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Experimenting with Magnetic Components
Blog Experimenting with Magnetic Components - Boost Converter part 3: Measure the Inductor in action
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  • Author Author: Jan Cumps
  • Date Created: 8 Oct 2021 12:54 PM Date Created
  • Views 1634 views
  • Likes 1 like
  • Comments 3 comments
  • experimenting_with_magnetic_components
  • magnetic_components
  • bourns
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Experimenting with Magnetic Components - Boost Converter part 3: Measure the Inductor in action

Jan Cumps
Jan Cumps
8 Oct 2021

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 Bourns RLB Series 1mH Radial Lead InductorBourns RLB Series 1mH Radial Lead Inductor.
image

 

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.

 

image

 

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.

 

image

 

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.

So in this blog's images and photo, the magenta UL signal is inverse of what it should be image.

 

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.

image

 

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

 

 

Related posts
1. Boost Converter part 1: Inductor and Calculations
2. Boost Converter part 2: Build
3. Boost Converter part 3: Measure the Inductor in action
4. LCR meter experiments
5. SMD transformers experiment gizmo part 1: Build
6. SMD transformers experiment gizmo part 2: Measure
7. Common Mode Choke
8. Make your own Inductor
9. Calculate your own Inductor
10. Boost Converter part 4: Efficiency
11. DIY Inductance Meter
Planar PCB Transformer: GaN Point of Load converter 48V to 1V 50A
Measure Unknown Inductor Value with Function Generator and Oscilloscope
Experimenting with Magnetic Components: About the Competition
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  • shabaz
    shabaz over 3 years ago in reply to Jan Cumps

    I'd forgotten about that! The frequency is not too bad, better than some ready-made clamp sensors. I have a ready-built sensor (Pico Tech) which is nice, but is limited to 20 kHz : (

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  • Jan Cumps
    Jan Cumps over 3 years ago in reply to shabaz

    shabaz  wrote:

     

    Another option might be to use a hole-in-the-middle hall current sensor since it's still 100 times cheaper than a sliding current probe, but they might not have the response at typical DC-DC converter frequencies.. plus there's a bit of additional inductance since they are large, so it won't work for all scenarios....

    There's one waiting to be used image :

     

    image

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  • shabaz
    shabaz over 3 years ago

    Hi Jan,

     

    Interesting blog post, I too had a similar type of issue recently, and it's a real dilemma how to do it without damaging anything : )

    I did a similar thing, use a resistor, but for my particular scenario I was fortunate I could fit it on the 0V side, and took measurements referenced from there knowing I would need to subtract the voltage across components or have errors (some of which I could reduce a bit, e.g. lower resistance current sensing, but that brought it's own issues).

    Another option might be to use a hole-in-the-middle hall current sensor since it's still 100 times cheaper than a sliding current probe, but they might not have the response at typical DC-DC converter frequencies.. plus there's a bit of additional inductance since they are large, so it won't work for all scenarios. In the end I figured resistor was 'good enough' and would not introduce many more difficulties other than the obvious : )

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