Author Topic: Why do I keep killing these MOSFETS?  (Read 5869 times)

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Offline CaptainNomihodai

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Why do I keep killing these MOSFETS?
« on: March 12, 2017, 10:26:37 pm »
Hi,
The circuit in the attached image should be a simple constant current load, but I'm having some unexpected problems.
1. I keep killing one of the MOSFETS. I know when this happens when all of a sudden the current jumps to over 12 amps. For what it's worth, I measured about 1k \$\Omega\$ gate to source and 2.5k \$\Omega\$ source to drain on the last failed one. The MOSFETS are mounted on a relatively large heatsink with a 12V DC fan. The heatsink gets hot, but not so much that I can't touch it. The MOSFETS are mounted uninsulated, but I don't think that should matter since the drains are tied together anyway.
2.  The current tops out at about 3.3A, corresponding to about 9V on the inverting inputs (forcing the output voltage to the rail since the non-inverting input voltage is greater). By my understanding it should top out at over 10A, since R1-4 in parallel, while hot, measures about 3.5 \$\Omega\$ and the Rdson of the MOSFETS is .04 \$\Omega\$, low enough to be trivial by comparison. I've since noticed that my supply is putting out 36V instead of 50V, probably a bad filter cap or something, but even then I figure I should be getting about 10A max.
3. The op-amp seems pretty unstable. This is the least of my concerns right now, since it seems to be working well enough that I can adjust the current in the circuit with the pot (up the 3.3A, at least), it just fluctuates a lot. I'm wondering if this has something to do with the fact that I'm using a breadboard for the op-amp part of the circuit? I don't think this is a defective op-amp, unless I have a handful of defective LM324s.

I suspect there may be a simple explanation for all this that will make me hang my head in shame once revealed, but for now I'm stumped (this is the beginner's forum, after all...). I'm also wondering if I shouldn't rebuilt this circuit with BJTs, since in my experience they are less fragile than MOSFETS.

Anyway, any input is appreciated. Thanks.

Edit: updated with revised circuit
« Last Edit: March 13, 2017, 04:09:44 am by CaptainNomihodai »
 

Offline pelule

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Re: Why do I keep killing these MOSFETS?
« Reply #1 on: March 12, 2017, 10:43:17 pm »
As two MOSFETs are never exactly the same (just near the same, when produced at the same lot), you need to align them with a smal resistor. See the International Rectifier AppNotes:
http://www.infineon.com/dgdl/para.pdf?fileId=5546d462533600a401535744b4583f79
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Offline CaptainNomihodai

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Re: Why do I keep killing these MOSFETS?
« Reply #2 on: March 12, 2017, 11:15:23 pm »
Thanks for the reply, but I'm not quite sure I follow you. I get what you're saying, that I can't assume the MOSFETS to be identical, but I'm not sure how to align them with a resistor (I'm still working through that linked doc, but some of it's a bit over my head I think). Anyway, I was trying to account for mismatch by driving them with different opamp outputs. I now realize that this didn't matter since I had the inverting inputs tied together, and have modified my design accordingly. Is this what you were talking about?
That all being said, does mismatch even matter that much here, given what the MOSFETS I'm using are supposed to be able to handle? They're rated for 50A continuous and 300W power dissipation. Like I said, the circuit is maxing out at 3.3A with a 36V power source, so that's about 110W dissipation from the MOSFETS (once you account for what's dissipated from the power resistors).
 

Offline rstofer

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Re: Why do I keep killing these MOSFETS?
« Reply #3 on: March 12, 2017, 11:28:58 pm »
What's your voltage between the gate and source.
Ideally, your MOSFETs would turn fully on and the dissipation would be Rds*Id which should be a very small number.

I seriously doubt that you can get up to 10 Vgs and get there quickly.

I would look at a MOSFET Gate Driver - they are designed to drive the gate hard enough (amps, not milliamps) to overcome the capacitance in a small amount of time and will provide sufficient Vgs to ensure that the device is fully on.
 

