Switch-Mode Power Supplies---SPICE Simulations and Practical Designs
Christophe P. Basso
5/10/2008 1:45 PM EDT
The following text, which focuses on feedback and control loops, is excerpted from Chapter 3 of the book Switch-Mode Power Supplies—SPICE Simulations and Practical Designs, by Christophe P. Basso. Reprinted with permission from McGraw-Hill, copyright 2008. McGraw-Hill offers our readers a 20 percent discount on this book. Click here for more information.—VJB
For nearly 100 percent of the applications, a switch-mode converter delivers a parameter—a voltage or a current—whose value must remain constant, independent of various operating conditions, such as the input voltage, the output loading, the ambient temperature. To perform such a task, a portion of the circuit must be insensitive to any of the above variations. This portion is called the
reference, usually a voltage source,
Vref, which is precise and well stable over temperature. A fraction (α) of the converter output variable (for instance, the output voltage
Vout) is permanently compared to this reference. Thanks to a loop that feeds the information back to it, hence the term
feedback loop, the controller strives to maintain the theoretical equality between these two levels (Eq. 3-1):
