How to modify DC Buck converter to provide more power?

Thread Starter

electronice123

Joined Oct 10, 2008
323
All,

I burned up several Buck Converters (SZBK07-uses LM2511). They're rated for 300W, 20A. Input is 21V adj to 7.5V out. Running brushless motors with changind load, 15A peak current from measurements. With electronic load the converter puts out 22A @ 7.5V but gets HOT.

I think my problem is HEAT. First attempt to make them more reliable was to add cooling fans and so far that seems to be working but maybe I just got a good one. Looking for advice/suggestions as I really need these to be more reliable and can't find anything better in the same size or smaller. Here are my thoughts/research so far on what might help:

1. Add cooling fans to heatsinks
2. Change the NCE8290's to IRFB3077's
NCE8290's rated for 82V, 90A and have an Rds of 7.5mΩ while the IFRB3077 is rated for 75V, 120A and has a much lower Rds of 3.3mΩ.
3. Change the R004 current sense resistor to an R005 to reduce peak current by about 2A

I have not been able to determine exactly why they are failing but I'm sure I'm overloading/overheating them. One had a shorted FET, another loses output voltage once I get to 13.5A draw, One has a Low Side FET that gets really hot @ no load and that one won't put out any voltage, and One lost all regulation-with 21V in output is 18.5V but still puts out good current?

Any help/suggestions appreciated.
 

LowQCab

Joined Nov 6, 2012
2,506
You are most likely becoming confused by the special Chinese-Power-Rating-System .........
You need to take off at least 2-zeros, or Decimal-Places, to get the actual Power-Rating numbers.

Heavy-Duty Power-Supplies, that are actually reliable, are not cheap or small in size.
Check out one of the major suppliers, like DigiKey, they don't sell cheap-Hobby-Toys.
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Thread Starter

electronice123

Joined Oct 10, 2008
323
I agree that Chinese supplies are not rated accuratly.
Also; how are you measuring load current? Motors often pull very high current for short amounts of time.

I Measured load current using a bench top power supply, most of the time it's under 8A, I know it's not 100% accurate and that the brushless motors pull much higher surge currents.

Problem is, some of them have ran great with no problems. I have one going on a year so I have hopes that if I change a few things they'll be more reliable. I really do think my biggest problem has been overheating as when they're running they get really hot.
 

Thread Starter

electronice123

Joined Oct 10, 2008
323
IMHO, running a motor from an SMPS is the height of folly. It seldom ends well.
I'm not driving the motor directly it's going through an ESC which has some high quality input caps.

Motors run great, ESC's run great, buck converters fail but not always. Some of them have lasted over a year with no problems at all.

I just need to find out what's causing the failures. I think heat is my biggest problem but none of the failures seem to be the same so it's hard to pinpoint what is actually failing.... which again leads me to think it's heat.

One thought is that when the FETs overheat they are allowing overcurrent which is damaging other components?
 

Thread Starter

electronice123

Joined Oct 10, 2008
323
You are most likely becoming confused by the special Chinese-Power-Rating-System .........
You need to take off at least 2-zeros, or Decimal-Places, to get the actual Power-Rating numbers.

Heavy-Duty Power-Supplies, that are actually reliable, are not cheap or small in size.
Check out one of the major suppliers, like DigiKey, they don't sell cheap-Hobby-Toys.
.
.
.
I agree with you. But I have tested these buck converters with an electronic load. They'll put out 22A peak at 7.5V which surprised me, granted they probably won't do it for long.

Realistically I'm pulling less than half that 80% of the time. I'm just going to do more testing with added cooling and change the FETs to get a lower Rds to reduce my heat and see if I still have problems.
 

LowQCab

Joined Nov 6, 2012
2,506
They are poorly designed Circuits using the fewest-number of the cheapest knock-off Parts,
with a completely inadequate Heat-Sinking-System.
You've been lucky ...........
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