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Registered Member #883
Joined: Sat Jul 07 2007, 01:02PM
Location:
Posts: 16
I was just wondering why almost all sstcs and drsstcs use GDTs instead of optoisolators. There has to be a good reason, but i cant come up with it on my own.
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Heh, dunno - I use optos :) Actually, they have some limit for DRSSTCs that are to be fed via a voltage doubler, but for off-line or alike (I use 240vac vs off-line 220 ones) they are pretty good - the rise/fall times are low and there seems to be less ringing, also optos are insensitive to high-range frequency float and allow you to drop gate voltage to a negative value immediately on the turn-off as they don't limit the waveform to a symmetric structure.
Registered Member #1535
Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117
I believe, from what I've read, that GDT's go negative output instead of just off. This will make the transistor turn off harder/faster... I suppose a cluster of opto's could accomplish the same task... I have boatloads of opto's, but would certainly go for a GDT based driver when I choose to go solid state...
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
TheBoozer wrote ...
I believe, from what I've read, that GDT's go negative output instead of just off. This will make the transistor turn off harder/faster... I suppose a cluster of opto's could accomplish the same task... I have boatloads of opto's, but would certainly go for a GDT based driver when I choose to go solid state...
But the GDT still has a long switch-off slope as the negative side is only achieved after the deadtime (if any). For the optos I use a -8v negative bias, so whenever a FET has to close it comes down to this voltage as soon as it can. Also there is no reason to prolong the turn-on slope as the deadtime is sufficient to exclude any shoot-through, so optocoupled drivers, if properly designed, can highly reduce the switching losses.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
But the GDT still has a long switch-off slope as the negative side is only achieved after the deadtime (if any).
Not true. The same current that is driving the other switch ON, is present to drive the switch OFF, even if its a "unipolar" drive temporarily because of some dead time insertion on the driver side.
GDTs absolutely rule if you happen to be using a symmetric drive scheme. I have also worked on GDT techniques that can drive the switch with an asymmetric waveform and maintain proper bias, but it requires some extra parts and adds some propagation delay.
The main draw back to optos is their fairly big delay times, often in the uS range. Ive used some expensive optocouplers in a class-D amplifier that had 60nS rise times. Now, though, i use some new isolators by TI (look up ISO721). They have only 10nS propagation delay, really pretty awesome parts.
But i still say GDTs are the best method for driving tesla coil bridges... even for big CM300s. HVGUY uses GDTs to drive CM1000s.
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Steve Ward wrote ... Not true. The same current that is driving the other switch ON, is present to drive the switch OFF, even if its a "unipolar" drive temporarily because of some dead time insertion on the driver side.
Well, not exactly what I meant :) No matter how rectangular the signal looks, it's just a set of exponential waveforms. Therefore, if you have a deadtime injected into the signal, it looks like an exp dropping from the peak voltage to zero, and only then, after the deadtime, the negative side is achieved, only working as a Miller peak clamp. However, if your driver applies the negative voltage to your gate resistor immediately, the exponent drops to the level of switch's on-threshold faster, slightly reducing the switching loss. Alike things are completely unnoticable in DRSSTCs where the ZCS is achieved, but in generic linear SSTCs this may reduce the heating a bit.
Steve Ward wrote ... The main draw back to optos is their fairly big delay times, often in the uS range. Ive used some expensive optocouplers in a class-D amplifier that had 60nS rise times. Now, though, i use some new isolators by TI (look up ISO721). They have only 10nS propagation delay, really pretty awesome parts.
Most highspeed optos still manage to achieve delays below 30ns still having a high enough CMR.
Steve Ward wrote ... But i still say GDTs are the best method for driving tesla coil bridges... even for big CM300s. HVGUY uses GDTs to drive CM1000s.
Heh, I wouldn't dare driving a 620vdc bridge with optos - but for small coils it's a matter of own preference in fact. In small constructs, the optocoupled drivers can outrun GDT-based ones in size - an 8A driver (I overloaded them a bit so they start at 8.5A) is a tiny board, while a 32amp (without taking the mag current in account) device to drive the same bridge via a GDT may get tricky. However yes, that's more of a trick to push a bridge to a higher limit then an optimal drive technique.
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
Any 10MBit/s coupler should go with overall delay way below 100ns, try looking at HCPL26xx or even 6n137 (this one highly varies for different distributors however).
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optoisolators vs GDTs
