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Offline LED driver at -40degc Ambient

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treez

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Hello,
We have an 40W offline LED driver which uses no electrolytic capacitors...only ceramic ones.
Also, it uses only sequence switched linear regulators, -no SMPS.
It is only surface mount components.
Optocouplers, resistors, caps, diodes, FETs, SFH5711 light sensor, MCP9701 temp sensor, MCP6001 opamp, BZX84C-XX zeners.
Do you think we will have problems at -40degC?
The PCB sits on a rubbery thermal pad which is on a heatsink

We are getting it in the oven at -40degc, but wonder if there are any long term effects.
The product is totally enclosed..no vent holes....a rubbery gasket makes the seal between the optic and the heatsink
 

Presumably all the parts are within their design ratings and operating temperature range so you shouldn't have any problem. The only thing I would be cautious of is expansion. When operating the temperature will obviously rise and probably to +50C or more so the PCB and components will expand quite a lot over the 90C increase. You would have to experiment but possibly a 'loose' PCB mount would be advisable to allow it to expand without buckling. Mounting screws with a short shoulder comes to mind that can be fully tightened but leave a small (PCB thickness) gap under their head so the board isn't held down tightly.

Brian.
 
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Thanks, that sounds good though our PCB has to be held down tightly, for those occasions when they get used in hot ambients...because the PCB sits on a heatsink (well... sits on a rubbery thermal pad on the heatsink)
 

How is the PCB held down?

If there is scope for mounting it in a recessed 'dip' in the heatsink it may be possible to secure it with pressure from above. Screw hole fixing is probably the worst method as it prevents expansion. Maybe screws outside the perimeter of the board that overlap the edges is an option.

Brian.
 
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How is the PCB held down?
Click to expand...
Thanks, ..screw holes in the PCB....going through a hole in the thermal pad, and threading into holes in the heatsink below.
We didnt think that FR4 PCB would expand much at all?

- - - Updated - - -

Thanks, it is held down with screws through screw holes
 

Too late then, unless you can enlarge the holes maybe.
FR4 is quite stable but you are talking about a large temperature range. I'm not sure what temperature the board reaches after prolonged operation but I'm guessing at least 50C so you need to check the expansion from -40 to +50C to see how much you should allow. The danger isn't to the FR4 as much as to the components mounted on it if it bends between fixing points. The cheapest, although not ideal fix is simply to move the mounting positions just off the edge of the board so it is held captive between the screws but it can still slide slightly under the screw head. It's a no-cost solution.

Brian.
 

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