Would anyone be willing to give me a couple minutes of help with a circuit?

[rhd]

I'm having trouble with the implementation of a MOSFET into a circuit that I otherwise know works properly.

If anyone is willing to glance at it and give me their thoughts, it would really be helpful. I'd be happy to toss a few fractions of a bitcoin in your direction as a thank you, lol

 

[NKO29]

Re: What anyone be willing to give me a couple minutes of help with a circuit?

What is the particular Mosfet in question? Pics of the circuit possible?

 

[rhd]

Re: What anyone be willing to give me a couple minutes of help with a circuit?

What is the particular Mosfet in question? Pics of the circuit possible?

Sure - and thanks.

This is the MOSFET:
http://www.diodes.com/datasheets/DMN2065UW.pdf

This is my circuit:

When I close the circuit (trigger the switch) the MOSFET burns itself out in a few seconds.

 

[NKO29]

Re: What anyone be willing to give me a couple minutes of help with a circuit?

I'm no electronics expert, so i may be totally wrong, but do you have your driver's negative side connected to the mosfet's DRAIN? even though it's the negative side, it is still a SOURCE for the mosfet... Just a thought, may be wrong

 

[rhd]

Re: What anyone be willing to give me a couple minutes of help with a circuit?

I'm no electronics expert, so i may be totally wrong, but do you have your driver's negative side connected to the mosfet's DRAIN? even though it's the negative side, it is still a SOURCE for the mosfet... Just a thought, may be wrong

Yes, it's connected to the drain.

I basically followed this circuit:

http://upload.wikimedia.org/wikiped...circuit.svg/571px-Mosfet_n-ch_circuit.svg.png

But instead of the load being a resistor+diode, the load is another circuit (a driver)

 

[paul1598419]

Re: What anyone be willing to give me a couple minutes of help with a circuit?

rdh, you have a simple switch circuit here and don't quite understand why you are using this circuit for a laser driver. It is an N channel enhanced mode MOSFET with the gate connected through a switch to the Vcc. This will make a switch, but nothing more. I did notice you have a schottky diode across the low side of the LD and ground. That won't give you any regulation though. There are probably dozens of ways to set this up to regulate the current flow, but can't do it for you this late. I will see what I can do tomorrow if you like.

 

[rhd]

Re: What anyone be willing to give me a couple minutes of help with a circuit?

This will make a switch, but nothing more. I did notice you have a schottky diode across the low side of the LD and ground. That won't give you any regulation though. There are probably dozens of ways to set this up to regulate the current flow, but can't do it for you this late. I will see what I can do tomorrow if you like.

That's all I'm looking for - a switch. Not current reg. I'm not looking to build a driver here, just a switch

You'll see that there are outputs labelled DRIVER- and DRIVER+ (they're not labelled LD). Imagine that those are the inputs to... a Flexdrive, or an X-Drive, or whatever you might be using for current reg.

This isn't a driver thread, just a switch thread. I can't see where I've gone wrong - this should work as a switch, no?

 

[paul1598419]

One problem that jumps out at me is the sum total of all your current, and by virtue all the power dissipated through your driver will go through your MOSFET. You may want to check the power dissipation of your entire circuit and see if your MOSFET can handle it for a lengthy duration. I'm assuming that for purposes of scale you are not using a heat sink. This is where I would start to look.

 

[rhd]

One problem that jumps out at me is the sum total of all your current, and by virtue all the power dissipated through your driver will go through your MOSFET. You may want to check the power dissipation of your entire circuit and see if your MOSFET can handle it for a lengthy duration. I'm assuming that for purposes of scale you are not using a heat sink. This is where I would start to look.

That's a good thought. I don't think that's the problem though.

Continuous drain current on this MOSFET is 2.3 to 2.8 A, and I was only running at ~1.5 A. Plus (and this is really the key) the MOSFET immediately burned up, and the circuit never powered up even for a second. If I was above current/heat dissipation tolerances (which I wasn't), it wouldn't immediately burn up. But, as mentioned, this MOSFET can handle the current I was feeding it, so I don't think that's the problem.

Could there be an issue powering the Gate with 8V ?

The Gate threshold voltage on this MOSFET is really low (like 1.5V I think). However, the Gate-to-Source voltage is 12V, so I thought I was fine.

 

[paul1598419]

rhd, it occurred to me that if you are trying to push all your power through this MOSFET you might be better off by using a DMOS instead of a CMOS FET. The DMOS is a double diffused metal oxide semiconductor as opposed to a complimentary metal oxide semiconductor and are used in circuits that need to displace a lot of power, which usually shows up in the form of heat. Look into it and see if one of these might be what you are looking for. Your gate set up looks fine to me for a switch. I don't think your problem lies there.

 

[NKO29]

Looking at the circuit you made, and the one in wikipedia i can see that you've probably connected the Drain and Source the wrong way

 

[rhd]

Looking at the circuit you made, and the one in wikipedia i can see that you've probably connected the Drain and Source the wrong way

The diode in that first photo is the LOAD, it's actually an LED. It is part of the circuit. It's not part of the MOSFET.

The diode you're circled in my design is just the MOSFET's internal diode. It is not part of my circuit, it's part of the MOSFET.

 

[NKO29]

Ah... But it still shows what way the current is flowing? The 2 internal diodes would not let that current pass would they? Just my opinion on this I can't see anything else that could be wrong do you have any spare mosfets to play with?

 

[rhd]

Ah... But it still shows what way the current is flowing? The 2 internal diodes would not let that current pass would they? Just my opinion on this I can't see anything else that could be wrong

That's the point of the internal diode. It's reversed biased. The current doesn't flow over the MOSFET's internal diode, it flows over the MOSFET. I think it can safely be ignored - it's just there.

 

[paul1598419]

The internal diode is a schottky diode and the drain is connected to the load while the source is connected to ground. This is connected properly to make a switch. I still think the problem is in the load. You may want to try a load resistor to limit the current. You can't see the initial load at the instant of conduction and that may be higher than the MOSFET's rating.

 

[rhd]

I don't want to limit current into a drivers input though. If that were the solution, then it would negate the benefit of a MOSFET's efficiency. It just seems wrong in principle to have to use a current limiting resistor on the input to a high efficiency switching current regulator (or whatever type of driver I power with this circuit).