Offline Benta

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Re: Why do I keep killing these MOSFETS?
« Reply #4 on: March 12, 2017, 11:36:30 pm »
I'm just wondering what on earth induced you to connect two opamps like this? Neither of them has a defined feedback path, they're competing and perhaps happily oscillating at a high frequency.

1: remove one of the opamps and drive both FETs from one. Keep the gate resistors and connect both to the (now single) opamp output.
2: limit the voltage from the pot by adding a series resistor, so the the input voltage to the opamp cannot be higher than +15 V - 2.5 V = 12.5 V

 
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Offline The Soulman

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Re: Why do I keep killing these MOSFETS?
« Reply #5 on: March 12, 2017, 11:42:56 pm »
What's your voltage between the gate and source.
Ideally, your MOSFETs would turn fully on and the dissipation would be Rds*Id which should be a very small number.

I seriously doubt that you can get up to 10 Vgs and get there quickly.

I would look at a MOSFET Gate Driver - they are designed to drive the gate hard enough (amps, not milliamps) to overcome the capacitance in a small amount of time and will provide sufficient Vgs to ensure that the device is fully on.


Yes in a switching application, this a linear dummy load.  ;)

Load sharing resistors for such a application are typically around 0,1 Ohm to keep size and power dissipation minimal.

 

Offline Benta

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Re: Why do I keep killing these MOSFETS?
« Reply #6 on: March 12, 2017, 11:45:32 pm »
What's your voltage between the gate and source.
Ideally, your MOSFETs would turn fully on and the dissipation would be Rds*Id which should be a very small number.

I seriously doubt that you can get up to 10 Vgs and get there quickly.

I would look at a MOSFET Gate Driver - they are designed to drive the gate hard enough (amps, not milliamps) to overcome the capacitance in a small amount of time and will provide sufficient Vgs to ensure that the device is fully on.

The MOSFETs are operating linearly. VGS is probably around 3...4 V.
Gate drivers are for switching, but not all MOSFET applications are hard switch types. Just sayin'
 

Offline CaptainNomihodai

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Re: Why do I keep killing these MOSFETS?
« Reply #7 on: March 12, 2017, 11:50:13 pm »
I'm just wondering what on earth induced you to connect two opamps like this? Neither of them has a defined feedback path, they're competing and perhaps happily oscillating at a high frequency.

1: remove one of the opamps and drive both FETs from one. Keep the gate resistors and connect both to the (now single) opamp output.
2: limit the voltage from the pot by adding a series resistor, so the the input voltage to the opamp cannot be higher than +15 V - 2.5 V = 12.5 V

My idea was to drive each FET from its own opamp because there is a full 2V range that the threshold voltage could fall in, according to the datasheet, so I didn't want one to be above Vt and the other not, etc. I noticed earlier that my design was wrong in that their inverting inputs were tied together. My modified design as attached to my second post has each of them with their own feedback loop to accomplish this.
 

Offline Benta

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Re: Why do I keep killing these MOSFETS?
« Reply #8 on: March 13, 2017, 12:01:32 am »
I'm just wondering what on earth induced you to connect two opamps like this? Neither of them has a defined feedback path, they're competing and perhaps happily oscillating at a high frequency.

1: remove one of the opamps and drive both FETs from one. Keep the gate resistors and connect both to the (now single) opamp output.
2: limit the voltage from the pot by adding a series resistor, so the the input voltage to the opamp cannot be higher than +15 V - 2.5 V = 12.5 V

My idea was to drive each FET from its own opamp because there is a full 2V range that the threshold voltage could fall in, according to the datasheet, so I didn't want one to be above Vt and the other not, etc. I noticed earlier that my design was wrong in that their inverting inputs were tied together. My modified design as attached to my second post has each of them with their own feedback loop to accomplish this.

