Plasma Rocket Engine

[plasmahunt3r]

Got Magnetic Induction running right on Class E Amp

I have been testing Class E all this month and finally got the result I was looking for.

First of all, I needed high current and not high volts. I rebuilt my power supply just for this purpose.

To get high current, I cut out the secondary of a microwave transformer and relaced the secondary with 16.75 turns of 14 ga. insulated wire. This produced 16.6 VAC. When I put in a full wave bridge and a 3300uF Capacitor, the unregulated came out to 22VDC. The unregulated 22VDC went to the Class E stage.

I built a regulator using a Zener Diode and a Series Pass Transistor (MJE13007) which gave me a regulated 13.2Vdc. The regulated 13.2VDC was used for the Crystal circuit and gate driver circuit.

Most of all, I switched to a 100V 180A MOSFET (IXTH180N10t). They were $4.025 each at Digikey (in quantities of 10). The 180 Amp was overkill, but gave plenty of margin, so I would quit blowing up transistors.

I also rebuilt the Choke Coil using less turns and a less permeable toroid. It read .37 mh (370 uH). I kept with the auxiliary coil since previous test worked well with one. I think the Auxiliary coil will become a standard on all my RF and SMPS projects.

Worked great. The frequency remained stable, even with a load. I put a clamp Ampmeter on the MOT secondary and no load draw was 4.5 amps. Then I put in a steel screwdriver as a load. The Amps increased up to 8 amps as the steel got hotter.

I think I finally understand Class E Amps. Here is my updated circuit:

http://i1351.photobucket.com/albums/...pse6360e7a.png

[plasmahunt3r]

How Chokes affect Class E Amps

I have had the most problem, figuring out chokes. I could not find any rules on the internet on choke sizing for Class E Amps or SMPS circuits, only that they are required and are a critical part of the circuit.

Chokes are vitally important in Class E power Amps. I have done some test, showing how different size chokes affect the circuit. Auxiliary coils were added to some chokes to make sure the choke discharges completely (each cycle). The circuit was run using crystal driver at 32.768KHz.

100 UH Choke (no Auxiliary coil): Transformer drew 8.4A and frequency jumped to 65.6 Khz.

1.88 MH Choke (no auxiliary coil): Transformer amps kept increasing and I turned it off at 10A. Frequency jumped to 57 KHz.

2.36MH Choke (with Auxiliary coil): Transformer drew a stable 2.6A and frequency was 57 KHz.

370UH Choke (with Auxiliary coil): Transformer drew a stable 4.2A and frequency kept a stable 32.8 KHz. "Correct Frequency"

The transformer was humming loudly when using standard chokes with no auxiliary coil. Transformer was quiet on chokes with Auxiliary coil.

I don't know a rule or calculation for sizing of chokes, but the correct size is required for the circuit to work correctly.

Some observations can be made from my testing:
a. In Class E, the choke is charged when the transistor is on, and when the transistor is off, the charged choke provides current for the induction coil.
b. In Class E, the choke is used as a current storage device similar to any SMPS circuit.
c. Standard chokes (no aux coil) draw more current and hence more current available to circuit. I believe the chokes are not discharging completely without an auxiliary coil, and the current builds up, until the transistor fails. One time, the capacitor leads burned off.
d. using the auxiliary coil on the choke draws less current than a choke with no aux coil
e. Only the 370 UH choke with the aux coil gave me the correct frequency.
f. Chokes too large (or too small) do not work correctly.

In some circuits, chokes are used to block frequency feedback, and sizing would follow a different set of rules.

In SMPS and Class E Power Amps, the choke is used as a current storage device, and a proper size must be found.

I think, as a general rule for SMPS and Class E chokes, they should:
a. Have auxiliary coils to completely discharge the coil on each cycle.
b. The higher the frequency, the smaller the choke size (uH) should be. 370uH worked well for low frequency of 32.8 KHz. Possibly 47 UH for MHZ circuits.
c. Pick a size and add/reduce turns until the circuit runs at specified frequency.
d. The smaller the choke size, the closer the choke is to a "Dead Short", and the higher the current draw.
e. On higher frequencies, the choke is on less time, which should limit the current draw (assuming the current is drained in each cycle by auxiliary coil).

If anyone has any experience with ckokes, your insights would be most appreciated. I can only do trial and error, until the circuit works as required.

[plasmahunt3r]

Switched to Half Bridge

Class E is "TOO Frustrating". The variables are complicated, and although I finally got it to work right after 2 months Trial and Error, I don't want to go through the same effort when I switch from 32 KHz to 13.56 MHz.

Switched to a Half Bridge Circuit to run the Magnetic Induction Heater. Worked right the first time. The frequency started at 90 KHZ, until the circuit warmed up, then stabilized at 32.8 KHz (with and without a steel rod load). This is the way to do it.

Attached is the pointer to the new Half Bridge Circuit:

http://i1351.photobucket.com/albums/...g?t=1391725323

[plasmahunt3r]

Gate Drive Transformer

Using a Gate Drive Transformer is common when isolating control circuit from a higher voltage/current drive circuit.

