Eliminating shunt capacitors in Class E Amplifiers

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Eliminating shunt capacitors in Class E Amplifiers

Postby WA2WVL » Sun Sep 03, 2006 2:48 pm

NO MORE DOORKNOBS. A spot is comming.
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Postby N3LRX » Fri Feb 09, 2007 6:47 pm

Yes??
We're waiting.. :)
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Postby kf1z » Fri Feb 09, 2007 7:29 pm

What he's doing Randy, is using a block of thick copper plate, under the mosfets with a sheet of Kapton dielectric between, to make as a capacitor.

Can actually be more expensive to do than buying capacitors, if you don't have milling equipment, or a friend who does....
And of course the copper flat stock....


Can be done if you have some lapidary, or other grinding equipment to milll the copper flat to mate to the mosfets, and heatsink....
I guess you may be able to find a source of stock, that is flat, and has
NO surface imperfections, that could puncture the Kapton.

It is of course a neater layout and probably eliminates a lot of "stray inductance"....
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Postby N3LRX » Fri Feb 09, 2007 7:37 pm

I've got access to milling machines at work.
My boss is an excellent machinist. I could get him to mill some stuff down for me.

They just installed all new equipment in the new shop.. :)
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Postby blackplasma » Fri Feb 09, 2007 9:58 pm

I have double-sided 3oz/sqft copper clad FR-4 which I put to good use ;-)

But at the high frequencies I work at, I usually only need < 250pF of added shunt capacitance.
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Postby kf1z » Sat Mar 17, 2007 12:03 pm

Can you use aluminum for a heat spreader...?

Or Brass ?

Both are way cheaper than copper....


I suppose aluminum you would need to torque down evenly, to keep the plate flat....

Seems like Brass would be a good choice....
That usually comes from the supply house already flat, and smooth.

I have a bunch of 2 mil Kapton....
I know it's kinda thin.... but the stuff is cheap....
Played with some, and found that it takes quite a bit to puncture a hole in it... tough stuff.

Anyhow, planning on trying out this homemade shunt cap idea soon....
Rebuilding my e-rig to neaten it up... and in a nice rack-mount box..

Thought this might be the time to try the Spreader/Kapton thing....

We'll see!
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Postby WA2WVL » Mon Mar 19, 2007 9:43 pm

I finally got around to reading the discussion on the subject.
First do not use brass since it is a poor thermal conductor. Aluminum is better but Copper is much better.
The mounting technique I was discussing was the same as that used by Frank Carcia in his QEX article. The FETS are screwed directly to the copper bar (no insulator) and the copper bar is insulated from the heatsink by 2 or 3 mil Kapton. With a bar 3/8 inch thick you can tap blind holes to screw down the FETs without thru holes. A 3/8 Copper bar, 2 inches wide and 12 inches long will mount 10-12 devices and is $38.38 from McMaster-Carr. In Push-Pull the 12 inch bar is cut in half to made two 6 inch bars. With the current cost of quality shunt capacitors $38 is not so bad.
It is importain to note that Push-Pull requires much less shunt capacity than single-ended since little stored energy is needed to comutate the output allowing a lower network Q hence higher efficiency.
I use holddown blocks to insure the best copper to aluminum heat transfer. No milling need be done with the bars mentioned above.

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Postby kf1z » Tue Mar 20, 2007 6:47 am

Thanks Floyd....

I actually ordered 1/2 copper .. 2"x6" (2) from an 'on-line' metal supplier for $22 for both...(+sh//h)
It was cheaper than 3/8" (only by a couple bucks..) I supposed it's not as in-demand as 1/2" ?

Overkill in thickness perhaps for my application, but oh well....

Bought some 2 mil Kapton on ebay for $9. (about 3 sq/ft)


I THINK that the resulting capacitance will be about right....

If it turns out I need more... I can "tack on" some more...
If less, then I'll be able to trim them a bit...



I used to buy a lot of stuff from McMaster-Carr....
They are very expensive... BUT, they have just about everything!! (even the shop sink!!

Last I knew, they wouldn't sell to individuals, only buisnesses, maybe they've changed that now.... it's been about 10 years since I ordered through them when I used my employer's account.


The quality of the Cu bars I get still remains to be seen....
But smoothing a 2x6" bar is no problem here with large Lapidary flat-laps..

I noticed though that the heatsink I was planning on using isn't smooth enough.... they used a large surface grinder on it....
And I guess the surface has about 1 mil arc-shaped 'grooves' in it...

So that's no good....I'll either have to try to smooth that some, or use a different sink.


Anyway, we'll see how it goes!

I'll post here my progress, or problems!

