Question: Is mounting a UIB2 enabled maxflex onto the led heat sink good or bad?

[iggs]

Thinking about heatsinking a UIB2 programmed maxflex for use in a bike light and I'm coming up against a clash of thoughts

1 - mounting the heat sink tab on the maxflex to the coolest part of the housing would be the bast thing to do to remove heat from the drivers board

2- mounting the heatsink tab onto the back of the same heatsink as the led's will bring the driver into the same heat path as the led's and enable the new heatsensing capabilities of the boards firmware to work. At the end of the day its going to get no hotter than the settings allow by being there.

Here's a photo taken from here showing a maxflex mounted to the led's heatsink

 

[georges80]

You really want the maxflex heatsink path to be the coldest part of the LED housing. Given it's all indirectly connected anyway, it will still sense the temperature. Maybe not the exact temperature - but all you need to know is the delta between the LED end and the maxflex end to set things up.

Case in point is the bFlex that senses temperature in the same way. What was done to set the temperature for the min-T housing was to run the min-T at high current and do the ouch test on the housing. The temperature of the bFlex was measured (about 15-20C less if I remember rightly) and that was then set as the forced dimming temperature. (we were a bit more scientific than that of course ).

maxflex being a boost converter will run considerably hotter as the input voltage drops (becomes less efficient) versus bflex. That means that building a bike light with maxflex requires more attention to detail in the thermal management area.

Also, with bFlex (buck) there really is no failure mode if the input voltage is kept below it's operating max. With maxflex (boost) it is possible to destroy the driver by running it with low input voltage and max current to the load and poor heatsinking. I've had folk destroy maxflex drivers because of that where with bflex it hasn't been the case. Of course both have been destroyed due to inexperienced soldering/assembly.

Just and example:

Lots of folk want to run 6 or 7 LEDs at 1A. There is no free lunch... 6 leds at 1A is 6 x 3.7 x 1 = 22.2Watts. With low input voltage the efficiency will drop (maybe as low as 85%) and now there's 22.2 x 15% = 3.3Watts of heat being 'lost' in the driver. That 3.3Watts is HEAT and that's a LOT of heat that needs to be removed from the driver. That's actually more than the IC package's thermal resistance will allow to be channeled out of the package and to the thermal path on the PCB. i.e. it won't work...

So, for maxFlex the temperature sensing in the uController (that will sense the PCB temperature), is more important for protecting the driver than for protecting the LEDs!

cheers,
george.

 

[iggs]

Thanks for the reply George, interesting stuff.

Going to ask a couple of daft questions as I work out a few things in my head, please bear with me. I'm really sorry if these have been answered else where and I've missed the information

What temperature did you find made you go ouch? I seem to remember from making coffee, well steaming milk actually for the coffee, that its around 60 deg C

You say that 3.3 watts is too much for the board to deal with. On the website you say that anything from 1 watt up should have thermal transfer off the board designed in. Whats the max wattage that you think should be realistically considered so that the board will also have a realistic working life too?

What sort of temperatures can boards actually cope with? What would be the maximum running temerature a maxflex can run at for a sustained period of time without doing any harm to the board, short term and or long term?

First of all I'm not planning on running 6 or 7 LEDs at 1A (at the moment), that would not balance the amount of light needed with the amount of battery and run time required, less power used = lighter battery and or/more run time.

But more led's = greater efficiency (well at the led end of things anyway) but greater Vf

But thats a side issue