I am working on a circuit, using a CLASS E Power Amp for Magnetic Induction Heater. The circuit works... for 3 or 4 minutes. Then the MOSFET fails (Fails "ON" which is a short, then sometimes catches fire). Here is the schematic for my circuit:

http://i1351.photobucket.com/albums/p784/plasmahunt3r/DC-Restored-Gate_zpsdbadf12b.png

The Power supply is built from a rewired microwave oven transformer. I produce 13.2vdc regulated (to the breadboard) and 22vdc unregulated to the Choke coil and Class E Amp.

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 fit 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...-Induction-Test2_zpsce631b1d.jpg?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 tried 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.

http://i1351.photobucket.com/albums/p784/plasmahunt3r/DC-Restored-Gate_zpsdbadf12b.png

The Power supply is built from a rewired microwave oven transformer. I produce 13.2vdc regulated (to the breadboard) and 22vdc unregulated to the Choke coil and Class E Amp.

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 fit 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...-Induction-Test2_zpsce631b1d.jpg?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 tried 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???**
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