What do I see in the current ?

Some times an engineer will ask: " ...why does my current look like that ? "

Each scenario is different; layout, the way you probe, what you use as a transducer - all affect the accuracy - or fantasy in some cases - of what is seen.

Take the above buck converter image: 2uS/div, 5A/div. and about 50Vp-p on the volt waveform - yellow

- if you just want a near enough look at the current - then this setup is likely good enough

- you can see the current peaks at about 7A or so and the ripple is therefore about +/- 3.5 amps, the current is a nice straight line - so there is no saturation concern.

But what about the "jump" in current followed by the ringing in current at each switching event ?

- what causes this ? is it real ?

Well, since the current in an inductor cannot jump instantaneously under the drive signal we see in yellow - we know straight away the jump in current is not real.

What is happening is:

- there is some inductance in the low ohm resistor being used for these measurements,

as the di/dt rolls over from being negative, thru zero, to being positive,

the voltage produced in the parasitic L of the sense resistor drops to zero for di/dt = 0 ( very briefly ), and then;

jumps to the value set by V = L. di/dt which in this case could be V = 100nH.7A /5uS = 140mV, or for 10nH of L, we would get 14mV.

You can see (just) that the jump up ( above true ) of the sensed shunt voltage on the up swing is larger ( due to the 50V applied ) with the di/dt being larger, and the jump down ( below true ) is smaller due to the reduced di/dt on the down-slope.

Given the shunt R could be as low as 10 milli-ohm ( 70mV for 7A ) it is easy to see how this di/dt effect across the parasitic L in the sense R causes wave-forms like the above that lead you to think the current might be jumping.

There are other artifacts as well,

for the above it certainly appears that the dV/dt from the switching node is causing a very narrow spike to be seen on the green current signal, by way of RFI into the probing wires.

There is, also, some small-ish but noticeable oscillation following a switch, this is AC current in the power circuit,

this ringing current causes the input DC bus to ring also, as can be seen in the top part of the yellow waveform - at the same time and in phase, ( look hard ).

This is a product of the wiring inductance of the power switching loop, and the self capacitance of the power inductor - being low loss the ringing continues for a few cycles as we see.

The ESR & ESL of the caps on the input bus allow us to see the HF current ripple as a voltage on this bus, in yellow ( thru the low on resistance of the top fet ).

The ringing current loop during turn on is through; the Vin caps, fet, inductor, output caps back via ground to the input caps, and involves all the wiring inductance of the pcb, the ESL of all the caps and the capacitance of the inductor.

During the off time the ringing loop is through the bottom mosfet, inductor and output caps, ( the input caps are not in circuit ) - so we have less wiring L, hence the slightly higher ringing frequency.

In the sense R we see a slightly amplified version of the ringing current - due to the ESL in the sensing resistor, as higher frequencies of current x any inductive Z ( 2.pi.f.L ) = higher voltages across the parasitic L of the sense R. So the VHF AC that we see or measure is a bit higher than true.

So, as often, there is more than meets the eye at first glance when trying to get a good look at your power switching circuit, be it watts or kilo-watts,

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