My switching mode power supply is running hot. - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: August 27, 2018, 06:04:32 am
content: This is about that NeoPixel strip I was talking about.Specs:Input: 12V 1.5A maximum (number marked on the power brick)Output: 5.3V 2.5A maximum (number obtained from stress testing)I have built the power module as a single-phase buck converter based on a SGM6130 controller. The controller has 90% efficiency at this power level, and I am running at capacity.When I run the load test (using a 12V 2A power brick for some additional power budget) the converter chip would get hot to the touch. I did properly solder the exposed pad of the chip to the continuous ground plane, and there is some via stitching beneath the chip.Will sticking a small heat sink on top of the chip help? Is there any alternative options available?

author: Phoenix
author_rank: Frequent Contributor
timestamp: August 27, 2018, 07:13:17 am
content: Assuming 90% efficient at 5.3V, 2.5A => 1.33WThe datasheet lists junction to ambient as 50K/W. That's a 66K rise above ambient... it's going to get pretty hot! But still within its rated 150ºC rated and 160ºC shutdown temperature.

author: jbb
author_rank: Super Contributor
timestamp: August 27, 2018, 08:13:16 am
content: Quote from: technix on August 27, 2018, 06:04:32 amUnfortunately our fingers aren't good temperature probes. A component can burn your finger and still be within acceptable temperature limits. Have you got a temperature probe?Also, the diode in the buck converter is a major area of losses. Assuming you use a Schottky diode (hint hint) the forward voltage is probably around 0.5V. Let's look at its current flow:Vin = 12VVout = 5.2VVout' = Vount + 0.5VD ~= Vout' / Vin = 0.475Idiode ~= Iout * (1-D) = 1.3APdiode ~= Idiode * 0.5V = 0.66WThis is probably half your losses. The good news is that it happens outside the buck converter chip.A heatsink on the top of the chip won't do much, because there's quite a lot of insulating plastic between the silicon and the top. You could try putting it on the other side of the PCB, to draw the heat out through the heatsink pad and via stitching. Will also help with getting heat out of the diode.If you are using battery power, you might consider using a synchronous buck converter - it will be more efficient and therefore a) cooler and b) make the batteries last longer.

author: technix
author_rank: Super Contributor
timestamp: August 27, 2018, 10:57:48 am
content: Quote from: jbb on August 27, 2018, 08:13:16 amI can not put anything on the back of the board, since I need to install the board to the underside of my new wooden desk shelf or the steel side panel of my server rack using adhesive. It draws power from a 12V power brick.

author: technix
author_rank: Super Contributor
timestamp: August 27, 2018, 10:59:27 am
content: Quote from: Phoenix on August 27, 2018, 07:13:17 amThat means 106ºC in a 40ºC ambient (it is Shanghai urban center here, with a record high of 42ºC in summer.) Sheesh.

author: ocset
author_rank: Super Contributor
timestamp: August 27, 2018, 03:51:31 pm
content: just my 10 cents.Are you too far into DCM?..(is your inductor too small).But if the top of the chip is the only place you have for a heatsink, then thats where it must go........shove some gap pad on it and press a piece of metal down onto that and find some way of affixing it.Have you got a scope?......i dont know what your controller is...but shove the scope across the inductor say and see if the controller is switching smoothly at max load.......non smooth switching can make a buck run hotter than it should.You can tell by viewing inductor voltage how far into dcm (if at all) the buck is running.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 27, 2018, 04:54:27 pm
content: Quote from: technix on August 27, 2018, 10:57:48 amUse a thermal pad and attach it to the server rack.

author: technix
author_rank: Super Contributor
timestamp: August 27, 2018, 08:37:10 pm
content: Quote from: treez on August 27, 2018, 03:51:31 pmI spec'd the inductor based on the application note from the chip vendor. And the inductor is not running hot.Quote from: NiHaoMike on August 27, 2018, 04:54:27 pmAs long as I don't turn the LEDs too bright the SMPS won't be hot anyway, so I am going to implement a software brightness cap.

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 27, 2018, 08:43:48 pm
content: incorrect inductor material.. probe the current ramp across inductor IL

author: technix
author_rank: Super Contributor
timestamp: August 27, 2018, 09:07:11 pm
content: Quote from: diyaudio on August 27, 2018, 08:43:48 pmHow do you probe that on a densely-packed PCB?

author: mikerj
author_rank: Super Contributor
timestamp: August 28, 2018, 10:05:07 am
content: Quote from: technix on August 27, 2018, 09:07:11 pmUsually by soldering some fine wire across the component of interest (e.g. fine gauge enameled copper).