In the first part of this Fly-BuckTM Topology Series blog, we introduced the intent and philosophy of the secondary compensation loop on the isolated side. In this article, we will review this compensation circuit and demonstrate the improvement of the secondary side regulation.
Figure 1. Fly-Buck converter with feedback compensation circuit at secondary output
Figure 1 shows the complete compensation circuit and prototype LM5017 application circuit. The external compensation circuit includes an optocoupler for feedback isolation and a shunt regulator LM431A [3] that provides a large gain for the error amplifier at lower frequencies. The feedback circuit contains a typical Class I compensation network (C1, R1) that determines the cutoff frequency. In addition, this Class I compensation also ensures high DC gain and reduces low-frequency DC regulation errors. When the phototransistor in the optocoupler turns on, the effective feedback ratio of one output will decrease.
Figure 2 is a modified circuit with the correct circuit parameters. When the compensation circuit is added, the apparent value of the resistor divider ratio (RFB2/RFB1) will change during operation. The high-side feedback resistor RFB2 needs to be readjusted by updating the output voltage once to set it slightly above the nominal 12V.
Figure 2. The LM5017-based Fly-Buck converter circuit provides optocoupler-based voltage regulator circuits
As shown in Figures 3a, 4a, and 5a (dotted lines), the secondary-side regulator circuit based on an optocoupler can significantly improve the secondary regulation effect compared to the uncompensated secondary output. In the prototype LM5017 circuit, the secondary output voltage has a negative gradient under load conditions, and the use of this circuit significantly reduces this negative gradient. As the input voltage increases, the secondary output voltage is constantly regulated to bring it close to the 5V rating. However, this improved voltage regulation on the secondary side is also traded off at the end of a single output regulation process, because the basic relationship between these two outputs still depends on the power stage. Figures 3b, 4b, and 5b are the actual one-time regulation in this latest configuration and the comparison with the corresponding initial circuit.
Figure 3a. Secondary-side load regulation
Figure 3b. Primary side load regulation
Figure 4a. Secondary-side load regulation
Figure 4b. Primary side load regulation
Figure 5a. Secondary-side load regulation
Figure 5b. Primary side load regulation
The main purpose of adding this isolated feedback compensation circuit in the initial Fly-Buck converter is to improve the secondary output voltage regulation over the entire load and input voltage range. The above results show that the isolated compensation circuit is very helpful for maintaining the isolated output voltage regulation.
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