If using a 1:1 Transformer, the output from the Gate Transformer is AC, at about 1/2 rail voltage. On a 13.2VDC regulated voltage, the output was 6.37v. A MOSFET generally requires at least 10V to turn on fully.

I had previously dealt with this by winding a custom toroid with 6T primary and 13T secondary, to get the voltage up to required levels.

I have another way to get the voltage up by using a standard "Off the Shelf" COMMON MODE CHOKE. These are 1:1 coils, generally with through-hole wires, that allow easy placement on a PCB. I used a Panasonic 3A Common Mode Choke, although, there are many on the market.

As with any 1:1 transformer, the output voltage is AC at 1/2 rail voltage. By adding a Capacitor and a Diode to the transformer output, it forms a Voltage Doubler that also rectifies the output from the Gate Transformer, into Pulsed DC at close to rail voltage. This is called "DC Restored Gate Drive Transformer".

I tested the change on my Class E Magnetic Induction Heater Circuit, and it worked well. Instead of Custom Gate Transformers using Toroids, I can now use 1:1 "Off the Shelf" Common Mode Chokes. For Half Bridge circuits, I can use standard 1:1:1 pulse transformers.

Here is my upgraded Class E Magnetic Induction Heater circuit using the Common Mode Choke as the Gate Drive Transformer. Notice the 1 UF Capacitor and Diode on the output side of the transformer. That is the doubler that rectifies the output into Pulsed DC at Rail Voltage. This technique is commonly used in Microwave Transformer Circuits.

http://i1351.photobucket.com/albums/...g?t=1391738996

[rosehillworks]

I am interested in what you are doing with your driver circuit but I cannot find your last diagram would you please repost it . thank you
My son and I want to use an electric induction heater as a heat source in a gasifier unit .

[plasmahunt3r]

Repost Picture

I think I was editing the post and updating the picture at the time you looked. I had forgotten the auxiliary choke coil on the Class E portion of the schematic.

Here is the updated picture:

http://i1351.photobucket.com/albums/...g?t=1391739098

I would like to see what you end up with and what circuit tweaks you make to suit your needs.

[antigraviticsystems1]

magnetohidrodinamica en plasma - YouTube

[plasmahunt3r]

Antigraviticsystems1

Cool Video Antigraviticsystems1. Do you have any schematics and diagrams???

[rosehillworks]

Thanks for the update. I will let you know what we did and how it turns out.

[plasmahunt3r]

Choke VS Snubber

I have a lot of burned up transistors testing Class E Power Amps. When I finally got the circuit to work, I had gone to a Choke coil with an auxiliary coil and a Shunt Capacitor of .039 UF.

Lots of frustration working with Class E. The half bridge circuit works without the choke coil complications.

I have been monitoring Amps during testing, up to the point the transistor fails. Current doesn't appear to be excessive. I am concluding that "Voltage Rise" from the inductor is the culprit, when the transistor is turned off.

Here is the Class E circuit again:
http://i1351.photobucket.com/albums/...psdbadf12b.png

I am now looking at the circuit differently, with a view of the auxiliary coil acting as a snubber. The Auxiliary coil, along the the shunt capacitor, are snubbers, each performing a different function.

The Auxiliary coil limits Current Rise from the choke inductor.

The Shunt Capacitor limits the voltage rise from the choke inductor, when the inductor is turned off.

During turn-on condition a series inductor "called snubber inductor" is used to limit the rate of rise of current (di/dt) through the device. The Auxiliary coil is limiting the current rise.

A shunt capacitor "called snubber capacitor" is used to limit the rate of rise of voltage(dv/dt) across the device at turn-off instant. The Shunt Capacitor, in the Class E stage IS a Snubber capacitor. I didn't realize Shunt and Snubber meant the same thing.

I found some rules of thumb for snubber capacitors sizes:
(0.01 - 0.069μF) small snubber
(0.07 - 0.14μF) normal snubber
(0.15 - 0.3 and > 0.3 – 5μF) large snubber

In a small snubber the capacitor voltage reaches quickly to the input voltage level during its charging condition. Therefore, it is not sufficient for turn-off protection.

The normal snubber provides sufficient turn-off protection.

Large snubber the voltage rises slowly to reach the input voltage, and thereby provides good turn-off protection with minimum turn-off energy loss.

The Class E problems arise from the Choke Coil Inductor, and careful tuning of Snubbers are the solution. I found a web site with Snubber Calculator which helps:

Snubber Circuit Design Calculators

[plasmahunt3r]

My Magnetic Induction Test Board

I thought I would post a picture of my Class E Test Setup. I created my power supply by rewiring a Microwave Oven Transformer. This MOT can deliver the Amps.

I mounted the transformer, along with a power switch, Full Wave Bridge, Capacitor, and Voltage Regulator on a polyethlyene cuttting board. I hot glued a solderless breadboard to the cutting board and connected it to the 13.2V regulated power.

