The Rat Flasher - Efficiency Measurements for Boost Convertor

It all seemed so straight forward. Move from two cob leds in series to 4 cob leds orientated every 90 degrees around a case, each with their own boost convertor. How hard could that be?

Trivial, until you try to get every last drop out of the boost convertor. The boost converter data sheet never really envisaged hitting leds so hard for such a short pulse. So when I measured efficiency I found the absolute best I could get was 72% at which point I was hitting the cob led with 250mA with a power dissipation of 2.1W. 

Four of those flashing together was going to be really bright. However, that meant a peak supply current of around 4 x 1 Amp, or 4 Amps.  And a 4 Amp surge meant I had to add larger decoupling capacitors and consider the micro browning out from the voltage dip.

I found the efficiency was dependent on the battery voltage. A drop of 0.5V meant efficiency fell from 72% to 64%. Which means I will have to revisit the PCB at some point to use some bigger traces and eliminate the headers used to connect the battery.  A voltage drop of 0.5V when drawing 4A is just 125mΩ!

A slight tweak in the code means it is now easier to lengthen the time between sets of pulses. The rat flasher had not been in the ceiling for 3 nights, and no vermin had returned, so I applied a cautious tweak. With the adjusted timing I expect I can get 3 nights of solid flashing before the LiFePO4 battery goes flat. Over 24 hours the average current drawn should be around 40mA. I have a solar cell ready to go on the roof which should re-charge the battery during sunny days. With the arrival of spring there will be plenty of sunshine.

Longer term I need to move to a better boost converter. The boost converter this project evolved from uses a NMOS switch with a Rds of 0.5Ω. I expect a step change in efficiency by moving to a boost convertor with on resistance of 80mΩ. And a revised circuit that essentially charges a large capacitor and dumps that through the leds at much higher current, and hence brightness. 

There's no point in showing you a circuit of how not to do this. Another iteration is warranted so please be patient. If your desperate to drive those critters out of your roof and are happy with a less than optimal solution then by all means contact me.