Tue Dec 21 23:20:22 EST 2010

Inductors for LED driver

I got the 470,1000,2200 uH power inductors from Futurlec in the mail
today.  Seems their series resistance is 1,2,4 Ohm respectively, which
means I can't do much with them for driving the LED lamp at its low
voltage, which has 14 white LEDs in parallel.

The idea seems to be to use these for larger voltages.  Series LED
circuit seems more appropriate.  This then allows a boost

  * The current will be lower (order of 20mA instead of 300mA for 15
    LEDs).  This is good for efficiency.

  * However, the charge cycle will take proportionally longer than the
    discharge.  This means the LEDs are pulsed with a short duty cycle
    and thus need larger currents.

This seems to be a case of the law of conservation of misery.  Maybe I
postpone this until I have some more info.  It seems that using
smaller inductors and higher switching speed is a good way to go.

With this information, it seems that higher efficiency is not really
going to be one of the properties of the controller, compared to the
1Ohm series resistor I have now.  However, what can be useful is the
output current regulation when battery voltage drops, and the
possibility to regulate the intensity.

Again, what are the variables:

   - Output voltage, current  (LED topology + dimmer)
   - Switch frequency
   - Inductance
   - Series resistance

Higher inductance, lower switching frequency.  There seems to be no
real disadvantage to high frequency apart from switching losses, which
are (I believe) mostly due to transistor speed and capacitive load.

The higher the inductance, the higher the resitance (for the same
inductor size).  Is there any advantage for higher inductances?

This[3] might be an interesting place to look.

[1] http://en.wikipedia.org/wiki/Boost_converter
[2] entry://20101216-004450
[3] http://www.coilcraft.com/led.cfm