Watts. This reduced power can easily be handled by on-board heat sinks with little or no air exchange to the outside of the chassis. The trick is to get the switcher output to track a specified output-voltage profile.
In addition to controlling the output voltage of single-chip switchers by adjusting the values of feedback resistors R1 and R2 (with point A grounded), the output voltage can be adjusted by using fixed resistors and adjusting the voltage at point A (Fig. 2).
The equation for the LM2575/76 is:
VA = 1.23 − R1 x (SOUT − 1.23)/R2
where VA is the voltage at point A, and SOUT is the switcher output voltage. Because this is a linear equation, finding two points that satisfy the equation will define the line. Referring to Figure 1, to maintain 4 V across Q1 for all REGOUT voltages in the 0-25 V range, the SOUT range must be 4-29 V (4 V above the output voltage). Determining the first point is easy since the maximum SOUT is achieved when VA = 0. Substituting, and choosing an appropriate value for R1:
0 = 1.23 − 1.2k x (29 − 1.23)/R2
solving, R2 = 27.1k (arbitrarily set R1 = 1.2k).
Now use the resistor values to determine the value of VA necessary to set the switcher output to 4 V:
VA = 1.23 − 1.2k x (4 −1.23)/27.1k
solving, VA = 1.11 V.
The pre-regulator tracking control circuit (outlined in Figure 1) provides the necessary inversions and scaling so that as the output control voltage goes from 0 to 2.5 V (causing REGOUT to go from 0 to 25 V), VA goes from 1.11 V to 0.0 V. This causes SOUT to go from 4 V to 29 V. Notice the bypass capacitor at point A, which maintains a good ac ground to keep the switcher stable.
To add current limiting to Figure 1, all that’s required is sensing of the output current, and then using it to modify the output control voltage. If the supply goes into current limiting and the control voltage is reduced, SOUT tracks it, and the 4-V differential across the MOSFET is maintained.
As further protection, the input voltage to the pre-regulator tracking control (VP at the far left of Figure 1) could be taken from the negative input of U2A instead of the positive input. Tracking will be the same, however, even if the U2A linear regulator fails to reach the commanded output voltage causing it to be unbalanced—the 4-V differential across the MOSFET will be maintained, and its power dissipation limited.