plasma (pics/video) with single IRFP450 in class-E

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plasma (pics/video) with single IRFP450 in class-E

Postby blackplasma » Wed Nov 07, 2007 11:41 am

Image click to enlarge

This class-E amplifier didn't even get past the gate-driver stage; a single IXDD414 in a TO-263 package is driving all of this. I quickly realised that there would be too little space between the flywheel/network components and the edges of the box; the inductor Q would be terrible, and I'm not convinced I could even fit capacitors in there.

But let's be positive! Whilst a pointless wreck, it's the first contraption I've ever really put into a box and called "finished"! I feel sad for not taking more photos of this whilst I was building it. For a sense of scale, the aluminium enclosure is 50mm x 50mm x 30mm.

Most of the bits (including the IXDD414 driver) are surface-mount. I'll never again attempt to solder down a TO-263 device with just a soldering iron; it was a pain in the arse, the copper on the board didn't have enough area to heatsink it properly, and was too thin to transmit much heat well, anyway. I'll have to buy some solder-paste and try my luck at toaster-oven reflow someday.

You just pop up to 40V in the side and that goes straight through the single IXDD414 into the primary. The secondary resonates at just under 4MHz, and there's an 8-SOIC regulator for the oscillator and the cliched blue LED.

I didn't have the guts to commit to fixed-value resistors on the PLL chip, but I guess I can always do it now since I'm not going to change the secondary in a hurry.

The box itself triples as RF shielding for the chips inside, as a capacitive counterpoise for the secondary, and as a heatsink for the transistors. It's connected to the base of the secondary and one end of the primary, so only one feedthrough is needed for the hot wire after the DC-blocking capacitor.

I've got surface-mount 1N5819 Schottky diodes clamping the IXDD414's output to the v+/gnd rails. I'm not sure how hot they get, since I can't fit my fingers down there to touch them.

You could be forgiven for being glad to live on a different continent, where 1/d^2 stops me from messing up too much of the band. ;-)

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Last edited by blackplasma on Sun Nov 18, 2007 10:18 am, edited 2 times in total.
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Curiosity...

Postby blackplasma » Wed Nov 07, 2007 12:42 pm

Just realised how I could actually fit a class-E driver into this box; if I omitted the matching network completely, I could put the primary coil in series with the drain and drop the shunt capacitor across as usual.

Now, can I fit a TO-247 package into this box?

Given a rainy day, I could go completely overkill and and fit a surface-mount TL494, two more MOSFETs and a single toroidal core transformer in there as the PWM audio-modulated power supply.
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Postby kf1z » Wed Nov 07, 2007 1:29 pm

Sure! Why not.....

You've gone this far....

Might as well see what else you can squeeze in there....

Need a shoe-horn?

:D
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Postby blackplasma » Fri Nov 09, 2007 1:36 pm

kf1z wrote:Need a shoe-horn? :D


A bigger hammer might come in handy! ;-)

Image click to enlarge

Too much plasma is never enough -- I've already started V2 knowing I won't have to fit a matching network in there. Proper MOSFETs and Class-E here we come!

The secret at the moment is to keep all the semiconductors on the edges (and the universal grounding sure makes the single-layer board layout neat).

The potentiometers allow for a frequency and duty cycle offset. I'm hoping to only ever have to set them once, at which point I left the legs long enough to fold them down sideways out of the way (the next secret!)

After all that, I've just physically confirmed that the donut core I intend to use on the modulator will fit down into the semiconductor sandwich. I may get away with heatsinking the TL494's push/pull MOSFETs to the lid.

(Otherwise we could end up with a Chinese puzzle of screws which cannot be tightened because there's a lid in the way, but removing the lid removes the objects which one wishes to tighten. =P

Long live the IXDD414, let's hope it never goes out of production.

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Postby blackplasma » Sat Nov 10, 2007 11:50 am

In order to screw the secondary down to the box, I had to modify this second PCB. Unfortunately, I drilled straight through the PLL chip without looking! WOOPS! Turns out I had *not* placed it off-centre as I had intended to. So I made another board with a few revisions, including allowing space for that screw in the middle.

Here's today's progress on the coil, including first breakout. After fitting in bypass and shunt capacitors, I can well and truly kiss that TL494 board goodbye. If I have to fit a single reactive component within the box, not to mention plugs/sockets, it quickly becomes impossible. I was too optimistic! =P

Just for interest, I've taken pics of the board after tinplating, and how I cut the odd profile so it fits snug into the box.

I'll take some photos of the drain waveform later.

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Postby blackplasma » Sat Nov 10, 2007 11:38 pm

Confident that things were running reliably and efficiently enough, I made the executive decision to increase the drain feed voltage to 120VDC. The little aluminium box gets a bit hot for my liking, but it's all in a day's work. What do you expect when you try to pump hundreds of watts from such a small package? =)

Image click to download the video (~3MB)Image click for just the still image

Something tells me this just isn't going to get FCC approval easily... =P
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