Simple mosfet question

I have a simple circuit where I want to convert a digital input from

0-5V to an inverted signal from 0-12V. ie, 0V into the circuit = 12V out, and 5V into the circuit = 0V out.

I don't have the means to get a schematic up, but I've tried to draw it with text below (but it probably won't show up right), so I will describe the simple circuit and I think it will be clear:

There is a 2N7000 N-ch mosfet transistor, with the source tied to ground. The gate is tied directly to the input. The drain is tied to a resistor, 10k, and the other side of the 10k resistor is tied to a battery, +12V. The output is connected to the drain, and is used to feed the clock inputs of 2 CMOS ICs. There is a 470pF filtering capacitor tied from the output to ground.

My question is, can anyone see how this circuit would blow the transistor? Is this an OK way to hook it up? Should I maybe add another 10k resistor between drain and output (before the cap) to better protect the transistor? I realize that doing this would bring the output voltage down very slighlty, but that's fine if it is needed to protect the transistor.

I'm asking because I've hooked up the circuit more than a few times, and it always works at first / for a while, but sometimes the transistor blows unexpectedly and I don't know why yet. I think it is because I was damaging the transistor during install (with ESD, I was not using protection at the time, but I learned my lesson!), but I want to make sure it isn't actually a circuit problem too.

Sorry that I can't get a schematic up, I don't have a place to upload files while I'm at work. I hope I have made it clear enough :)

+12V | | / \ 10k / |_______OUTPUT __| | | INPUT _____| |__ | | | GND
Reply to
Andrew
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Make your pictures with a fixed-width font, then we'll all be able to read them.

Putting a resistor in series with the cap will spare your transistor, but you don't need to make it 10K. You could probably get away with 200 ohms or less if I recall the capabilities of that transistor correctly.

If you're driving it from CMOS a small resistor from gate to input would spare the driver from the capacitive load, but may not be necessary.

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Tim Wescott
Control systems and communications consulting
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Reply to
Tim Wescott

There's nothing wrong with the circuit, something else is blowing the FET. There must be a transient occurring on one or more of the three FET terminals that is causing the damage. The gate terminal is most susceptible and this is handled by placing a zener in parallel with it.

Reply to
Fred Bloggs

mosfets are very sensitive to gate drive and spurious signals / static electricity etc.. A small transient in the gate lead can eat a mosfet where a BJT will shrug it off. You are doing something that kills the transistor - bread boarding? Keep the gate lead short to minimize inductance. Protect the gate and source/drain if there is an inductance in the load. Use a scope and watch the transients.

Look at the data sheet for the 7000 - particularly look at the safe operating area and you see how fragile they can be.

A good source for the use of mosfets:

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Reply to
Andrew

Sorry, I made a mistake. On the PCB the trace from input to gate is

12mil wide and only about 0.5 inches total length. I was thinking of a different portion of the circuit, sorry for the confusion.
Reply to
Andrew

I'd want to know more about that . . . Servo motor or something like an RC servo? This thing has a motor connected to it? Intercepting makes it sound like it does. If that's the case, you need look no further, that is probably the cause.

Motors are notorious for inductive transients - even the brushless ones can be pretty bad. If the driver is applying PWM or switching from nothing to all out, the wonder is it works at all.

Gate resistors help but it all depends on the energy and persistence of the transient. The mosfet gate is a small capacitor (with an equally fragile dielectric). The resistor in series with the signal to the gate uses the gate capacity to make a low pass filter. Hopefully, the transient will develop across the resistor and not punch through the gate insulator.

In the case of the 2N7000 it is only 20 picofarads - so that isn't much of a low pass filter.

Talk of motors and intercepting signals (one presumes that the signal you are intercepting also has wires running about) would lead me to think you will need something a little more robust than a series gate resistor.

If you need speed (2N7000 has to follow the signal with minimum phase shift) something like a bipolar might be better suited or if speed isn't too important a simple RC low pass filter might be better. Zeners are good but they have to be fast too - There are circuit/transient protectors that specify switching times and those may be the better choice.

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default

Reply to
Andrew

Well that doesn't sound too bad. RC servo is something like a one millisecond pulse (if my memory is working) that is pulse position modulated. 0-4.5 or 5 volt pulse. That shouldn't be a problem and I wouldn't think 6" of wire would be either.

If you have a scope, look at the signal on the 10X probe range. And look at the output (drain) too for ringing.

Reply to
default

The RF modulation is PPM, but the signal between the receiver and servo is PWM, with a 1500us pulse corresponding to neutral. The pulse frequency isn't significant.

Reply to
Nobody

You are right. It has been awhile since I messed with them. Pulse frequency is around 80 HZ

To control the servo, you command it to a certain angle, measured from

0 degrees. You send it a series of pulses. The ON time of the pulse indicates the angle to turn to; 1ms = 0 degrees, 2ms=max degrees(about 120) and anything in between gives a proportional output angle. 1.5ms is generally considered to be the "center". The 1~2ms limit is manufacturers' recommendations; you can usually use a wider range around 1.5ms for grater throw. I did not do this, but you can use pulses of less than 1ms, and more than 2ms for an output angle of 180degrees or more. The limiting factor is the feedback pot and the mechanical limits built in the servo.

Years ago I built a little joystick pan and tilt from a pair of servos to aim a tiny camera. I used a quad op amp to generate the pulses for both servos.

