December 4, 1997

±12V supply accepts 9 to 30V inputs

Ron Young, Maxim Integrated Products, Sunnyvale, CA

For applications that require symmetric supply voltages from a wide input range, the switch-mode supply in Figure 1 delivers more than 3.6W at ±12V and operates from inputs of 9 to 30V. Over this input range, efficiency varies from 74 to 80%. The step-down switching regulator, IC₁, includes an internal switching transistor that drives the primary side of the flyback transformer. Matched output windings produce the ±12V outputs. Resistor-divider feedback by R₂ and R₃ regulates the positive output, and tight coupling between the output windings regulates the negative output.

With a 20-mA load on the positive 12V output, the ­12V output changes less than 7% in response to a load change of 0 to 300 mA. In other words, cross-regulation is less than 7%. You can adjust the feedback resistors for different output voltages and optimize the R₁/C₁ compensation network for your application and layout.

The close-coupled transformer windings provide another benefit: The primary winding's low leakage inductance re-quires no snubber circuitry for control of flyback voltage to protect the internal transistor switch. The switch voltage flies back to the reflected output plus a diode drop, or to a total of ­13V, which is well within the IC's absolute maximum voltage rating.

The primary winding comprises two windings in series to achieve its nominal 44-µH inductance. To maintain a 1-to-1-to-1 turns ratio, each of the output windings comprises two windings in series. (DI #2121)

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