Author Topic: Why does the current keep increasing when using an N-mosfet ?  (Read 14217 times)

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Offline DuncanSteelTopic starter

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Why does the current keep increasing when using an N-mosfet ?
« on: October 22, 2016, 02:14:16 pm »


Hi

I watched the Dave make the dummy load & made my own, now the problem is that the current keeps increasing with time, even tho I am not increasing the voltage with potentiometer. Why is this happening?

Another question i have is that if I apply 0.5V to the n-mos source & replace the resistor with 0.2 Ohm value, I should be getting 2,5 A. In reality Im getting only barely an Amp. How  comes?
« Last Edit: October 22, 2016, 02:15:54 pm by DuncanSteel »
 

Offline CJay

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #1 on: October 22, 2016, 02:30:08 pm »
Because your MOSFET isn't turning on, it won't be fully turned on until the gate is around 10V, you need a logic level MOSFET as Dave very clearly mentions right at the start of the video, those should be fully turned on at 5V.
 

Offline DuncanSteelTopic starter

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #2 on: October 22, 2016, 02:39:23 pm »
I am a little confused, I thought that Gate Threshold voltage means that the mosfet will turn fully on at this voltage typical 3V for my IRF540N?  what kind of parameter should I be looking at in the datasheet?

IRF540N Datasheet
 

Offline bktemp

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #3 on: October 22, 2016, 02:53:48 pm »
I am a little confused, I thought that Gate Threshold voltage means that the mosfet will turn fully on at this voltage typical 3V for my IRF540N?  what kind of parameter should I be looking at in the datasheet?
It is called threshold, because it is the threshold value were the mosfet starts to turn on.
Note the test condition: ID = 250uA.
FIGURE 7. TRANSFER CHARACTERISTICS shows current vs. gate voltage.
IRF540 needs at least 5V to be almost fully on.
NE5532 also does not like a low operating voltage: It can't drive neither to negative nor positive rail.
 

Online Kleinstein

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #4 on: October 22, 2016, 03:05:01 pm »
The NE5532 is not really happy with the 0/5 V supply. It is not a single supply type. So it does not work right with a voltage below about 1 V (the datasheet even wants more than 2 V, worst case even 3 V). Also the output will go to only 3.5 to maybe 4 V. So this 3,5-4 V might be just enough for 1 A , depending on the specific FET sample. There is no need to fully turn on the MOSFET, so no need for 10 V but something like 5-6 V or a MOSFET with lower threshold would be a good idea.

So using a more suitable OP (e.g. LM358 or TLC272 - single supply, but not very accurate) would be the first step. Still the output voltage is rather low. For higher output voltage a Rail-Rail OP like MCP6002 would be better. 

The current is expected to increase as the FET gets warmer.
 

Offline DuncanSteelTopic starter

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #5 on: October 22, 2016, 03:12:53 pm »
It is called threshold, because it is the threshold value were the mosfet starts to turn on.
Note the test condition: ID = 250uA.
FIGURE 7. TRANSFER CHARACTERISTICS shows current vs. gate voltage.
IRF540 needs at least 5V to be almost fully on.
NE5532 also does not like a low operating voltage: It can't drive neither to negative nor positive rail.

So they let through 250uA from drain to source at typical 3V & mosfet will start letting opening?

How do you know its 5V it needs to be fully open? I cant see it to be written anywhere. From figure 7 i see that at 5V from gate to source the output is 30A, but all I need is 2Amps. So how comes 3V is not enough? (Rather I dont know where to look for it in the datasheet.)

How do you know the NE5532 also does not like a low operating voltage: It can't drive neither to negative nor positive rail. ?? where is this written?

 

Online Mechatrommer

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #6 on: October 22, 2016, 03:14:28 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline DuncanSteelTopic starter

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #7 on: October 22, 2016, 03:17:28 pm »
The current is expected to increase as the FET gets warmer.

But this is a problem, lets assume I make the led dimmer & drive it with the mos, the leds will just burn out as the current will increase & the mosfet gets hotter.