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default

Reply to
Andrew

I was reviewing my older schematics and PCB designs, and I just realized something.

I originally had a 2 layer PCB version of this circuit that worked and went through rigorous testing great! Unfortunately it was not a great (PCB) design and was noisy. I had filtering caps here and there that made it work, but I worried that this noisy design was going to cause issues later, so I switch to a 4 layer board which used a power and ground plane. When I switched to this *better* board, I was able to remove some filtering caps as it was a much better design and was significantly less noisy.

One of the filtering caps that I removed was a 10nF cap that went from the gate to the source, to filter the input signal.

Was this filtering capacitor also inadvertently "protecting" the transistor and that is why that design never failed, but now, the new, less noisy design that does not have this capacitor is more susceptible to noise/transients/spikes that could kill the transistor?

Or does the removal of this capacitor theoretically have nothing to do with it because it was not "protecting" the transistor in any way?

I'm guessing that it has alot to do with it, and the capacitor was protecting the gate input from transients and/or spikes.

What do you guys think?

Reply to
Andrew

You do have a flyback diode on the relay coil, right?

Reply to
Nobody

No, I don't have a flyback diode on the relay, but it is in a completely different portion of the circuit that I haven't had any trouble with yet?

I didn't understand the idea of a flyback diode at the time I first designed this circuit, and I do have a little better understanding of it now, and how to use it, I just don't fully understand what the voltage spikes can do if they are not controlled with a flyback diode. It seems to me that the spike would just be across the relay coil for a short period of time? I suppose that this voltage spike could kill the transistor (another 2N7000)? It hasn't happened yet on any of the prototype boards, but I suppose it is a real possibility.

I think I will add one at any rate. Does it have to be a schottky in the case of a relay, or will any old diode suffice?

Any any thoughts on the 10nF capacitor from gate to source possibly protecting the transistor?

Reply to
Andrew

The relay coil isn't connected to the transistor that's blowing?

You cannot instantaneously change the current through a coil. If the coil is in series with a transistor, and you turn the transistor off, the current will keep flowing through the transistor anyhow. The effect of "turning off" the transistor is that the voltage drop across it will increase to whatever figure is necessary to keep the current flowing.

E.g. if you have 1A flowing through the coil and transistor, and you instantaneously increase the collector-emitter (or drain-source) impedance to 1MOhm, you would get 1MV across the transistor. In practice, you can't get "instantaneous" turn-off, but you can still generate very high voltages; you could easily get 1KV from a relay coil if the switching is fast enough.

Recovery time won't matter here. It only applies if you switch the relay off then switch it on again before the coil current has dropped to zero (which is likely to be in the millisecond range unless you have a really large coil).

It will typically extend the turn-off time, potentially allowing the inductor's current to drop to zero before the transistor's impedance gets too high.

Reply to
Nobody

I like gate transient as the cause but . . .

Looking at the characteristic curves for a 2N7000, you might just be blowing it because you aren't turning it on with enough voltage. Don't know what your load resistor is, but with 4.5 volts on the gate, your drain current has to be relatively low to avoid cooking the transistor.

Look at the chart that shows "On-Resistance Variation with Gate Voltage and Drain Current" then calculate the load current you are using and compare that to the absolute maximum values. Little buggers are only good for 200 milliamps and 400 milliwatts

The gate voltage has to be high enough to turn on the transistor fully or you end up generating heat in the linear range. The gate threshold for a 2N7000 (point where it begins conducting not where it is fully saturated) is three volts - you only have four point five - that might not be enough with a batch of marginal spec transistors.

Reply to
default

Nope. There is a relay with a 2N7000 in a different portion of the circuit. I haven't had any trouble with it, but I'm going to add the flyback diode for good measure.

The circuit in question is the one described in the first post. The cliff notes, though, are that the circuit works great for a while, but after a varying amount of time, for an unknown reason, the transistor will blow. I'm guessing it is transients or noise of some kind. The exact same circuit, except WITH a 10nF cap from gate to source, was tested rigorously in four separate PCBs for many many hours in hot/ cold temperatures and had zero problems.

Since the time varies, I'm wondering if I was just lucky with those 4 boards that they worked so great for so long, or if it is possible that the 10nF cap from gate to source was acting to block transients that otherwise would have killed the transistor?

And thanks for the additional help on flyback diodes and the description of what can happen!

Just one question on that matter..

This is only for a short period of time though? Or will current flow through the transistor indefinitely (voltage across the transistor)?

Reply to
Andrew

Yes.

No; that would give you free energy.

The current will drop at a rate proportional to the voltage across the coil (minus the voltage drop for the coil's resistance) and inversely proportional to the coil's inductance. dI/dt = V/L.

Reply to
Nobody

My transistor is a fairchild (if it matters) and from what I can tell, with 4.5V Vgs, I don't have to worry about on resistance until ~

400mA. The load resistor for the circuit that is blowing the transistor is 10k when the transistor is on. With a max voltage of around 14V, that is 1.4mA Id, way way way under the worry zone as far as Rds on, right?

Can anyone comment one way or the other on whether a 10nF cap from gate to source should have any real effect on protecting the transistor? I keep thinking yes, but it seems you guys don't agree? Wouldn't it limit potentially damaging spikes to the gate?

Reply to
Andrew

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