Bruce
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Postby WA2WVL » Tue Mar 20, 2007 4:01 pm

Bruce, I also used 1/2 copper. Somewhat easier to tap blind holes.
I don't know how large your sinks are but I bought some nice sinks from Melcor, 7 3/8x 7 3/8 with 1 inch fins. #EXT-301 @ 26.50 ea.

My experience is that a total C equal to the C out of the devices is more than enought for Push/Pull. Single ended would need more.

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Postby kf1z » Tue Mar 20, 2007 4:45 pm

My heatsinks are 7 1/4" x 13" .. 1" fins....
Bought 6 of them on ebay for $15 each + shipping...

Also a guy on the good ole ebay that has 5 1/4" wide... and as long as you want for cheap... brand new....

That's what I use right now...
The 5 1/4" wide x 16 3/4" long was about $22.

I may get another of those, so I have a virgin surface to work with.


On the capacitance....
You're saying take the 'rated' output C, add them together, and that is the approximate shunt C needed ?

didn't realize that....


All I know right now is, I'm using (3) 470pf metal clad micas per 5 FETs..
and I believe the case/heatsink C through the sil-pad is about 40pf each...

For a total of about 1610pf...

Planning on my total C being about 1800pf with the heat-speader arrangement...

Won't know for sure 'til it's built!

Thanks again,
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Postby N3LRX » Wed Mar 21, 2007 2:43 pm

I got my heat sinks from this guy..
HeatSinks on eBay
I've ordered 2 from him both are high quality shipped fast. He'll cut them to size free as well.

What differences do the shunt capacitor values make? The Schematics say 2000pf for 75 per module and 3000pf for 160 per module. Some have used more, some have used less. I'm wondering because I got a bunch of 1200pf/5kv's cheap. I was thinking of using 2 on each module. What difference would the extra 400pf make including the added capacitance from the sil-pads?

I'm still waiting for the PWM boards. Maybe one of these years I'll have something built.
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Postby WA2WVL » Wed Mar 21, 2007 4:34 pm

Randy, the Push/Pull amp I built for W1TY had 5 IRFP32N50K FETs per side (2750pf) and capacitance added by the 5 mil Kapton insulator was only 720pf. This amp ran at 90% on 40 meters.
Adding shunt capacitance across the FET output raises the Q of the output network. A minimum Q of 1.8 is needed for single ended designs but the Q needed in Push/Pull is less than 1. This means higher efficiency.

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Postby kf1z » Wed Mar 21, 2007 5:12 pm

Yes Randy, that's the guy I get mine from....
Ordered 2 last night ....

2 pcs... 16" long just under $60. including shipping....
He's gone up a little since last year..... to be expected I guess....

I am no expert, but I think you're going to run into trouble with those caps.....
Maybe ok for the "extra" shunt C for 160 meters....

The ATC 100C or 100E caps are good....($9. each... )
I'm now using the Metal clad micas from digikey....( $3.75 each ? for 470pf 1000v)
They have been working fine....

But now have (2) 2" x 6" x 1/2" copper sitting here in front of me...
Just about to clamp one over a piece of Kapton, to make sure I'll have the capacitance I'm looking for.....



You may find that with doorknobs (I assume that's the 1200pf caps you have) you need more capacitance than with the MLCCs .

I think that 1800pf is close for knobs
and 1400+ is close for the smaller, lower ESR higher Q caps....



One way to find out Randy..... build it, and find out.....
You may think ahead to the "what if.." in case you need to change them....
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Postby N3LRX » Sat Mar 24, 2007 5:42 pm

The Dorkknobs I have are Ceramite #715C 1200pf-Z 5KV

I was going to order some ATCs but these came up. I may try those clad micas from Digikey since they're allot easier to order from and cheaper.

The question is how do I know if I've got too much or too little?

It's going to be a while till I can test it I guess I still don't have a modulator yet. Don't understand what the delay is for the PWM PCB's but oh well. Just annoys me I've got a bag full of all the parts for the PWM minus 2 IC's and nothing to solder them too It's only been 4mo. now since I ordered them. I'm at a blockade now where I really can't plan anything further because I have no modulator!

I may just say screw the PWM and build an H-Mod because that's the easiest to bread board together and since I should have all the parts I need to build a 10 FET Class H. I don't have the BUF's but I've got an older schematic with 10 FETs I'll build that instead if I have too.
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Postby kf1z » Sat Mar 24, 2007 5:57 pm

You know you have too little shunt C, when your peak drain voltage is more than about 3.5 times the input voltage....

So, if you're modulator is providing 40 volts to the amp., your drain voltage would be about 140 volts.
Or, I should say , no more than that...

So I understand anyway....


As for how to tell if you have too much....

Not exactly sure...

But I believe you would see the drain waveform (VOLTAGE) NOT reach 0 volts before the FETs turn ON again....
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