24V SMPS gets really hot, swap the MOSFET ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 31, 2018, 12:00:43 am
content: I have 2 or 3 24V SMPS power bricks for LCD monitors. And all 3 get pretty darn hot. So they aren't very efficient. I've opened 2 of them and they look nice and tidy at least, and pretty straight forward.So would it be the main MOSFET is likely cheap, and has a high Vds resistance and so consumes that much more energy ? It would be fun to swap a MOSFET, nicer than adding a fanI think these use a PWM chip, IDK if I can change the frequency or anything. Soon I'll play around with the 2 unused ones.(I'm assuming they can deliver power required, and aren't over driving or something)

author: strawberry
author_rank: Super Contributor
timestamp: March 31, 2018, 02:19:20 am
content: transformer is designed to specific frequency

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 31, 2018, 04:10:25 am
content: Of all the power brick SMPS I can remember in the last 10yrs, these 3 for 27" Korean IPS monitors all ran hot.So why tho ? I know some equations for this stuff, what's the weak efficiency link usually?Soon I shall scope this thing

author: KL27x
author_rank: Super Contributor
timestamp: March 31, 2018, 04:48:59 am
content: QuotePlease be careful. Voltage dividers... Differential probes... proper precautions. You and your scope are at risk of dying, here. Personally, I'm more worried about your scope. But your priorities are probably different.

author: killcode
author_rank: Contributor
timestamp: March 31, 2018, 05:32:35 am
content: Keep in mind that Rds isn't the only criteria for 'FET selection. Qg, Ciss etc all must be considered. Swapping in a lower resistance 'FET only to increase switching losses (assuming gate driver can even cope with it) because of higher gate charge is pointless.And to echo others, be careful with mains voltage...

author: Jwillis
author_rank: Super Contributor
timestamp: March 31, 2018, 07:19:22 am
content: You could probably replace the MOSFET for a beefier one easily.And if there is enough room ,some added heat sinking would help. Consumer goods these days are made just barely with a certain life expectancy. It's not a manufacturers interest to produce products that last. Not to start a argument about "the good ol' days" but My parents bought a Hitachi color TV back in the mid seventies.That TV ran for over 40 years. I doubt that any newer would last that long.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2018, 11:38:06 am
content: Don't make assumptions.Test!Open it up, run it under full load for 10 minutes, then measure temperatures. Is the transistor actually putting out that much heat? How about the output diode, or capacitors, or the transformer?Typically, power is evenly distributed through all these components, plus snubbers and other minutiae. You can reduce loss in any one component, but mind the sausage effect: example, a bigger transistor has less Rds(on), but more switching loss and more drive power. More drive power means more auxiliary power (controller) dissipation. More switching loss means more snubber loss.The transformer will be your elephant in the room. You can't really change it, due to footprint and form factor; and even if you find another the same size, makes you think the new one will have any lower loss? (You can purchase new cores of lower-loss materials, if you're willing to go that route -- but if you're limited to salvaged parts, who knows?) You can use more turns (less flux density --> less core loss), but you have to use thinner wire (or fewer interleaved layers) to fit it on the same bobbin --> more copper loss. Again, the losses (core and copper) are normally distributed evenly, so there isn't much to save here.If you put it in a larger enclosure to accommodate all these changes (beefed up parts, maybe even a chopped up PCB to make everything fit), you still have the problem that, after all this screwing around, EMI is probably worse. (There are very real cases were switching loss is traded for EMI, so if you're strictly optimizing for efficiency, you will end up with EMI being much, much worse!) If nothing else, you're changing the components and layout, which is likely to change EMI by -- who knows how much. Safest is to put it inside a metal enclosure, with line filters on both ends, so the noise stays inside.Tim

author: janoc
author_rank: Super Contributor
timestamp: March 31, 2018, 01:24:50 pm
content: A much simpler (and cheaper + safer too) solution would be to replace the power bricks for ones that are rated for higher current.Poking around a mains SMPS is not really a job for a beginner, IMO.

author: wraper
author_rank: Unknown
timestamp: March 31, 2018, 01:53:04 pm
content: Opening post makes me cringe. They get hot not because of low efficiency but because of lack of cooling. How much heat do you think can dissipate tiny heatsink in closed plastic brick?

author: strawberry
author_rank: Super Contributor
timestamp: March 31, 2018, 04:10:13 pm
content: DCM (discontinuous conduction mode) utilize components at max and generate heat. Increase primary inductance and core size can fix thisReduced primary filter capacitor (perhaps for better PF) cause more ripple therefore more loses in MOSFETRC and C snubbers across secondary diode and primary MOSFET could cause heat. Without them could drop ~10degC

author: james_s
author_rank: Super Contributor
timestamp: March 31, 2018, 06:26:41 pm
content: How about measure the efficiency and find out how much loss they actually have? Measure the input power with something like a Kill A Watt and measure the output power with a multimeter, a power resistor or incandescent lamp as a test load may make this easier. The efficiency may not be as bad as you think.