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Old 04-20-2010, 11:59 PM   #7 (permalink)
NiHaoMike
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I don't think there's any point in isolation since it is expensive, inefficient, and does not really provide much of a safety advantage since the output is still high voltage. If anything, isolation may make for a false impression of safety in a high voltage circuit. (I know of an electrical engineer who got zapped by an isolated 300v power supply because one side was grounded by the scope probe. She lived to tell the story, but it shows that isolated high voltage power supplies are not safe!)

PFC is normally unnecessary for home use, but it is easy to integrate it into a charger design. Just have rectified AC supply a buck/boost converter (buck followed by boost, with a shared inductor), with a few small, high ripple current capacitors to ensure a low source impedance. It will support a very wide range of output voltages. up to 450v or so is reasonable depending on the rating of the components. Regulation compliance can go all the way to 0v.

There will definitely have to be some analog control circuits. A good and simple way to control a buck/boost converter is to use double hysteresis current mode control. Basically, the buck part has a slightly higher setpoint than the boost part, such that the hysteresis ranges do not overlap. The setpoints are determined by more control circuits. There will be a comparator to set a "dead band" during which the converter stops operating, since efficiency is poor during low instantaneous input voltage. A clamp circuit can limit maximum input current as desired by limiting maximum control voltage. Another circuit will limit maximum output voltage.

The charge control can be either analog or digital. For lead acid, simply adding a timer to reduce the output voltage a time after the setpoint voltage is reached is enough. For Lithium or other chemistries, a microcontroller can be used to provide primary charge control. It can also use a control signal from the BMS.

One of my current projects is a high efficiency inverter-based power supply/float charger for a home network. It operates at much lower voltages and power levels, but the basic concept is the same.
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