Simplified PWM (or PDM - the same thing) Using UCC25701 Chip

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Simplified PWM (or PDM - the same thing) Using UCC25701 Chip

Postby steve_qix » Sat Apr 19, 2003 12:13 pm

Hi!

I have been trying to come up with a way to simplify the process of generating high quality Pulse Width Modulation (PWM).

I may have reduced the process to two (2) ICs. I am currently experimenting with the Unitrode (Owned by Texas Instruments) UCC25701 PWM controller IC.

After quite a bit of research, I chose this IC for the following reasons:

1) The device will generate waveforms between a 1% and a 99% duty cycle. This is very important for utilizing the full power supply voltage availble, and acheiving full modulation capabilities. Many PWM chips will only generate waveforms with a 50% or 80% duty cycle, maximum.

2) The device will operate properly up to 700kHz. This indicates a high internal bandwidth and good resolution of small pulses.

3) The device is available in a plastic 14 pin DIP package, in addition to surface mount.

4) The price is very reasonable - about $5.00 per device.

Experiments thus far are very promising. The ramp generator produces an extremely linear ramp with very sharp points at the transition points.

I constructed a simple PWM generator switching at 110kHz, and a used 400 Hz triangle wave as the modulating waveform. I constructed a simple R-C integrator and observed the output. The reconstructed waveform was every bit as linear as the original modulating triangle wave. The only observable difference were the peaks, which were not as sharp as the original waveform, due to the effects of the integrating network on the frequency response of the system.

Based on these experiments, I am constructing a prototype PWM generator using this device, and a TL074 as an audio filter and negative peak limiter.

If this works as expected, I believe we will have a simple, effective, high quality PWM generator circuit which will be easy to implement, making building a PWM transmitter a much simpler task.

More later :-)

Regards,

Steve, WA1QIX
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Simplified PWM (or PDM - the same thing) Using UCC25701 Chip

Postby Todd WD4NGG » Tue Apr 22, 2003 10:52 pm

Great idea Steve! I have been working on a similar solution using one of the "All-in-one" PWM chips for our use as a PDM generator. One of the all-in-one chips should inherently be a better way to go as the triangle wave or ramp generator should be optimized to the comparator with almost zero lead lengths and all functions on one chip. Not all power supply PWM chips are optimized for our use as a PDM generator though. As you mentioned many of them do not cover the full range of duty cycles from 0-100% . Also low distortion may not be a priority in a power supply and the triangle-wave or ramp generator may not have the best waveform. There are several Class-D hi-fi audio amplifier chips on the market that show promise for our use as a low-distortion PDM generator. 73 Todd WD4NGG.
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Postby frank carcia » Wed Apr 23, 2003 11:41 am

Todd,
I like your idea. I wonder if anyone knows of a chip that will interface to high power FETs to build a big amplifier. It will need a DC coupled output
that swings 0 to about 100 VDC. fc
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Good ideas on this site - Thanks Blain!

Postby N9NEO » Wed Apr 23, 2003 6:25 pm

Yep, lots of great ideas here!

Frank - I just ordered a UC3875N Phase shift controller to run the modulator, and they will do dc. I'm only shooting for 250w PEP, some might say QRP, but mosfets up to you. Chip is expensive @ $8.74 each, but will do 200kHz no prob. I prefer the 3879, but digi-key no have.

I'm using 4 of the IRGI644G mosfets 250v/8amp for the modulator, and a pair for the rf deck. Chose them because small package with low parasitic inductance.

take a peek at the parts.

73
Bob
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Postby frank carcia » Thu Apr 24, 2003 9:41 am

Hi Bob,
Interesting PWM. I wonder if you will be able to swing it 0 to 100 %.
I would think you would do an off line converter with a step down transformer to get 0 to 120 or so volts out. This would eliminate the big 60 Hz. transformer. The problem becomes the switching transformer which will have to work over the pulse width of 0 to 100%. Leakage inductance can be a real problem with this design. I tried this a while back
with discrete parts with straight current mode control. I had a large! pot core but a big ratio. I was trying to produce B+ for a tube. Still you will need a lot of primary inductance . What will the transformer do when you
try to put a 100 ns pulse through it and then 10 Us. This woulld be the range required it the clock was 100 kHz. Then you have to rectify the output with some mofo fast high current diodes. Still sounds like a cool project if the magnetics are possible. Good luck!!!!!! 7 octive transformer
now that is broad band! fc
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Magnus Opus de Modullatti Ether

Postby N9NEO » Thu Apr 24, 2003 5:29 pm

Frank,

Glad you asked.

Modulator is off line with only small ferrite transformer carrier is 200kHz. I can use EE/EP, or high perm ferrite EMI cores I have. I'm starting with a 250w design. Ratio will be around 2:1 or 3:2 So max voltage at 100% duty near 90-120v I haven't calculated output filter to gnat's ass, but I'm sure I can modulate a 50ohm antenna load to 5kHz with 100% modulation. Going to use the class E/F design. Leakage inductance is actually necessary with this design as it charges mosfet output capacitance. I've used coaxial transformer windings in the past to minimize the problem, but I'll save that technology for the rf deck.

