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Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
Not to toot my own horn or anything, but I built Steve's mini SSTC just a few weeks ago, so you can see the construction part of the way down.
Be very weary of is poking around on the high side with the oscilloscope during operation - *don't do it* unless you know what you are doing. Run the TC off of a low voltage DC source first, and keep the ground lead away since you can't connect anything to ground on the high side.
I've also learned that once you modify the diameter and height of the secondary, you will have to recalculate the primary windings to keep things happy. Start with a higher number of windings to keep the primary current down, and the MOSFET's from dying, use a benchtop PSU or a variac when running off of mains and monitor the current! (DC ammeter, or AC clamp meter). I killed a ton of fet's cause I followed the original instructions of 4-5 turns, but with 10+ the FET's were able to take full continual power just fine.
One last thing - I omitted the protection diodes on my halfbridge, sometimes you don't need them, but you can experiment. One thing you can't forget are the freewheeling diodes that run parallel to the MOSFET's. It is common place to put zener diodes and TVS (just 1500W) across the gate and source, so you could try that if you keep frying the gates on your MOSFETs.
PS: Nicko - awesome variac setup you have there, breakers on main, input and output of the variac all mounted down, must have cost alot.
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
aonomus wrote ...
...PS: Nicko - awesome variac setup you have there, breakers on main, input and output of the variac all mounted down, must have cost alot.
Not too much - I look upon it as an investment in my future well-being About USD 150 - the Hagar box was on offer if you bought the bits for it at the same time - the RCBO was about 15 bucks and the MCBs about 5 bucks each. The LCD volmeter was about 6 bucks from eBay (but is easily disturbed by c**p from the coil, so the coil needs its own filter otherwise it reads 900VAC ! ). The 15A 230VAC Variac was another eBay special for about 80 bucks, and the 16A commando & mains sockets were about another 20 bucks... There is a really nice big 30A TDK line filter that I got but had not arrived when I took the photo - it was about 20 bucks and lives next to the input socket. The bit of board was lying in my scrap pile in the shop...
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Nicko wrote ... Unsolder & check the outputs of the GDT - they should be in anti-phase with a good square-wave shape and about 10V peak to peak with not too much ringing or roll-off.
Just do NOT unsolder it - the FETs make the GDT signal way-way worse ;) Also 10v is just too low, 12 for higher current direct drive and 15 for GDT is a good decision.
Nicko wrote ... The signals on the gates of the FETs MUST be in anti-phase (thet's why you need a 2 channel 'scope)
Actually they will if you have connected the GDT properly so there's no reason to check - but if you want to, you can check it even with a single trace by rigging the scope gate-to-gate, you should see a nice 24-30v square.
Nicko wrote ... Remember, white plastic throughout, no black cable ties or pipe. Secondary was 14" on a 2.5" former.
No reason actually - the coal particles causing arcovers is a myth. As a proof, check my coil - it is just spraypainted with back paint (surely a lot of coal).
Nicko wrote ... DO CHECK THAT THE FETs & DIODES ARE INSULATED FROM THE HEATSINK! If they are shorting to each other, they will die.
Bad idea - better just rig fets on separate heatsinks - way better from any point.
Registered Member #989
Joined: Sat Sept 08 2007, 02:15AM
Location: São Paulo, Brazil
Posts: 476
The Ideal voltage to Mosfet gets is 15V, so, use 15V clamping zeners at gate and get the GDT Voltage Output more than 15V. The Gate Drivers should be supplyed with regulated 15V, the 1:1:1 GDT will do peaks about 20V at gates, so you will have desired voltage.
