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In the moment I am just completing my 3rd SSTC. It is full bridge of IRFP460 driven by the 74hc14. The GDT is driven by a discrete driver (because I can't find hire in Bulgaria the UCCs), So here is the waveform on the gate of the FET (all 4 MOSFETs are connected to the GDT, and the resistor is 4.7 Ohms ). The rise and fall time is something like 150nS, but i am concerned about the ringing. Will it be a problem ? . The scope is set to 0.5micro second and 10v divider.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
If peak voltage doesn't exceed the maximum gate voltage, it shouldn't be problem.
You can decrease ringing with bigger gate resistor reistance, but it will increase rise/fall times.
The another way, and better, to minimise it, is to make GDT better (wires twisted in nice way, good quality of GDT core, wires' insulation, shorter lead wires etc.)
For more details, look at Richie's pages: , and
By the way, would you give a scheme? I'm building a discrete driver, too :)
Registered Member #1025
Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566
I solved similar problems with ringing by using two bidirect TVS (20V 5W) in parallel and feeding the gate by more than 30V. The TVS get quite hot, but it cuts-off the signal very nicely thus no more weird shapes and everything works fine for me…
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
TVS can often have as much capacitance as the MOSFET gate did. They get rid of the ringing, but the capacitance slows down the gate drive even more, and makes your gate driver get even hotter. I've always preferred a pair of ordinary zener diodes back to back.
I have been trying lots of different things with the GDT. The leads are very short (not more than 20mm in the worst case (to the mosfets) and the one to the driver is about 50mm long). The wires are very tightly twisted together. I use CAT5 cable. Also if i remove the gate resistors the ringing gets enormous - the whole plateau become like a sinusoid. Also i have experimented with the number of turns and now i use about 8. I don't have any idea why it looks so bad. The waveform at the output of the driver looks very good, no crappy waveforms. The only thing left is to change the ferrite core .
Here how it looks the signal of the whole thing (this is the 555 startup oscillator for Steve's SSTC). Why on the down side there is no such ringing. Can this be caused by the non 50% duty cycle ??? ( I use DC blocking cap on the primary side of the GDT and changing it's value doesn't help. )
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
wrote ...
I have been trying lots of different things with the GDT. The leads are very short (not more than 20mm in the worst case (to the mosfets) and the one to the driver is about 50mm long). The wires are very tightly twisted together. I use CAT5 cable. Also if i remove the gate resistors the ringing gets enormous - the whole plateau become like a sinusoid. Also i have experimented with the number of turns and now i use about 8. I don't have any idea why it looks so bad. The waveform at the output of the driver looks very good, no crappy waveforms. The only thing left is to change the ferrite core .
It's not bad as you think, heck, it's a quite good waveform and you're close to target. If those ringing don't exceed gate maximum voltage, you can even let it. Did you try with increasing resistance of gate resistors? It's a method I do use, when I can't get rid of those ringing in other way than tuning GDT.
wrote ...
Why on the down side there is no such ringing. Can this be caused by the non 50% duty cycle ??? ( I use DC blocking cap on the primary side of the GDT and changing it's value doesn't help. )
I don't know how to explain in scientific way, but when rise/fall times are larger, ringings are smaller - there's corellation between them. Fall time is larger than rise, and ringings are smaller, as you can see on waveform. From my experience, duty cycle doesn't change much ringings (changing duty cycle changes frequency and it would cause changes with ringing, but it hard to see visible differents.)
EDIT: And please read Richie's notes - they're easy to understand and contain great informations! :)
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Waveform looks good, although overshoot is a bit too high (almost 40V from the way i look at it)
Actually, i'd just scale down the voltage using less turns on the secondary of your gate transformer. Overshoot isn't a problem as long as it doesn't exceed max ratings of device, and any ringing doesn't go less than the threshold voltage of the MOSFET. At steady state, the waveform is almost 30V. You can bring total voltage down to say 15-20V, and overshoot would be about 30V and you'll be fine.
Hi guys for the gate voltage it is clamped by 18V zeners and they don't see to be conducting so everything is allright then . Also I've tryed the whole coil and at 30V input it gave about 2sm of sparks but I am using 74LS14 in the moment (waiting to receive the HC-version) so when i pull out an arc it go out of resonans. At 30v the mosfets aren't even heating up (no heatsinks for now )
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Question about gate waveform
. The scope is set to 0.5micro second and 10v divider.
. 
. Also I've tryed the whole coil and at 30V input it gave about 2sm of sparks but I am using 74LS14 in the moment (waiting to receive the HC-version) so when i pull out an arc it go out of resonans. At 30v the mosfets aren't even heating up (no heatsinks for now )