DC-DC Boost Converter MOSFET is overloading once the power is turned OFF ?!?

Thread Starter

-Ty-

Joined Feb 5, 2017
83
Hey all,

I'm trying to build a photography light and I've been running into an issue with my DC-DC boost converters burning out for a reason I can't identify.

The circuit is pretty simple: A 120W AC-DC power adapter is outputting 12V to the boost converter (LINK), which is bumping it up to 32V, with a constant-current limit of 3A. The boost converter is then outputting to a 100W LED chip rated for a forward voltage of 30-34V and a max current of 3.5A. The power adapter is also feeding into a dc-dc step down module which is regulating the current to under-volt a 12V computer case fan, for cooling.

Now, despite burning out the first board for a reason I think i understand, I got the second board to work steadily and without failure. That was, until I turned it off one time.

I had been running the LED for about a half hour (and this was the fifth time ive done so), and then when I was done, i flicked the switch in the circuit to cut the power between the laptop adapter and the rest of the equipment, and this is when I noticed that the powder adapter's LED light was flickering, as it had done when the last board burned out and was shorting. Realizing what was about to happen, i went to pull the plug, but I was too slow and the second board burnt out.

The component that is burning out is the NCE6075K N-Channel Enhancement MOSFET (DATASHEET), and I don't know enough about electronics to figure out why exactly.

I find it very strange in my mind that the board only burnt out after the switch was turned off (and i think the switch is working fine, it had been up until then).

I need to figure out what the problem is so that I can avoid it with the replacement boards.... I figure maybe the capacitors in the system are discharging through the MOSFET after the switch is turned off, but I don't know enough about electronics to know if this is an issue.

Unfortunately, I can't provide a circuit diagram for the boosting boards because I can't seem to find one anywhere on the internet. Instead, please take this crudely-drawn illustration of the basic circuit.

Circuit Diagram.jpg

Any help is greatly appreciated, thank you.
 

#12

Joined Nov 30, 2010
18,217
I don't know the answer, but I can start with some basics.

If there is no connection in one of the wires back to the first buck converter, the capacitors in the other converters can not discharge back into it. This is a solid fact.
Therefore, your first converter must be getting its power to burn out from the 120 VAC power line.
Your 120 watt power budget is just barely enough, so the first converter must be running near its maximum capability (above 90%). That is going to leave it thermally hot when you shut off the loads. Check to see if it requires some minimum load.
Your 32 volt converter is rated for 250 watts and you are running 96 watts into the LED. That looks OK.
Computer fans do not respond well to reduced voltage control because they internally chop the input voltage to run the fan with AC current.

These are only clues.
I suggest you turn off the power in the 120 volt power line instead of turning off the loads.
 

Thread Starter

-Ty-

Joined Feb 5, 2017
83
I don't know the answer, but I can start with some basics.

If there is no connection in one of the wires back to the first buck converter, the capacitors in the other converters can not discharge back into it. This is a solid fact.
Therefore, your first converter must be getting its power to burn out from the 120 VAC power line.
Your 120 watt power budget is just barely enough, so the first converter must be running near its maximum capability (above 90%). That is going to leave it thermally hot when you shut off the loads. Check to see if it requires some minimum load.
Your 32 volt converter is rated for 250 watts and you are running 96 watts into the LED. That looks OK.
Computer fans do not respond well to reduced voltage control because they internally chop the input voltage to run the fan with AC current.

These are only clues.
I suggest you turn off the power in the 120 volt power line instead of turning off the loads.
I should specify, in case i didnt make it clear, its not the AC-DC power adapter from the wall that's overheating or overloading, its the MOSFET on the DC-DC Boost converter downstream. That being said, I like your comment about moving the switch... I was wondering if that would do anything but I can't figure out why it would matter. If one of the cables leading to the boost converter is cut, it shouldn't matter that the other (shown as red in the drawing) is still connected, should it? Im no electrical engineer but even i know a circuit has to be completed for electricity to flow... (though i dont know if the internal circuitry in the boost converter somehow bypasses this).
 

