[myrio] switching frequency problem in boost converter

[nim21kr]

I am designing 'hands-on Boost converter' circuit by using Myrio in the college. I generated PWM signal in myrio for switch(mosFet) of my Boost converter. however, I don't know why maximum frequency which I can control is only 40kHz. If I increased the switching frequency, then output voltage waveform on the ocsilloscope is dissappeared. Is there any way to increase my switching frequency in boost converter? Please help me. thanks.

[Kristoff2]

Hey nim21kr,

As you mentioned the maximum frequency supported by the myRIO PWM express VIs and Advanced IO VIs is 40KHz.  This is a result of the way the PWM frequency is generated using a counter, target count, and clock divider in the myRIO FPGA personality.

If you need to generate a faster PWM frequency you will need to modify the myRIO FPGA using LabVIEW FPGA.

Please let us know if you have any more questions about this.

Thanks!

-Sam K

LabVIEW Hacker

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[Kristoff2]

Hey,

I thought I'd add some more detail here (and sort of a correction).

In the myRIO FPGA code there is a 16-bit counter.  It uses the 40MHz clock and supports clock dividers of 1, 2, 4, 8, 16, 32, 64. The fastest frequency you can use is when using the lowest divider (fastest clock).  The (default) myRIO software guarantees 1% duty cycle resolution, so each period is divided into 100, so the maximum output frequency the default FPGA personality can achievable is 400KHz.  

However the wider the supported frequency range the harder it is to achieve specific frequencies (the frequency resolution is reduced).  So a design decision was made to support 40Hz - 40KHz to provide a wide range of frequencies while still being able to accurately achieve the user specified frequency.  This is evident when you click the validate frequency button in the myRIO express VI.  It takes your target frequency and tells you what frequency it can actually produce.

You can try the following to get a wider range of PWM frequencies (fair warning I haven't tried this so you might want to back up the myRIO VIs before you change anything.)

For the myRIO-1900:

Open <LabVIEW>\vi.lib\myRIO\Common\Instrument Driver Framework\myRIO v1.0\PWM\vis\myRIO v1.0 Generate Register Values PWM.vi

Change the Max Frequency constant (shown below) to something greater than 40KHz (but less than 400KHz).  Save the VI.  

Try your myRIO code again, with a PWM frequency greater than 40KHz.  The Express VIs might prevent you from using higher frequencies, but if they do you can just switch to the Advanced IO VIs...they'll let you do whatever you want 

Let us know if that works and remember, your PWM frequencies might be less accurate as a result.

-Sam K

LabVIEW Hacker

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[Kristoff2]

I decided to type some of this up so I could reference it in the future.  You can find the info here.  The LabVIEW Hacker website is a wiki so feel free to add on to it, fix any mistakes or let me know if you want me to add more detail on any of that.

Thanks!

-Sam K

LabVIEW Hacker

Join / Follow the LabVIEW Hacker Group on google+

[nim21kr]

Thank you for your reply. so, you mean that I need to use FPGA of PWM to increase the frequency above 40k Hz right?

Then, Could you please recommend me any basic reference or example to learn abour FPGA with myRio?

Thank you.

[nim21kr]

Also, your suggested topology is the example from the myRio module? Could you please upload the file(PWM generated by FPGA level) so that I can download and implement it? I tried to approach through your linked address but, I could not find the sample. 

[Kristoff2]

You should be able to increase the PWM freqeuncy beyond 40KHz by modifying the VI as described above without using LabVIEw FPGA.  The VI is part of the myRIO Module and can be found at:

 <LabVIEW>\vi.lib\myRIO\Common\Instrument Driver Framework\myRIO v1.0\PWM\vis\myRIO v1.0 Generate Register Values PWM.vi

-Sam K

LabVIEW Hacker

Join / Follow the LabVIEW Hacker Group on google+