Power tip: Weighted feedback in multiple-output supplies
Regulation requirements for this design are ±5% for the 3.3V and ±10% for the 5V. A few techniques to improve the cross-regulation between outputs are included in this design. The transformer uses a single-tapped secondary winding to provide the 3.3 and 5.0V. This helps in two ways.
The cross regulation between the two outputs is influenced by the leakage inductance between them. Using a tapped-winding helps minimise the leakage inductance. If the 3.3V is the regulated output, the resistive drop in its winding is regulated out and only the voltage on the remainder of the 5V winding varies. Essentially, you get a variation on a 1.7V winding instead of a 5V winding.
Figure 1: Resistor R18 provides the ability to weight output voltage regulation (Click on image to enlarge.).
R6 and D5 help the regulation when the 5V winding is lightly loaded and the 3.3V winding is heavily loaded. During this case, when the primary FET turns off, there is a lot of energy in the leakage inductance that can be peak-detected in a lightly loaded secondary. R6 and D5 provide a switched-load that draws little current until the voltage approaches the maximum regulation point.
Figure 2 shows the cross regulation of the output voltages as a function of load current with R18 of figure 1 open. The regulation of the 3.3V output is precise over all load combinations. The load line is flat with output current and the accuracy is set mainly by: the resistor tolerances, the reference tolerance, and its temperature characteristics.
Figure 2: 3.3V regulation only can force 5V out of the specification.
However, in this particular case, the 5.0V output is not well regulated and exceeds the specification. There are a number of factors leading to this poor performance. The first is the open-loop performance of the 5V output by itself. There are resistances outside of the regulated control loop on the 3.3V, output as well as the leakage inductance between the secondary windings that degrade the 5V regulation. These are the reasons that there are two 5V output regulation curves for a given 3.3V current.
In addition, loading the 3.3V output forces the 5V output higher. One example of such an effect is the 3.3V diode. At higher output currents, its drop is higher and the control loop forces the transformer voltage higher to compensate. This increased voltage proportionally raises the 5V output, which is why the 5V output curves slope up.
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