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Old 02-24-2009, 11:23 AM   #28 (permalink)
Join Date: Feb 2009
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Just found this thread...

... and i've been thinking of this project for a while now since I got my car working and am unsatisfied with the high power onboard chargers that are either massive in weight or massive in dollars.

The ultimate goal: I want something that'll pump 20+ amps into my 144V nominal pack when running on 220V, full 3 stage charging that is adjustable for the number of batteries I have or ambient temp, and will run on 110V without changing anything. Well, maybe a current knob to adjust the current down when on lower power lines.

I got so far as drawing up a design using the TI BQ2031 chip to run a power section as a buck converter with PWM, but have no EE basis to validate it (my work/job is all mechanical engineering).

But, I did order all the power section components and have laid them out. My plan is to test this with just a 555 pwm setup sending a signal to the mosfet driver, so essentially I'd have a fancy 'dumb' charger for now if it works. I attached the schematic below - I think it's virtually the same as a motor controller except that it has coil components for added inductance.

There's some big caps for input voltage smoothing from the bridge, another cap across the battery, some large inductors for current smoothing, a diode for clamping the voltage spikes when the FET closes, a large FET acting as a switch, and a shunt to measure current.

The components:

Big 400V caps: $10 each
Big 30A Inductors: $27 each
600V 37A diode: $5
500V 60A FET: $18
600V 40A bridge: $6 (from a surplus site)
Various heatsinks: Maybe $20 worth. I went overkill with just things I had around the house and found at a local surplus shop.

So that's about $140 worth of stuff. Everything else - the control board - shouldn't cost anywhere near that, so we're talking a fairly inexpensive high power charger if this works.

I went overboard with some of the specs since I see nothing wrong with having margin. Also, I can see this project running in stages:

1. Get the powerstage working with just a generic PWM signal (555 timer).

2. Get the control board to successfully control a charging algorithm. (the BQ2031 seems overly complicated - maybe Paul can help with the programing of a custom controller?)

3. (and this is where the high voltage components come in) Build a 300V (or so) Power Factor Corrected power supply that'll supply 300V (or so) to the charger from an AC input from 100 to 250V. This'll allow the charger to run from any typical voltage source AND maximize the power drawn from the power source.

But first things first. Anyone have any comments on this power section schematic? I read up a little on snubbers, but not sure if they're needed or how to implement/design one. At some point, I'll wire all of these up, borrow a scope, and see what happens (i'll start small, maybe just charge a 12V battery running off a 24V transformer).

But, any input is much appreciated!

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