Power Conversion Projects
The pages here describe a number of power conversion circuits that have practical value as well as providing an introduction to switch mode power conversion. All circuits are intended for use in a raw 12V dc battery power environment (i.e 11.5V-14.5V). The circuits converge towards finding efficient ways to manage a remote 12V power system that derives power from sources such as wind and solar, and using battery storage.
has been noted in various publications that when working with switched
power circuits, it is essential that well laid out printed circuit
boards be used, with a great deal of care taken to minimize stray
inductance and capacitances, to minimize the length of paths for
current flows and to isolate sensitive control logic from the high
current parts of the circuit. The aim of the circuits described here is
to be able to explore some simple SMPS circuits without the expense and time delays
of PCB manufacture.
The circuits described here have made use of pre-drilled prototyping board. PC boards will be made towards the end of the design process. If care is taken this type of board may work for low power designs and low frequencies. Tracks can be thickened for lower resistance by soldering lengths of wire across them, or heavy wire can be used in place of circuit tracks to carry large currents. Careful layout can minimize the length of these interconnections and avoid cross coupling. Some circuits can even be assembled for initial testing at low powers on plug-in prototype systems. Frequencies used in the projects described here are below 100kHz and testing is done at 5 to 10 watts.
experimental purposes inductors can be scrounged from dead power
supplies and other equipment. Old monitors are a rich source of these.
The inductor saturation tester
by Alan Yates has proved invaluable in measuring the maximum current
before core saturation takes place and also whether the saturation is
hard or soft. It will measure values around 10µH to 100µH with saturation currents from 1A to 10A, but the circuit could be modified to extend these ranges if necessary. This
range is suitable for many of the circuits here using a switching
frequency upward of 50kHz. A small number of inductor values are
available from electronics distributors, although information about
core properties is often not provided. A third option is to make up an
inductor, and for this purpose cores can be purchased or scrounged
inductors could be modified. This would of course be impractical for
toroids if the number of windings is large. Some valuable advice is
provided in this thread.
For SMPS work microprocessor control allows a great deal of flexibility. Most general purpose microcontrollers however require an amount of external supporting circuitry such as high speed A/D and D/A converters and feedback amplifiers depending on the circuit configuration.
STM ARM Cortex M3
The ARM Cortex M microcontrollers have a number of advantages for SMPS work, notably speed and a rich set of peripherals. The STM32 series have 12 bit A/D at 1MS/s, and 12 bit D/A converters which provide good dynamic range. The STM32F3 family also has built-in analogue comparators necessary for rapid response to current control events. They also have very richly featured timers with comprehensive PWM modes.
some of the projects described here the Atmel AVR microcontrollers
provide A/D conversion at only 8 bit resolution and a maximum rate
of 70,000 samples per second, which is somewhat low for SMPS work.
This is aggravated if only one A/D converter is present and several
feedback signals are to be measured. In addition the sample and
hold amplifiers have poor frequency response. Some of the AVR
range provide useful PWM features such as deadtime control between
complementary PWM output pairs, useful for driving synchronous
switching circuits. The AVR microcontrollers however are well
supported by open source software development tools, and
programming the devices is very simple. The PDIP package formats allow for simple prototyping.
There are a number of microcontrollers specially designed for SMPS applications. Suitable choices for the more advanced SMPS projects include the Microchip dsPIC30F1010, dsPIC30F2020 and dsPIC30F2023. These have an on-board D/A converter attached to the analogue comparator, and 2 megasample per second A/D converters. Microchip has produced a very large range of devices providing a rich choice for any application. They also provide a development environment at no cost, having only some limitations on code optimization. The IDE is unfortunately not open source and is only available for Windows operating systems. However it will run under Linux (and presumably OS/X) using recent versions of Wine. Microchip's own device programmer only works under Windows, but open source programmers are available.
First created 5 August 2010
Last Modified 29 October 2015
© Ken Sarkies 2010