Electronic Design

Averaging Improves Regulation Of Second Power-Supply Output

A switching power supply's main output usually will have a load regulation of better than 0.1%. However, what if it doesn't need regulation that tight on the main output, but instead needs better regulation on a second output? The averaging technique described here eliminates the need for adding linear regulators to achieve better regulation on the second output, reducing cost and parts count.

Figure 1 shows a typical switching regulator. The 12-V output will be tightly regulated, but the 100-V output's regulation is around 5%. By averaging the regulation of the two outputs, you can improve the second output's regulation at the expense of the first.

Figure 2 shows the technique. For example, say the internal reference is 1 V. Then, 1 V/R1 = 1 mA. Thus, resistors R2 and R3 must supply the 1 mA to maintain 1 V at the feedback pin (FB). To equally weight the output regulation, R2 and R3 will sink 0.5 mA each. That is,

(100 V - 1 V)/R3 = 0.5 mA

and

(12 V - 1 V)/R2 = 0.5 mA

Choosing standard-value resistors, R2 = 22k and R3 = 200k. If in this example the second output had a regulation of 5% and the first output 0.1%, now both outputs will have a regulation of about 2.5%. By changing the weighting between R2 and R3, the improved regulation of the second output at the expense of the first can be controlled.

However, be aware that if there are step loads, the output without the step load will have a step change because of the averaging technique. How fast the output will change depends on the bandwidth of the power supply. So if you use this technique with step loads present, make sure your circuitry can tolerate the sudden changes in voltage.

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