You can see the .68 mH choke coil with the auxiliary coil. It gets the unregulated 22V power. The red and black wires twisted together. The red is the primary coil and the black is the auxiliary coil. The aux coil is connected to a 20A 1000v Diode (anode on case) which is connected to the negative supply. The combined red/black twisted wires connect to positive in. The single red out goes the the Class E Amp stage.

The Class E Amp is on the lower right and all it's components fir onto a 1/75 x 1.75 inch PCB. The Induction coil is connected to the PCB.

Here is the pic of my Class E, Magnetic Induction Test Setup:
http://i1351.photobucket.com/albums/...g?t=1392510743

I still have a problem with the circuit. The Amp load keeps rising. Eventually, the Transistor catches fire. I suspect that the Voltage is rising also, but I don't know how to measure it. Maybe I am getting RF feedback from the Induction coil. Don't know the solution for Class E.

I have done a temporary solution by limiting the current that can be provided by the rewired Microwave Oven Transformer. I placed a 30-36 UF 250v Start Capacitor on the AC in Hot Wire (Black), which limits the current to the Primary using Capacitor Reactance:

Capacitor Reactance in Ohms = 1 / ( 2 * Pi * Frequency * Uf / 1000000)

So a 36 Uf cap on a 60Hz circuit would be:
1 / (2 * 3.14 * 60 * 36 / 1000000) = 73.72 Ohms

120Vac / 73.72 Ohms = 1.628 Amps to the AC primary.

So the 36 UF circuit limits the current to the primary coil to 1.628 Amps. With the Class E running, the secondary current topped out at 8.93 Amps. Limiting the current available to the Class E is a temporary solution for now. 9 Amps at 22 volts is about 200 Watts.

The current ratio between the secondary and the primary is about 5.5 to 1 (8.93A / 1.628A = 5.485). If I had 10 amps available to the primary, the secondary would produce 55 Amps. More power than I need. So, limiting the current available is probably a wise precaution when using powerful transformers.

The MOSFET still fails after 3 or 4 minutes. When the MOSFET fails, it remains in an "ON" state (which is a short). I have added a fan to the heatsink thinking of a thermal failure, but it has no effect. I have tried a 180A/100V MOSFET, 20A/500V MOSFET (irfp460), and a 20A IGBT. They all fail after a few minutes. Does anyone have any idea how to stop the MOSFET failures???

[plasmahunt3r]

Right Capacitor and Autotransformer Test

Class E MOSFET Failures is solved. The MYLAR Capacitors were the problem.

The MKP (Metalized Polypropylene Film) Pulse Capacitors came in. I updated the circuit with one 2.2 uF pulse capacitor and the frequency was stable at 32.8 KHZ and it worked like a champ. To think, all the testing and Transistors I blew, was because of improper type of capacitor. Live and learn.

Now that that problem is solved, I did some experimenting with the homebuilt Autotransformer I am using.

It is a 39mm Toroid with Bifilar coils wound just like a Joule Thief Circuit. It reads 1.54 mH on each coil.

In the first test, the CT is connected to the MOSFET Drain. The Current Draw was around 7.49A without the Steel Rod in the Induction Coil, and around 7.58A +/- .4A when the rod is inserted. Circuit maintained a constant 32.8 KHZ, with/without the steel rod load. Here is the updated circuit:

http://i1351.photobucket.com/albums/...ps4ecfdad9.png

The second test was done utilizing the same 39mm Toroid, but I swapped the output to use the CT with amazing results. While each coil still read 1.54 mH, the combined coils read 6.26 mH. A higher mH reading meant the the circuit would draw less current from the power supply, but the CT Amps Out would be double while the Voltage Out would be halved. This is more efficient because Magnetic Induction Heating is mainly a function of Frequency and Current, and not Voltage.

Using the CT as output, the current draw without the Steel rod placed in the coil is 3.28A. When the rod is placed in the coil the current draw on the power supply jumped to 4.65A, then slowly rose to +5A. Since the CT Amps Out is double, the CT Out provided 10A to the Induction Coil, while the previous version only utilized 7.58A. More Amps to Coil made it run hotter, quicker. I think this is amazing because it utilized less power from the power supply but did a better job of heating. Here is the updated circuit using the CT out from the autotransformer:

http://i1351.photobucket.com/albums/...ps150502a0.png

Now that I have Class E Magnetic Induction Heater problem solved, I would like to change the circuit to have an Antenna out. I am not quite sure how to do this. Any Radio guys out there know how to configure an antenna as output. It seems like I could use the CT out with the 2.2 uF capacitor and put AC out to a wire antenna. Don't now for sure if that is enough or how to test it. Any Help would be appreciated.

I notice in many antenna circuits on the interweb, that there is an inductor between the antenna and the ground. Since I already have an inductor connected to ground (the 13T Magnetic Induction Coil), would it be possible to extend an Arial Antenna off the point where the 2.2 uf capacitor and Inductor connect together. How can I verify the signal frequency and power??? Is there some king of RF meter that I can buy???