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PWM Current Capability

PostPosted: Tue Apr 15, 2014 11:09 am
by Brian K1LI
I really appreciate the knowledge collected on these pages. I've learned a lot from them.

I don't understand, though, why the PWM needs to have so much power-delivery capability. The description says that each transistor - capable of delivering 35A continuous - can provide 200W of modulated DC to the amplifier. Each transistor appears to be capable of providing 1400W, so why are multiple transistors needed?

Thanks,

Brian K1LI

Re: PWM Current Capability

PostPosted: Wed Apr 16, 2014 2:07 am
by Gilly
Hi Brian

The maximum current rating can be a bit misleading.....

There is a big difference between switching a FET fully on once allowing current to flow though to a load ( like a linear power supply ) and switching the FET on / off at a high rate......
Imagine a Class D/E TX at 7Mhz thats 7 Million times a second the gate capacitance has to be fully charged and discharged...a lot of work .....
So thats the problem really.....if you read Steves WEB site he mentions terms like Miller Effect / transition times etc: all these things and others contribute to the device operating well under their maximum ratings.....


Wayne

Re: PWM Current Capability

PostPosted: Wed Apr 16, 2014 6:18 am
by Brian K1LI
Wayne,

I understand that both the MOSFET's input capacitance and output capacitance must be charged and discharged each cycle and that the challenge of doing so challenges the device's ability to supply current at higher frequencies. But I don't see how that problem is addressed by adding devices in parallel, which increases both the input and the output capacitances proportionately.

Would you, please, point me to the part of the web site that deals specifically with this issue? I thought I had read through the PWM material carefully, but didn't see it.

Thanks,

Brian

Re: PWM Current Capability

PostPosted: Wed Apr 16, 2014 8:36 am
by Gilly
Hi Brian

Its just like a linear Power Supply....
To increase the current capacity you use more pass transistors to share the current....
With the PWM paralleling the FETS gives more current capacity too. It does increase the total gate capacitance but at 160Khz a IXDD414 can drive the 5 FETs OK.....
Using 5 FETs is probably an overkill for your legal limit but its there just in case :D

Perhaps paralleling the FETs is in the Transmiiter part of Steves WEB site....somewhere.....

You can parallel as many FETs as you like I suppose as long as the driver chip can handle the current required etc etc......

Hope I have answered your question OK


Wayne

Re: PWM Current Capability

PostPosted: Wed Apr 16, 2014 12:03 pm
by Brian K1LI
"Overkill" is precisely the question I'm trying to answer, Wayne. What I'm now thinking is that, because AM requires the modulator to deliver so much "idle" power, plus 50% more power on modulation peaks, the "extra" devices are meant to reduce the temperature rise in each individual MOSFET which would otherwise begin a thermal spiral by raising the "on" resistance. Conversely, with the modulator operating at 90% efficiency, this should not be a problem in the first place!! I hope I'm making clear the apparent contradictions I'm trying to resolve.

Mainstream manufacturers of ham gear typically prescribe ~50% output power reduction of their Class AB power amplifiers when running in AM or other high duty-factor modes. It's clear that they have to do this because Class AB amps are lucky to achieve 55% overall efficiency, so they produce a lot of heat which must be tolerated by the amplifying devices and removed by sinks, fans, etc. I would not expect that this kind of derating would be needed for circuits, like Class D PWMs and Class E amps, that run between 80% and 90% efficient.

Re: PWM Current Capability

PostPosted: Wed Apr 16, 2014 7:04 pm
by Gilly
Hi Brian

I wouldn't worry too much about this overkill thing.....the more FETs there are the more power you can deliver :oops:
At 90% efficiency heat is very easy to control.......
Order a kit from Steve and make up one of these AM systems as they do give results that are very satisfying.....
The idle current is what AM is all about.....nothing like tuning the band and finding a strong carrier with modulation...without that carrier what have we got :lol:
In one of my modulators I used just one FET....it looked lonely so in the others used more.....it looks better thats all ! We run lower power here in Australia.....


Wayne

Re: PWM Current Capability

PostPosted: Wed Apr 16, 2014 8:01 pm
by n1ps
Well nice to see some activity on this site again. It seems of late people have been posting questions on amfone. I don't completely follow your concerns Brian, but in terms of the series PWM modulator like what is on Steve's site, the headroom is in the power supply. The modulated DC VOLTAGE is typically run at about 1/3rd the power supply B+. But some of the gurus can better answer that like Steve, Frank, Bruce etc.

If you want to venture into an E rig, this is a great site to get help, along with amfone. For a design and parts, I concur with Wayne.... Steve has made the process greatly simplified with a number of boards and parts locators and a bulletproof design. Here in the northeast many have built E and D transmitters.

Peter

Re: PWM Current Capability

PostPosted: Sun Apr 20, 2014 12:36 pm
by kf1z
Steve's PWM is "adjustable"

Meaning, you may or may not populate all the mosfet/diode pairs as needed for your RF deck.

In most cases, on two are needed.
I built one with 3 once, but was not needed

Re: PWM Current Capability

PostPosted: Sun Apr 20, 2014 12:40 pm
by kf1z
Overkill is always a good thing... :-)

You'll find, that some of Steve's designs are based on devices "that are available at the time..."
Meaning he has them in stock.

Also, most of his designs for PWM, Class H modulators, and the Class-E RF decks are designed to
be nearly bullet proof/ indestructible

So if the design calls for a device that is to normally see 200 watts , he will use a device that is rated for a MINIMUM of 2-3 times that rating.


And why not, these things are cheap! The mosfet used in the pwm modulator are only $3 - $3.50 each.... so why not add an extra or two,
instead of using a $2 device, that is barely overrrated, and have to replace it ( and other supporting components ) if there is some mis-hap?


If you are looking for a more technical reasoning why he says that each one is capable of delivering 200 watts of modulated DC, when it appears that it could handle 1400 w....
Well, hopefully he'll come in and explain it.... :-)

Re: PWM Current Capability

PostPosted: Mon Apr 28, 2014 1:27 pm
by Diego-CX4DI
I am using only one IRFP250 for to modulate a 400W class e amplifier(50Vdc/9amp.) during the last 4 year less fails.
ref.: Mr. todd roberts WD4NGG article.

best 73's
Diego
CX4DI

Re: PWM Current Capability

PostPosted: Thu Dec 31, 2015 9:05 pm
by steve_qix
The repetitive peak current in the modulator MOSFETs is approximately 3x the DC output at carrier. This is due to the duty cycle being quite a bit less than 50%, which it should be. Even at 50% the peak is twice the DC.

So, with a 10A load at 45V - typical 450 watt carrier input, the repetitive current is 30A. At full output, the DC is 30A. What happens if there is an overload? The peak current rises rapidly. There isn't enough headroom in the design to be safe with only 1 modulator device. Two are minimum and three are better.

Also, the R-DS on values are paralleled, increasing the efficiency of the modulator somewhat.

ALWAYS overdesign. The initial investment is super-small - a whole lot smaller than having to fix the modulator later because the implementation was too close to the edge.