PWM Mod Problem

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PWM Mod Problem

Postby yb0djh » Sat Nov 19, 2011 6:48 pm

Hi ALL

Continuing my previous posting First Class E from Indonesia, I would like focus to find out what is my problem with my PWM Modulator

I will post the schematic of my modulator. I m using TL494 and IRS2025 highside driver to drive pairs of IRFP250. The voltage input is 120 volt. the carrier voltage (no mod) i set to 43 Volt. The waveform i took when i tune RF to 8 Amps.

Here are the pics:

Carrier Waveform

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I made the sweep rate slower to see the modulation

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Inserting low modulation

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Inserting medium modulation, and i have distorted on lower peak of waveform, you can see below

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and inserting full modulation (100%)

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Previously i suspect the same as Wayne thought, about run out of PSU Capacity. Later i suspect the problem is laying on tinny square wave from PWM that creating distortion, but i cannot pointing it where. :(


Need your assistance. Thank you for responses

BR. Agus-yb0djh
Last edited by yb0djh on Wed Dec 14, 2011 12:41 am, edited 1 time in total.
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yb0djh
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Re: PWM Mod Problem

Postby Gilly » Sat Nov 19, 2011 8:47 pm

Hi Agus

Was interested in seeing your PWM. Congratulations as I have only copied Steves design so you are one up on me :!:
What is the frequency of the PWM is it in the region of 90-200Khz ?

Hopefully Steve or Bruce can comment too...

Well your TX is OK ... although I would not push the power to your levels.
The waveforms from the PWM are definitely distorted.
Disconnect the TX at the moment and use the dummy load to the PWM...
What I would do is check the analog section up to the input of the TL494. First with no power to the output FETs and then with power just to see if all OK and no feedback exists.
Are a bit concerned with the Output Filter Circuitry and layout though.
Unfortunately my ideas will probably create more work for you.. :(
I would move the Power Transformer and filter capacitors well away.
Also .... to test that your design is all going well ... replace the inductors with Powered Iron Cores similar to what I did some time ago..
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If these changes prove to be successful you can then replace the components with higher power components...etc:

But run these tests not at 50Volts but low power say 30Volts no more....

Hope you understand everything Agus.....

Wayne
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Re: PWM Mod Problem

Postby yb0djh » Sat Nov 19, 2011 11:11 pm

I HAVE TO MOVED THIS POSTING HERE


The switching freq is 100 KHz




====================
Dear Wayne

Now, this is the report of trouble shooting PWM Modulator.

First step i reduce it to 30 volt. Loaded with 6 ohm 400 watts resistor.

The modulation i started from light medium and then i make it over.

Started light modulation

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Then, medium

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i make it over (before reaching zero, the waveform is distorted

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After over modulated

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Second Step I made it 40 Volt (no Mod) before reaching zero the waveform are rounded as below

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Before reaching zero

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over modulated

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This is the modulator. See the LCLC Filter, L1 is air core, L2 is Toroid

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Thank you for your thought of this problems


BR
Agus - YB0DJH
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yb0djh
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Location: Indonesia Jakarta

Re: PWM Mod Problem

Postby Gilly » Sat Nov 19, 2011 11:27 pm

Hi Agus

Can you check your Analog waveform at the input pin of the TL494....with No power to the FET and then with Power....

Just to make sure that the Analog signal is all OK and that no clipping / distortion is happening there...

Also does your circuit have an Audio Filter say with a cutoff of 6Khz ?

Let me know as we are on appox the same timezone....I think you are about 2 hours behind me !! 3.30pm now ...


Wayne
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Re: PWM Mod Problem

Postby Gilly » Sun Nov 20, 2011 4:25 am

Also Agus


How are the output filter capacitors grounded....looks like braided cable running somewhere ?


Wayne
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Re: PWM Mod Problem

Postby Diego-CX4DI » Mon Nov 28, 2011 6:49 am

Could you post the schematics of your switching modulator?

What type of toroidal cores are you using in the PWM filter?

73's
Diego
CX4DI
Class E with PWM modulation
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