Dave also made the led dimmer with 555 timer & a mosfet, how comes his leds didnt burn?

About logic lvl mosfet, so I should be looking for a mosfet that is fully turned on at 3V (or If I use an railtorail opamp 5V), but how is this characteristic called in the datasheet?

So basically I need an op amp that can output say from 0V to 5V and a mosfet that is completely turned on at 3V-5V ?
« Last Edit: October 22, 2016, 03:19:46 pm by DuncanSteel »
 

Offline DuncanSteelTopic starter

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #8 on: October 22, 2016, 03:25:19 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...

Can you explain  peak-to-peak output-voltage swing please?

I do understand the peak to peak part, but if it can input +-15V how can it output between 24 to 26V ?
« Last Edit: October 22, 2016, 03:27:22 pm by DuncanSteel »
 

Offline rob77

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #9 on: October 22, 2016, 03:30:14 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...

Can you explain  peak-to-peak output-voltage swing please?

I do understand the peak to peak part, but if it can input +-15V how can it output between 24 to 26V ?

+-15V is 30V ...
 

Offline DuncanSteelTopic starter

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #10 on: October 22, 2016, 03:32:39 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...

Can you explain  peak-to-peak output-voltage swing please?

I do understand the peak to peak part, but if it can input +-15V how can it output between 24 to 26V ?

+-15V is 30V ...

And if I ask it to output 1.2V then its within +-15V isnt it? even if I ground the - port?
 

Offline rob77

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #11 on: October 22, 2016, 03:42:36 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...

Can you explain  peak-to-peak output-voltage swing please?

I do understand the peak to peak part, but if it can input +-15V how can it output between 24 to 26V ?

+-15V is 30V ...

And if I ask it to output 1.2V then its within +-15V isnt it? even if I ground the - port?

in your schematic you have 0V and +5V so 1.2V is not at least 2V above 0V.
+-15V means you have -15V : 0V : 15V  where 0V is ground -15V is 15V below ground and +15V is 15V above ground so the +-15V supply gives you a total potential diference of 30V. in that case if you substract the 2V from top and bootom - you will get -13V : 0 : 13V interval for output signal - gives you the 26V.

in your schematic with the 0V : 5V sypply you have a total potential differentce of 5V - substract 2V from top and bottom and you will get an interval of 2V : 3V  so output voltages between 2V and 3V are possible.

if you use a rail-to-rail opamp then you will lose just some milivolts on the output - not the 2V like with the NE5532.

you can use the NE5532 but you have to increase the supply voltage and add a negative supply as well, you can use asymetric supply like +12V on positive rail and -5V on negative rail (or even -2V - just enough to overcome the output's limitation).
 

Online Mechatrommer

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #12 on: October 22, 2016, 03:47:40 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...
Can you explain  peak-to-peak output-voltage swing please?
I do understand the peak to peak part, but if it can input +-15V how can it output between 24 to 26V ?
it means, if you power 5532 from ±15V, +15V on Vcc+ pin and -15V on Vcc- pin, that is called 30Vpp power supply. if you supplying the 5532 from 30Vpp, it onyl capable at outputting 26Vpp that is ±13V. meaning it can only go to +13V from +15V rail, that is 2V lower. same thing on the other side, it only can go -13V from -15V rail, that is 2V higher, however voltage you give the 5532 at the input pins. in your case, you are powering the 5532 from 5V and GND, that is 5Vpp, it only can output 2 to 3 volt, however the input voltages are. in order to command nfet to switch off completely when the nfet source pin at 1.2V, the 5532 must be able to command the nfet gate 1.2V, give or take, which is out of its range from 2-3V.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Online Mechatrommer

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #13 on: October 22, 2016, 04:01:26 pm »
and then on the nfet side, since it cannot completely off, it generate heat as current flowing through it, as it heated up, more and more current will flow until it possibly smoked. notice the diagram left side from the datasheet how current will rise as temp rise to 175degC.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Online Kleinstein

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #14 on: October 22, 2016, 06:31:18 pm »
If the input would be in the valid range the NE5532 still has a chance to turn off the MOSFET at something like 2 V gat voltage.