Sorry, didn't see your second schematic before I posted last. Yes, that solves the feedback problem.

Did you test it yet?

 

Offline raspberrypi

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Re: Why do I keep killing these MOSFETS?
« Reply #9 on: March 13, 2017, 12:10:42 am »
Dont you want to tie pin 1 and 2, and 13 and 14 together with a resistor in series? Every op amp I hae seen has this set up to keep it balanced.
I'm legally blind so sometimes I ask obvious questions, but its because I can't see well.
 

Offline David Hess

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Re: Why do I keep killing these MOSFETS?
« Reply #10 on: March 13, 2017, 12:33:13 am »
The MOSFETs are not sharing current at all; only one is being used at at time.

 

Offline AndersJ

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Re: Why do I keep killing these MOSFETS?
« Reply #11 on: March 13, 2017, 12:44:01 am »
The two feedback lines are crossed.
The op-amps have no feedback.
"It should work"
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Offline rstofer

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Re: Why do I keep killing these MOSFETS?
« Reply #12 on: March 13, 2017, 01:47:05 am »
Having the load resistors in the bottom side means the source voltage is increasing as the load increases.  As a result, the gate voltage needs to go even higher to get Vgs into range.

Ordinary MOSFETs take a LOT of gate current to switch in small time.  Small time is required to keep the device out of its active region which is where all the heating occurs.  Sometimes logic level MOSFETs will work with lower gate voltages but when you see specs like Vgs=10V at 28A versus 3V at some low number of mA, you know you have to drive the gate hard and high.
 
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Offline T3sl4co1l

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Re: Why do I keep killing these MOSFETS?
« Reply #13 on: March 13, 2017, 04:00:37 am »
This is not how you parallel MOSFETs in linear operation.

LM324 is also a particularly poor choice.

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Offline CaptainNomihodai

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Re: Why do I keep killing these MOSFETS?
« Reply #14 on: March 13, 2017, 04:29:46 am »
This is not how you parallel MOSFETs in linear operation.

LM324 is also a particularly poor choice.

Tim

Can you (or anyone) elaborate on this? Please note that I have fixed the error in the original schematic (now removed) where the feedback lines were shorted together. Also what's wrong with an LM324? I mean, all the opamp has to do here is match the voltage across the resistors, it doesn't need to be particularly fast or anything, right? I picked it because it could run single supply.

Having the load resistors in the bottom side means the source voltage is increasing as the load increases.  As a result, the gate voltage needs to go even higher to get Vgs into range.

Ordinary MOSFETs take a LOT of gate current to switch in small time.  Small time is required to keep the device out of its active region which is where all the heating occurs.  Sometimes logic level MOSFETs will work with lower gate voltages but when you see specs like Vgs=10V at 28A versus 3V at some low number of mA, you know you have to drive the gate hard and high.


Well, since this is a dummy load, I want my MOSFETS in the active region. That being said, I think your first sentence answers my question as to why I'm topping out at 3.3A... I was failing to account for those resistors pulling the source voltage up and assuming that my Vgs was the gate-ground voltage.

The MOSFETs are not sharing current at all; only one is being used at at time.



I'm pretty sure this is false. I don't see how that could be the case, and my measurements indicate otherwise. Even if it were true, though, these FETs are rated for much more than I'm giving them.


So, question #2 is answered, but it's still not clear as to why my FETs keep dying.
 

Offline Mechatrommer

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Re: Why do I keep killing these MOSFETS?
« Reply #15 on: March 13, 2017, 05:19:59 am »
The two feedback lines are crossed.
The op-amps have no feedback.
yup and further investigation the logic reveals a nightmare. when a mosfet flows too much current, it will turn off the other mosfet. this is race condition not sharing, who is infront will further push its contender to the back, making him alone win the race = burnt. or maybe a rescue condition, you shutdown let me burnt. except when the the first one died in open/disconnect mode, the other will follows. but if the burnt one got shorted, the other will be spared.
« Last Edit: March 13, 2017, 05:25:15 am by Mechatrommer »
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Offline MK14

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Re: Why do I keep killing these MOSFETS?
« Reply #16 on: March 13, 2017, 05:25:03 am »
How well heatsinked are the Mosfets ?

