Re: a lot of switching mode power supply design questions


"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@xxxxxxxxxxx> wrote in message
news:g60%j.109$k67.0@xxxxxxxxxxxxxxx
bigcaboy@xxxxxxxxx wrote:



This design aims to keep current adjustable yet constant with low
ripple. I couldn't use low freq. dimm. adjustment.
I wonder if I use a much bigger inductor, e.g. a 1000 uH compared to
a theoretical 330 uH, may I get much cleaner
current? Large inductor means large ESR, ESC. I'm using a homemade
toroid inductor. 1000 uH with 176 ohm and
26 uF @1k Hz.

Trying to figure out what you're doing?
wouldn't a photo feed back work better like used in laser diode systems
to maintain a desired level of luminance? current monitoring may not
be such the idea.


These figures don't make much sense, and the exact intent and purpose of
the circuit, and its complete design specifications, are still a mystery. A
toroidal inductor with 176 ohms ESR is nearly impossible unless it is wound
with Nichrome, and I don't know what ESC is (maybe equivalent series
capacitance?) You could use a 26 uF capacitor as part of a filter for the
output, but I don't know where your 1 kHz comes from. And also how do you
come up with a 330 uH theoretical inductance?

I found this Maxim circuit that can drive 30 amps of LED load with 5000:1
dimming, but it still uses PWM and will have high ripple:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5266

If this is a laser diode, there are many factors to consider, and I don't
think it is possible to adjust output over more than a limited range with a
low ripple DC current. This seems helpful:
http://www.ilxlightwave.com/appnotes/overview_laser_diode_characteristics.pdf

Here is a very extensive overview of laser diodes, from a Wiki link:
http://www.repairfaq.org/sam/laserdio.htm

I really don't think this is a laser diode, as it would be about 20 watts
(2V at 10 amps), which is generally only possible for extremely fast pulses
or very high-end (and expensive) industrial lasers. And if it is, it must
be controlled with photo feedback, and the output light power varies from
0-100% over a narrow range of current, and is highly variable with respect
to temperature and composition.

If this is a high power visible LED, the highest currents I know of for
single units is about 1 ampere continuous. If this is a cluster of ten or
more such LEDs in parallel, then the mismatch in Vf and variations in
temperature will result in current hogging and destruction of several of
the devices. Standard practice is to put the LEDs in series and generate a
higher voltage to drive the entire string.

Paul


.

Relevant Pages

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