Battery charging with intermittant power source

[[email protected]]

 On a sunny day (Mon, 12 Jan 2009 13:42:24 -0800 (PST)) it happened


mm
Here is a simple example for a MOSFET low dropout regulation I just composed:
 ftp://panteltje.com/pub/MOSFET_low_dropout_regulator_with_short_circu....

Output is over R7, so the regulator is in the *minus* lead.
Q1 is for start up, to hold current limit inactive for about 1.5 ms (C1 +R8).
The current is then measured by using the voltage drop over the MOSFET.
If it exceeds .75 V then Q1 starts conducting, and the MOSFET is switchedoff,
resulting in the voltage over it to become much higher, and Q2 conductingmore,
output goes then to zero permanently.
It will, depending on the type of MOSFET, easily do 45A..... before it either evaporates
(too big a value for C1, too long current sense inhibit), or melts (huge heatsink needed).
You can limit much lower, I use a LM324 in my other project to sense the voltage over the MOSFET,
but did not find a spice model for the LM324 for LTspice, so maybe later.
C2 is needed to keep if from going bad if entering current limit.

You can leave Q1, Q2, and associated components out, and use a fast fuse I guess....
Good thing is that it seems stable with about any MOSFET I select.
Ripple rejection is about 10x for small signals..

I would use a controller chip over discretes. For one thing, the long
tail pair matching in the error amp will be better over discretes.

 

[[email protected]]

That is absolutely true.
However we should never forget what we use it for,
and high accuracy is not really a big thing here.
What counts is that I have the transistors laying about
I am actually working on something like this, as a ripple filter
stage coming after a switch mode computer supply for 25A or so.
The idea is to do AC peak detection on the _output_, and then
use that detected voltage to create a voltage drop (low) over
the series regulator, so that drop is just enough to filter out the ripple.
Dunno if that ever has been done, you cannot go for 'zero ripple',
but set for example for 1 mV or so.
Computer switchmodes are really really noisy, and this way you minimise
losses, so for 1V ripple drop the 12V PC supply to about 11V ripple free.
Will try some simulation tomorrow perhaps, and yes, that uses a LM324 opamp,
not a long tailed pair.

The nice thing about controller chips is a thousand customers have
flogged the design by the time you get to use it. There are always
gotchas that one person may not consider in the design until a
customer does something you as the designer didn't anticipate. There
are always design elements in a chip that you don't reveal in the
datasheet. As a chip designer, I am always hesitant to roll my own
for stuff I build for myself. There is a lot of screw up derived
knowledge in every chip.

LDOs are funny things. Too much gain can be a problem. Generally a
single gain stage works best since you have to deal with pole shifting
due to the external load. So beware of using multistage op amps in
such circuits.

Wouldn't you just pick a voltage and linear regulate to it? The
trouble with your sensing scheme is it will need to take into account
spikes on the supply due to load variations. While you are thinking of
a feedforward design, there is feedback from the load variance to
consider.

 

[[email protected]]

"The nice thing about controller chips is a thousand customers have
flogged the design by the time you get to use it."

Yeah, but don't you think that about 900 of them probably just used the exact
example circuit shown on the first page of the data sheet?

What kind of ICs do you design, Miso?

---Joel

MIxed mode analog.