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Design A Bench Power Supply

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I've been thinking about designing and building my own bench-top power supply for general use. I envision the primary use to be with prototyping low-power analog/digital circuits with the occasional small DC motor and other electro-mechanical devices. The primary reason I'm doing this is to become more familiar with power supply design, and having a useful bench-top supply is an added bonus.

Target specs and device goals so far, recommendations/suggestions are welcome:

1. Powered from mains electricity (I live in the US, so I'm targeting ~120VAC@60Hz).

2. Mains isolated.

3. Variable voltage and current output, at least 1A max output current and 12V max output voltage. Preferably I'd like a 15V or 24V max output voltage as well as split supply, though these aren't critical specs.

4. Decent efficiency. I haven't quite spec'd out a target efficiency yet, but I would expect greater than 50% efficiency. I've heard of SMPS supplies going up to ~80-90%, possibly higher.

Since this is taken as primarily a learning exercise, I've been seriously considering if I should turn this project into one where I can learn more about using mains electricity directly. I'm hesitant to do so because I understand the very real danger involved with getting something wrong on mains and I could use a wall adapter (either AC-AC or AC-DC) or pre-built constant voltage unit to power my supply to reduce the risk associated with working with mains. I'm curious as to what you guys think about this, as well as any recommended resources I should check out before working with mains directly. If you have any suggestions for projects which would be better suited for a beginner working directly with mains that'd be great, too.

If I did choose to go with the direct mains method I think I'd starting out with a basic transformer KY286-53030C(datasheet:http://www.kynix.com/uploadfiles/pdf/53030C.pdf) to provide isolation and reduce the input voltage, then use some circuitry to get adjustable voltage/current.

The basic interface with the mains:


The main side is tied across the one side of the transformer, and the powered circuit is tied to the other side. The signal is then rectified through a diode bridge, and the output is filtered using a large capacitor in parallel with the load. If I wanted to, later down-stream I could use circuitry to further regulate the output voltage and current (something similar to what is done here).

The voltage across the load should be (assuming a sufficiently small load or large capacitor, as well as a near-ideal transformer):


Where Vmain is the amplitude of mains electricity (~170 V in the US).

This is a basic design which works in theory, but what real-world considerations do I have to take into consideration when designing the actual interface with the mains electricity? I know that a highly capacitive or inductive load will result in a poor power-factor, but since this is a general purpose power supply I'm assuming that whatever load gets attached down-stream will be designed to compensate for that rather than try to have the power supply do the compensation.

An alternative approach is to use a switched-mode design, but I haven't found many useful resources on how to design/implement one. From what I understand a SMPS uses much higher frequencies so the transformer can be smaller while still providing a semi-stable DC output. What I don't understand is how you're suppose to take the 60Hz signal from the wall and up that to a ~10kHz or even ~1MHz input for the SMPS transformer.


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