The reflected voltage, V P, is proportional to the output voltage following the equation: This improvement significantly reduces the complexity in the transformer design and construction, and PCB layout. It eliminates issue (a) mentioned earlier by sensing the primary side voltage directly, thus not requiring a third winding in the power transformer. No-opto flyback topology: The No-Opto Flyback DC/DC is a variation of the primary side control method. This primary side control method offers poor output voltage regulation and thus is not practical in many applications, forcing the designer to use a post regulator, which adds even more cost and increases the total solution size. Traditional isolated DC/DC converter using optocoupler and associated feedback circuitry This produces sensed output voltage error.Ĭ) The leakage inductance ringing on V W further adds to the reading error of the sensed output voltage.Ībove: Figure 4. Also, V F varies with load and temperature. While this method effectively eliminates the optocoupler, it creates a new set of issues:Ī) Adding a third winding complicates the transformer design and construction, adding more cost.ī) The reflected voltage involves the output rectifier diode voltage, V F. Where V O is the output voltage, V F is the output rectifier diode voltage drop, N a is the third winding number of turns, and N S is the secondary winding number of turns. The reflected voltage, VW, is proportional to the output voltage following the equation: In this scheme, a third winding on the power isolation transformer is used to measure the output voltage indirectly during the OFF cycle. Primary side control topology: One way to eliminate the optocoupler is to employ the primary side control method. On top of that, this CTR drops 30% to 40% over time. ![]() Figure 6 shows a typical optocoupler’s Current Transfer Ratio (CTR) that varies 270% over the −60☌ to +120☌ temperature range. The optocoupler faces another major issue: its performance varies with temperature and degrades over time, which can cause reliability issues for certain applications. Naturally, there comes a quest to eliminate this circuitry. The optocoupler, error amplifier, and voltage reference circuit have 12 components, which substantially add to the total design component count, and take up large board space (Figure 5). This solution serves its purpose very well until the equipment shrinks and leaves little room to fit it. The information is passed across the isolation barrier through an optocoupler to the primary side, where the control circuit modulates the power stage to regulate the output voltage.Ībove: Figure 3. Isolation for level shifting In this implementation, the output voltage is sensed by an error amplifier, then compared to a voltage reference. The solution uses an optocoupler, error amplifier, and a voltage reference to form a feedback loop across the isolation barrier. Isolated DC/DC converter basicįigure 4 illustrates a traditional isolated DC/DC converter. Level shifting: Occasionally, a system with a mixture of many power rails uses isolated DC/DC to generate multiple isolated positive and/or negative output voltages. Isolation is used here to avoid disruptive ground loops, and also isolate digital noise from the precision analogue system. Isolation to avoid ground loopĪvoid ground loops: In a large or complex system, grounding potential differences exist at different areas. The isolation barrier here routes the dangerous surge energy back to the primary ground, thus avoiding it passing through the operator.Ībove: Figure 2. Without proper safety isolation, a lightning strike could send a very high voltage surge through the equipment to the operator and ground. Figure 1 illustrates a power system with the main power source isolated from the secondary side, where a human operator can have physical contact with it. Safety: To prevent current surges from damaging equipment and protect humans from the main power source. Many power systems in a variety of industries such as factory automation, building automation, e-mobility, automotive, avionics, medical, commercial, and others employ isolated DC/DC converters for one or more of the following three reasons: Where and why isolated DC/DC converters are used? The new solutions also provide new benchmarks for output voltage accuracy. The system engineer needs a small, low-cost, highly reliable, and easy-to-design solution.įortunately, there are new no-opto flyback solutions that eliminate the optocouplers, associated feedback circuitry, and the need for third transformer winding. The system engineer must overcome all these challenges when starting a new isolated DC/DC design. ![]() ![]() Furthermore, end equipment form-factor is shrinking, leaving limited space for power supply, and adding thermal management challenges. The optocouplers degrade over time, reducing system reliability.
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