I have designed numerous isolated power supplies over the years. As you might suspect, the majority of these supplies tend to have output voltages of 3.3V or higher. When I was asked to design a 2.5V isolated supply, my initial thought was that there was no way that I would be able to use a TL431 with this low of an output voltage. I cringed at the thought of having to generate a higher voltage auxiliary output just to power the error amplifier, reference, and opto-coupler. After a little brain storming about how to make better use of the voltage that was available, I came up with a simple little solution by just adding a PNP transistor. To understand the problem and the solution, you must first know what is inside the TL431 and how it is used to regulate a power supply.
The TL431 shunt regulator is perhaps the most common IC found in isolated switching power supplies. It provides a cost effective and simple way to accurately regulate the output voltage. Figure 1 shows a block diagram of the TL431 and the typical application circuit used to regulate an isolated power supply output. The TL431 incorporates an internal reference and an amplifier in a single, three-terminal device. The R3 and R5 resistor divider and internal reference voltage of the TL431 set the output voltage. Inside the TL431, The output of the error amplifier drives the base of a transistor. The collector of the transistor is tied to the K (cathode) pin of the TL431 and is capable of driving an optocoupler, which sends the error signal across the isolation boundary to the primary controller. The frequency response of the feedback loop is shaped by the compensation components placed between the cathode and REF pins of the TL431.
Figure 1. The TL431 circuit commonly used to regulate the output voltage of isolated supplies.
When the converter’s output voltage is less than 5V, this circuit begins to run into some limitations. The minimum recommended operating voltage on the cathode is equal to the reference voltage, which is 2.5V on the standard version of the TL431. Photo emitters inside optocouplers have a maximum forward drop of around 1.5V. If the output voltage is less than 4V, the optocoupler may not be sufficiently forward biased. Furthermore, extra voltage margin needs to be allocated for the drop across the biasing resistor (R1). This limits the practical use of the standard TL431 to output voltages greater than 4.5V. There are lower voltage versions of the TL431, such as the TLV431, which provide a 1.25V reference. This allows enough head room to drive an optocoupler with a 3.3V output. To regulate lower output voltages with this part requires modifications to the standard circuit.
By simply adding a PNP transistor as shown in Figure 2, the low voltage TLV431 can be used to regulate output voltages less than 3.3V. In this circuit, the cathode of the TLV431 drives the base of the PNP transistor, which is configured as an emitter-follower. This allows the optocoupler to be moved between the collector of the PNP and ground, where there is adequate headroom for the forward voltage drop of the photo emitter. With a 1.25V minimum cathode voltage and a 0.7V typical base-to-emitter potential, the minimum voltage at the emitter of Q1 is around 1.95V. This leaves over 0.5V to drop across the biasing resistor with a 2.5V output.
This simple modification extends the range of the TLV431 regulating circuit to include the 2.5V rail. However, for output voltages below 2.5V, the modifications required to the standard regulating circuit become increasingly more complicated. Eventually, it becomes necessary to generate a higher voltage auxiliary supply rail just for powering the TLV431 and driving the optocoupler.
Figure 2. Adding a transistor allows the TLV431 to drive an optocoupler to ground for a 2.5V output.
For some example supplies that use the TL431 and TLV431 as described in this blog post, check out the following designs in PowerLab…