Registered Member #1632
Joined: Mon Aug 11 2008, 08:53PM
Location: Plainfield, IL
Posts: 12
WOW a lot happened while I was sleeping! Thanks for the information guys, there is definitely a wealth of it in this thread now! I remember seeing some ringing on the output of my GDT, it was kinda strange, but I racked it up to the fact that i picked this 100Mhz scope out of the trash!! The poor scope has seen better days and strange things happen when I turn knobs, so I thought the ringing I saw may be internal to the scope or the fact that there is no option to terminate with 50 ohms, just 100Mohms. I guess I should have realized that when I used the scope on the logic signals there was no ringing and the square waves looked very nice!! I think that is where I will focus the first part of my search since I do think there was some problem with it. I had some ferrite cores laying around, which I thought were good, but perhaps they are indeed junk! I replaced all damaged components last night except the damaged fast diodes across the MOSFETs. Need to order them. I'll let you know what happens, and i'll take a couple pics to share too.
Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
LithiumLord wrote ...
Nicko wrote ... Unsolder & check the outputs of the GDT - they should be in anti-phase with a good square-wave shape and about 10V peak to peak with not too much ringing or roll-off.
Just do NOT unsolder it - the FETs make the GDT signal way-way worse ;) Also 10v is just too low, 12 for higher current direct drive and 15 for GDT is a good decision.
My bad on that - should have been 2 x the driver voltage, i.e. peak-to-peak of about 24V in the miniSSTC. Indeed, leaving the GDT soldered is a good idea to check the GDT when loaded - this will degrade the waveform - do this with no HV... however, its a good sanity check to look at the unloaded GDT too.
LithiumLord wrote ...
Nicko wrote ... The signals on the gates of the FETs MUST be in anti-phase (thet's why you need a 2 channel 'scope)
Actually they will if you have connected the GDT properly so there's no reason to check - but if you want to, you can check it even with a single trace by rigging the scope gate-to-gate, you should see a nice 24-30v square.
If you've connectect everything correctly, obviously there's no need to check the GDT, however the OP has problems, so checking is always a good idea, and its cheaper than more FETs...
LithiumLord wrote ...
Nicko wrote ... Remember, white plastic throughout, no black cable ties or pipe. Secondary was 14" on a 2.5" former.
No reason actually - the coal particles causing arcovers is a myth. As a proof, check my coil - it is just spraypainted with back paint (surely a lot of coal).
Not all black is carbonaceous, e.g. aniline black which is used a lot in matt black paint.
LithiumLord wrote ...
Nicko wrote ... DO CHECK THAT THE FETs & DIODES ARE INSULATED FROM THE HEATSINK! If they are shorting to each other, they will die.
Bad idea - better just rig fets on separate heatsinks - way better from any point.
Very odd comment, that. Bad idea to check? No. A lot of people will mount all their devices on a single heatsink for this small SSTC, so if there is more than one device on a sink its plain common sense to check that they are isolated from each other. With bigger SSTCs with higher drive voltages, other risks come into play, such as breakdown of the thermal insulating pads, so that would be a good argument for individual headsinks but not here...
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Mestre Racerxdl wrote ...
The Ideal voltage to Mosfet gets is 15V, so, use 15V clamping zeners at gate and get the GDT Voltage Output more than 15V. The Gate Drivers should be supplyed with regulated 15V, the 1:1:1 GDT will do peaks about 20V at gates, so you will have desired voltage.
Naah! First, you should set up 18v clamps as 18v is still way below the maximum gate voltage, but you'll not get any accident currents through them in a normal operation. As for GDT, I'd advice to use a slightly step-up one to feed the drivers with generic 12v (as far as most use UCCs, it's not a good idea to feed them with 15v anyway).
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Steve Ward's Mini SSTC problem
About USD 150 - the Hagar box was on offer if you bought the bits for it at the same time - the RCBO was about 15 bucks and the MCBs about 5 bucks each. The LCD volmeter was about 6 bucks from eBay (but is easily disturbed by c**p from the coil, so the coil needs its own filter otherwise it reads 900VAC ! ). The 15A 230VAC Variac was another eBay special for about 80 bucks, and the 16A commando & mains sockets were about another 20 bucks... There is a really nice big 30A TDK line filter that I got but had not arrived when I took the photo - it was about 20 bucks and lives next to the input socket. The bit of board was lying in my scrap pile in the shop...
