In general, isolated power supplies often use optocoupler isolation feedback due to its simplicity and low cost. However, there is a lack of in-depth research on the various connection methods of optocouplers and their differences. Many engineers still face confusion when selecting the right configuration for optocoupler feedback, which can lead to circuit failures. This article aims to provide a detailed analysis of how optocouplers work and compare several common feedback methods.
Optocouplers like TLP521 and PC817 are commonly used for feedback. The TLP521, for instance, functions by converting the input current into light, which then controls the output transistor. The current gain (Ic/If) varies with temperature, making it important to design the surrounding circuitry to operate within a stable linear range. Without proper design, the system may become too sensitive, leading to instability.
The most common feedback method involves connecting the optocoupler with a TL431 voltage error amplifier. The TL431 acts as a programmable reference, and the feedback loop adjusts the duty cycle based on the output voltage. In one typical setup, the optocoupler's primary side is connected to the error amplifier, while the secondary side controls the chip’s feedback pin. When the output voltage increases, the optocoupler’s current increases, reducing the duty cycle and stabilizing the output.
Another configuration connects the optocoupler directly to the error amplifier’s output. In this case, the optocoupler’s current must not exceed the amplifier’s output capability, or the output voltage will drop. This method is more suitable for systems with smaller duty cycles, as larger currents can cause instability.
A third variation adds an external resistor to ensure sufficient current flows through the TL431, preventing it from entering an unstable region. Similarly, a fourth configuration uses a resistor to adjust the operating point, improving stability under varying conditions.
To better understand these configurations, characteristic curves of the TLP521 were analyzed. It was found that the optocoupler should operate with a current between 5–10 mA to avoid saturation and maintain a stable transfer function. At higher temperatures, the Ic value decreases, but the Ic-If ratio remains consistent.
Through experiments on both half-bridge and flyback power supplies, the impact of duty cycle on feedback performance was observed. For example, using the second feedback method on a half-bridge circuit caused oscillations due to high gain at large duty cycles. In contrast, the flyback circuit, which operates with a smaller duty cycle, showed stable behavior.
In conclusion, choosing the right optocoupler feedback method depends on both the optocoupler’s characteristics and the system’s duty cycle. Methods 1 and 3 are more versatile, while methods 2 and 4 perform better in low-duty-cycle applications. Understanding these factors is essential for reliable and stable power supply designs.
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