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  • Author Author: Jan Cumps
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TI-PMLK Buck Experiment Board - part 1c: 1st Experiment Measure

Jan Cumps

I am a Road Tester of the TI-PMLK Buck Experiment Board: TPS54160 & LM3475.

It's an educational kit - board and book - to learn buck converter theory and practice.

Because it's an educational kit, I give minus points each time there's vendor lock-in image .

 

 

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I applied for the Road Test to check the educational value of the kit. The focus in this blog series will be on the Lab Manual and exercises.

In this blog, I measure the efficiency for Exercise 1.

 

Doing the measurements

 

This is where the educational value of the kit is really excellent. The book doesn't discuss Buck regulator theory or measurement principles.

It assumes you have those skills. I think that's a good starting position in this case.

If you don't understand Buck converters or lack the skill of using multimeters and oscilloscopes, it doesn't really make sense to do these experiments.

They build upon your knowledge. They are not the base for  it.

 

 

Goal of Experiment 1:

The goal of this experiment is to investigate how the efficiency of a buck regulator depends on the line and load conditions and on the switching frequency.

 

 

The document reviews the parameters that play a role in efficiency, shows the components that have a loss independent of load and components that have load dependent losses.

image

 

 

The exercise is to measure input and output power with varying load, for an input voltage of 6 and 24 volt and calculate efficiency.

Then you have to compare that with the efficiency calculation. You get enough help in the doc to get that calculated.

Simulation affectionados (yes you Jon) may want to run it through Spice or the likes.

 

Then follow the steps to set everything up and perform the exercise (click to enlarge).

 

imageimage

 

You record both calculated and measured values in a table, for those efficiency and loss tests.

 

imageimage
imageimage

 

Here's a few of my measurement recordings and some scope captures:

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  • Input 6 Output current 0.8 A

image

 

  • Input 24V Output current 0.8 A

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Here's what the manual shows for 24V 1A. It's obvious that my current measurement method (blue in the capure above vs. green in the TI manual) is not up to the game.

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What I haven't done yet is to measure the behaviour when the regulator operates in non-continuous mode.

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That's a todo for the next article. I'll also comment on educational value and vendor lock-in score of the kit (spoiler: high and low image)

 

 

Related Blog
1a: 1st Experiment Set-up
1b: 1st Experiment Lab Setup
1c: 1st Experiment Measure
2: Educational Value

Road Test: TI-PMLK Buck Experiment Board: TPS54160 & LM3475

  • in reply to jc2048

    I was using the probes in 10X - so 20 pF is close.

     

    image

  • in reply to Jan Cumps

    I estimated 2.1MHz from the trace, calculated about 320pF using the formula you give for a parallel LC circuit at resonance, and then knocked off 20pF for the scope probe (which wouldn't normally be there). [It's perhaps worth noting that the ringing frequency with the scope probe present is different to what it will be without the probe being there.]

     

    As well as the diode and the MOSFET, there's also a contribution from the pcb, particularly with a multilayer board with a plane close under the switch-node copper.

     

    As you now show, the diode capacitance figure varies according to the reverse bias, so you can't do a simple calculation with a single figure (particularly not when the figure you're giving is the worst-case for 40V of reverse bias).

     

    I doubt you'll find anything on the internal MOSFET.

     

    Good discussion there that you link to. It's just basic physics, really - once the coil can't keep the diode on, the remaining residual energy sloshes back and forth between the coil and the parasitic capacitance until it has all been dissipated. The extra thing, that isn't necessarily obvious, is that if the supply rails are well decoupled then, to ac, it looks like there is a short between the rails, so the diode capacitance to ground, the MOSFET capacitance to the input voltage rail and the coil connecting to the output rail, all look like they are in parallel.

     

    Now you're in a position to work out what the total resistance in the LC circuit is. [It won't just be the coil resistance. The diode and MOSFET semiconductor materials will have some resistance too that's effectively in series with their capacitances. There's also the ESRs of the decoupling components, though that won't be much if they're ceramic caps.]

  • in reply to jc2048

    According to the doc:

    The ringing is determined by the resonant loop formed by the inductor and by the parasitic capacitances of

    the MOSFET and of the diode

    Period of ripple is 500 ns (1 graticule): f = 2 MHz

    The inductor is 18µH.

    The N-channel MOSFET is inside the TPS54160 - couldn't find the parasitic capacitance of that FET  yet

    The diode is a Schottky B260-13-F: 200 pF typ/max according to the specs (according to the TPS54160 datasheet, the typical capacitance of a B220 is 120pF).

    image

    from Bourns B220 - 260 datasheet

     

     

     

    Stole this formula from wikipedia: image

     

    my calc-fu says approx 350 pF parasitic capacitance.

     

    discussion about the ringing in discontinu mode: https://groups.google.com/d/msg/sci.electronics.design/uClujub7998/lbTFGZgyzegJ

  • in reply to Jan Cumps

    Very nice - they're the kind of pictures that would resonate with any true engineer.

     

    So, about 300pF, then?

  • Here are switch node captures in non-continuous mode:

     

    Switch frequency 500 kHz

     

    image

     

    Switch frequency 250 kHz

     

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