The main difference between this approach and discreet ps/modulator is that there is nothing driving the voltage on the output capacitor (RF deck supply cap.) down but the output power transmitted. Where a discreet audio amplifier can drive the voltage up or down. This is what will limit the absolute power-bandwith envelope. Cap is looking to be in neighborhood of 1uf. Just a big bypass cap. heheh. Inductor will be chosen using same rational - How fast do I need to change the loop current to maintain a decent power bandwith product.

Volt-second balancing may or may not become an issue, I don't think 3875 chip will take care of. Usual solution is to place a cap in series with transformer primary. A rather messy solution. I'm sure I can throw in an extra winding or two on the transformer, integrate this, and sum with current amplifier to keep I mag under control.

Output diodes are big dual 16amp ultrafast devices with snubber. On larger design syncro-rectification might be approach to take.

One thing which keeps cost down is able to use small dc bus filter off line. Since bandwith of modulator is high, especially with feed forward desigm, that any ps ripple is rejected. A pair of 1000uf bus caps should do.

I'm thinking of a modulation down scheme so carrier sits at about 60%-70% with no modulation. Will implement with compressor. Simulations are pretty much done on rf deck, just playing with output transformer and some Fourier stuff with output filter. Have the preamps and compressor simulated, just gonna wing the switcher.
Hope this answers questions. If you have any input I'd really like to hear. I don't know if any others have experience with this topology. Please Chime In if you do.

regards,
Bob
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Postby N9NEO » Thu Apr 24, 2003 8:25 pm

After posting my reply I read it . Bassackwards. Shoulda read and then...

In first paragraph I said to effect leakage inductance is needed with this design. I was refering to the smps modulator, not the rf deck.

bob
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Postby N9NEO » Thu Apr 24, 2003 8:36 pm

Ok, Ok, I know. Just one more thing and I'll hang up. Didn't address the narrow pulse issue.


This particular SMPS topology uses phase shifting to generate narrow pulses. Just changes the orientation between the two phases. So each pole is actually switching at 50% duty cycle. If both phases in sync then output is zero. If poles 180 out of phase then you get 100% out.

I just heard Steve's new modulator. Sounds Awsome.

73
Bob
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Here it is!

Postby steve_qix » Thu Apr 24, 2003 11:10 pm

I have completed the simpllified PWM generator. The audio results are superb! The PWM generator is small and uses only two ICs.

Here is the schematic.

Comments?

Regards,

SteveImage
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Postby N9NEO » Fri Apr 25, 2003 4:30 am

Steve,

Nice job! Looks really simplified. I did hear your audio early last evening and it really sounded crisp.

Did you get my email? I sent to bicnet earlier in month. I think resonant link with pdm may be possibility.

73
Bob
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Postby frank carcia » Fri Apr 25, 2003 7:31 am

Hi Bob, Steve,
Bob just consider what your transformer has to pass at 1 % duty cycle.
200 KHz is 5 uS pulses at 100 % so at 1 % 50 nS. Consider a power transformer that works from 200 KHz to 20 MHz. Yes a BB RF transformer
is the way to go but the high number of turns will effect the high end
response. I understand the phasing of the primary but when they get close in pahse the component the transformer sees is the short pulse. Still I like your idea. Heck we have a couple vicor 500 watt modules in the lab. At some point I will connect one to a function generator to see what they do. I've done a lot of work with BB RF transformers and you might get lucky. The trade off will be core loss VS leakage inductance. I'm thinking a core with Ui = 2500 or so. That stuff isn't too bad at 20 MHz.
Steve, nice to see your parts count coming down. I knew you would see the light after you tried newer PWMs. Good luck. fc
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Single chip PDM

Postby Dan Brown » Fri Apr 25, 2003 10:44 am

Hi Steve:

Great circuit!
Glad you found a chip that has the best of all worlds.

FYI: My PDM generator (your "ultimate PDM generator" descrete design) has balanced line level in and no negative clipping. I do this becaude I am using a broadcast processor.

Also, keep in mind the International Rectifier IR2110 FET driver. A simple circuit to drive the modulator power fets. However, this chip will not pass extremely small pulses. But this works for me as a negative clipper.

73
Dan W1DAN
73
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Postby frank carcia » Fri Apr 25, 2003 10:49 am

Hi,
Be careful with the 2110 as it needs a continous stream of pulses and has limited drive. I would use one per FET. Great simple isolator though. fc
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IR2110

Postby Dan Brown » Fri Apr 25, 2003 2:09 pm

Hi Frank:

You are indeed correct on both. One circuit I saw shows a small external power FET used to drive multiple power FET's.

Right now I am driving only one IRFP450LC with the IR2110.
When I get the 6 FET amp going I will hit the limited drive problem as I will have to beef up the modulator.

The opto-isolator system is good as well. It may be more adaptable.

Dan
73
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Postby VK3KRI » Fri Apr 25, 2003 5:46 pm

Steve,
If you re-arrange things a bit, can you feed audio and DC 'carrier' level through a pot, allowing simple power output control ie, the carrier and the audio level are varied ath the same time?
For instance, if you add a pot between the last fiter op-amp and the PWM chip, you should be able to control the output without fiddling audio gain and carrier level.
( Assuming, of course you don't wat to run flat out all the time)
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