Thermal runaway can be a problem with MOSFETs, if used in linear operation like for a current sink. Especially at higher voltages this can be a problem inside the chip as well, especially with modern types.
 

Offline Zero999

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #15 on: October 22, 2016, 06:43:39 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...
Actually I think the problem is more likely because the common mode range is being exceeded, rather than the output voltage swing. The NE5532 will behave unpredictably when the input is taken below 2V from either power supply rail.
 

Offline rob77

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #16 on: October 22, 2016, 06:53:17 pm »
http://www.ti.com/lit/ds/symlink/ne5532a.pdf
output swing 26Vpp from 30Vpp supply, meaning it cannot get any closer than 2V from either rail. you asked the 5532 to output 1.2V which is 1.2V from lower rail (gnd) which is a fail...
Actually I think the problem is more likely because the common mode range is being exceeded, rather than the output voltage swing. The NE5532 will behave unpredictably when the input is taken below 2V from either power supply rail.
might be, or both ;) .. but in any of the cases, increasing the positive rail voltage and introducing a negative rail at least -2V (while keeping the sensing and input ground referenced) will solve the issue.
 

Offline DuncanSteelTopic starter

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #17 on: October 22, 2016, 07:29:21 pm »
and then on the nfet side, since it cannot completely off, it generate heat as current flowing through it, as it heated up, more and more current will flow until it possibly smoked. notice the diagram left side from the datasheet how current will rise as temp rise to 175degC.

That was so much information that it made me confused, lets just do 1 step at the time. Starting with n-mos.

1) I understand now when my IRF540N starts opening VGS(TH) = from 2V to 4V. (Gate to Source Threshold Voltage) correct ?

2) How do i know when its fully open, which parameter should I look up in the datasheet? IRF540N Datasheet




 

Offline rob77

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #18 on: October 22, 2016, 07:43:07 pm »
and then on the nfet side, since it cannot completely off, it generate heat as current flowing through it, as it heated up, more and more current will flow until it possibly smoked. notice the diagram left side from the datasheet how current will rise as temp rise to 175degC.

That was so much information that it made me confused, lets just do 1 step at the time. Starting with n-mos.

1) I understand now when my IRF540N starts opening VGS(TH) = from 2V to 4V. (Gate to Source Threshold Voltage) correct ?

2) How do i know when its fully open, which parameter should I look up in the datasheet? IRF540N Datasheet

it's not defined, the only clue is the Rdson - in the datasheet you can find at what gate voltage they guarantee the lowest Rdson. for example your mosfet has 40miliohm (0,040 ohm) Rdson at 10V Ugs.. it says if you apply 10V to gate (relative to source) the mosfet will be fully open with a resistance of 40miliohm maximum. but th emosfet might be open at lower gate voltage already (but the resistance might be higher yet)
 

Offline DuncanSteelTopic starter

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #19 on: October 22, 2016, 09:53:25 pm »
it's not defined, the only clue is the Rdson - in the datasheet you can find at what gate voltage they guarantee the lowest Rdson. for example your mosfet has 40miliohm (0,040 ohm) Rdson at 10V Ugs.. it says if you apply 10V to gate (relative to source) the mosfet will be fully open with a resistance of 40miliohm maximum. but th emosfet might be open at lower gate voltage already (but the resistance might be higher yet)

I see. Now my goal was to make a current load that is constant & will not change. So, now I found this n-mos  BSR802N that will start opening at typical (0,55V) & will be completely open at 2.5V.

So If I understand this correctly now I need to use an op amp that can deliver more than 2,5V. And inputs of this op amp must be in range of lets say 0V-0,021V so I can use the pot to adjust from 0V to 0,021V so I can use the 10 mOhm resistor as load & be able to send 2.1 Amps through the gate.