The 300 Watt rating, is done by keeping the Heatsink (with zero thermal resistance between case and heatsink, in practice it won't be zero!) at a constant 25 deg C, while soaking up 300 Watts of heat. I'd be interested in knowing how you are doing this ?
Hint: It can only cope with a lot less than 300 Watts in practice.

As others are trying to explain to you. You seem to have the op-amps the wrong way round, in that they are controlling the wrong/opposite Mosfet.
I.e. Each Op-amp is sensing the current and then controlling the mosfet, connected to the OTHER pair of 10 Ohm resistors.
This could be fixed by swapping the output connections around (pins 1 and 14).

Possible causes of Mosfet destruction could be if they are overheating or if the gate voltage is (even for a tiny amount of time, like  a microsecond), exceeding around 20 volts.

There are much better op-amps, than the LM324.
« Last Edit: March 13, 2017, 05:30:18 am by MK14 »
 
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Offline Paul Moir

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Re: Why do I keep killing these MOSFETS?
« Reply #17 on: March 13, 2017, 05:53:30 am »
Quote
I picked it because it could run single supply.

This is a myth I have personally suffered which must die.  There is no difference between a "single supply opamp" and any other opamp.  The claim comes from that the LM324 could operate within ~1.5V of V+ and right down to about V- so could be useful with a +5V/0V supply, while the ancient dinosaurs it was competing with like the LM741 couldn't operate within something like 5V of V+ or V-.  Otherwise same thing.  Of course with today's low voltage designs, most modern opamps are "rail-to-rail" or near enough that single supply operation is no problem at all.

 
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Offline CaptainNomihodai

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Re: Why do I keep killing these MOSFETS?
« Reply #18 on: March 13, 2017, 06:21:05 am »
How well heatsinked are the Mosfets ?

The 300 Watt rating, is done by keeping the Heatsink (with zero thermal resistance between case and heatsink, in practice it won't be zero!) at a constant 25 deg C, while soaking up 300 Watts of heat. I'd be interested in knowing how you are doing this ?
Hint: It can only cope with a lot less than 300 Watts in practice.

As others are trying to explain to you. You seem to have the op-amps the wrong way round, in that they are controlling the wrong/opposite Mosfet.
I.e. Each Op-amp is sensing the current and then controlling the mosfet, connected to the OTHER pair of 10 Ohm resistors.
This could be fixed by swapping the output connections around (pins 1 and 14).

Possible causes of Mosfet destruction could be if they are overheating or if the gate voltage is (even for a tiny amount of time, like  a microsecond), exceeding around 20 volts.

There are much better op-amps, than the LM324.

I've got them on a relatively large heatsink with a fan. It gets warm but not too hot to touch. Really good to know that the 300W figure is unrealistic, though.

And, oops, those feedback lines are indeed crossed. I think that was just a mistake on the schematic, but I'll check the physical circuit tomorrow.

Regarding gate voltage exceeding ~20V as a means of failure, is there any way, in this case, that that could happen since the opamp driving the gate is powered by only 15V? I don't see how, but then again I didn't notice that those feedback lines were crossed either...
 

Online BrianHG

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Re: Why do I keep killing these MOSFETS?
« Reply #19 on: March 13, 2017, 06:25:41 am »
Doesn't the LM324 latch up if either of the inputs ever reach the rails, ie 0v, or 15v.
I would use an opamp which supports rail-rail inputs at least in this circuit, or, add series resistors feeding the op-amp inputs, namely the feedback - input, to prevent internal latchup.

