Why not take advantage of the micro-controller included in many systems
these days? This design uses microcontroller feedback to produce a ?50-V, 1-W
power supply (see the figure). It leverages
an unused analog-to-digital converter (ADC) input and a pulse-width-modulation
(PWM) output to close the loop on a basic inverting single-ended primary inductance
converter (SEPIC) topology.
Varying the duty cycle of the waveform that drives the pass-FET (Q1) controls
the circuit's negative output voltage. A simple voltage translator solves the
problem of feeding a negative voltage back to the microcontroller's 12-bit ADC,
which has only a positive-voltage common-mode range. The npn transistor (Q2)
and R1, R2, and R3 form the voltage translator for converting the negative output
voltage to a proportional positive feedback voltage for the ADC.
Within the microcontroller, a simple Proportional Integral Plus (PIP) software
routine closes the loop (see the code listing; for a downloadable version,
see the online version of this article). The code for controlling the loop
is written for a Freescale DSP56827 digital signal processor, but it could be
adapted to any microcontroller with a multiply function. Typical integral gain
is 0.01, and proportional gain is 0.2, but some experimentation will be necessary
to determine optimal values.