I am experimenting with a simple fixture for oscilloscope made for measuring inductor saturation current. The principle is using a scope to observe how current passing through an inductor changes over time. The point where the rate of change increases (usually quite prominently) is I
sat. Helper components here are a current sense (shunt) resistor, a MOSFET used as a switch, a gate driver circuit and a source of short pulses to turn the switch on and off.
Here's the schematic:
I built it on a solderless breadboard first, then made a proper PCB to verify that general behavior does not change and it wasn't caused exclusively by the breadboard's parasitics.
The effects that I am curious about and some of which I do not understand are not visible in SPICE (ngspice) simulation, so they must be coming from something unaccounted for.
Here's what I see on the scope. Control pulse is a 0..5V signal coming from a signal generator. The inductor is a drum-type (no idea what core material) 47 uH.
First, control pulse (channel 1, green) and VSENS (channel 2).
...and, respectively, SW_GATE and VSENS:
Everything before the turn-off edge is expected, and then we see a huge reverse current spike. Let's have a closer look.
1) Ch1: control pulse, ch2: VSENS
2) Ch1: SW_GATE, ch2: VSENS
(note that the vertical scale of ch2 is different between the two.)
Part 1: the big negative spike.
Part of what I'm seeing here can probably be explained by the return current of the discharging gate-source capacitance of the switch transistor (2.2nF). However, the charge it stores is very far from being able to produce such a big spike. I tried to connect a 15nF capacitor between gate and source: it made this spike higher, but narrower. This makes me think that this effect is a combination of multiple factors, one if which, but not the major one, is the gate capacitance.
So, what is responsible for the majority of this energy flowing in the reverse direction? Inductance of the sense resistor itself? I doubt it, it should be very small. It's coil-like, yes, but it's only 10 turns of wire without any core (I don't have other suitable shunts at hand to confirm this).
...another question is how to suppress it, but the answer to this should come with the understanding of what causes it.
Part 2: things that happen before going below zero.
There's that drop at VSENS which aligns with the falling edge of the control pulse. Why does it happen exactly then, instead of when the switch transistor's gate voltage drops below ~Vth? Instead, it first drops to zero, following the control pulse, then bounces back (why?) to where it was, and then, following the gate voltage fall (and that's when I would expect it to), finally falls to zero, which is followed by the negative spike.
Why does this happen? I have verified that it's not some kind of scope inter-channel interference by disabling channel 1 and detaching the probe.
Two more screenshots:
1) Control pulse + MOSFET drain:
2) MOSFET gate + MOSFET drain: