Keeping driver temp down?

[blackizzz]

Hello DIYers!

I'm in the process of starting a new tank, and doing so I decided to use the DIY LED fixture I built a while back.

I had a custom driver PCB and controller PCB done, which works great, the driver PCB basically consists of 7 CAT4101's.

Both cards are in a little box, with a lot of small holes drilled in it, and a 40mm fan which supposedly pushes a lot of air for its size.

They're also passively cooled by a small heatsink, taken from an old computer, the fan blows across this heatsink.

Now to the "small" problem:

When running the LED's at 50% for a long time I had no problems with temperature in neither the driver/controller box or fixture, but when I cranked it up all the way to 100% the driver box ran for 2 minutes, then the lights started flickering, one channel at a time.

This is the result of the Automatic temperature safety on the CAT4101's which is triggered at 150° celsius. At the same time this happened (and the drivers supposedly were 150° hot) I held the driver box in my hand and sure it was warm, but maybe 45°C warm, not near 150°...

But, even so, I need to keep the temp down in the box. What can I do?

Is it a matter of throwing another fan or 3 the box all blowing across the heatsink?

Scrap the box and get a larger one and a larger heat sink?

Drill more holes in the box and hope for a better air flow?

Grateful for all answers

Cheers!

:wave:

 

[mcgyvr]

"IF" you are sure that its the drivers overheating (sounds like you are just guessing though) then increasing the airflow and possibly increasing the size/amount of vent holes in the box will help.

40mm means NOTHING as far as the performance of the fan.. Find the specs for the fan and look at the CFM rating and find a fan with a higher CFM output.

 

[karimwassef]

Electronic devices get hot at key locations inside their structure when conducting. This "junction temperature" is what the device self protects to. The fact that your fixture is warm is no indication of the actual device junction temperature. A junction is very small and if the silicon is not adequately heat sunk to your frame, you can easily create a hotspot while the frame may be freezing.

Follow the thermal conduction path from the device to your heat sink. If you have a thermal probe, measure along the path and see where the heat flux is trapped (high temperature gradient).

If these drivers are in a closed plastic box, they may simply derate and you can't get that much current out of them.

 

[jedimasterben]

Add more fans if you're wanting to keep them enclosed. 40mm fans push very, very little air, especially if it is a quiet fan. Even moving to an 80mm low speed fan will increase airflow exponentially.

 

[mcgyvr]

Add more fans if you're wanting to keep them enclosed. 40mm fans push very, very little air, especially if it is a quiet fan. Even moving to an 80mm low speed fan will increase airflow exponentially.

again..The physical size of a fan says very little about its actual performance..
A 40mm fan can do as little as 1CFM up to around 40CFM.
There is a HUGE difference in evacuating heat from a box with 1CFM vs 40..

 

[jedimasterben]

again..The physical size of a fan says very little about its actual performance..
A 40mm fan can do as little as 1CFM up to around 40CFM.
There is a HUGE difference in evacuating heat from a box with 1CFM vs 40..

Of course - but a 40CFM 40mm fan will be akin to a jet turbine

 

[blackizzz]

Okay, so a little more info may be necessary:

I have identified the problem, which is the drivers and I'm sure they're the problem.

The drivers, CAT4101, are attached to a heat sink using silver thermal paste.

The heat sinks are 2 units, one smaller one that only cools ONE of the CAT4101's. The heatsink has a thermal resistance of 20°C/W. The load on this CAT is about one watt, so the temperature SHOULD be 20°C above air temp, IE around 45° approx.

The other heat sink is an unknown one, taken from an old computer. So I have no idea what the thermal resistance is, but my guess is that it's too "weak" for the amount of watt that it's loaded with, (which should be around 6 watts) without getting too hot.

I'll just have to find a "better" heatsink?

The fan is a fractal design 40mm silent series, air flow: 3,8 CFM or 6 m³/h Which isn't much, but it should be enough to get the air flowing when it's such a small area.

 

[karimwassef]

The formula is Tmeas = Tjunc - Q x R
Where Q is power dissipated as heat (not pass through) and R is the thermal resistance.

