Traditional converters with higher price are often used in power converter design. This article mainly shows how to achieve the same function with a cheaper three-terminal shunt regulator.
The popular multi-source TL 431 IC, developed as a three-terminal shunt regulator, offers designers a number of interesting application possibilities in addition to their original applications. The TL431 includes a precision voltage reference, an operational amplifier and a parallel transistor (Figure 1a). By connecting two resistors, RA and RB, in a typical regulator application, the shunt regulated output voltage can be set at the low end of the load resistor Rs (Figure 1b).
In today's power market, most designs are cost-driven. For example, some Asian power supply manufacturers use single-sided printed circuit boards to save every penny for their power products. This design example illustrates how a three-terminal shunt regulator can be used in a power converter design to replace a more expensive conventional op amp. The switching power supply uses the electrically isolated feedback portion of the PWM circuit (Figure 2). In the design of the elimination of the voltage amplifier, the shunt regulator can be used as an inexpensive op amp. The resistors RI and R set the DC output voltage of the power supply, while the optocoupler IC2 completes the electrical isolation function. Resistor R1 provides a bias for the optocoupler and TL431, IC1. Resistor R3 and Zener diode D1 establish a fixed bias to ensure that bias resistor R1 does not form a feedback path. Resistors R1 and R2 control the gain on the optocoupler. In most designs, the ratio of R2 to R1 is approximately 10:1.
Elements CP, CZ and RZ provide frequency compensation for the control loop. The optocoupler contains a high frequency pole, fp, in the frequency response, which is ignored in most optocoupler data sheets. You can use a network analyzer to determine the location of the high frequency pole or estimate its production to be around 10 kHz. The following equation describes the small signal transfer function of the compensation network:
It is worth noting that in some cases, a bypass capacitor is added to diode D1 to reduce output noise.
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