Power Supply Design

Question: How do I sense multiple outputs?

Original Question: Please, help me to obtain particular information about closing the feedback loop in an SMPS design. The SMPS design is a two transistor forward converter using planar cores for output transformer and coupled inductor. It has four outputs:

+5 V/4.4 A
-5.6 V/1.8 A
+3 V/3 A
+2 V/2 A

Which of the four outputs must be sensed for feedback control? T. J. 05/22/2001

Answer: Normally you would sense the winding with the maximum power letting the other windings cross-regulate through the transformer turns ratio. There are exceptions to this rule-of-thumb. If the maximum-power load has large transients on it, then you may want to sense an output with a steadier load. Also, if one winding has to have tighter regulation than the others, then you might want to sense it.

Another approach is to sense all windings and set the weighting that each winding has on regulation by the values of the summing resistors.

Note that it is very difficult to obtain cross-regulation better than 5% and it takes some effort to get this over a production run. For example, you have to have excellent control over the production of the transformer and coupled inductor. Small variations in the exact geometry of the windings and placement of tape can have an impact. Also, if the effective turns-ratio of the transformer and inductor don't exactly match, it can affect the power stage transfer function and power supply stability. Of course resistance and diode variations also have to be minimized. The lower the voltages, as in this case, the harder it is because of diode variations. There is an old saying that all cross-regulated outputs are +/-10%. The designer tried for +/-5% but was never able to hold it due to manufacturing and lifetime variations.