If the measured temperature is your aluminum plate at 40C (warm), and R is 20C/W and assuming that the dissipated power is 1W, then Tjunc = 40 + 1 x 20 = 60C

That's not enough to hit the 150C limit. You'd need a thermal impedance of 100C/W or 5W of dissipated power to do that.

Adding more fans to keep the plate cooler by 10C may get you from 150C to 140C but the life of those drivers will be significantly reduced if you're running anywhere north of 125C.

Then again, maybe you just got bad drivers that trigger their protection prematurely or dissipate 5x their normal W.

 

[O2Surplus]

Have you checked to see if the CAT4101's are being forced to drop more voltage than is necessary? If they're dropping more than .5 - .8V over their respective led string Vf, that's why the CAT4101's are getting hot.

 

[blackizzz]

The formula is Tmeas = Tjunc - Q x R
Where Q is power dissipated as heat (not pass through) and R is the thermal resistance.

If the measured temperature is your aluminum plate at 40C (warm), and R is 20C/W and assuming that the dissipated power is 1W, then Tjunc = 40 + 1 x 20 = 60C

That's not enough to hit the 150C limit. You'd need a thermal impedance of 100C/W or 5W of dissipated power to do that.

Adding more fans to keep the plate cooler by 10C may get you from 150C to 140C but the life of those drivers will be significantly reduced if you're running anywhere north of 125C.

Then again, maybe you just got bad drivers that trigger their protection prematurely or dissipate 5x their normal W.

Cheers for the information! That'll definitely come in handy!

You seem knowledgable about this, so I might as well ask

If I have 7 drivers, all driving 5*3.5V LED's.
That should equal 17.5V in all 7 strings.
The CAT4101 draws approximately 0.5V (18V)

My Powersupply is rated at 19V, IE 1V above the needed.

This should be what causes the heat, right, that 1V?

1volt * 7 strings, at max 1A
This equals 7W
IE heat equal to 7W

So, as long as the heatsink can handle 7W's without getting too hot I should be golden?
What temperature resistance should I aim at as an absolute max?

Sorry for the questions

Cheers mate :dance:

 

[blackizzz]

Have you checked to see if the CAT4101's are being forced to drop more voltage than is necessary? If they're dropping more than .5 - .8V over their respective led string Vf, that's why the CAT4101's are getting hot.

Read my previous post

 

[karimwassef]

Have you measured your current? Have you measured your voltage drops across the driver and LEDs? Have you measured your total circuit voltage from the power supply?
Do you have a schematic or circuit layout you're using?

It's not the overall power lost that matters. It's the power dissipated in a small piece of silicon that gets no thermal path for relief.

 

[mcgyvr]

You have NOT really found the problem..
A cat4101 should be able to dissipate 2-3W without any heatsinking at all.
Not to mention 20degC/W is the thermal resistance of the heatsink alone.. That does not include junction to case thermal resistance of the cat4101 itself..

remove the case.. if it doesn't show any problems then you just have insufficient airflow through your enclosure. Additional fans(more CFM) and larger/more vent holes will fix that.

 

[blackizzz]

You have NOT really found the problem..
A cat4101 should be able to dissipate 2-3W without any heatsinking at all.
Not to mention 20degC/W is the thermal resistance of the heatsink alone.. That does not include junction to case thermal resistance of the cat4101 itself..

remove the case.. if it doesn't show any problems then you just have insufficient airflow through your enclosure. Additional fans(more CFM) and larger/more vent holes will fix that.

So I did as you said, removed the case and now it has been running at 100% on all channels, for 30 minutes and the heatsink is just slightly warm to the touch, not much warmer than what it is in the room tbh. And the light is shining like this was it's last day on this earth (lets hope not lol)
This is with the fan lying on top of the heatsink and pushing air down on it.

No problems have presented themselves during this time, so I'm pretty sure your right :beer:

Now I just have to "fix" my case and make the air more free flowing, and if that doesn't work I'll have to get a better case and tidy up the wiring even more. :headwally:

Cheers for all the help guys!