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Regulating multiple loops in power supply

Posted: 10 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:power supply? LED drivers? voltage? battery chargers? boost converter?

This circuit has the same advantages and disadvantages of the first case. Of particular note, because this is a boost converter, the compensation can be especially tricky with multiple loops. Also in this case since the current is not referenced to GND, we need to add an additional current sense amplifier. Figure 3 shows how accurately this circuit can regulate the output voltage and the input current. The output voltage is set to regulate at 9V from an USB input. Four curves are shown for different input current regulation settings (500 mA, 1.5A, 1.8A and 3A).

The final example is a circuit that limits the input current during startup and then regulates the output voltage. This type of circuit can be very useful when it is necessary to charge a large output bank of capacitors. The previous two circuits used multiple external amplifiers to regulate the current and voltage loops. Most controllers include an integrated voltage loop amplifier that can still be utilized. Figure 4 shows an example of how you could use the built in error amplifier and the output of the compensator to reduce the number of external parts necessary. The basic operation is that current loop pulls down the output of the voltage amplifier when it is active. When the current loop is not active, its output goes high and it doesn't affect normal operation. There still is the drawback of needing an external reference.

Figure 3: Input current and output voltage are both tightly regulated.

Figure 4: Buck converter to charge super caps.

Reference
John Betten, LED buck regulator with current-mode control simplifies compensation

About the author
Robert Taylor is an applications manager of power design services and a member group technical staff at Texas Instruments. Taylor received his MSEE and BSEE from the University of Florida, Gainesville.


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