Re: 20-60A adjustable ~200vdc current mode buck design


"Genome" <mrspamizgood@xxxxxxxxxxx> wrote in message
news:JiNCh.8926$fa.8795@xxxxxxxxxxxxxxxxxxxxxxx
<<SNIP>>

Oh..... OK, fiddle fiddle fiddle. Try one of these,

http://www.genomerics.org/laser/pfcpar.asc

That's two stages operated in parallel with the clock waveforms shifted.
The idea is sort of that the current error amplifiers receive the same
demand signal from a single or master voltage error amplifier and program
the same current in their respective power stages. You have to figure out
how to tie the available parts in the ICs together to do the job.

One thing you are forced to do is use current transformers to recreate the
inductor current. At high powers this would be done anyway but at low
powers you would use a sense resistor in the return lead to the input
rectifier. Unfotunately when you try to operate things in parallel you
can't do that any more.... they just won't connect together proper like.

I've added some source sense resistors to the 'mosfets' but not
implemented a current limit with them. Problem is that the drain current
transformer is operated at high duty cycles and might saturate. You lose
the signal and things go bang. Resistors don't, lose signals, so it's a
kind of 'oh ***!' protection.

The peak limit is simplified a bit. It really needs slope compensation but
the IC does not make that easy to add, that's why those transistors were
in the previous one. Here I've just thrown in some appropriate current
sources.

I've diddled it to do 1KW per section from a 220V line..... I suppose you
will need 8 of them because you are some sort of cross dressing freak.
There are ways of synchronising these ICs by shoving a pulse up their
oscillator. I'm sure if you read some data sheets you'll get some ideas.

In the old days a 4017 Johnson Counter clocked at N (stages) times the
desired base frequency with the reset pin on the appropriate tap did the
job. These day I suppose you would use a PIC.

Really all you are getting is something to practice your sums on. You will
probably find some of the values I've left in there are sub-optimal. You
can try putting in 'real' components to get some guesses about power
losses. I'd recommend you grab hold of some SiC diodes for the main boost
diodes.

Have Fun and try not to go too fruit pie.

DNA


I've already located inexpensive isolated current sensors with models for 50
or 100 amps and 3000v isolation, so I can easily do high side current
sensing -- just gotta do a spice model for it, which looking at your models
has helped me figure out (a voltage source defined via a formula referencing
the load current)

I'm seriously looking at the PFC chip you mentioned, and some others (I've
already tried to model out one using the LT1248 but its not working right
yet)

You want fruit pie ?? here is fruit pie -- what would it take if I wanted a
true earth ground on the output of this mess -- I've considered running
seperate boosts on the poitive and negative outputs from the bridge, using
the isolated current sensor to get EVERYTHING out of the ground path.. one
of them inverting to get a positive voltage (add more in pairs if 2 wont get
me enough current at reasonable ripple)

is that fruit pie enough for you ?? or is there a better way ??


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