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Offline blueskull

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Re: Why do I keep killing these MOSFETS?
« Reply #20 on: March 13, 2017, 06:30:53 am »
Two small caps (say, 1nF) across R5 and R6?
 

Offline Mechatrommer

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Re: Why do I keep killing these MOSFETS?
« Reply #21 on: March 13, 2017, 06:34:39 am »
Regarding gate voltage exceeding ~20V as a means of failure, is there any way, in this case, that that could happen since the opamp driving the gate is powered by only 15V? I don't see how
possible if you have inductive path to the gate. when opamp is slewing off hard, your gate path become boost converter. put fast acting diodes from gate to 15V and from gate to gnd to show some respect to the mosfet.
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Offline T3sl4co1l

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Re: Why do I keep killing these MOSFETS?
« Reply #22 on: March 13, 2017, 08:20:13 am »
This is not how you parallel MOSFETs in linear operation.

LM324 is also a particularly poor choice.

Tim

Can you (or anyone) elaborate on this?

Certainly, :)

1. You need separate source resistors.  Otherwise there is no way to sense the currents of the individual transistors.  (This has been fixed.)
2. Feedback needs to be to each respective op-amp.  (This is a new bug that has been introduced.)  As shown, you have a bistable flip-flop, which will hog as much current as possible through one transistor.
3. Your transistors are fuckoff massive, as far as most anything in this circuit is concerned, except for power dissipation.  (This is the price to pay for using ancient switching transistors in linear applications.)

The "size" of a transistor refers to the gate capacitance (which the op-amp output pin must drive), its current capacity and Rds(on), the V*I capacity of its SOA, etc.

Modern transistors are about 5x smaller on gate capacitance, for the same V*I capacity (of course, they're about that many times lower in power dissipation, too).

If you've learned op-amps in school, I'm sorry to break it, but you've been lied to.  There is absolutely no such thing as an ideal voltage source.  No op-amp has one attached to its output pin.  This isn't a terrible approximation at DC, but when things start moving, it's a preposterous error.  A reminder that: a circuit is always moving, and even if your intent is to have it doing absolutely nothing, sitting statically at DC, it still must reach that operating point first, and if it spins out of control on its way there, that's your problem.  There is no such thing as an "intent sensing circuit". ;)  The circuit does precisely what it does, no matter what you wanted it to do...

Since op-amps are nonideal, we must consider the effect of the transistor's gate capacitance on its output pin.  Capacitive loading is almost always a bad idea, and LM324 falls into the set of op-amps where this is true.  Most likely, this circuit will oscillate, just from capacitive loading, without any other help from the transistor and load.

To prevent oscillation, we must compensate the amplifier.  The first step is to add a series gate resistor Rg (which you have -- a good start).  The next is to add a series resistor R to the feedback path (i.e., from shunt resistor to -in).  Finally, a C or Rz+C is connected between op-amp output, and -in.  The R*C time constant must be on the order of the Rg*Ciss time constant, and Rz (on the order of R) can be used to adjust damping.



It looks like this, except with an optional resistor in series with C1, and with the junction between R2 and R3 being the MOSFET (in which case, C3 represents its gate capacitance, which won't exactly be going to ground, but the shunt resistors are on the small side, so it's close).

Measure damping by making step changes to the reference potentiometer (most often, by replacing it with a square wave generator), and observing the stability of the output current (rise time, undershoot / overshoot, ringing).

Note that stability will vary with what kind of load is attached, as well as the current setting.

4. LM324 is bad.  Just, in general.  It's okay for slowly varying servo loops, and poor quality audio applications, but not much beyond that.  It has much more noise than most op-amps, mediocre input offset, slow response, and worst of all, a class B output stage.  What does that mean?  The exact output voltage isn't quite equal to the voltage "intended" by the op-amp's inner workings (namely, the voltage on its internal gain node), but has some "slop", just the same way a shaft, that fits loosely in its bearing, has some slop against rotation.  It can be locked to a position or angle at one end, yet is still free to move some at the other end.

