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Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
Yeah, with 300VDC 2A it gets hot, but I am using no coolers on the heatsinks, and the heatsink are too small (Using some PSU Heatsinks for each 2 mosfets) I will try to buy a good heatsink (or find one here) and try to modulate audio. Other thing I need to do is to solve the problem with the interrupter, that stops to work when coil is on.
Yesterday I putted a heatsink on top of the coil and I get impressive big sparks (Running at 150VDC 2,5A) After sometime the bridge blew one mosfet ~~ I will buy some more mosfets for try to modulate audio, that IRF740 is cheap :D
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Your coupling looks way too low - use a much wider form for primary coil (to prevent flashover between primary and secondary), so that primary coil is at least like 2 cm apart from secondary. Put the primary between 1/2 and 1/3 of secondary length.
Use these calculators
To determine the reactance of your primary without secondary present. Divide the supply voltage to get the lowest value of magnetizing current - for IRF740 I wouldn't go over 4 amps.
Also, try running with unfiltered or just slightly filtered input. This will make your spark longer, reduce power consumption and increase your power factor a lot (that way you will be able to tell the actual real power going into the coil by measuring input current).
You should get at least 20cm sparks from mains with such setup.
If you are buying heatsinks, absolutely buy 4 and use them without sil pads. This is critical for reducing temperature of the mosfet's. It would also be proffered to clamp the mosfets instead of using a single bolt, although it will probably be just OK if not too much torque is used.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Mestre Racerxdl wrote ...
Are you sure that can be 20cm from this 8cm coil? Here is a complete photo from my coil:
Yeah, that coil is tiny indeed - wouldn't be surprised if your output was ultimately limited by overheating of that secondary rather than power devices. If I see right it is wound on PVC pipe, and thus it's temperature should not exceed more than about 80degrees C. That's why I would always suggest using cardboard tubes for secondary forms for CW coils.
IRF740 should do well at 1.5 Mhz. At about half that frequency I observed no significant switching loss with IRFP450's - the smaller mosfets are even faster and all they need to have is enough magnetizing current and deadtime to ZVS properly.
You can check what your inverter can do by feeding the input by a signal generator at 1.5Mhz, putting your primary coil on the output and loading with light bulbs. This should give more or less realistic load.
Do not attempt to drive light bulbs at so high frequency without any commutating inductance, as this will cause very high losses and possibly death of the mosfets.
Make sure your gate drive is OK (GDT's can be troublesome at over 1Mhz, wind them with shielded cables instead of twisted wires), and use reverse diodes across gate resistors to allow for some more deadtime.
And I found a bunch set of 100 small heatsinks for TO-220, I dont know if I can use it one for each mosfet (And the four with a 8x8 cooler in the top)
My primary coil have 4cm diameter 7 turns, but its not turn by turn side by side, its one over other turn.
Those heatsinks look rather small, maybe you could use two of them per mosfet (back to back with thermal grease in between), and give them lots of air flow.
For mosfets, apply the grease in thin uniform layer and do not screw them down too tightly.
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
My GDT is working very fine, that is the first GDT that I have winded with a PSU Transformer core with copperlayer grounded arround it. In this week I will buy a 10Mhz Scope to get more accurate the waveform of gate drive and for my other sstc's too.
About the heating on the secondary, yeah, I noticed some heat on the secondary. When I turn off it was a little bit hot, but the plastic is not PVC, its CPVC (For High Temperature Water Pipes) its not much more heat capacity but it will handle better.
I have too big problems with noise at my stuff here, I think its because the bottom of the secondary is grounded at - of the supply ( I dont have mains ground yet), I think its better to I remove the secondary from where is it, and make it bipolar (I have half centimeter from the borders, so I can do a support to stand the secondary.
About the mosfets, its dead here, and I need to buy some more mosfets, its better to I stay using IRF740 or its other better mosfet for that use?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Cool coil and good job getting it to work at 1.5MHz. What kind of driver are you using, just a fixed frequency oscilliator?
About mosfets buy the best you can afford. (should maybe do that after the testing period.) Look for low on resistance, short delays and low gate charge. IRF740s will be good for further testing.
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MicroSSTC - 1.5Mhz