#12

Joined Nov 30, 2010
18,217
If one of the cables leading to the boost converter is cut, it shouldn't matter that the other (shown as red in the drawing) is still connected, should it?
That's what I said, but I was focused on the first converter.
its not the AC-DC power adapter from the wall that's overheating or overloading, its the MOSFET on the DC-DC Boost converter downstream.
That is a part I did not understand. The 32 volt converter is still connected to the fan controller when you open the ground to the first converter. Try removing the fan controller. It's not the right thing to do to a computer fan and it might be part of the problem.
 

Thread Starter

-Ty-

Joined Feb 5, 2017
83
That's what I said, but I was focused on the first converter.

That is a part I did not understand. The 32 volt converter is still connected to the fan controller when you open the ground to the first converter. Try removing the fan controller. It's not the right thing to do to a computer fan and it might be part of the problem.
Yeah, i was wondering if the fan controller might have something to do with it..

I'll consider removing it, but i still need a way to power the fan to cool the system, and not at full speed (12V). Do you know of any better way to do this?
 

#12

Joined Nov 30, 2010
18,217
a way to power the fan, and not at full speed (12V).
Use a smaller fan or choke the intake air with a mechanical baffle.
Computer fans need pulse width modulation to slow them down. They just get annoyed with DC dimming.
It works, somewhat, but it's not the right way to do it.
Besides, you don't need to slow the fan down during this troubleshooting activity.
 

ian field

Joined Oct 27, 2012
6,539
Hey all,

I'm trying to build a photography light and I've been running into an issue with my DC-DC boost converters burning out for a reason I can't identify.

The circuit is pretty simple: A 120W AC-DC power adapter is outputting 12V to the boost converter (LINK), which is bumping it up to 32V, with a constant-current limit of 3A. The boost converter is then outputting to a 100W LED chip rated for a forward voltage of 30-34V and a max current of 3.5A. The power adapter is also feeding into a dc-dc step down module which is regulating the current to under-volt a 12V computer case fan, for cooling.

Now, despite burning out the first board for a reason I think i understand, I got the second board to work steadily and without failure. That was, until I turned it off one time.

I had been running the LED for about a half hour (and this was the fifth time ive done so), and then when I was done, i flicked the switch in the circuit to cut the power between the laptop adapter and the rest of the equipment, and this is when I noticed that the powder adapter's LED light was flickering, as it had done when the last board burned out and was shorting. Realizing what was about to happen, i went to pull the plug, but I was too slow and the second board burnt out.

The component that is burning out is the NCE6075K N-Channel Enhancement MOSFET (DATASHEET), and I don't know enough about electronics to figure out why exactly.

I find it very strange in my mind that the board only burnt out after the switch was turned off (and i think the switch is working fine, it had been up until then).

I need to figure out what the problem is so that I can avoid it with the replacement boards.... I figure maybe the capacitors in the system are discharging through the MOSFET after the switch is turned off, but I don't know enough about electronics to know if this is an issue.

Unfortunately, I can't provide a circuit diagram for the boosting boards because I can't seem to find one anywhere on the internet. Instead, please take this crudely-drawn illustration of the basic circuit.

View attachment 120613

Any help is greatly appreciated, thank you.
My first thought is back emf.

A manufactured converter should have whatever precautions it needs already built in.

If I ended up desperate enough, I'd examine parasitic inductance of external wiring - but the converter should have input and output filter caps that take care of that.
 

ScottWang

Joined Aug 23, 2012
6,988
Just don't trust what the seller claimed that the DC-DC 250W Boost Converter really could provide the power that high.

The Vds and Ids of NCE6075K N-Channel is ok for your application, but you can try some others that the Rds <= 2.4mΩ.

You should desolder the mosfet from PCB and put it on a heat sink, and the fan no needs the DC-DC step down module, if you want maybe you can use a 555 pwm to adjust the speed or just connects the fan to +12V directly.
 
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