Hidden Applications of Three-Terminal Shunt Regulators

We can add an optocoupler to the circuit to achieve a degree of galvanic isolation (galvanic isolation). Figure 3 shows a schematic diagram of the isolated feedback circuit. Resistor R1 is used to bias the optocoupler and TL431. Resistor R3 and diode D1 provide a fixed bias to ensure that the bias resistor R1 does not form a feedback path. Resistors R1 and R2 are used to control the gain across the optocoupler. In most designs, the ratio of R2 to R1 is set roughly ten to one. Optocouplers have high pole frequencies (fp). Optocoupler data sheets generally do not provide information about the pole frequency. Using a network analyzer, we can find that the pole is about 10kHz in many applications.

In switching power supply designs, the PWM IC is usually powered by an auxiliary winding, as shown in Figure 4. Starting this circuit requires a continuous replenishment charge resistor (Rt) and a holding capacitor (Ch). To minimize power dissipation, we want to make the replenishment charge resistor as large as possible. The holding capacitor should also be large because it provides energy to the PWM before the power supply starts switching.

In some power supplies, the PWM is powered by an auxiliary winding similar to the circuit shown in Figure 4. The problem with this circuit is that under light load operation, the energy stored in the auxiliary winding is not enough to power the IC. The operation of the power supply even becomes difficult to predict because the PWM will be constantly switching on and off. The circuit shown in Figure 6 provides a solution to this problem by using a series bypass regulator that starts up under light load conditions and shuts down when the bias winding can power the PWM controller.

Ra through resistor Rd and diodes D1 and D2, together with transistor Q1, form a low-power bias power supply. The low-power bias power supply can adjust the voltage according to the setting so that it is higher than the turn-off voltage of PWM and lower than the rated voltage of the auxiliary winding (Vaux_nominal). This enables transistor Q1 to act as a diode or circuit. If PWM is powered by the auxiliary winding, the Vaux voltage will be reverse biased, turning off transistor Q1 and saving energy. If the Vaux voltage drops due to insufficient energy, Q1 will become forward biased to provide the necessary energy to the PWM controller.

Setting up a low-pass series bypass regulator is not difficult. Resistor Rc should be sized just enough to provide bias current to D1, resistor Rd should be sized just enough to keep transistor Q1 within its SOA, and resistors Ra and Rb should be sized to regulate the voltage of the low-power series bypass regulator. The voltage provided by this low-power series bypass regulator should be set higher than the start-up voltage of the control IC and lower than the nominal voltage provided by the auxiliary winding (Vaux_nominal).

The following equations are used to adjust the resistor divider formed by Ra and Rb. The voltage set at the emitter of Q1 should be lower than the nominal auxiliary voltage (Vaux_nominal) provided by the secondary winding of transformer T1. Vref is the internal reference voltage of the shunt regulator D1. Vd2 and Vbeq1 are the voltage drops of diode D2 and the base-emitter voltage of Q1, respectively.

A three-terminal shunt regulator like the TL431 is useful in many applications. This three-terminal device is inexpensive and versatile. This regulator can be configured to perform a variety of functions in a switching power supply. This device can be used as a precision reference or as an inexpensive op amp for feedback control. This regulator can also be used to quickly bootstrap a power supply, unlike traditional methods. A shunt regulator used with an NPN transistor can also be used to implement a low-power bias supply that starts up at light load conditions and shuts down when the auxiliary winding can provide enough power for the PWM.