Adjustable shunt regulator based control systems

Adjustable shunt regulators known as TL431 are extensively used to build the control systems in switched-mode converter applications. The resulting control system is typically a two-loop system, where one loop defines the low frequency behavior, and the other loop the higher frequency behavior, respectively. The shunt regulator is regarded to possess characteristics similar to an operational amplifier, even if it is basically a transconductance amplifier having current as an output signal instead of voltage as well as positive feedback instead of negative feedback. The small-signal response of a shunt regulator shows that it may be modeled as a voltage-controlled current source with a small transconductance gain and an equivalent capacitor as output impedance, which may be extracted from the open-loop frequency response by inspection. The variance in the parameters may be high depending on the manufacture, and operating point, i.e., cathode current, which is normally chosen to be close to 1 mA due to the specifications. The analysis shows, however, that the cathode current should be at least 5 mA for stable equivalent circuit parameters. If low cathode current is used, the control system may not be better than a Zener diode replacing the shunt regulator.