TL431 Shunt Regulator Circuits
by Lewis Loflin
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While the TL431 is an excellent shunt regulator, its current range is limited to 100mA. The addition of a PNP shunt transistor greatly increases power handling capability.
In simple terms the TL431A acts a temperature compensated variable/adjustable Zener diode. It can also act as a voltage reference. In addition to good temperature compensation and stability, the TL431A has a voltage range of 2.5 volts to 36 volts and a current range from 1mA to 100mA.
For basic information on the TL431 see Experiments with TL431A Shunt Regulator
Fig. 2 Basic TL431 shunt regulator circuit example 1.
The TL431 is an excellent shunt regulator, but its current range is limited to 100mA. The addition of a PNP shunt transistor greatly increases power handling capability.
The output voltage on the TL431 cathode voltage (TP1) is determined by 2.5 * (1 + R1/R2). The voltage difference between TP1 and Vout determined the current flow shunted though Q1 to ground.
The current from Rin is split between the TL431 (Ik), Q1 (Ice), RL (IRL). Rin should be carefully chosen as I will illustrate.
Fig. 3 Basic TL431 shunt regulator circuit example 2.
Fig. 2 reveals the problem created when Rin is too large. With a 30-ohm 10-watt resistor the maximum current this could supply with 10-volt Vout is 200mA. With the load consuming 167mA (IL) this leaves little current to regulate the output voltage.
A little heavier load or voltage drop in the input voltage would mean no voltage regulation from the TL431-Q1 combination. The voltage drop across the 330-ohm resistor controls the current through Q1.
Fig. 4 Basic TL431 shunt regulator circuit example 3.
Fig. 3 shows a correct value of Rin at 20-ohms. With Vout adjusted to 10-volts IRin (with 16V input) =~ 6 / 20 = 300mA available. The load IL is still 167mA while the remaining current is split between Ik and Q1.
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