The TL431 and SMPS
About the writer: Harvey Morehouse is a contractor/consultant with many years of experience using circuit analysis programs. His primary activities are in Reliability, Safety, Testability and Circuit Analysis. He may be reached at firstname.lastname@example.org. Simple questions regarding my articles for which I know the answer are free. Complex questions, especially where I am ignorant of the answers, are costly!!!
Summary: The TL431 device has come to play an important role in SMPS designs. Here is how it is used in SMPS designs both in isolated and non-isolated versions.
A Block diagram of the TL431 device is shown in Figure 1, taken from reference 1, a Fairchild device datasheet.
TL431 Block Diagram
In Figure 1 are shown two different representations of the device, both as an error amplifier and as a switchable zener diode.
In the top diagram it is shown as an error amplifier. The Reference input is compared to an internal voltage reference value of 2.5V, and an output NPN transistor turned on when the reference input exceeds 2.5V.
In the lower diagram it is shown as a controlled SCR. The SCR is enabled when the Reference input exceeds 2.4V. It is not really internally an SCR, and the upper representation is more accurate, however the lower symbol of Figure 1 is most often used.
Now the device is very interesting, incorporating in one package a voltage reference, an operational amplifer and an output transistor. This has some package count, assembly, footprint and cost implications. The device is not as precision as would be an implementation with a discrete operational amplifier, however in many instances it is more than sufficient to do a good job.
Now there are some performance issues to be considered in using this device within an SMPS in place of a conventional error amplifier. Reference 2 written by Chris Basso covers some of these issued nicely.
Here we are concerned mostly with the analysis issues of using this device.
Now referring back to Figure 1, the opamp representation shows that the device is really a transconductance amplifier. It is presumed that the device is used within a closed loop. Hence as the error input increases or decreases with respect to the opamp reference voltage, the output current will correspondingly increase or decrease.
Non-isolated Error Amplifier Implementation:
Non-Isolated TL431 utilization
Figure 2 was taken from Reference 3. Here it can be seen how the TL431 can be used in a non-isolated configuration. It does require a separate DC supply shown here as Vcc. Now could the output voltage replace Vcc? Providing that the converter had a M(D) = 1/(1-D) function to self start itself, yes. Other alternatives include bootstrap methods to get the switching regulator started, after which the output voltage would replace Vcc. But note that there would be an additional feedback path from the output to the modulator input through the effects of Vout and R5. Now one could always regulate the output voltage and use that with R5, but this would add complexity.
Isolated Error Amplifier Implementation:
To use the TL431 in an isolated configuration requires that an output isolated from the input be provided. A good way to do this is with an optical isolator.
Isolated TL431 utilization
Figure 3, also taken from reference 3, shows how an isolated error function could be achieved. Looking back to Figure 2, it can be seen that a type 2 error amplifier configuration was implemented. A type 2 amplifier is one where the required phase boost is less than 90° (less than 70°, in practice). The gain may be adjusted up or down.
Now the configuration in Figure 3 takes advantage of the opto-isolator and the TL431characteristics to achieve type II error amplifier performance. Type 3 performance is also possible. A type 3 amplifier is one where the required boost is less than 180°. The gain may be adjusted up or down.
There are some variations in possible implementations with some special considerations. In Figure 3, if one were to determine loop gain, one could just place a generator in series with Vo , common to R1 and R5. However, one might wish to connect R5 not to Vo, but before the output filter inductor. In that event, determination of the closed loop gain-phase response could be difficult, requiring several simulations/tests to verify and validate the answers.
This topic is covered nicely in reference 3.
The actual design of compensation for the TL431 circuits is covered in the referenced articles as well as in the second SMPS book by Chris Basso, mentioned prominently on the home page of Beigebag Software. In the book is a spreadsheet for error amplifier compensation, including the TL431 circuits.
The book files at the Beigebag web site contain a TL431 model in B2SPICE format.
The TL431 device is understandable and usable in the design and analysis of SMPS systems. Even though it is shown as a programmable SCR, it is closely akin to a conventional error amplifier in operation