I am experimenting with an inductor to see what is happening when it gets saturated and at which current it happens.
So I set up the attached test circuit and checked the current on the 1Ω resistor. I expected the current to ramp up until the inductor gets saturated and then to increase at even higher rate, because as far I know after saturation the inductive behavior is lost, and current is only limited by the DC resistance.

But instead I get what is seen attached, the probes are on the source and drain of the transistor. The function generator is sending a 320µs pulse on the gate. During the first 100µs everything as expected, but then the FET voltage rises and the current is steady around 150mA. Aftter about 50µs it starts ringing somehow. When the pulse is over and the FET switches off I can see the extra voltage on the freewheeling diode and its resistor. Eventually the inductor is depleted and it starts ringing, which is fine.

First I thought the DC resistance took over, but the inductor DC resistance is about 4Ω, and also the on resistance of the FET is quite low. I also increased the voltage to 10V, but I get the same 150mA current limitation (much earlier in the pulse though).

So what is happening after 100µs? I am not an expert, but up until now I thought I roughly knew what was going on...

 

what happens to the gate voltage ?
what happens if you put the 1 ohm on the other side of the FET

 

You have succeeded in making a constant-current sink.
I = (Vg - Vgs(th))/Rs.
where Vg is the output from the generator, Vgs(th) is the gate-source voltage at which the FET turns on and Rs is the resistance in the source lead.
To investigate what is happening as it saturates, you need a larger input voltage, so that the FET turns fully on.

 

What I think is happening is that you do not have enough Gate Voltage. Double the Gate drive.

 

The source resistor is likely limiting the current.
As suggested, use a higher gate signal voltage.
That transistor needs a Vgs of 10V to fully turn on (below):

 

Probably your signal source can not drive the 51 ohm load. Try 1k or 10k. You need a good solid 7 volts of drive. Maybe 10V.

 

The source resistor is likely limiting the current.
As suggested, use a higher gate signal voltage.
That transistor needs a Vgs of 10V to fully turn on (below):

View attachment 257830

Thank You All for your responses, it was indeed gate drive problem. To ensure I would not kill a FET for each test I make I chose a big fat MOSFET for the purpose. I also checked the Vgs requirement, but when the spec says Vgsmax = 5V, I would expect 5V to be able to drive it. Nobody sees stuff like Vgs=Vds, right?

Anyway, I replaced the 51Ω to 100Ω and gate voltage went up to like 7V. The signal generator has 50Ω output impedance, this is why I used 51 initially.

I attached also an image of the 5V gate voltage, it looked allright, this is why I did not suspect that. With using 5V as main power the current diagram is probably gets limited by the resistances. When I switched to 10V I could get the increasing current rate I have been looking for at about 0.8A.

 

I also checked the Vgs requirement, but when the spec says Vgsmax = 5V, I would expect 5V to be able to drive it.

You are misreading the data sheet.
5V is the maximum Vgs threshold voltage, which has a manufacturing range of from 3V to 5V (the voltage where the MOSFET just starts to turn on @ 250µA Ids).
You need a Vgs of 10V to fully turn it on and get the minimum ON resistance, as shown in post #5.

The absolute maximum Vgs is 30V (below):

 

The current waveform in TEST4_10V.png looks like about the right shape, except the inductance change at saturation would probably be greater. Note: When the inductor becomes saturated, the windings still have inductance but the inductance is more like that of an air core inductor of the same dimentions. It is only the core that saturates.

 

We haven't seen the inductor. It could be an iron powder inductor which goes into saturation much more gradually than an inductor with a ferrite or laminated core.

 

You are misreading the data sheet.
5V is the maximum Vgs threshold voltage, which has a manufacturing range of from 3V to 5V (the voltage where the MOSFET just starts to turn on @ 250µA Ids).
You need a Vgs of 10V to fully turn it on and get the minimum ON resistance, as shown in post #5.

The absolute maximum Vgs is 30V (below):

View attachment 257849

You are right, I tend to discard checking the conditions when reading the parameter. It is getting even worse when I am focusing on different topics. (Eventually it was an experiment about the inductor, not the MOSFET...)

 

We haven't seen the inductor. It could be an iron powder inductor which goes into saturation much more gradually than an inductor with a ferrite or laminated core.

It is an inductor taken from a light bulb. It _looks_ like an EE core, but I could not tell if the material is iron powder or ferrite.

 

It is an inductor taken from a light bulb. It _looks_ like an EE core, but I could not tell if the material is iron powder or ferrite.

Probably ferrite with an air-gap.