Power Tip 70: Use weighted feedback in multiple-output supplies to meet regulation specs

April 07, 2014 // By Robert Kollman
Power Tip 70: Use weighted feedback in multiple-output supplies to meet regulation specs
Robert Kollman, Senior Applications Manager, Texas Instruments expalins that many times you are asked to design a multiple output power supply for a minimum cost yet with somewhat stringent output voltage regulation requirements.

A good example is the low-power offline flyback shown in Figure 1, where both a 3.3- and a 5-volt output are required.

Figure 1: Resistor R18 provides the ability to weight output voltage regulation.

Regulation requirements for this design are ±5% for the 3.3 volts and ±10% for the 5 volts. 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 5.0 volts. 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 minimize the leakage inductance. If the 3.3 volts is the regulated output, the resistive drop in its winding is regulated out and only the voltage on the remainder of the 5-volt winding varies. Essentially, you get a variation on a 1.7-volt winding instead of a 5-volt winding.

R6 and D5 help the regulation when the 5-volt winding is lightly loaded and the 3.3-volt 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 3.3 volt 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: 3.3-volt regulation only can force 5 volts out of the specification.

However, in this particular case, the 5.0-volt output is not well regulated and exceeds the specification. There are a number of factors leading to this poor performance. The first is the

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