LM2567 Switching Regulator - Heat Sink required...?

Thread Starter

danielantonic

Joined Sep 22, 2019
26
Hello

I am making a power supply to power a Raspberry Pi/camera/touch screen (5V @3A), and some LEDs, servos/motors (12V LEDs and 9V servos/motors). I am using a 12V/5A adapter to supply power, and an LM2576 switching regulator to take the 12V down to 5V @3A for the pi etc. and a 7809 to drop 12V to 9V for the servos.

lm2576.png
20210908_154518.jpgWhen I did my first test with the Raspberry Pi attached to 5V, it worked fine... beside the smoke starting to come from the regulator! I immediately turned it off and it still provides 5V at low loads, so I assume putting a heat sink on will fix my overheating problem... but I don't know if the ones I have are sufficient (recovered from old equipment). Could someone point me in the right direction please?

heat sink.jpg
 

ericgibbs

Joined Jan 29, 2010
14,183
hi Daniel.
I would use this quick method.
Fix the LM2576 to the finned alloy heat-sink using leads as short as possible.
Run up the PSU, put it on load and measure the rise above ambient of the heat-sink.
From that information you can finalise the size of the sink. [hack saw]
E
 

Thread Starter

danielantonic

Joined Sep 22, 2019
26
hi Daniel.
I would use this quick method.
Fix the LM2576 to the finned alloy heat-sink using leads as short as possible.
Run up the PSU, put it on load and measure the rise above ambient of the heat-sink.
From that information you can finalise the size of the sink. [hack saw]
E
Thanks Eric. I gave that a try and just using that heat sink as pictured.
20210910_185807.jpg
I hooked up my multimeter and measured the temperature as the Raspberry Pi booted up. The low power message kept popping up but the multimeter showed 5.08-5.12V the whole time, so I think there might be an issue with the Pi's onboard voltmeter.

The temperature slowly rose above ambient, and after 2 mins it was about 10 degrees celsius above ambient according to my laser thermometer (both on the regulator and the first capacitor), but touching it felt hotter... Reckon I should use some thermal paste or not really needed on something this small?
 
Last edited:

ericgibbs

Joined Jan 29, 2010
14,183
hi Daniel,
Thermal paste does help with the heat transfer from the Chip to sink, without paste there is trapped air, which is a poor heat conductor.
A 10Deg C rise is OK.
Also if possible, mount the fins vertically, improves the cooling air flow.
E

BTW: The LM2576 TAB body is connected to 0V, so no insulation for the heatsink is required,
 

Thread Starter

danielantonic

Joined Sep 22, 2019
26
Thank you very much Eric! I am redoing my prototype board today, but I have found on the forums etc that the Raspberry pi 3 that I have takes 2.5A on its own, plus the 7" touchscreen that takes 550mA. Since this ads up to 3.05A, and I have other 5V peripherals to add, is there a way to get any more current from the LM2576, or is this a situation where you would get a second LM2576 and set it up in parallel...?
 

ericgibbs

Joined Jan 29, 2010
14,183
hi Daniel.
I would suggest a second LM2576 to supply a second 'group' of peripherals, also power up both LM's with the same power ON switch.
I would not connect the Vouts of the LM's together.

Ensure that LM's and all the peripherals have a common 0V.
If the heatsink can handle it, mount both LM's on the same sink.

E
 
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