I'm reviewing a set of inductors for the Experimenting with Magnetic Components design challenge.
Because it's a design challenge, I'd like to start with a working product. A switch mode DC converter.
It's one of the standard circuits: the boost converter. A design that increases a DC voltage.
In this post, I measure the current and voltage of the circuit's .
Measure without creating a short circuit
I want to measure voltage over the inductor, current through the inductor, and show the PWM control at the same time.
The voltage is measured over the inductor.
I've added a shunt resistor in series with the inductor, to be able to show the current going through it.
When measuring points in the circuit that have a different ground reference than other measure points or a different ground reference as your LAB instruments, you have to be careful.
There's a decent risk that you (or in this case I) will create undesired short circuits via the ground paths.
The easiest and safest way in this case (different when dealing with utility lines!) is to have all things isolated.
In my case, scope, PSU and generator are isolated from each other. That's a good start. I don't have to worry about a bogus current path via their power line ground.
The only one that's connected to my house grounding is the oscilloscope.
There's a second risk: All oscilloscope inputs share a ground. So I only can put the ground clips at a single point in the circuit.
I only have one isolated probe. It would have been easier if I had at least two of them.
If I had 2 isolated probes, I could use 1 to measure current and one for the voltage over the inductor. These probe locations are both not referenced to the circuit ground.
A 3rd unisolated channel could be used to measure the PWM signal that's referenced to the circuit ground.
But as indicated, I don't have a second differential probe.
Luckily, there is one point in the circuit that's shared: the node where the current shunt connects with the inductor.
I can use that node as the common ground for the current and voltage probes.
And use the isolated differential probe to sample the PWM control signal.
In this scenario, I'm getting the inverse of the current, because the measure point is before the shunt and the ground after it.
Fortunately, my oscilloscope has an inverse function, so I can show the signal correctly on the oscilloscope screen.
Unfortunately, I'm not always thinking. I also inverted the voltage signal, while that was not needed.
I only detected it while writing the blog, and I had dismantled the test setup.
The Inductor Voltage and Current
The input is 9 V DC, PWM 50 kHz, 75% duty cycle.
Output was 30 V.
The serial resistor has some influence on the circuit's performance. It's better to use a current probe than a shunt.
If you have a 2 channel oscilloscope or don't own a differential probe, it's still possible to show the current and voltage together.
And if you use the "save trace function", it's even possible to first store the PWM input and recall the trace when showing the two signals.
Just take care that you use the same trigger event to capture results in both cases so that they line up in the end result.
Maybe an exercise for an aside article, I can at the same time correct the silly inverse UL goof-up)
Next article I'll measure efficiency.
edit: I've done that now: Experimenting with Magnetic Components - Boost Converter part 4: Efficiency