Low voltage isolated power supply output voltage regulation solution

Perhaps the most common IC in isolated switching power supplies , the TL431 shunt regulator provides a simple, low-cost way to accurately regulate output voltage. Figure 1 is a block diagram of the TL431 and a typical application circuit for regulating the output of an isolated power supply. The TL431 combines an internal reference and an amplifier in a single three-terminal device. The R3 and R5 resistor divider and the TL431's internal reference voltage set the output voltage. Inside the TL431, the error amplifier output drives the base of the transistor. The transistor collector not only connects to the TL431's K (cathode) pin, but also drives an optocoupler that sends the error signal across the isolation boundary to the host controller. The frequency response of the feedback loop is formed by the compensation components located between the TL431's cathode and REF pins.

　　Figure 1. The TL431 circuit commonly used to regulate the output voltage of an isolated power supply.

　　The circuit starts to show some limitations when the converter output voltage is less than 5V. The minimum recommended operating voltage of the cathode is equal to the reference voltage, which is 2.5V for the standard TL431. The optocoupler internal photoemitter supports a maximum forward voltage drop of about 1.5V. If the output voltage is less than 4V, the optocoupler may not be fully forward biased. In addition, additional voltage margin needs to be allocated for the bias resistor (R1). This limits the practical output voltage of the standard TL431 to above 4.5V. There are lower voltage versions of the TL431, such as the TLV431, which provides a 1.25V reference. This provides ample performance headroom for driving optocouplers with a 3.3V output. Modifications to the standard circuit are required to regulate lower output voltages using this part.

　　As shown in Figure 2, the low voltage TLV431 can be used to regulate output voltages less than 3.3V by simply adding a PNP transistor. In this circuit, the cathode of the TLV431 drives the base of the PNP transistor, which can be configured as an emitter follower. This allows the optocoupler to be moved between the collector of the PNP transistor and ground, where there is enough room for the forward voltage drop of the photoemitter. With a minimum cathode voltage of 1.25V and a typical base-to-emitter potential of 0.7V, the minimum voltage at the emitter of Q1 is about 1.95V. This allows a 2.5V output with 0.5V left for the bias resistor.

　　This simple modification expands the TLV431 regulation circuit range to include a 2.5 V rail. However, for output voltages below 2.5 V, the required modifications to the standard regulation circuit are much more complex. Ultimately, a higher voltage auxiliary rail must be generated specifically to power the TLV431 and drive the optocoupler.

　　Figure 2. Adding a transistor helps the TLV431 drive an optocoupler to ground for a 2.5V output.

　　For power supply examples using the TL431 and TLV431 devices described in this blog, see the following designs in PowerLab:

　　PMP6886—Class 3 36V-72V Input 2.5V/5A Active Clamp Forward; 1/16 Brick;

　　PMP8790—Active Clamp Forward, Quarter Brick (18Vdc-60Vdc Input, 3.3V/12A);

　　PMP9203 - Universal AC input; 5V/2A USB adapter supporting fixed frequency operation.