Is my understanding correct?
« Last Edit: October 22, 2016, 09:56:52 pm by DuncanSteel »
 

Offline rob77

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #20 on: October 22, 2016, 10:07:01 pm »
correct, the opamps common-mode input voltage must reach to 0V.  and as you're going to use a very small shunt and having only 21mV  full-scale signal, it has to have a VERY low input offset voltage as well. a typical generic opamp has upto 3-5mV input offset, that would mean 25% error in your case. so you need one with a very low input offset voltage or you could also use a 100miliohm shunt and have  a 210mV full-scale signal.
 

Offline JS

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #21 on: October 22, 2016, 10:09:12 pm »
  You have to consider closely the opamp you are using, you should look some that would allow inputs beyond ground from a single supply, you just need to work at ground but too close to it so better allow beyond. There are quite a few that can go 0.3V or so below ground, that's what you are looking for.

  Working with 1? resistance makes higher the dissipation in the resistor of course but bit lower on the transistor, and also makes the working range much more usable. As you are usually drawing some power with this you expect to have a decent voltage across it, so "burden voltage" isn't a problem other than the power at the resistor. The drift with self heating of the resistors might be a consideration if going too crazy in the expected precision. I don't know what usable range you are expecting from it. Noise might become an issue as well working with a resistor too low for the expected currents.

  I'm just saying, check your options and consider the best value for this resistor, it will define the behavior of the device and is quite easy to miss the point, but also easy and cheap to correct if you get it wrong the first time.

JS
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Offline Audioguru

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #22 on: October 23, 2016, 02:28:08 am »
The graphs on the datasheet for a Mosfet are for a "typical" device that you cannot buy. You get whatever they have and yours might have minimum or maximum spec's.
Yours might have a threshold voltage of 4V when it conducts only 0.25mA. The output of the NE5532 opamp might go as high as only 3V with a 5V supply which is not be enough. 
 

Offline JS

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #23 on: October 23, 2016, 04:06:26 am »
Why are we discussing NE5532 here, that opamp doesn't make sense for this application...

JS
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Online Mechatrommer

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Re: Why does the current keep increasing when using an N-mosfet ?
« Reply #24 on: October 23, 2016, 04:43:59 am »
1) I understand now when my IRF540N starts opening VGS(TH) = from 2V to 4V. (Gate to Source Threshold Voltage) correct ?
it's not defined, the only clue is the Rdson
the threshold voltage and Rdson is only for nfet working in saturation mode. i'm not an expert of nfet in linear mode, but i'm suspecting you are running the nfet in linear mode based on your circuit setup similar to bjt high side emitter follower. so Vg (linear) doesnt have to take full Vgth to be working in lieanr mode and Rds (linear) will be much much greater than Rdson.

if you look at the diagram below. Vgth is only to overcome the "capacitance charge resistance" (or somesort of term) of the gate (horizontal line), once you get passed that threshold, nfet will be in full saturation mode, or very close to it... since you havent passed that threshold, i suspect you are working in fet linear mode. you'll be blessed if mosfet datasheet provides greater details about linear mode working condition, just as how bjt's datasheets have. but as usual, IRF540's nmosfet datasheet doesnt.

edit: its ok with your emitter follower circuit, except you are using n-mosfet for it instead of a npn power bjt. your circuit is intended to be working in linear mode anyway, so yeah, as far as the nmosfet linear mode and the datasheet is concerned? the specification of your circuit is basically.... "undefined".

and as Hero999 suggested, 5532 may go into unpredictable output when given not the right input (beyond common mode range of the working condition) such as phase reversal. i'm not sure 5532 if it has phase reversal problem i forgot, but opamp with phase reversal is something that i want to get rid of in anything, or else great care must be taken on its inputs.
http://www.analog.com/media/en/training-seminars/tutorials/MT-036.pdf
« Last Edit: October 23, 2016, 07:13:58 am by Mechatrommer »
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