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Power tip: How to compensate isolated power supplies

Posted: 12 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:isolated switching power supply? TL431? optocoupler? Zener? feedback loop?

If ever you have designed an isolated switching power supply, you've probably come to the realisation that compensating isolated supplies is more complex than non-isolated supplies. Isolated supplies that contain a TL431and an optocoupler are complicated by the fact that there are two feedback loops in the circuit.

While many papers have been written on the topic, there are not many resources to simply explain how you can select resistor and capacitor values to shape the compensation and total loop response. The easy way out is to eliminate the inner loop with a Zener clamping circuit. However, this unnecessarily increases the component count. With a little understanding of the underlying equations, choosing compensation values around a TL431 can be nearly as easy as compensating a buck.

Figure 1 shows the feedback system. The inner feedback loop is formed by the pull-up resistor, R1. This loop is often referred to as the fast loop, as any perturbation in output immediately affects the optocoupler current in this path. The outer loop is the path back through the resistor divider and TL431 compensation. This is the slower loop, as the compensation components in this loop affect the output voltage's response.

Figure 1: This common TL431 circuit contains two feedback paths.

First, let's consider what appears to be a simple integrator. To implement this, we simply set R4 to zero Ohms in our circuit. The resulting transfer function and gain plot from "VOUT" to "FEEDBACK" is shown in figure 2. Interestingly, we have a pole at DC, and a zero formed by R3 and C1. The zero is somewhat counter-intuitive due to the presence of the inner loop. The gain at frequencies above this zero is simply the ratio of the two resistors, R6 and R1, multiplied by the optocoupler's current transfer ratio (CTR). Above 10kHz the optocoupler bandwidth introduces a pole that limits the gain.

Figure 2: Integrating capacitor around the TL431 introduces a single zero.

Notice that there is no way to take gain out of the circuit by changing the component values around the TL431. This limitation can become a problem in power supplies with low-output voltages, where the power stage gain tends to be high. We could change the ratio of R6 and R1 to lower the gain, but these resistances are usually determined by how much current is required by the optocoupler. If there is too much gain in the plant, the gain is most easily attenuated by adding a capacitor and resistor in parallel with R6. This introduces a pole-zero pair that must be placed at frequencies well below the crossover frequency of the entire loop.

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