Re: High brightness white LEDs damaged by custom switcher


"rebel" <me@xxxxxxxxxxx> wrote in message
news:mt0m331s7q1fishrbqr9il5ffjcki79r45@xxxxxxxxxx
On Fri, 4 May 2007 03:17:54 -0400, "Paul E. Schoen" <pstech@xxxxxxxxx>
wrote:


I'm sure they would "stand by their chips", but I think the concern was
that the software could go awry and damage could be done before it could
react. That's why I am considering a separate fail-safe mechanism, such
as
a fast acting fuse, or an NPN transistor that would turn on, when the
voltage on the 1 ohm sense resistor reached Vbe, and turn off the MOSFET.
Maybe an SCR would be better, requiring a complete power down to reset.

Why not use something like an LM317 in the (series) feed to the LEDs as a
programmable current source. That would facilitate (a) avoiding possibly
damaging current surges (sorry, tongue in cheek), and (b) allowing the
stepping
of the LED current through 0/100/700mA or whatever steps you want.

Pierre (who loves 34063's and 317's)

Thanks for the suggestion, but the power required for the clusters is at
least 21 watts for 7 and 39 watts for 13. Even if the device could handle
the voltage and power, it would be inefficient and unusable in the
application, which is a diving flashlight. The efficiency appears to be in
the order of 90%, and 4 watts is not excessive, especially when the water
acts as a huge heat sink. However, efficiency and battery life are
important.

I think the additional transistor or SCR will work. I can't add too many
parts, as the entire board is only 0.95" x 2". The transistor would
effectively form a linear current regulator, but would subject the MOSFET
to excessive power. If I can get a small SCR, or make one from two
transistors, it would latch the MOSFET off, but then the 12 volt gate drive
would be applied across the 20 ohm gate resistor.

Maybe I will use the TI UCC27321 MOSFET driver which has an enable line
with a Schmitt trigger. Luckily it is an active High with internal pull-up,
so the saturated NPN transistor would disable the drive. Adding a capacitor
should hold the output off for long enough to produce a "flashing" effect,
which would be very noticeable.

It needs a redesign anyway, and I'm a little leery of the Microchip
drivers. The TI products have a combination MOSFET and bipolar output
designed to switch more efficiently through the on threshold.

Additional suggestions are welcome!

Paul


.

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