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Old 11-04-2014, 02:52 AM #1
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Default Killed diode when Mosfet on FLexMod P3 died

I have fried NDB7875 now, because of ded Mosfet on FlexMod P3
I want to do problem analysis to make sure I not going to repeat this again.

Setup:
FlexMod P3.3 with Bigger heatsink then original and Fan blowing on it
NDB7875 in copper AIXIZ module inserted in 12mm heatsink housing, fan blowing on it as well
Current 2.4A
Vin 24V
Vmod 5V (full power)
Not even 5 minute of playing with wood burning, I hear crack.... and LD is off. No smoke no smell. Driver heatsink was hot, but I could hold my hand on it for some time.
Diode was warm

In result, mosfet has crack on it!
Diode fried inside all leads to central crystal are melted (checked under microscope)
T fuse is blown.

I don't think I really pushed it (I mean driver) 2.4A for this LD (480mV on power resistor of FlexMod) is normal according to dtr-lpf

Everything was soldered, nothing shorted.
Diode reads infinity right now.

Potential cause: Mosfet overheat and short inside, sending some massive voltage to LD? My PSU is 24V 20A so it can fry stuff....
I dont have another ideas. May you be guys have ideas/suggestions?

PS. Where can I see schematic of FlexMod P3.3?


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Old 11-04-2014, 03:14 AM #2
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Read the instructions next time. That package is only designed to dissipate an abolute max of about 25W. You had it dissipating twice that. Lower your voltage from 24V to 7-9V, or it WILL happen again.

According to this guy, The FET is: IRLIB9343
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Old 11-04-2014, 03:29 AM #3
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Reading doesn't mean understanding
I though Maximum continuous power dissipation, is output power, in my case I would have been less then 25W.
Can you please share how did you estimated 50W of dissipation?

Thank you for mosftet info!
Also kinda sucks that there is no LD protection, having schematic would help alot.
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Old 11-04-2014, 03:42 AM #4
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Vin = 24V
I = 2.4V
Vo = 6V (estimating generously)

Pin = 24V*2.4A = 57.6W
Pout = 6V * 2.4A = 14.4W

Pdis = Pin - Pout = 57.6W - 14.4W = 43.2W disipated.

That's my rough calculations.
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Old 11-04-2014, 03:54 AM #5
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Default Re: Killed diode when Mosfet on FLexMod P3 died

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Pdis = Pin - Pout = 57.6W - 14.4W = 43.2W disipated.
That's my rough calculations.
make sense, but that means this drive physically cannot drive stuff at 24V 4A period. This assumption was my mistake, without thinking.
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Old 11-04-2014, 04:02 AM #6
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Default Re: Killed diode when Mosfet on FLexMod P3 died

You can apply 24V to the input of the driver if you have enough diodes in series to provide adequate voltage drop across the output.
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Old 11-04-2014, 04:57 AM #7
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Quote:
Originally Posted by xchg View Post
Also kinda sucks that there is no LD protection
Wat? There IS protection. The driver protects the laser diode. You blew up the driver

It's like cutting your seatbelt in half and setting fire to the airbag, then complaining the car is unsafe when you go flying through the windshield.
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Old 11-04-2014, 04:58 AM #8
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Default Re: Killed diode when Mosfet on FLexMod P3 died

So what would be the right solution - can I stick befoer FMod DC-DC switch mode converted like some of those one hand low from ebay, providing it can handle 4A?
LM2576 based for example?

Edit: LM2576 is 3A max, some other regulator like this

Last edited by xchg; 11-04-2014 at 05:11 AM.
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Old 11-04-2014, 05:08 AM #9
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Quote:
Originally Posted by Cyparagon View Post
Wat? There IS protection. The driver protects the laser diode. You blew up the driver

It's like cutting your seatbelt in half and setting fire to the airbag, then complaining the car is unsafe when you go flying through the windshield.
I don't think this is relevant comparison.... Overheating of mosfet would be critical failure, and when this happens it shorts and sent all Vin to diode... where is protection from driver failing?

But lesson learned - and lesson is - LEARN The hardware you work with.

I guess for schematic I would have to reverse engineer it. I hate this black glossy solder mask so much
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Old 11-04-2014, 05:20 AM #10
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Remove components and lightly sand the mask away. I do that all the time. Sure it's not usable after but way easier that probing pad's if the driver isn't to expensive.
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Old 11-04-2014, 06:27 PM #11
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Default Re: Killed diode when Mosfet on FLexMod P3 died

What about idea of replacing: IRLIB9343 by
this - http://www.irf.com/product-info/data...rf9530npbf.pdf
I'e checked specs - seems good too me.
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Old 11-04-2014, 09:13 PM #12
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Default Re: Killed diode when Mosfet on FLexMod P3 died

the right solution would be to not use a 24v dc psu on it.

why are you doing that?

Quote:
Originally Posted by xchg View Post
So what would be the right solution - can I stick befoer FMod DC-DC switch mode converted like some of those one hand low from ebay, providing it can handle 4A?
LM2576 based for example?

Edit: LM2576 is 3A max, some other regulator like this
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Old 11-04-2014, 09:26 PM #13
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Quote:
Originally Posted by swamidog View Post
the right solution would be to not use a 24v dc psu on it.

why are you doing that?
This is contraption attached to 3D Printer which has only 24V... and 5.
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Old 11-04-2014, 10:41 PM #14
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Here is post Laser Cutter addon for RigidBot 3D Printer explaining stuff
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Old 11-04-2014, 11:02 PM #15
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Quote:
Originally Posted by xchg View Post
where is protection from driver failing?
"I didn't know you couldn't give a rabbit a bath in the dish washer. Now drain is clogged from rabbit guts. Why did the dishwasher not have protection?"

You abuse it, it dies. You can try to blame that on poor dishwasher design, or you can take responsibility for your mistake.

Quote:
Originally Posted by xchg View Post
This is contraption attached to 3D Printer which has only 24V... and 5.
Neither will work. Add a DCDC converter to get 7V, or install a 7V power supply.
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Last edited by Cyparagon; 11-04-2014 at 11:04 PM.
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Old 11-05-2014, 01:56 AM #16
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Default Re: Killed diode when Mosfet on FLexMod P3 died

Quote:
Originally Posted by Cyparagon View Post
"I didn't know you couldn't give a rabbit a bath in the dish washer. Now drain is clogged from rabbit guts. Why did the dishwasher not have protection?"

You abuse it, it dies. You can try to blame that on poor dishwasher design, or you can take responsibility for your mistake.
I got it already
thank you, geez your analogies make my skin crawl
Quote:
Neither will work. Add a DCDC converter to get 7V, or install a 7V power supply.
[/QUOTE]
Looks like DCDC is the easiest way.

Last edited by xchg; 11-05-2014 at 03:01 AM.
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