Many semiconductor companies make threeterminal shunt regulators. These devices have an internal precision reference, operational
amplifier and an internal shunt transistor to control a precision voltage source. The typical circuit application is featured in
Figure 1.
The three-terminal shunt regulators are inexpensive semiconductor devices that have other useful power supply design applications other than a shunt regulator. The semiconductor devices can be used as inexpensive operational amplifiers for control loop feedback. The device can be used in conjunction with a transistor and passive components, the device can be used for fast boot-strap circuits. These devices can also be configured to work as a low power auxiliary supply to power the pulse width modulator (PWM) controller under light load operations. These circuits will not be found in the shunt regulator’s data sheet but are useful applications.
Operational amplifier
In power supply designs that contain PWMs without a voltage amplifier, a system designer can use a shunt regulator as a cheap
operational amplifier. Figure 2 contains a functional block diagram of this application. The small signal transfer function for this compensation network can be explained with Equation 1.
An optocoupler can be added to the circuit to provide some galvanic isolation. Figure 3 shows the schematic of the isolated feedback circuitry. Resistor R1 is used to bias the optocoupler and the TL431. Resistor R3 and diode D1 provide a fixed bias point to ensure that 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 10 to one. The optocoupler has a high frequency pole (fp). The optocoupler’s data sheet typically does not provide information on fp. Using a network analyzer, one can find the pole in many applications of approximately 10 kHz.
Boot-strap circuit
In switching power supply design, the pulse width modulator IC is typically powered by an auxiliary winding (Figure 4). To start this circuit requires a trickle chargeresistor (Rt) and a hold-up capacitor (Ch). To keep power consumption to a minimum, the trickle charge resistor is chosen to be as large as possible. The hold-up capacitor also needs to be large as well, because it supplies energy to the PWM until the power supply starts switching. The shunt regulator can be configured using a bipolar transistor and a few resistors to speedup the boot-strap time (Figure 5). Electrical components C, D1, Q1 and Ra through Rd form the boot-strap circuit. At power-up, capacitor C will be completely discharged and the voltage at the PWM’s power input (Vaux) will be determined by the series passregulator that is controlled through Q1 and D1. The Vaux voltage at turn “on” will be at its peak voltage (Vaux_peak) and is determined by theresistor ratio of Ra and Rb. Capacitor C and resistor Rc are used to determine the timing and turn “off” voltage of the boost-strap circuit toconserve energy. Resistor Rd supplies bias current to the TL431, while resistor Re limits the current to keep transistor Q1 in its safe operating area (SOA).
Setting up the circuit is not that difficult. Resistors Ra and Rb are selected to set the peakcharging voltage (Vaux_peak).
Resistor Rc is selected to lower the shunt voltage below the nominal Vaux voltage (Vaux_nominal) that is supplied by the auxiliary winding.
Capacitor C sets the boost strap time (Tboot).
Low power PWM bias supply
In some power supplies, the PWM is powered by an auxiliary winding similar to the circuit presented in Figure 4. The problem with this circuit is under light load operation there is not enough energy stored in the auxiliary winding to power the IC. The behavior of the power supply may even become erratic because the PWM will be turning on-and-off. The circuit presented in Figure 6 shows how to overcome this problem with a serie-pass regulator that turns “on” under light load conditions and turns “off” when the bias winding can supply the energy to the PWM controller.
Resistors Ra through Rd, Diodes D1, D2 and transistor Q1 form the low-power bias supply. This low- power bias supply is setup to regulate a voltage above the PWM’s turn-off voltage and below the nominal auxiliary winding voltage (Vaux_nominal). This allows for transistor Q1 to act as a diode OR circuit. When the PWM is powered by the auxiliary winding, the voltage at Vaux will be back-biased turning “off” transistor Q1 and conserving energy. When the voltage at Vaux drops due to lack of energy, Q1 will become forward-bias to deliver needed energy to the PWM controller.
Setting-up the low-pass series pass regulator is not difficult. Resistor Rc is sized to supply biascurrent to D1, resistor Rd is sized to keep transistor Q1 within its SOA, and resistors Ra and Rb are sized to regulate the voltage of the lowpower series pass regulator. The voltage provided by this low-power series pass regulator needs to be set at voltage above the control ICs turn “on” voltage and below the nominal voltage supplied by the auxiliary winding (Vaux_nominal).
The following equation is used to adjust the resistor divider formed by resistors Ra and Rb. The voltage set at the emitter of Q1 needs to be below the nominal auxiliary voltage (Vaux_nominal) supply by the secondary winding from transformer T1. Vref is the internal reference of shunt regulator D1. Vd2 and Vbeq1 are the voltage drops of diode d2 and the base emitter voltage of Q1 respectively.
Summary
The three-terminal shunt regulator, such as the TL431, is useful in many applications. These three-terminal devices are inexpensive and versatile. The regulators can be configured to perform many functions in switching power supplies. The devices can be used as a precision
reference and can function as an affordable operational amplifier to provide feedback control. The regulator can be used for quickboot
strapping of the power supply compared to conventional methods.
The shunt regulator in conjunction with a NPN transistor can provide a low-power bias supply, which turns “on” under light load conditions and turns “off” when the power from an auxiliary winding is sufficient enough to supply energy to the PWM.
Author Information
Michael O’Loughlin is a customer applications engineer at Texas Instruments, where he is responsible for PSCP (power-supply-control products) customer support. He holds a bachelor’s degree in electrical engineering from the University of Massachusetts (Lowell) and enjoys sailing and mountain biking.
