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Power measurements and testing Voltmeter DVM  ---R&S RTM3K Oscilloscope Road-Test Review

snidhi

 

Power measurements and testing Voltmeter DVM

This blog is in continuation from the Rohde & Schwarz Oscilloscope Kit RTM3K-COM4 - Review

 

Here I setup an experiment to measure the ripple and do power analysis of the DC/DC TPSM84A21 Power Module. It is an integrated power solution that combines the TPS54A21, a 10-A, DC/DC, synchronous, step-down converter with power MOSFETs, shielded inductors, input and output capacitors, and passives into a package. The tests were done with 10V and 12V input voltage and outputs in the range of 0.6V, 0.8V,1V, 1.1V, 1.2V were measured.

Datasheet TPSM84A21 Power Module

 

A detailed description of the TI SWIFTTm Power Module EVM can be seen here. I will focus on the measurements with the R&S RTM 3004 Oscilloscope.

 

Power measurements in the RTM 3004

 

Fig: How to Set up the RTM 3004 for Power Measurements

 

The R&S RTM K31 Power Analysis bundle offers different options to measure input and output of (Switched Mode Power Supplies) SMPS and DC/DC Converters. The TPSM84A21 Power Module is a high current DC/DC converter. The power analysis of the DC/DC converter is presented below.

 

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Fig: R&S RTM K31 Power Analysis bundle offers

Setup of the TPSM84A21 Power Module

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Fig: Test setup for the Power Measurements

 

Measurements with the TPSM84A21 Power Module

 

The below measurements were done using the automated power analysis tool in the RTM 3004 with the DC/DC TPSM84A21 Power Module.

 

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The aim is only to present how this automated button of power noise ripple works and the below mentioned tests were done with no load in the DC/DC converter output.

 

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Fig: Power Analysis of the 0.6V output of the DC/DC TPSM84A21 Power Module

 

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Fig: Power Analysis of the 0.8V output of the DC/DC TPSM84A21 Power Module

 

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Fig: Power Analysis of the 1V output of the DC/DC TPSM84A21 Power Module

 

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Fig: Power Analysis of the 1.2V output of the DC/DC Converter Module

 

Fig: Power Analysis of the DC/DC Converter

 

The tests here will be done with fixed resistance load in the DC/DC converter output.

 

tbd.. new addition of test on popular demand

 

Spectrum analysis of the Output voltage

 

The settings to measure are as below. The Vin = 12V and Vout = 0.8V of the DC/DC TPSM84A21 Power Module. The frequency measured of the output signal is as measured above is 2MHz

 

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Fig: Settings for spectrum analysis

 

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Fig: The spectrum output for Vout = 0.8V

 

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Fig: Detailed look into the measured data

 

The minimum horizontal measurement setting for RTM 3004 is 8MHz hence the spectrum behavior immediately after the switching of the device cannot be analyzed here as it is around 2MHz.

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Fig: Detailed look into the measured data

 

Transient Analysis of the Output voltage

 

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Fig: Measuring the transient response on the output signal for Vout = 0.6V

 

Measuring with Digital Voltmeter in RTM 3004

 

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Fig: DVM features of RTM 3004

 

Below, the DVM can be seen live in action and measuring the input signal voltage of the DC/DC TPSM84A21 Power Module.

 

Fig: Measuring with the DVM in RTM 3004

 

 

In conclusion the power measurement features of the RTM 3004 are quite powerful & allows results to be obtained with minimum effort. By one click of the button the scope settings are done and one can do fast measurements instead of investing time for finding the correct settings for the tests.

  • in reply to jc2048

    But how can you impede the air flow if there are no hole on the top. This device has only holes on the sides and behind.

    Due to the rug, there can be heat which may not be dissipated via the PSU housing body from the top-side because the rug is blocking it perhaps. Is this what you mean?

     

    And of-course better cautious than sorry image

  • in reply to snidhi

    Speaking for myself, if I saw ventilation holes on a piece of equipment, and a heatsink on the back, I wouldn't impede the airflow around them as a matter of course, whether or not I knew it would have a negative effect. Perhaps I'm just over-cautious.

  • in reply to jc2048

    Hi Jon,

      wrote:

    BTW It might not be a good idea to run your analogue PSU with a rug blocking the ventilation for the power transistor heatsink inside. Won't matter for the situation in the picture, where it's only supplying 20mA, but it might matter at higher output currents. You might know to remove it under those circumstances, but it sets a bad example to others who don't.

    The ventilation for this PSU is only on the sides and the behind.  

    On top there is only a holding handle so I guess it won't make a difference for this PSU.

    I know for some old devices the ventilation is on the top but not for this one I have. Do you think it still makes a difference with the rug on top?

     

    Cheers

  • The tools that the oscilloscope provides look really useful for doing these measurements. Thank you for showing us this in addition to your main review.

     

    A couple of questions for you.

     

    I don't know much about spectrum analysers (never having used one), but don't you need to set a RBW of much lower than 4MHz to pick out a 4MHz fundamental and its harmonics sensibly? The RBW defines the resolution, doesn't it?

     

    With the transient test, what was the transient that you were measuring? Did you have a switched load on the output? The results look much too good to me (although a 4MHz converter is going to be fast, you wouldn't expect the control loop to recover things that quickly - a rough 'rule of thumb' is that the control loop bandwidth might be something like a tenth to a twentieth of the switching frequency). Or were you just showing us how the measurement might work by simply letting it measure the rise time of the ripple?

     

    BTW It might not be a good idea to run your analogue PSU with a rug blocking the ventilation for the power transistor heatsink inside. Won't matter for the situation in the picture, where it's only supplying 20mA, but it might matter at higher output currents. You might know to remove it under those circumstances, but it sets a bad example to others who don't.

  • in reply to rachaelp

    Oh and to add insult to injury you also need the "deskew fixture" in order to calibrate the setup before taking measurements which you can only get from Keysight and you guessed it, it costs a pretty penny too!