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Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I got my coil to work at full power today with no mains isolation. It worked for about 30 seconds, then the mosfet exploded. I assume it overheated because i see a melted insulator. I have a mosfet with less than half the on state resistance than this one on the way. My coil appears to be in tune. I may try to limit my power level tho by adding primary turns and decreasing my shunt capacitor value.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Nice setup, and lively sparks too!
That MOSFET looks very small for a Class-E amplifier running this amount of power though. Class-E is typically about 90% efficient so a 400W CW coil will still dissipate about 40W in the MOSFET. Most of this is dissipated as conduction losses because of the ZVS/ZCS behaviour of Class-E switching amplifiers. 40W of heat is far too much to dissipate in a small TO-220 device, particularly if it is not clamped down directly onto a large metal heatsink. I wouldn't try to dissipate more than 15-20W through a mica-washer using a standard TO-220 device. You might also consider forced air cooling if you want to keep the heatsink volume small.
Since your operating frequency is quite low, you could change the MOSFET for a larger die device which will have lower Rds(on). Since most losses are conduction losses, it makes sense to use the biggest MOSFET that you can still drive effectively at your operating frequency.
If you want to decrease the power throughput then increase the number of primary turns, decrease the d-s shunt capacitance, decrease the pri-sec coupling and re-adjust the drive to the new operating frequency.
Another observation is that the green-blue iron-powder radio-frequency choke is likely to overheat quickly. Type-52 material is really only suitable for low frequency opeation up to a few hundred kHz and with a small flux-swing. It is really intended for things like buck chokes in SMPSUs. You might get away with it at 1.5MHz for short runs, but longer runs or higher frequency are likely to make that green-blue toroid turn crispy brown quite quickly! Type-2 material (red/clear) is the material of choice for Radio Frequency Chokes in HF Class-E amplifiers. Suitable toroids are often for sale on Ebay for radio amateur use. (ebay items: 390047924056 and 390037077195)
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3mhz Class-E coil try 2