The op-amp's input side eventually sees the error, and corrects for it, but this takes time.  The output is the time-integral of the input, so over a short period of time (a couple microseconds), you can change the output by a quite noticeable amount (fractional volt), with very little current (indeed, this effect is strongest around 0mA output), until the op-amp corrects the error (10s or 100s of us later).

By the way, lie #2: if you were taught op-amps are infinite gain, they aren't.  They have high gain at DC, but having high gain at AC would be not only absurd, but physically unrealizable, and impossible to use (if not other things, like violating causality itself!).  No, an op-amp is an integrator: its output changes over time, based on an input.  An integrator has infinite gain at DC (ideally), so this works identically at DC, but the gain drops smoothly with frequency, eventually dropping below 1 at a well-defined frequency (beyond which it isn't useful; for the LM324, this is a couple of MHz).

One more rub: the output pin, besides having nonzero impedance, and a really messed up impedance that depends on output current -- it can only source so much current, around 10mA.  And when it's delivering a large current, it's not able to pull all the way up (only maybe 2-3V below +V), or down (maybe 1V above GND).  This fundamentally limits how fast the gate voltage can be changed.  This also provides a rationale for choosing Rg: it should be on the order of Rg = (+V) / (Iout(pk)).  Or about 1kohm in this case.  (That beefy >10nF gate will therefore have a time constant in the >10us range, which honestly isn't terrible considering that's only a little slower than the poor op-amp can go, anyway.)

Now, with enough Rg to allow the output pin voltage to move freely, and an awareness of the limitations of the LM324, and external compensation to stabilize it, you have a hope of getting useful operation out of the circuit. :)

Tim
« Last Edit: March 13, 2017, 08:25:26 am by T3sl4co1l »
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Offline Mechatrommer

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Re: Why do I keep killing these MOSFETS?
« Reply #23 on: March 13, 2017, 10:01:10 am »
it can only source so much current, around 10mA.
to be precise, LM324 short circuit and source current spec is 40mA.

to the OP, your gate resistor is 100 ohm, at some points you are burdening the opamp since you theoritically try to pull 15V / 100 = 150mA from it way beyond its spec. dont be surprised if your LM324 will burn as well soon. set the burden current half of spec, 20mA or 30mA if you are too desperate for current juice, that makes Rgate = 15 / 0.02 = 750 ohm. btw forget the LM324 bashing, people have made usefull stuffs out of even an UA741 chip. LM324 is certainly perfectly fine for you application, given that you correct the surrounding details explained such as feedback path., and especially the feedback path.
« Last Edit: March 13, 2017, 10:03:41 am by Mechatrommer »
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Offline MK14

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Re: Why do I keep killing these MOSFETS?
« Reply #24 on: March 13, 2017, 10:29:44 am »
it can only source so much current, around 10mA.
to be precise, LM324 short circuit and source current spec is 40mA.

To be even more precise, his 10mA is apparently actually bang on!
If you are going to correct someone else, then it is best to get your facts right (I may regret saying that, at some point!).

The 40mA on the datasheet seems to be a typical figure at room temperature.

https://www.onsemi.com/pub/Collateral/LM324-D.PDF (I started using the TI datasheet, but it seemed to contain a technical error/mistake (as regards the output current), so used the onsemi one).

But 10mA is guaranteed as a minimum (and across the full temp range). (Or 20mA at room temp, but best to allow for the internal case temp to get quite warm, so the 10mA is still the better figure to use).

There are a number of limitations with the LM324, which might make choosing a better op-amp, a good idea. Such as its limited voltage output swing, compared to more modern parts (e.g. full output rail to rail). They (modern/better/rail-to-rail-output) are still relatively cheap, so why not use them ?
« Last Edit: March 13, 2017, 10:43:59 am by MK14 »
 


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