SFT Power Supply Project

A photograph of the completed prototype

The SFT Power Supply is a switching power supply prototype that tests a number of 'Proof of Concept' ideas regarding the use of an AVR microcontroller, some of which may be patentable. The basic design uses the PWM outputs and analog inputs that are built into the AVR microcontroller to monitor a number of parameters. The voltage is controlled using 0-5V output from 4 wirewound potentiometers on the front panel next to the banana jacks. The left-most potentiometer controls the +/- 0 to 12V output setting, and the right 2 potentiometers control the voltage on the nearby banana jack (-15 to -75V, 36 to 400V).

The low voltage output uses 2 simple direct coupled transistor circuits to convert the 0-5V PWM output of the AVR to an appropriate high current PWM output, into a 100uH load coil and a pair of filter capacitors. Each output has a separate PWM signal controlling it. The AVR microcontroller's internal timers are operated at much higher than normal frequencies to obtain better results with smaller components.

The high voltage outputs are driven by an internal 'charge pump' voltage converter, consisting of a similar direct-coupled transistor circuit with a 0-5V PWM input, to create an internal +45V 1A power supply. When coupled with the -15V power supply another direct-coupled transistor circuit can generate an approximately 60VDC p-p PWM signal that drives the voltage multiplier for the 36V to 400V output. The minimum of 36V is coincidental to voltage drop across the multiplier circuit (9 diodes in series). A similar voltage multiplier circuit drives the -HV (-15 to -75V) output (using less expensive components).

The prototype internally has a simple +/-15V @2A unregulated power supply using a split-phase 120/24VCT transformer, and the output from the unregulated power supply is available via jacks on the front panel. These output jacks were intended for use by the calibration process but could still be used to provide power.

The true 'secret sauce' of the design involves the manner in which the software controls the voltages. Although nothing is really all that complicated as far as the hardware is concerned, the software algorithm is a completely different story. In short, it simulates a hardware device with predictive capability and alters the PWM output as necessary to control the voltages. Many different voltage parameters, including the unregulated power supply voltage and current, are measured and acted upon by the code in a continuous loop. The LCD display continuously displays the 4 measured output voltages, temporarily switching to a different mode that displays the appropriate voltage setting while you rotate one of the setting potentiometers. If an overcurrent condition is detected, the PWM output is shut down, placing the unit in a safe condition (indicated by the LCD display). The 'overload' condition can be reset by power-cycling the device.

Because of all of the internal measurements, and the use of NON-precision resistors and components (for lower cost), a calibration process is needed. When performed manually, it takes a few minutes, using a command-line utility and an RS232 adaptor (an RS232 connector is mounted on the side of the device for this purpose). The calibration utility has you take specific measurements and record the voltmeter readings. Then it makes some calculations and saves the calibration data into the AVR microcontroller's NVRAM.

Because the power supply is controlled by a microcontroller, MANY application possibilities exist. Several weeks of careful research and design went into the firmware, as well as my own experience in control systems. Without going into a lot of detail as to how I envision its use, there are several kinds of circuits in which a variable voltage or current monitoring capability would come in handy. (I already have a lot of good application ideas, but I'd really rather not mention any of them until I have working protoypes). But as a 'test bench' power supply for electronics research, it already has a lot of value to me.


Project Status

Additional Photos

Back view of the prototype, showing the cooling fan

A view of the inside - multiple proto-boards and lots of wires!

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Last Update: 9/17/2014

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