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Sep 18 2020

TL431 Shunt Regulator: 8 Simple Circuits for Beginners

I Description

This blog will introduce 8 simple and easy to understand circuits that using TL431 as the main component. Such as TL431 Precision Reference Voltage Circuits, TL431 Adjustable Regulated Power Supply Circuit, etc.

Hope this blog can help beginners to better understand TL431 Shunt Regualtor.

This Vedio Introduces Circuits with TL431 Regulator

I Description

II TL431 Precision Reference Voltage Source

III TL431 Adjustable Regulated Power Supply

IV TL431 Overvoltage Protection Circuit

V TL431 Constant Current Source Circuit

VI TL431 Comparator

VII TL431 Voltage Monitor

VIII TL431 Controllable Shunt Characteristics

IX TL431 Switching Power Supply

FAQ

II TL431 Precision Reference Voltage Source

The precision reference voltage source circuit has good temperature stability and large output current. But when connecting capacitive loads, we should pay attention to the value of CL to avoid self-excitation.

tl431 circuitFigure 1. TL431 Precision Reference Voltage Source

III TL431 Adjustable Regulated Power Supply

As shown in the figure, Vo can be adjusted between 2.5~36V.

V0=Vref(1+R1/R2)(Vref=2.5v).

Since the withstand voltage is related to (Vi -Vo), when the voltage difference is large, the power consumption of R increases.

tl431 circuit
Figure 2. TL431 Adjustable Regulated Power Supply

IV TL431 Overvoltage Protection Circuit

As shown in the figure, when Vi exceeds a certain voltage, TL431 triggers. At this time, the thyristor is turned on and generates a large instantaneous current, which blows the fuse, thereby protecting the rear circuit. V protection point = (1+R1/R2)Vref.

tl431 circuit

Figure 3. TL431 Overvoltage Protection Circuit

V TL431 Constant Current Source Circuit

As the picture shows. The constant current value is related to Vref and the external resistance, and the margin should be considered when selecting the power transistor. This constant current source can be used as a current limiter if it is connected to a stabilized circuit.

tl431 circuit

Figure 4. TL431 Constant Current Source Circuit

VI TL431 Comparator

As shown in the figure, it cleverly uses the critical voltage of Vref=2.5v. Due to the small internal resistance of the TL431, the input and output waveforms track well.

 

tl431 circuitFigure 5. TL431 Comparator

VII TL431 Voltage Monitor

As shown in the figure, use the transfer characteristics of TL431 to form a practical voltage monitor. When the voltage is between the upper and lower limit voltages, the LED power and upper and lower limit voltages are (1+R1/R2)Vref and (1+R3/R4)Vref respectively.

 

tl431 circuitFigure 6. TL431 Voltage Monitor

VIII TL431 Controllable Shunt Characteristics

It can be seen from the functional module diagram of TL431 that when the voltage at the REF terminal changes slightly, the shunt from the cathode to the anode will change within 1-100mA. With this controllable shunt feature, small voltage changes can be used to control relays, indicator lights, etc., and even directly drive audio current loads. The picture shows a simple 400mW mono power amplifier circuit for this application.

 

tl431 circuitFigure 7. TL431 Controllable Shunt Characteristics

IX TL431 Switching Power Supply

In the past ordinary switching power supply design, the output voltage is usually fed back directly to the input terminal after error amplification. This voltage control mode can also work well in some applications. However, with the development of technology, most of the world's power supply manufacturing industry has adopted a scheme with a similar topology.

The switching power supply of this kind of structure has the following characteristics:

The output is fed back by TL431 (controllable shunt reference) and the error is amplified. The sinking end of TL431 drives the light-emitting part of an optocoupler. The feedback voltage obtained by the photosensitive part of the photocoupler on the main side of the power supply is used to adjust the switching time of a current mode PWM controller. Thus, a stable DC voltage output is obtained.

tl431 circuitFigure 8. TL431 Switching Power Supply

The picture above is a practical 4W switch type 5V DC regulated power supply circuit. This circuit adopts this kind of topological structure and uses TOPSwitch technology at the same time.

In the picture:

  • C1, L1, C8 and C9 constitute an EMI filter;
  • BR1 and C2 rectify and filter the input AC voltage;
  • D1 and D2 are used to eliminate the spike voltage caused by transformer leakage inductance;
  • U1 is a current mode PWM controller chip with built-in MOSFET, which accepts feedback and controls the operation of the entire circuit;
  • D3 and C3 are the sub-pole rectifier filter circuit;
  • L2 and C4 form a low-pass filter to reduce the output ripple voltage;
  • R2 and R3 are output sampling resistors, and the divided voltage of the output is controlled by the REF terminal of TL431 to control the shunt of the device from the cathode to the anode.
  • This current directly drives the light-emitting part of the optocoupler U2.

Then when the output voltage has a tendency to increase, the Vref will increase and the current flowing through the TL431 will increase. Therefore, the light-emitting of the optocoupler is strengthened, and the feedback voltage obtained by the photosensitive terminal is also greater. U1 will change the switching time of the MOSFET after receiving this increased feedback voltage, and the output voltage will drop with the change. In fact, the process described above will reach equilibrium in a very short time. When balanced, Vref=2.5V, and R2=R3, so the output is stable 5V.

It should be noted here that the output voltage can no longer be changed by simply changing the values ​​of the sampling resistors R2 and R3. Because, the parameter of each component in the switching power supply will have a great influence on the working state of the whole circuit. According to the parameters shown in the figure:

  • The circuit can be within the input range of 90VAC~264VAC (50/60Hz);
  • Output +5V;
  • The accuracy is better than ±3%;
  • The output power is 4W;
  • The maximum output current can reach 0.8A;
  • The typical conversion efficiency is 70%.

FAQ

  • What is the use of tl431?

The TL431 is a "programmable precision reference" and is commonly used in switching power supplies, where it provides feedback indicating if the output voltage is too high or too low. By using a special circuit called a bandgap, the TL431 provides a stable voltage reference across a wide temperature range.

  • What is tl431 transistor?

The TL431 is a regulator diode whose output voltage can be programmed by changing the value of resistors connected to it. It acts almost like a Zener diode except for that the voltage rating of this IC is programmable. It is commonly used to provide negative or positive voltage references.

  • How does a shunt regulator work?

The shunt regulator or shunt voltage regulator is a form of voltage regulator where the regulating element shunts the current to ground. The shunt regulator operates by maintaining a constant voltage across its terminals and it takes up the surplus current to maintain the voltage across the load.

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