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Switch Mode Power Supply Implementation

Study Project
Advanced Electronics Project II
November, 2001

The purpose was to construct a step-down switch-mode power supply with an output of 5v DC, and operative for current range of 0.1A to 1.0A. The design involved conversion, reduction, rectification, filtration and regulation of the standard AC voltage into the required output. A transformer was used to convert and reduce the AC input, and diodes were used for rectification. A RC network was used for filtering, and circuitry based on an IC pulse width modulator was used for voltage regulation. The circuit was implemented and tested in stages; during the implementation a higher than analytically required short circuit resistance was used. The circuit operated and produced a 5V output for the current range of 0.1A to 0.3A. By reducing the short circuit resistance the design requirements should be met.


Voltage Conversion: The high ac input voltage of 120 Vrms, at 60 Hz is reduced to appropriate voltage for the design. A transformed is used for this purpose.

Rectification: A central-tap transformed with half bridge diodes or transformed with full bridge rectifier can be used to convert the ac into DC or ripple voltage.

Filtering: Using a capacitor the ac component is reduced or filtered out. Voltage Regulation: Maintaining a required voltage for a given current range.











First, the unregulated part of circuit was designed, and 20 V rectified input was verified. Then, the circuit was fully connected and tested. It functioned properly as expected from the start. The suggested Rsc=0.1W was not available, and R=0.5W was used. This reduced the power supply current range. Furthermore, it is noteworthy that a resistance in the order hundred should be connected in series to the power diode to initiate a current flow.


In short, the step-down switch-mode power supply was operative at the required voltage of 5v DC between 0.1 A and 0.3 A. The design failed to provide the current range of up to 1A. However, by reducing the Rsc from 0.5W to 0.1W the requirement should be easily met.

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