I am studying the similarities and differences in the circuits of these Switch Mode Power Supplies from a 5 Volt, 6 Volt, and 12 Volts wall warts. I want to find out how they create their output voltages and how they regulate these voltages. One of the similarities that I noticed right away was that each unit had an Opto-coupler between the primary high voltage side and the secondary low voltage side. In my study of the three boards two had a TL431 Precision Programmable Reference and one had a Zener Diode tied to the LED side of the Opto-coupler. Here are rough approximations of the two variations of the circuit that I found:
The variation of the circuit that uses the Zener diode has a current limiting resistor in series with the Zener which is also in series with the LED side of the Opto-coupler. As the voltage on the output rises and exceeds the value of the Zener plus the forward junction voltage drop of the LED current begins to flow and the LED turns on. This is turn causes the photo transistor in the Opto-coupler to begin to conduct. The conduction of the photo transistor must be tied back to the primary side of the SMPS and provide negative feedback to the circuit generating the primary current.
The SMPS that use the TL431 instead of the zener have basically substituted the TL431 for the Zener. The TL431 is quite a versatile component but in this application it is being used as a Zener which can be programmed by the resistor voltage divider between its Cathode, Reference, and Anode terminals. I decided to look a little more closely at the TL431. Here is a Data Sheet for the component and a link to Newark's listing of the part:
You will note in my schematic fragment I have drawn the resistor voltage divider as a potentiometer. On the actual boards the resistor are simply small surface mount resistors but since I wanted to experiment with the part I used a 10K potentiometer on the bread board where I began to test one of these cool little components.
My first experiment simply involved setting up the circuit as shown in the above schematic. I applied an input voltage of 24 volts and chose my R1 so that the 20 mA tolerance of the Opto-coupler could not be exceeded. The TL431 itself can handle a Max current of 150 mA. I placed my voltmeter between anode and the cathode of the TL431. True to the Data Sheet specs the voltage on the cathode began at 2.5 volts when the reference pin was tied to to the cathode and the voltage continued to rise as the potentiometer moved the reference voltage closer to the anode. Whenever I stopped on a specific regulated voltage I found that I could then adjust the input voltage up or down without any appreciable effect on the voltage across the TL431. It was doing a good job of providing a stable voltage reference. I next put an ohmmeter on the photo transistor terminals of the Opto-coupler and watched as it began to conduct as the input voltage surpassed the set voltage of the TL431 plus 1.2 Volts for the forward junction drop of the coupler's LED.
Note: The TL431 has an internal 2.5 volt base reference which keeps it from working below this level. The Max input voltage is listed at 37 volts and the data sheet says that it can provide a reference voltage between 2.5 V and 36 V.
The next step in my experiment was to see if I could build a simple linear regulator using the TL431 and the Opto-coupler as feedback. I bread boarded this circuit and began to test its limits.
Here is a picture of the bread board and shots of the meters involve in the test:
You can see that I am supplying an input voltage of 20.9 volts and my output voltage from the regulator is 6 volts. The 10.7mA input current of the circuit is displayed on the meter on the left. I was able to select output voltages from 4 volts up to 13 volts by adjusting the potentiometer and changing the voltage of the TL431. This experiment provided me with verification of my understanding of the TL431 in this application and proof of concept in how the output of the Opto-coupler could be used to control other components to regulate an output voltage. On the actual wall wart circuit I am sure I will find a small dedicated IC that is providing PWM current to the primary of the transformer. The IC will likely be controllable by the output of the opto-coupler. I will continue to further investigate this to verify the exact mechanism involved but that will be for another day. One question that occurred to me was why would a manufacturer use the TL431 and support resistors when they could just use a zener. It was apparent that the circuits that I am looking at were designed and built with the minimum cost necessary. My only thought is that the TL431 itself costs only pennies and perhaps the ability to fine tune its voltage with resistors and not have to buy an custom voltage zener is actually cost effective. I have ordered a bunch of the TL431s as I can see that they will be useful in a variety of applications in the future.